4

For help in preparation of this article, thanKehrer, NLfB-KTB, Hannover, Germany; HJochen Kück, Manfred Neuber, Thomas RöSowa and Klaus Sturmeit, Windischeschenmany; Karl Bohn and Ignaz de Grefte, SchWireline & Testing, Windischeschenbach,Howard, Schlumberger Wireline & TestingTexas, USA; Jörn Lauterjung, GFZ InstituteGermany; N. Riquier and Alain Poizat, SchWireline & Testing, Abbeville, France.In this article CSU (wellsite surface instrumFMI (Fullbore Formation MicroImager), FoMicroScanner, GLT (Geochemical LoggingLitho-Density, NGS (Natural Gamma Ray and Sidewall CoreDriller are marks of SchVAX is a mark of Digital Equipment Corpo

The drill bit has stopped turning and the KTB project is winding down.

complete. How and why was it drilled?

ieved so far?

Kurt BramJohann DraxlerGottfried HirschmannGustav ZothNLfB-KTBHannover, Germany

Stephane HironClamart, France

Miel KührWindischeschenbach, Germany

The KTB Borehole—Germany’s Superdeep Telescope into the Earth’s Crust

essnd total ofin- asck

idengicsndngns

specifically for scientific research only withinthe last thirty years.

The internationally funded Ocean DrillingProgram (ODP) was started as part of aworldwide effort to research the hard outerlayer of the Earth’s crust called the litho-sphere.1 Results from this project have beendramatic, providing real evidence of conti-nental drift and plate tectonics. The litho-sphere is made up of six major and severalminor rigid moving plates. New oceaniccrust is formed and spreads out at mid-oceanridges and is consumed at active plate mar-gins—subduction zones—where it sinks

annover

SCIENTIFIC DRILLING

ks to Peterorst Gatto,ckel, Michaelbach, Ger-lumberger Germany; Pete, Houston,, Potsdam,lumberger

entation), rmation Tool),

Spectrometry)lumberger.ration.

Germany’s superdeep borehole is

And what have the scientists ach

Thermal gradients, heat production, strfields, fluid transport, deep seismics adeep resistivity are all of great interestearth scientists. Studying these fundamentopics helps them unravel the mysteriesweather fluctuations, the distribution of meral resources, and natural disasters suchearthquakes, volcanoes and floods. Rooutcrops, river gorges and cliff faces provvisual evidence to interpret deep probimeasurements such as seismics, magnetand gravimetrics. Commercial mining adrilling have also guided scientists, givitangible connections to surface observatio

Hamburg H

Oilfield Review

(above). However, drilling has been used back into the Earth’s mantle. This processtakes up to a few hundred million years.

Continents are different. They are made oflighter rock and are not easily recycled,

5

■■The lithospherebeneath Germany.Without superdeepboreholes earthscientists rely onsurface observa-tions to paint a pic-ture of the structureof the lithosphere.

1. Anderson RN, Jarrard R, Pezard P, Williams C andDove R: “Logging for Science,” The Technical Review36, no. 4 (October 1988): 4-11.The lithosphere forms the hard outer layer of the earth.The rocks below are hot enough to deform freely andcomprise the asthenosphere. The lithosphere slidesover the asthenosphere with little resistance. As therocks of the lithosphere cool, they thermally contract,increasing their density. At some stage the lithospherebecomes denser than the asthenosphere and starts tobend and sink back into the Earth’s interior. A subduc-tion zone occurs where the descending lithospheremeets an adjacent overlying lithosphere.

2. Turcotte DL and Schubert G: Geodynamics: Applica-tions of Continuum Physics to Geologic Problems.New York, New York, USA: John Wiley & Sons, 1982.

allowing them to achieve ages of 4 billionyears.2 They also provide the vast majorityof the world’s resources, so it is vital tounderstand their structure and development.One way of doing this is to extend the workstarted by ODP to the continent. KTB—which stands for Kontinentales Tiefbohrpro-gramm der Bundesrepublik Deutschland, orGerman Continental Deep Drilling Pro-gram—is drilling one of a handful of bore-holes specifically for continental scientificresearch. This article looks at the majordrilling achievements of KTB, at the Schlum-berger wireline logging contribution and atsome of the main areas of research.

The project was initiated in 1978 by aworking group of the German GeoscientificCommission of the German Science Foun-dation. The group discussed more than 40

possible drillsites in Germany, eliminatingall but those with the broadest possibleresearch potential. Two sites were chosenfor further studies: Haslach in the Black For-est region of South Germany and Windis-cheschenbach 80 km [50 miles] east ofNürnberg in Bavaria, southeast Germany. In1985, the Federal Ministry for Research andTechnology gave the final approval for theKTB deep drilling program and both siteswere comprehensively surveyed.

Both geology and the expectation of alower formation temperature gradientfavored the Windischeschenbach site. Thesite is located on the western flank of theBohemian Massif about 4 km [2.5 miles]

Frankfurt KTB Munich

east of a major fault system—the Franconianline (left).3 Scientists also believe it lies atthe boundary of two major tectono-strati-graphic units in Central Europe—theSaxothuringian and Moldanubian.4 Thisboundary—which they hoped to cross—isregarded as a suture zone formed by theclosure of a former oceanic basin 320 mil-lion years ago. This process gave rise to acontinent-continent collision—forming amountain chain and the present-dayEurasian plate. The mountains have longsince eroded away, exposing rocks that wereonce deeply buried. Therefore, this area isideal for the study of deep-seated crustalprocesses. In addition, geophysical surfaceexperiments have shown that the areaaround the drillsite has unusually high elec-trical conductivity and strong gravimetricand magnetic anomalies, which deservecloser investigation.

The scientific challenges for the KTB pro-ject all contribute towards understandingthe fundamental processes that occur incontinental crust. Among these are earth-quake activities and the formation of oredeposits. The primary objectives, therefore,were to gather basic data about the geo-physical structure below the KTB site, suchas the magnitude and direction of stresses,so that the evolution of the continental crustmight be modeled. Information about ther-mal structure—temperature distribution,heat sources and heat flow—was alsoneeded to understand chemical processessuch as the transformation to metamorphicrock and the mineralization of ores. Fluidsalso play an important role in temperaturedistribution, heat flow and the variouschemical processes, so measurements ofpressure, permeability and recovery of fluidsfound were also important.

The overriding goal of the KTB projectwas to provide scientists with a permanent,accessible, very deep hole for research.With a budget of 498 million DeutscheMarks [$319 million]—provided by the Ger-man government—the initial target was todrill until temperature reached about 300°C[572 °F]—expected at a depth of 10,000 mto 12,000 m [32,800 ft to 39,370 ft]—theestimated limit of borehole technology. Thisincludes drilling hardware, drilling fluidchemistry, cementing as well as the down-hole instrumentation required for the vari-ous scientific experiments. Many technicalspin-offs developed from the project.

