Charles R. “Rick” Stone, George H. Medley, Dennis D. Moore, Signa Engineering Corporation

Kyle Fontenot, ConocoPhillips

DURING RECENT ATTEMPTS tolower drilling costs in several fields, apractical blend of several oilfield tech-nologies was developed that may set asubstantial, cost-effective trend in theevolution of drilling techniques. Theprocess involves combining underbal-anced drilling (UBD) with casing drilling(CD), while simultaneously using man-aged pressure (MP) techniques to widenthe window of pore pressureranges that can be addressed inan open hole section.

The inventive method, dubbedUBCD-MP, is especially excitingbecause it has the potential toeliminate the need for one ormore intermediate casing strings.This is a major milestone in thedrilling process that could dra-matically reduce cost, especiallyin areas where casing stringswere historically seen not as anoption, but as a costly necessity.

The UBCD-MP blend drawsadvantages from several tech-niques to create a unique, dynam-ic application that can bedesigned in the field for maximumefficiency. By harnessing the lat-est technology to analyze data, the tech-nique has proven effective for drillingthrough formations where conventionalmethods are not practical, or have failedin the past.

The blend synergistically combines themost powerful and efficient aspects ofUBD, CD and MP to reduce weaknessesthe techniques exhibit when used inde-pendently. In essence, the sum is greaterthan its parts, parts already remark-able, since individually they are consid-ered to be on the forefront of technology.

Despite the engineering challengesinvolved, the economic benefits haveproven to be worthwhile. Cost savingscan range from 40-60%, including well-head cost savings, reduced pipe costs(no intermediate casing string) and thetime saved to run and cement the inter-mediate casing string and rig down/up

blowout preventer stack. Mud andcement costs also are potentiallydecreased by downsizing the hole whiledrilling a longer section.

C A S I N G D R I L L I N G

Casing drilling was developed through ajoint effort between ConocoPhillips andTesco in an effort to make drillingfaster, safer and more cost efficient.Over the last five years, $50 million hasbeen spent in the evolution of casingdrilling from an experimental processinto a proven technique.

The major benefit of CD is the avoidanceof trips during drilling. The techniqueallows operators to drill and case thewell simultaneously, using standard oil-field casing that is permanentlyinstalled in the well. The casing pro-vides hydraulic and mechanical energyto the drilling assembly in lieu of con-ventional drillpipe.

Typically in CD, drilling fluid is circulat-ed down the casing and back up theannulus similar to conventional drilling.The casing can be rotated from the sur-face, and with one system, a bottom holeassembly (BHA) is latched to the bottomjoint of casing and run/retrieved viawireline, making bottom hole retrievaland directional drilling possible.

By eliminating drillstring tripping, andall the problems associated with trips,CD can speed up the drilling process by20-30% and allow savings on operating

costs and capital investment. The tech-nique also improves safety because CDeliminates drill pipe handling. The smallannulus characteristic of casing drillingallows for drilling a given section of holewith less mud weight than with conven-tional drillpipe. The increased annularfriction, or Equivalent Circulating Den-sity (ECD), developed at typical circula-tion rates replaces some mud weight todrill a section of hole when trips arerequired.

There are two main technologies for CDapplications. The first employs a BHA

comprised of a positive displace-ment motor (PDM), drill bit and ahole opener. The assembly islatched to the first joint of casing.The hole is drilled with thisassembly while the casing is low-ered into the hole, either in a stat-ic or rotating mode. Upon reach-ing total depth (TD), the latch-onbottom-hole assembly is recov-ered with a wireline conveyedretrieval tool. A valve system isrun and installed before cement-ing occurs.

The second technology utilizesonly casing to transmit rotarytorque and weight to the drill bit.A drill bit and valve assembly ismade up and run with the firstjoint of casing. The casing stringis rotated during drilling, typical-

ly with a spear assembly that providesrotation to the casing. Upon reachingTD, there is no need for an additionaltrip and the casing can be cementedeasily.

U N D E R B A L A N C E D D R I L L I N G

Underbalanced drilling offers advan-tages in reduced drilling problems,reduced formation damage andimproved penetration rate. As familiari-ty with these benefits becomes morewidespread, UBD is quickly becoming astandard application for drillers to con-sider rather than an obscure option.

Much of its popularity has stemmedfrom an intensified interest by oil andgas companies in improving reservoirrecovery and maximizing project eco-nomics. The process involves drillinginto any formation where the pressureexerted by the drilling fluid is less than

Blending technologies can eliminate casing strings

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The rig choke manifold may be adequate for managed pressuredrilling, but for underbalanced drilling, a separate flow choke mani-fold is desirable.

56 D R I L L I N G C O N T R A C T O R September/October 2004

the formation pressure. Consequently,the formation pressure will cause per-meable zones to flow if other conditionsallow flow at the surface. An underbal-anced condition may be identified byformation influx or annular pressurerecorded at the surface. Some circulat-ing media used for underbalanced oper-ations include gas, air, foam, clear flu-ids, oil or weighted muds.

Other advantages of UBD are minimiz-ing lost circulation, real-time formationproductivity evaluation, reduced costdue to increased ROP, reduced drillingproblems, improved bit life and lowermud costs.