6 Oilfield Review

■■Location of KTB wellsite. The site is located on the western flank of the Bohemian Mas-sif about 4 km east of the Franconian line. The local surface geology is shown alongwith the three-dimensional (3D) surface seismic time slice beneath the KTB site. Thiswas generated from data recorded after drilling the pilot hole (KTB-VB). Labeled aremajor seismic reflectors—SE1 corresponds to a major fault zone associated with theFranconian line. This was crossed by the superdeep hole (KTB-HB) at the interval 6850to 7300 m [22,575 to 23,950 ft]. The inset shows Variscan basement outcrops in MiddleEurope, including the Saxothuringian and Moldanubian units, and the position of theKTB site in the southeast corner of Germany close to the Czechoslovakian border.

Basalt

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ZEV gneisses

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Bohemian Massif

Drilling the Vorbohrung—Pilot HoleIt is not common to drill through surficialcrystalline rock especially when the drillingconditions are unknown. Kola SG 3, on theKola peninsula near Murmansk, Russia, isone exception. It is the world’s deepest bore-hole, but not an ideal role model (right).After 15 years of drilling, at an untold cost,the borehole reached a depth of 12,066 m[39,587 ft]. Years later it was deepened to12,260 m [40,200 ft].

The project management team, havingstudied the Russian project, decided to firstdrill a pilot hole—KTB Vorbohrung (KTB-VB).5 This was spudded on September 22,1987. The objectives for the pilot hole wereas follows:•Acquire a maximum of geoscientific data,

from coring and logging the entire bore-hole, at low cost and minimum risk beforecommitting to an expensive heavy rig andsuperdeep hole.

•Minimize core runs and logging in thelarge-diameter, straight vertical upper sec-tion of the superdeep hole.

•Analyze the temperature profile for plan-ning the superdeep hole.

•Obtain data about problem sections withinflow or lost circulation, wellbore insta-bilities and breakouts.

•Test drilling techniques and logging toolsin preparation for the superdeep hole.

To accomplish these objectives, a newdrilling technique was developed that com-bined rotary drilling and sandline coreretrieval techniques (right). A modified landrig used a high-speed topdrive to rotateinternal and external flush-jointed 51/2-in.outside diameter mining drillstring in a 6-in.borehole. This drillstring provided enoughclearance inside to allow 4-in. cores to becut and pulled up to surface through thedrillpipe by sandline—eliminating roundtrips to recover cores. A solids-free, highly

7January 1995

■■Depths of histori-cally importantboreholes in Ger-many and aroundthe world.

■■Sandline coring technique. Four-in. cores are cut with high-perfor-mance, impregnated diamond core bits (1). As the cores are cut,they slide inside the core catcher barrel (2 and 3). When coring iscomplete, the drillstring is picked up off bottom to break free thecore (4). A rig line and latching tool are run through the drillstringand latched onto the core barrel. The barrel is pulled out-of-holethrough the internally flush mining drillstring (5). A new core barrelis run back into the drillstring and landed above the core bit to con-tinue coring.

3. Massif is a block of the Earth’s crust bounded by faults or flexures and displaced as a unit without internal change.

4. A tectono-stratigraphic unit is a mixture of lithostrati-graphic units resulting from tectonic deformation. TheSaxothurigian is a low pressure-high temperature unitstill showing sedimentary structures. The Moldanu-bian is a low pressure-high temperature unit withrelics of two older phases of higher pressure. The sedi-mentary structures have almost disappeared.

5. Rischmüller H: “Example for Advanced Drilling Technology,” Oil Gas 16, no. 4 (1990): 16-20.

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KTB Logging ToolsAuxiliary Measurement Sonde1

Borehole Geometry Tool1

Colliding Detonation Drill Collar Cutter2

Fluid Sampler

Formation MicroScanner*1

Gamma Ray1

Self Potential

Single Shot

Temperature Tool

6-Arm Caliper

* Mark of Schlumberger1 Bought from or developed by Schlumberger for KTB2 Developed by SwETech (Bofors) for KTB

lubricating mud system had to be used,because of the small clearance between theflush external surface of the drillstring andthe borehole wall. This coring methodworked well until February 1989 whenexcessive corrosion in the pipe jointsrequired replacing the mining string withconventional 31/2-in. externally upsetdrillpipe and core barrels.

Coring operations had to be interruptedon other occasions—three times for direc-tional drilling to bring the hole back to verti-cal and twice for sidetracking, because oflost bottomhole assemblies after unsuccess-ful fishing. However, a total depth (TD) of4000 m [13,124 ft] was reached for KTB-VBon April 4, 1989, after 560 days of drillingand logging. More importantly, 3594 m[11,790 ft] of cores were recovered—arecovery rate comparable to those achievedworldwide in easier formations—and thehole was extensively logged with many dif-ferent instruments (left and next page).

The drilling experience in KTB-VB provedinvaluable to the planners of the superdeepborehole. For example, they encounteredareas of borehole instability across faultzones; they had to modify the mud systemto account for water influx and water-sensi-tive rock; they had numerous breakoutscaused by the relaxation of stressed rock;and the formation dipped more steeply thanpredicted making it difficult to keep the holeanywhere near vertical. In total, the pilothole presented a greater challenge fordrillers than expected.

For the next year, many experiments andmeasurements—such as hydrofracs, produc-tion tests and extensive seismic work—were carried out in and around KTB-VB. InApril 1990, the hole was finally cased andcemented. Meanwhile, plans continued forconstruction of a new rig to drill the super-deep borehole about 200 m [656 ft] away.

■■ Logging tools used by KTB. The loggingtools listed in the tables were run in thepilot hole (KTB-VB) and the superdeephole (KTB-HB) and were provided by vari-ous logging companies, universities andinstitutes. KTB bought several tools, someof which were developed for the project.

Oilfield Review

Schlumberger Logging Tools

Tool

Acoustic TeleScanner* ✓

✓

✓

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AIT* Array Induction Imager Tool

ARI* Azimuthal Resistivity Imager

Array-Sonic*

Back Off

Casing Collar Locator

Cement Bond Log

CERT* Correlated Electromagnetic Retrieval Tool

CET* Cement Evaluation Tool

CNL* Compensated Neutron Log Tool

DIL* Dual Induction Resistivity Log tool

DLL* Dual Laterolog Resistivity tool

DSI* Dipole Shear Sonic Imager

Eccentralized Dual Axis Caliper

FGT* Formation Gamma-Gamma Tool

FMI* Fullbore Formation MicroImager

Formation MicroScanner*

FPIT* Free Point Indicator Tool

GCT* Guidance Continuous Tool

Germanium Detector Tool

GLT* Geochemical Logging Tool

GPIT* inclinometry tool

High Resolution Temperature

HLDT* Hostile Litho-Density Tool

Litho-Density*

MicroSFL* tool

Multifinger Caliper Tool

NGS* Natural Gamma Ray Spectrometry tool

RFT* Repeat Formation Tester

SAT* Seismic Acquisition Tool

Severing Tool

Sidewall CoreDriller*

SLT* Sonic Logging Tool

Spontaneous Potential

Stratigraphic High-Resolution Dipmeter

TDT* Thermal Decay Time

Variable Density*

KTB-VB KTB-HB High-Temp.

9January 1995

University and Institute Logging ToolsUniversity or Institute

Berlin University, Berlin, Germany

Braunschweig University, Braunschweig, Germany

Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Hannover, Germany

Deutsche Montan Technologie (DMT), Bochum, Germany

Eötvös Lorant Geophysical Institute, Budapest, Hungary

Frankfurt University, Frankfurt, Germany

Geostock, Malmaison, France

Göttingen University, Göttingen, Germany

Los Alamos National Laboratory, Los Alamos, New Mexico, USA

Munich University, Munich, Germany

Niedersächsisches Landesamt für Bodenforschung (NLfB), Hannover, Germany

Petrodata, Zürich, Switzerland

Scientific Academy, Swerdlowsk, Russia

Heat Conductivity Logging Sonde

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Fluxgate Magnetometer

3D Magnetometer

Borehole Televiewer

Induced Polarization

Dipole-DipoleRedox Potential

Geophone

Gradient Magnetometer

Geophone

Fluid Sampler

Magnetic Susceptibility

Induced Polarization

Variable Amplitude Logging

Magnetometer

Tool KTB-VB KTB-HB High-Temp.

Lawrence Livermore National Laboratory, Livermore, California, USA

3 Now part of Schlumberger Geco-Prakla

Other Companies’ Logging ToolsCompany

EDCON

Prakla-Seismos3

Leutert

PREUSSAG

Scientific Directional Drilling Company

Western Atlas

Borehole Gravity Meter ✓ ✓

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Geophone Survey

Moving Source Profile

Vertical Seismic Profile

Fluid Sampler

High Temperature Fluid Sampler

Fluid Sampler

Gyroscope

Steering Tool

Gyroscope

Multi Shot

Multiparameter Spectroscopy Instrument - Carbon/Oxygen Tool

Pulsed Neutron Decay Time

Segmented Bond Tool

Z-Density Tool

Tool KTB-VB KTB-HB High Temp.

Drilling the HauptbohrungThe superdeep hole—KTB Hauptbohrung(KTB-HB)—was spudded on October 6, 1990,and reached a TD of 9101 m [29,859 ft] onOctober 21, 1994. To drill to this depth inonly four years required the design and con-struction of the largest land rig in theworld—UTB 1 (left).6 This rig could handle12,000 m of drillpipe that required a maxi-mum hook load of 8000 kN [1,800,000 lbf]—more than three times that of the rig usedto drill KTB-VB. Mechanical wear and tearwas expected to correspond to drilling 30deep conventional wells, with over 600round trips. Reducing trip time called forradical rig design. Using 40-m [130-ft]stands of drillpipe instead of the standard27-m [90-ft] stands saved 30% of the time.However, long drillpipe stands meant the righad to be 83 m [272 ft] high.

To further increase trip efficiency, an auto-mated pipe handling system was installed(right ). This consisted of a 53-m [174-ft]high pipehandler that grasped and liftedstands of drillpipe between the rotary tableand star-shaped fingerboard for stacking inthe derrick. Pipe connections were made byan iron roughneck that replaced traditionalpipe-spinners and make-up tongs. Auto-matic slips gripped the pipe in the rotarytable as connections were made andhydraulically operated elevator clamps didthe derrick man’s job. More time was savedas pipes were connected or disconnected byretracting the traveling block out of the wayas it traveled up or down the derrick. Thisallowed the pipehandler to operate whilethe traveling block was moving.

The entire operation was controlled by adriller, pipehandler operator and two floor-men. Only the floormen worked outside onthe rig floor—the other two sat inside a con-trol room at consoles equipped with videoscreens and gauges (next page, top). Com-puters controlled many of the operations ofthe pipehandler. Using a pipe conveyor tolift single pipes between rig floor and piperacks saved additional time.

Borehole torque and drag as well as thestrength of the drillpipe are decisive factorswhen it comes to reaching great depthsquickly and safely. Torque while drilling andexcessive hook loads when pulling thestring are caused by lateral forces and fric-tion between drillstring and borehole wall.These two factors are increased by theweight of drillstring, borehole inclinationand severity of any doglegs. Borehole trajec-tory influences not only drillstring design,

but also any proposed casing scheme. Aslim-clearance casing scheme cuts down onrock volume drilled, but also requires a nearvertical borehole to minimize friction.7

Without active steering the borehole wouldstart to build angle as was proved in KTB-VB.So KTB commissioned Eastman Christensen—now Baker Hughes Inteq—to develop aself-steering vertical drilling system (VDS)(next page, bottom).8

10 Oilfield Review

■■Pipehandling. Fast round-trip operationwas one of the main specifications for UTB 1. Three mechanical aids helped.First, the 53-m [174-ft] high pipehandlertakes stands of drillpipe and stacks themin the derrick. Second, to allow continu-ous pipehandler operation during roundtrips, the traveling block is retracted outof the way as it moved up or down thederrick. Third, a pipe conveyor picks upand lays down single joints of drillpipebetween rig floor and pipe racks.

■■UTB 1. This is the largest land rig in theworld, capable of drilling to 12,000 m[39,500 ft] and of handling extra-lengthdrillpipe racked in stands of 40 m [130 ft].The rig has many advanced features,meets high environmental standards,including full electric drive, a largedegree of automation and soundproofing.

6. Wohlgemuth L and Chur C: “The KTB Drilling Rig,”Oil Gas 16, no. 4 (1990): 42-46.

7. Sperber A: “The Casing Concept for the Main Well,”Oil Gas 16, no. 4 (1990): 21-23.

8. Chur C, Engeser B and Oppelt J: “ Vertical DrillingConcept for the Main Well,” Oil Gas 16, no. 4 (1990):26-29.

Pipehandler

Pipemanipulatorarm

Traveling blockwith retractorsystem

Pipe conveyor

53meters

19meters

11meters

11January 1995

■■Armchair drilling. The driller, surrounded by instruments and video monitors, controlsdrilling operations from the comfort of a control room overlooking the rig floor. The drilleris joined by the pipehandler operator when pipe is tripped in or out of the borehole.

■■Vertical Drilling Systems (VDS). The VDS systems consist of positive-displacement motors to drive the drillbits, battery-powered inclinometers to measure deviation, and hydraulic systems to steer the drill bit. Anydrift away from vertical is measured by the inclinometer and fed back to the hydraulic system. Two steeringsystems were developed. The first holds extendable ribs against the formation using mud pressure (left). Devi-ation corrections are made by releasing pressure in one rib, causing the whole assembly to move towardsthat rib. The second system is steered internally (center). Pistons move the gimbal-mounted rotating shaft,which is connected to the bit, to make any deviation adjustments. Another variation, VDS-4, reverted back toextendable ribs (right). (Courtesy Baker Hughes Inteq.)

Drilling instrumentation

Driller Pipehandler

DC motor controls

Pump volume andpressure gauges

Drilling parameterssuch as hook weight,weight-on-bit, rotaryspeed and torquePipehandler

Motor drive sub

Internal shockabsorber

Rotating part

Bearing assembly

Top stabilizer

Rotating shaft

Sensors, electronics,valves, batteries

Extendable ribs

Drill bit

VDS-1 / VDS-2

Motor universal joint

Rotating shaft

Motor drive shaft

Sensors, electronics,valves, batteries

Internal steering

Drill bit

Nonrotating housing

VDS-3 / II

Data pulser

Motor section

Top stabilizer

Sensors, electronics,valves, batteries

Extendable ribs onbearing assembly

Drill bit

VDS-4

12

■■Lithology com-parison betweenKTB-VB and KTB-HBand an overview ofdrilling and casingKTB-HB. The differ-ences in lithologybetween the twoboreholes—whichare only 200 mapart—highlightthe complex struc-ture being drilled(left). Drilling diffi-culties requiredcement plugs to be set to correct for deviation andfor sidetracks to be drilled afterunsuccessful fish-ing operations(middle). Extra casing strings hadto be run to protectcrumbling borehole(right).

The VDS system consisted of a positive-displacement motor to drive the drill bit, abattery-powered inclinometer to measuredeviation and a hydraulic system to adjustthe angle of the drill bit to correct for devia-tion. Two hydraulic systems were used: thefirst system operated external stabilizer ribsthat pushed against the borehole wall mov-ing the whole VDS assembly back to thevertical; the second system used internalrams to move the shaft driving the drill bitback to vertical. As long as battery powerwas maintained to the inclinometer, bothsystems operated automatically. Inclination,and other parameters such as temperature,voltage and systems pressure, were trans-mitted to surface by a mud pulser to moni-tor progress.

The first 292 m [958 ft] of KTB-HB weredrilled with a 171/2-in. bit and opened up to28 in. before setting the 241/2-in. casing(previous page). To meet the requirementsof a vertical hole, a 2.5° correction to devia-tion was made as the hole was widened.The next section was drilled with a 171/2-in.VDS system to 3000 m [9840 ft] and com-pleted at the end of May 1991. Teethingproblems with prototype VDS systemsmeant using packed-hole assemblies (PHAs)during maintenance and repair. Even so,average deviation for this section was lessthan 0.5°.

The same strategy was used for the 143/4-in.hole—alternating between the improvingVDS systems and PHAs. A high deviationbuildup from 5519 to 5596 m [18,107 to18,360 ft] during one PHA run led to theborehole being pulled back and a correc-tion made for deviation. The hole contin-ued on course to 6013 m [19,728 ft] where133/8-in. casing was set in April 1992—hor-izontal displacement at this stage was lessthan 10 m [33 ft].

Drilling continued with VDS systems andPHAs and 121/4-in. bits. Within this section45.7 m [150 ft] were cored, including 20.7 m[68 ft] with a newly developed, large-diam-eter coring system that gave 91/4-in. diame-ter cores. However, in July 1992 at 6760 m[22,179 ft], the bit became stuck. Eventually,after an unsuccessful fishing operation, thehole had to be plugged back to 6461 m[21,198 ft] and sidetracked. In March 1993,over an interval of 6850 to 7300 m [22,474to 23,950 ft], a major fault system wascrossed. The VDS system could not controldeviation over this interval and another cor-rection had to be made. This system wasthought to be an extension of the main faultthat lies along the boundary between sedi-

ments to the west and metamorphic rocks tothe east—the Franconian line. Along thisfault system a displacement of more than3000 m occurred, showing a repetition ofdrilled rock sequences. This signaled thestart of the most difficult drilling yet andadditional funds had to be provided by theGerman government to complete the project—bringing the total cost to DM 528 million[$338 million].

At 7490 m [24,573 ft], when the horizon-tal displacement was only 12 m [39 ft], theVDS system was abandoned, as boreholetemperatures became too high for the elec-tronics. The hole then started to deviatenorth (below). Within the main fault systemthe borehole became unstable and morebreakouts occurred. While tripping out-of-hole from 8328 m [27,323 ft], the drillpipebecame stuck at 7523 m [24,682 ft]. Jarringeventually broke the downhole motor hous-ing allowing the pipe to be pulled out butleaving behind a complicated fish. Severalattempts to retrieve the fish failed and the

hole was finally plugged back to the verticalsection—at 7390 m [24,245 ft]—and side-tracked. Drilling again proved difficult andso a 95/8-in. liner was set at 7785 m [25,541ft] in December 1993 to protect this hard-won section of hole.

Difficult drilling continued with a 81/2-in.bit down to 8730 m [28,642 ft]. Boreholeinstability prevented further progress and a75/8-in. liner was set in May 1994. To bypassthe unstable section, a sidetrack was made at8625 m [28,297 ft] through a precut windowin the liner. Funds to continue drilling werenow running low and a decision was madeto stop 476 m [1561 ft] later on October 12,1994. More than four years after spudding,the hole had reached 9101 m with a final bitsize of 61/2 in. However, the borehole hadnot finished with the drillers yet. Attempts tolower logging tools into the open hole failed.The last section had to be re-drilled and a51/2-in. liner set, leaving only 70 m [230 ft]of open hole for the wireline loggers andother scientific experiments.

13January 1995

■■Superdeep borehole projection. The main graph shows the horizontal projection of thefinal three openhole sections drilled to complete KTB-HB—inset are the vertical projec-tions. Down to 7490 m [24,573 ft], the horizontal projection wanders around the origin,indicating that the hole is nearly vertical. At this depth, the borehole temperaturebecame too high to continue with the VDS drilling system and deviation built up rapidly.The hole followed a course labeled Hole 4 until drilling difficulties caused it to be aban-doned. A new hole was started in the vertical section—Hole 5. After the casing was set,the last openhole section was drilled—Hole 6. The final directional survey, conducted at9069 m [29,754 ft], showed a horizontal displacement of only 300 m [1000 ft].

Data Collection and AnalysisThe main center of scientific activity at KTBwas the field laboratory with a staff of 40including resident scientists and technicians(see “KTB Logging Center,” page 16). Here,experiments were performed on cores—mainly from the KTB-VB—drill cuttings andgas traces from the shale shakers, sidewallcores from the Schlumberger SidewallCoreDriller tool, rock fragments from thedrillpipe-conveyed cutting sampler and fluidsamples collected during pump tests anddownhole. The field laboratory providedcataloging and storage facilities and a database of basic information such as petrophys-ical properties, mineralogy and lithologyneeded for further experiments (above).9More detailed long-term experiments wereconducted at universities and research cen-ters in 12 countries.

Nearly 400 logging runs were made inKTB-VB—the pilot hole—with every avail-able borehole instrument (page 8). And 266runs were made in KTB-HB—the superdeephole.10 The wealth of data acquired in thefield lab allowed a rare opportunity to cali-brate borehole log responses to core data incrystalline rocks—as opposed to sedimen-tary environments where their response iswell known—satisfying one of the mainobjectives of KTB-VB.

The formations that were cored anddrilled consisted of metamorphic basementrocks—principally gneisses and amphibo-lites.11 Initially cores and rock fragments—from cuttings—were photographed and cat-aloged according to depth recovered. Micro-scopic analysis of thin sections assistedrecognition of mineralogy and microstruc-

14 Oilfield Review

■■Field laboratory core measurements—detailed density profile (left) and electrical conductivity measurements (right).

■■Post-orientation of cores. The cores are released from the core barrels on the catwalk,reassembled in storage boxes and marked by a black solid line and a red dashed lineto show which end is top. They are then photographed in the field laboratory (far left). A photocopy is then taken by rolling the cores over the copier (left). This photocopymay then be compared to borehole images recorded by the Borehole Televiewer(BHTV) (right) or the Formation MicroScanner tool (far right) and the orientation of theblack line, drawn on the core, calculated.

BHTVTransparency overlay

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180N E S W N NE S W

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Core wrapping BHTV image FormationMicroScanner image

ture and assignment of rock type.12 By map-ping the macroscopic structure and orientingit with borehole logs such as the FMI Full-bore Formation MicroImager image or Bore-hole Televiewer (BHTV) image, a structuralpicture of the borehole was gradually builtup (previous page, bottom).

Petrophysical parameters, such as thermalconductivity, density, electrical conductivity,acoustic impedance, natural radioactivity,natural remanent magnetism and magneticsusceptibility were also routinely measured.In addition to determining the strength ofrock samples, scientists made highly sensi-tive measurements of expansion of the coresas they relaxed to atmospheric pressure.

Geochemists at the field laboratory per-formed detailed core analysis using X-rayfluorescence for rock chemical compositionand X-ray diffraction for mineralogy.13 Thisanalysis allowed a reliable reconstruction ofthe lithology.

After comparing logs with cores, scientistsat the Geophysical Institute at the Universityof Aachen were able to distinguish 32 dis-tinct electrofacies corresponding to 32 min-erals. This enabled borehole logs to con-tribute to and refine the lithological profileof the superdeep borehole, established fromcutting samples and the limited cores avail-able (right).

One contributor to the success of the log-ging operation was the GLT GeochemicalLogging Tool. This provided concentrationsof 10 elements present in rock: silicon, cal-cium, iron, titanium, gadolinium, sulfur, alu-minum, potassium, uranium and thorium.Another tool with a semiconductor detec-tor—germanium—was also used, whichgave a higher sensitivity and provided theadditional elemental concentrations ofsodium, magnesium, manganese, chromiumand vanadium.14 By combining the GLTresults with other measurements, mineralssuch as pyrite, pyrrhotite, magnetite andhematite could be quantified.

Older logging techniques also provedinvaluable. Abnormal Spontaneous Potential(SP) deflections occurred across mineralizedfault systems. Other SP deflections com-bined with low mud resistivity readings fromthe Auxiliary Measurement Sonde (AMS)occurred at zones of water influx. When theAMS resistivity showed only mud and theSP showed a deflection, this was regardedas an indicator for mineralization. Uraniumtends to concentrate at graphite accumula-tions so the uranium reading from the NGSNatural Gamma Ray Spectrometry tool wasused as a graphite indicator.

15January 1995

■■Comparison ofElectro-FaciesAnalysis (EFA) logand lithology fromcuttings. The EFAlog was developedby scientists at theUniversity ofAachen by com-paring core datawith logs from KTB-VB. They wereable to distinguish32 electrofaciescorresponding to32 rock types. The EFA log—pro-duced from logsfrom KTB-HB—com-pares well with thelithology profilefrom cuttings.

9. Emmermann R, Lauterjung J and Umsonst T: KTBReport 93-2: Contributions to the 6th Annual KTB-Colloquium. Geoscientific Results. Stuttgart, Ger-many: Schweitzerbart, 1993.

10. Bram K and Draxler JK: KTB Report 93-1: BasicResearch and Borehole Geophysics (Report 14).Stuttgart, Germany: Schweitzerbart, 1993.

11. Gneisses are banded rocks formed during high-graderegional metamorphism. Included in this group are anumber of rock types having different origins. Gneis-sose banding consists of the more-or-less regularalteration of schistose and granulose bands. Theschistose layers consist of micas and/or amphiboles.The granulose bands are essentially quartzofelds-pathic and may vary from 1 mm up to several cen-timeters in thickness. There are various types ofgneiss depending on the rock origin. Paragneiss isfrom a sedimentary parent and orthogneiss from anigneous parent. Amphibolite is a metamorphic rockcomposed mainly of feldspars and amphibole, agroup of inosilicates.

12. For more detail: Emmermann et al, reference 9: 67-87.

13. Emmermann et al, reference 9: 529-534.14. Grau JA, Schweitzer JS, Draxler JK, Gatto H and

Lauterjung J: “Elemental Logging in the KTB PilotHole—I. NaI-based Spectrometry,” Nuclear Geo-physics 7, no. 2 (1993): 173-187.Schweitzer JS, Peterson CA and Draxler JK: “Elemen-tal Logging with a Germanium Spectrometer in theContinental Deep Drilling Project,” IEEE Transac-tions on Nuclear Science 40, no. 4 (August 1993):920-923.

Cutting EFA log Cutting EFA log Cutting EFA log

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Paragneiss

Catacl.paragneissMetabasite

Catacl.metabasitePlagioclasegneissCataclasite

Alternationgneiss/amph.HornblendegneissCalc-silicaterockLamprophyre

Resistivity Gammaray

(continued on page 19)

The KTB logging center is every wireline engi-

neer’s dream come true. Situated 60 m [200 ft]

from the rig, the logging unit is housed in a cov-

ered enclosure, providing space for calibration

and operation checks of logging tools (above).

Portable units off the enclosure provide mainte-

nance workshops for electronics and hydraulic

sondes. Several offices are provided for the KTB

logging staff and Schlumberger personnel as well

as a computer room equipped with a micro-VAX

III for interpretation and presentation of results.

The offices also provide workplaces for scientists

from universities who run their own logging tools

into the borehole using the logging unit.

The winch unit houses the CSU wellsite surface

instrumentation computer and a silent power

pack, which provides hydraulic power for the

winch and electrical power for the instruments

(next page). The winch is extensively modified to

cope with the high cable tensions encountered

during logging. A high-strength drum holds

around 9500 m [31,200 ft] of cable, and a cap-

stan at the foot of the rig reduces cable tension

from a maximum of 90,000 N [20,000 lbf] to a

normal spooling tension of 4500 N [1000 lbf].

Other modifications allow additional tensiome-

ters and depth measuring systems to monitor the

cable at different points of the rig-up.

The logging cable is permanently suspended in

the derrick to save as much rig-up time as possi-

ble. The winch is housed in a cellar in front of the

logging unit allowing logging cable and ancillary

wiring to run through a tunnel below the rig yard

exiting at the capstan unit. The cable passes

around the capstan before going up the outside of

the rig. Here it passes over the upper sheave

wheel attached to a retractable jib. When logging

is required, the jib is extended out over the rig

floor so that logging tools may be connected to

the cable and lowered into the borehole. Built

into the rig floor is a series of tool magazines.

These contain logging tools in a state of readi-

ness to run into the borehole.

Logging at depths of 9100 m [30,000 ft] and

temperatures approaching 260 °C requires spe-

cial hardware—cable, logging head and logging

tools—to withstand ultrahigh temperatures and

pressures. The cable also has to have the

16 Oilfield Review

■■KTB logging centerand Schlumbergercrew. The logging cen-ter (top left) comprisesan enclosed area—housing the loggingunit—numerous officesfor KTB, guests andSchlumberger person-nel, a computer roomand various workshops,including a well-equipped electronicslaboratory (bottom left)and sonde laboratory(bottom right). Schlumberger EngineerMiel Kühr, OperatorKarl Bohn and Electron-ics Engineer Ignaz deGrefte were assigned toKTB operations for theentire seven-year pro-ject (top right).

KTB Logging Center

strength to pull the logging tools back to surface.

At 9100 m, the normal logging tension is 67,000

N [15,000 lbf], right on the maximum safe pull for

the special high-strength cable used. This cable

has high-tensile steel armor wires and is thicker

than standard to provide the strength. It has spe-

cial insulating materials at the business end that

allow logging at temperatures up to 260 °C for

short periods of time.

If the borehole had gone any deeper, a two-

cable approach to logging would have been used.

At 10,000 m, the high-strength logging cable

would be in danger of breaking under its own

weight—even accounting for the buoyancy effect

of the mud. To reduce cable weight, a smaller

diameter, lighter, high-temperature cable hooked

up to a second winch would have been used to

lower logging tools into the borehole for the first

3200 m [10,500 ft]. At this depth, the small-

diameter cable would be connected to the high-

strength cable of the main winch and the journey

into the hole continued. With this tapered cable

configuration, the overall weight of cable is

decreased, reducing the tension, and the high-

strength cable is at the top of the hole where the

tension is greatest. This technique was used

twice, but only with 1500 m [4921 ft] of small-

diameter cable. This approach was taken by Rus-

sian well loggers to log the 12-km [7.4-mile]

Kola borehole using a three-conductor cable.

A special oil-filled logging head was developed

by Schlumberger for the KTB project. This had

high-temperature feed-throughs and special O-

rings, and provided the connection between cable

and logging tool.

Although there are several standard high-tem-

perature logging tools available, tools were

upgraded especially for KTB. One example is the

high-temperature Formation MicroScanner tool,

which was upgraded to 260 °C (next page, left).

The first task in modifying this tool was to pro-

duce a list of components to upgrade. Several

components, such as the pads containing the

button electrodes, were not changed, but could

be used only once. Other components, such as

the hydraulic motor that opens and closes the

sonde calipers, could still be used more than

once. Mechanical maintenance of such high-tem-

perature tools has to be meticulous—using even

one component that should have been changed

could result not only in a malfunction but also in

destruction of expensive equipment.

Temperature limits on the mechanical aspects

of the tool were relatively straightforward to over-

come. However, the electronics were of major

concern. Normally these operate up to 175 °C

17January 1995

■■Permanent loggingrig-up. The loggingcable is rigged up per-manently to allow log-ging at short notice andsave rig time. The cabledrum is in a cellarbelow ground level, sothat the logging cableand ancillary cables canpass through a tunnelto the rig. At the side ofthe rig, the cablepasses around a cap-stan—to reduce cabletension—and then upthe outside of the der-rick to a retractable jibholding the uppersheave wheel. Loggingtools are pulled up tothe rig floor duringother rig operationsand lowered into toolmagazines. When therig is ready for logging,the jib is lowered, align-ing the logging cablewith the borehole. Thenlogging tools are con-nected to the cable andwinched out of themagazines beforeentering the borehole.

Capstan andhydraulic motor

Upper sheavewheel onretractable jib

Capstan

Yard

Logging unit

Cable

Logging unithousing CSUand silentpower pack

[350 °F]. To keep the temperature within this

limit meant housing them inside a Dewar flask

(below, right). The outside temperature could be

as high as 260 °C with the inside remaining

below 175 °C for up to 8 hours.

The cooling effects of mud circulation during

drilling were calculated to be about 50 °C [90 °F]

at TD. When circulation stopped, the temperature

would gradually climb, giving a window of 36

hours for logging before it exceeded tool ratings.

On the first logging run at TD, the maximum tem-

perature recorded was 240 °C [464 °F] and on

the last run, this reached 250.5 °C [483 °F]—

confirming earlier calculations. At the end of

each logging run the Dewar flasks were cooled

down slowly by blowing air through to avoid

thermal shock.

Apart from Schlumberger logging tools, sev-

eral universities developed equipment for their

own experiments in the borehole (see page 9).

18 Oilfield Review

■■High-temperatureFormation MicroScan-ner tool. The tool wasdeveloped for KTB andhas a temperature rat-ing of 260 °C. Standardelectronics—rated to175 °C [350 °F]—areprotected inside Dewarflasks and sealed at theends by thermal stop-pers. Standardmechanical compo-nents may withstandthis temperature, butthe button electrodepads are changed aftereach logging run.

■■Temperature profile inside Dewar flask. Logging tools are heat-tested inovens. With the oven temperature maintained at 260 °C for 2.3 hours, thetemperature remained below 120 °C [248 °F] inside the Dewar flask.

External temperature = 260 °C for 2.3 hours

200

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Logging cable

Telemetrycartridge

Auxiliarymeasurementsonde

Auxiliarymeasurementelectronics

Gamma Ray

Controllercartridge

Dewar flask

Inclinometer

Acquisitioncartridge

Caliper withbuttonelectrode pads

Insulatingsleeve

Nonmagnetichousing25 kpsi at 300 °C

Dewar flask

Standard175 °Celectronics

Thermalstopper

Dewar flask

Dewar flask

Dewar flask

Surprises—Some Welcome, Some NotBoth boreholes yielded unexpected resultsfor the scientists. Geologists had formed apicture of the crust at the Windischeschen-bach site by examining rock outcrops andtwo-dimensional (2D) seismic measure-ments. At a depth of about 7000 m[22,966 ft] they had expected to drillthrough the boundary between two tectonicplates that collided 320 million years ago,forming the Eurasian plate. However, thisboundary was never crossed, and the geolo-gists have had to redraw most of the subsur-face picture.

Other unexpected results include coreand log evidence for a network of conduc-tive pathways through highly resistive rock,and in rock devoid of matrix porosity, anample supply of water. Look at these find-ings in more detail:

Seismic Investigations—During the project,surface and borehole seismic measurementshelped visualize the structure below theKTB site. The original picture had beenformed from 2D seismic work undertakenbefore drilling. But the structural profile ofKTB-VB showed a more complicated sub-surface. Instead of a nappe unit, the forma-tion followed a more tortuous path (right).15

After KTB-VB was completed in April1989, a year was spent on major seismicevaluation. The seismic work, under thejoint responsibility of KTB and DEKORP—German Continental Reflection SeismicProfiling—was performed by Prakla-Seis-mos—now part of Geco-Prakla. Thisincluded a 3D survey over an area of 19 by19 km [11.8 by 11.8 miles], vertical seismicprofile (VSP) and moving source profile(MSP), using geophones in KTB-VB, and twowide-angle 2D seismic surveys with an off-set of 30 km [18.6 miles] using vibratorsand explosives as sources. The evaluation,conducted by a number of German univer-sities and their geophysical institutes, uti-lized acoustic impedance calculated fromborehole sonic and density measurementsand the acoustic measurements made oncores in the field laboratory. The seismicprocessing was complicated by the tortuous

structure and the large seismic anisotropy.The results, however, gave a much clearerpicture than the earlier 2D work and accu-rately predicted the major fault systemdrilled through between 6850 to 7300 m(page 6).16

It is now known that the boreholeremained inside the Zone of ErbendorfVohenstrauss (ZEV), a small crystalline unittectonically placed between theSaxothuringian and Moldanubian units.There are indications that these metamor-phic units of the Bohemian Massif have beenuplifted 10 km [6.2 miles] since Variscantime—about 300 million years ago—anderoded to the present day surface.17

Future experiments have been designed tomeasure seismic anisotropy at greaterdepths, the spatial extension of seismicreflectors—such as the “Erbendorf” structureat a depth of about 12 km [7.4 miles]—andJanuary 1995

nLithology andstructure revealedby KTB-VB. Simpli-fied lithology ofKTB-VB (left) showsthe alternating lay-ers of metamorphicrock. The structuralmodel (right) showsformations dipping50° to 75° south-southwest over thefirst 3000 m, fol-lowed by a rotationof the dip to theeast with a muchshallower dip of25° in the foldhinge. These mod-els were built frominterpretation ofcore data andborehole imagessuch as FormationMicroScannerimages. The forma-tion appears tohave twisted andpiled up.

15. Wall H, Röckel T and Hirschmann G: “StructuralInformation from FMI in Crystalline Rocks. Resultsfrom the KTB Drilling Site,” Document du BRGM223 6th International Symposium—Continental Sci-entific Drilling Programs—Paris, France, April 1992.Nappe is a sheet-like rock unit emplaced by thrustfaulting, recumbent folding or both.

16. Emmermann et al, reference 9: 137-164.17. Indications come from the analysis, for example,

of detrital muscovites from the sedimentary basinwest of the Bohemian Massif and the study oferoded sediments. Typical techniques used ingeochronology are the determination of coolingages by radiometric dating.

the detailed velocity distribution betweenthe two boreholes using seismic tomogra-phy. Seismologists will also take advantageof the superdeep borehole KTB-HB byrecording downhole seismic waveformsemitted by earthquakes. In this way, surfacenoise will be reduced and the frequencycontent of the signal preserved.

19

N

460 m

1160 m

1610 m

2470 m

2580 m

3575 m

Alternating layers of gneisses and amphibolites

Paragneisses

Metabasite

Amphibolite and hornblende gneiss

1000 E

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Referenceelectrode KTB-VB

KTB-HB

E1

Naabdemnreuth

Nottersdorf

Electromagnetics—One of the reasons forchoosing the Windischeschenbach site wasto investigate the origin and nature of a low-resistivity layer recorded by surface measure-ments that appeared to be 10 km below theEarth’s surface. This is not unique to southernGermany, as similar layers are found in manycontinents around the world.

To unravel the mysteries of this conductivelayer, scientists pursued many differentangles. Conductivity measurements on coresfrom KTB-VB showed high resistivity asexpected in crystalline rocks. But then highlyconductive graphite-bearing faults and cata-clastic zones were found at various depthsup to 7000 m [22,970 ft].18 These were alsoseen on borehole logs where abnormal SPdeflections of more than 200 millivolts (mV)coincided with the graphite. Other logs,such as induced polarization—where thedecay of a voltage applied at a surface elec-trode is measured downhole—showed con-ductive pathways potentially formed byveins of graphite and/or sulfides.

At a much larger scale, when the KTB-HBwas at a depth of 6013 m [19,730 ft] adipole-dipole experiment was carried out.This consisted of using the casing from bothholes to inject current into the formation(above, right). The resulting potential fieldwas measured around the borehole. Anychanges in potential indicated a connectionof an electric conductor to one of the cas-ings, supporting the theory for a conductinglayer extending over a distance of severalhundred meters. The results showed thatthe conducting layer coincided withgraphite deposits in a north-south strikingfault system—the Nottersdorf fault zone.The faults from this system crossed KTB-VBat about 250 m [820 ft] and KTB-HB atabout 1500 m [4921 ft].

Further experiments are planned to inves-tigate the depth, thickness, electricalanisotropy and source of the high conduc-tivity layer still believed to be at 10 km.

Stress and Deformation—One of the goalsof earth science is earthquake prediction,and ultimately reduction in earthquake risk.The physics of earthquakes requires anunderstanding of the movement of tectonicplates, the forces involved and role the crustplays in transmitting those forces. Many sci-entists think that the top 10 km of crust isbrittle and carries most of the stress thatmoves the entire 100-km [62-mile] thickcontinental plates. They also believe that,with increasing depth, the crust becomesductile and cannot support the stress. KTBresearch may help clarify the transition frombrittle to ductile.

Preliminary work in the two KTB bore-holes has already determined the orienta-tion of the local stress field.19 The four-armcaliper, resistivity imaging tools, such as theFormation MicroScanner tool, and acousticimaging tools, such as the BHTV, were usedto calculate the stress direction from analy-sis of two types of failure: shear failure ofthe borehole wall—called breakouts—anddrilling-induced tensile failures. The formeroccur at an azimuth orthogonal to the orien-tation of the maximum horizontal stress. Thelatter are near-vertical fractures in the bore-hole wall in the direction of the maximumhorizontal stress (next page, left). These frac-tures were easily identifiable on the corescut in the KTB-VB and were oriented usingFormation MicroScanner and BHTV images(page 14, bottom). The maximum horizontalstress is oriented to N 150° ± 10° E fromsurface down to 6000 m [19,685 ft].

To obtain the stress magnitude, hydrofracexperiments were carried out in both bore-holes at various depths in conjunction withgeoscientists at the Universities of Bochum,

20 Oilfield Review

■■Dipole-dipole experiment. Current is injected into KTB-VBand KTB-HB casing. This sets up a potential field between thetwo. Surface electrodes (E1, E2, E3, E4) positioned hundreds ofmeters away are used to measure the distribution of thepotential field with respect to a reference electrode. This dis-tribution indicates positions of conducting layers.

18. Cataclastic rock is rock that has undergone mechani-cal breakage usually by dynamic metamorphism or faulting.

19. Emmermann et al, reference 9: 165-218.

+–

–– ++

–+

MPa

■■Formation MicroScanner images beforeand after a hydrofrac in KTB-VB. The For-mation MicroScanner image recordedafter the hydrofrac clearly shows near-vertical induced fractures around anazimuth of about 150°.

and Karlsruhe, Germany and at StanfordUniversity, California, USA. By fracturingthe formation, the minimum and maximumprinciple stresses were determined.

These and earlier tests in KTB-VB con-firmed that the strength of the rock wasincreasing with depth, supporting the theorythat the upper crust is strong enough tocarry most of the stress of tectonic move-ment. Very recently, a hydrofrac experimentwas carried out at 9000 m [29,528 ft] and isbeing evaluated.

Thermal Studies—Of the many processesoccurring within the continental crust,most are temperature dependent. Mappingthe temperature distribution and measur-ing heat production, heat flow and thermalconductivity are therefore a vital part ofunderstanding these processes. During theinitial temperature mapping, KTB-VB heldthe unwelcome surprise that the formationtemperature gradient was higher thananticipated. This disappointing result

21

■■Temperature pro-files recorded inKTB-HB. The dashedline shows theextrapolated forma-tion temperatureestimated from borehole tempera-ture profiles. Esti-mated bottomholetemperature (BHT) is between 260 °Cand 270 °C [500 °Fand 518 °F] at 9101 m [29,859 ft].BHT calculations atother depths aremarked. The bore-hole was cooled bymud circulation inthe lower part of thehole and heated upin the upper sec-tions. Time-lapseprofiles, such asthose recorded inMarch 1992 whenthe hole was at 6013 m [19,728 ft],show temperatureincreasing.

100 °C

200 °C

300 °C

meant that 300 °C—the set limit of currenttechnology—would be reached at about10,000 m—much shallower than origi-nally predicted.

Temperature measurements were carriedout in the two boreholes during regular log-ging campaigns (above). These were used toestimate true formation temperature. Theborehole is cooled during drilling, by up to70 °C [158 °F] in the deepest sections ofKTB-HB. Formation temperature is obtainedby recording several temperature profiles atpreset time intervals as the hole heats upagain and extrapolating these profiles to infi-nite time on a logarithmic plot.

Each temperature profile was recordedduring the first wireline logging run. Thishelped avoid another complication, disturb-ing the mud temperature profile by the log-ging tools. A wireline tool was even modi-

Orientation NorthAfter HydrofracBefore Hydrofrac

0 360180Orientation North

0 360180

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Dep

th, m

Depth m

Temperature, °C

10,000

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6000

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03001500

Temp. grad.= 0.0276 K/m

January 20-21 1990

March 2-4 1991

June 1-4 1991

October 11-17 1991

March 13-20 1992

February 7-12 1993

February 17, March 2 1993

July 27 1993

August 12 1993

August 15 1993

January 2 1994

February 28 1994

Stationary temperature measurements

BHT calculations

fied at KTB with the temperature sensormounted on the bottom of the tool to pro-vide the least disturbance and give the bestpossible result.

Temperature data provided an opportunityto measure heat production and conductiv-ity. In addition, thermal conductivity mea-surements were carried out in the field labo-ratory on cores cut from the boreholes.From the NGS and Litho-Density data, heatproduced by radioactive decay was calcu-lated—for metabasites the results were 0.5micro-Watts per cubic meter (µW/m3) andfor gneisses 1.6 µW/m3.

The final temperature profile has yet to beextrapolated from the data obtained so far.Experiments will continue to examine tem-perature distribution, heat production, heatflow and thermal conductivity.

Fluids—The scientists at KTB expected deepcrystalline rock to be bone dry, but to theirsurprise, water influx occurred at severaldepths from open fractures.

Sonic, Formation MicroScanner and BHTVdata were used to detect the fractures. Asfresh mud was used for drilling, any salinewater inflow would cause a decrease in mudresistivity. This could easily be seen frommud resistivity measurements made by theAMS tool (above right). These zones wereallowed to produce by dropping the mudlevel, enabling a fluid sample to be collectedby a wireline-conveyed sampler run in com-bination with the AMS tool. Tests showedthe water had not leached down recentlyfrom surface. Further tests will be performedto ascertain the origin and composition andinvestigate fluid-rock interaction.

During a two-month pumping test 275 m3

[1730 bbl] of salt water were produced froman open fracture system at the bottom ofKTB-VB. Further evidence showed theextent of the fluid network. During a pro-duction test at 6000 m in KTB-HB, the fluidlevel in KTB-VB dropped. When the 133/8-in.casing in KTB-HB was cemented, there wasan increase in fluid level in KTB-VB. Thesetwo events confirmed hydraulic communi-

cation and allowed an estimate of perme-ability of the fracture system between thetwo boreholes.20

Natural causes of fluid movement becameapparent when pressure sensors deployed inKTB-VB recorded changes in pressure dueto earth tides caused by the gravitationalpull of the moon.

Fluids play an important role in the chem-ical and physical processes in the Earth’scrust, influencing mineral reactions, rheo-logical properties of rocks and melting andcrystallization processes. To aid further sci-entific research into these processes long-term pumping tests are planned betweenKTB-HB and KTB-VB to measure hydrauliccommunication, identify fluid pathways andcollect additional fluid samples.

The Future for Superdeep BoreholesIn 1996, the KTB boreholes will be handedover to GeoForschungsZentrum (GFZ), aGerman government-sponsored geoscien-tific institute based in Potsdam, Germany.

GFZ will continue the work started by KTBand the site will become a laboratory fordeep measurements. Although the rig willbe dismantled, the derrick will remain as amonument to KTB’s achievements. Theseachievements have inspired scientists allover the world to look again at superdeepboreholes with renewed enthusiasm. Manypotential sites whet the appetite: the SanAndreas fault zone, California, tops the listfor studying earthquake activity; for volcanicstudies, the Novarupta Vent in Alaska, USA;subduction zones at the Izu peninsula, Japanor the Hellenic subduction zone, Crete; andthere is no larger continental collision zonethan the intracontinental thrust in the NangaParbat region of the Himalayas. It is now upto scientists to convince governments tosupport international continental scientificdrilling with the necessary funding. —AM

22 Oilfield Review

■■Fluid detectionand sampling.Pulling the drillpipeout of hole withoutrefilling the boreholedecreased thehydrostatic head,provoking inflowfrom the formation.A fluid sampler (FS)—run in combina-tion with the Auxil-iary MeasurementSonde (AMS) tool—collected samples.The AMS tool wasused to detectchanges in mudresistivity (track 2).Where this occurred,a fluid sample wastaken (track 2). Also displayed are holesize (track 1) andlithology (track 3).All curves displayedwere acquired inopen hole.

Ca, Na, Cl N, He, CH4

Salinity

Dep

th, m

7000

6000

5000

4000

3000

300 800

Caliper

Max. Calipermm 10000 18000

23.3.92ppm

NaCl equivalent

10000 18000

24.3.92ppm

10000 18000

1.4.92ppm

10000 18000

28.12.92ppm

300 800Min. Caliper

mm

300 800Bit size

mm

FSFSFS

FS

FS

FS

FSFS

FS

Casing shoe Casing shoe

LithologyHornblende gneiss

Alter. Seq.

Musk bio. gneiss

Gran. sill. bio. gneiss

Catacl. gneiss

Amphibolite

Catacl amph.

Lamprophyre

Catacl.

Alter. Seq.

20. Bram and Draxler, reference 10: 337-365.