Allowing a Class 4 or 5 well to flow withdrill pipe out of the hole may becomehazardous to the drilling crews workingthe job. Re-entering a live well with adrilling assembly may be difficult,depending upon the magnitude of annu-lar pressure at the surface. It is not pru-dent to incur such a situation unless aviable plan has been conceived to con-tain surface pressures within a limit

that allows reentry with the drillingassembly. Consequently, UBD opera-tions are most vulnerable during drillpipe trips.

It is difficult to maintain a state of UBduring trips due to vertical migration ofhydrocarbons. Managing a live well dur-ing UB operations is dynamic in everyaspect. Conditions rarely stabilize andquite often the driller finds himself deal-ing with changing conditions thatsteadily worsen. Efficient managementof UB conditions is, among other things,a matter of timing.

M A N A G E D P R E S S U R E D R I L L I N G

Managed pressure drilling attempts toprecisely control the annular fluid pres-sure profile within a wellbore. Theprocess allows the driller to deal withpore pressure variations in the wellboreand has proven effective in extendingcasing seats and avoiding problemsresulting from excessive mud weight.The pore pressure profile in a givenopen hole section must be determined

as closely as possible prior to drillingthe section. This may be accomplishedby a detailed pore pressure analysisusing log data, seismic data, RFT pres-sures and virgin production pressures.Leak off tests and formation integritytests may be used to verify the fracturegradient profile near casing seats.

Consequently, this need for detailedpore pressure knowledge may excludeexploratory wells from managed pres-sure applications. One possible excep-tion is deepwater applications.where-many are exploratory in nature.

The relative hydrostatic “margin”between pore pressure and fracturegradient in the normal pressured hori-zons of a deepwater well may be small.Many deepwater types of sediment willsimply not withstand the column of mudrequired to return the cuttings to thesurface of the sea. Break down of adeepwater wellbore can result in severeloss of circulation, kicks from exposedpermeable zones, stuck pipe and associ-ated drilling down time.

Managed pressure techniques endeavorto use a mud weight that is “balanced”to the greatest “permeable zone” porepressure required in the exposed wellbore.

In the UBCD-MP blend, specifically theapplications that have recently proveneffective for Signa Engineering, a mid-range mud weight can be used betweenthe lowest and highest pore pressures inan exposed wellbore. One must firstdetermine the location and extent ofpermeable and impermeable rock. Thefailure mechanics of impermeable rockmust be addressed to avoid boreholecollapse.

The pore pressure of permeable rockmust be addressed according to theintent of the drilling operation. In man-aged pressure drilling, influx from per-meable rock must be controlled with abalanced or slightly overbalanced equiv-alent hydrostatic column. Using thistechnique, onshore operations havebeen successful at extending or elimi-nating casing seats, avoiding mudweight related drilling problems andminimizing drilling flat time.

P R O B L E M S F O R P R O F I T S

As mentioned, the UBCD-MP blendextracts the best qualities from the trioof aforementioned unconventionalmethods to create a unique process thatcan remove typical vulnerabilities whenUBD, CD and MP are used independent-ly. For instance, the most vulnerabletime relative to cost and safety for oper-ators drilling underbalanced is duringtrips, so obviously a technique fordrilling and isolating a wellbore withouttripping can eliminate a major difficultyof UBD.

The higher annular friction while circu-lating with CD allows greater underbal-anced margins with a given mud weight.In other words, the mud weight may bemaintained at a density lower than typi-cal with a drill pipe job.

Additional density would then be simu-lated with pump rate and associatedECD to satisfy the needs of the wellboreat the bit. If pore pressure increaseswith depth, the formations above the bitmay be effectively overbalanced by thesame mud weight that cause an under-balanced environment at the bit.

Depending upon rock strength and col-

lapse issues, the UB margin can beadjusted at the bit to allow for less“overbalanced” stress on a weak rocksection in the upper part of the exposedbore hole.

This adjustment can be affected byeither mud weight adjustments or bychanging the pump rate. It is often sim-plest to change the pump rate. Manag-ing pressure in the upper part ormechanically weaker part of an exposedhole section may be the most importantaspect of extending or eliminating a cas-ing seat.

The UBCD-MP blend is not applicable inevery formation. In many cases, such aswhen drilling highly fractured gas for-mations with a high productivity index,special considerations must be given toincreased formation influx rates anddecreased latitude between kicks andlosses.

This blend of technology sets has provenideal for use in tight gas sands and sim-ilar formations, and for areas where thepressure regime in a vertical holechanges significantly with depth. How-ever, as the blend is still in its infancy,those considering the technology cannotarbitrarily exclude (or include) applica-tions that could possibly benefit.

A P P L Y I N G T E C H N O L O G Y

As with most unconventional methods,determining whether wells are goodcandidates for UBCD-MP varies witheach scenario. Application of this tech-nology is well specific and no all-inclu-sive formula or specifications can beused as a diagram to insure the tech-nique is a success.

However, thoughtful planning and man-agement in several applications hasshown the technology blend is a power-ful new weapon in the expanding arse-nal that drilling engineers can use totackle difficult drilling situations andlower operational risk.

Drilling dollars are budgeted accordingto economic parameters that include therisk of exposing a producing formationto an acceptable conduit for production.Economic viability and risk assessmentare often at odds in making this deci-sion. Assigning risk to an economic pro-gram is often a personal assessmentbased on historical challenges in a givenfield or region, as well as the experience

of the one making those assignments,and the level of confidence in the abilityof new technology to reduce risk.

Assignment of risk may also be theresult of personal and/or corporatemotives and policies. Regardless of themethod used for assignment of risk,often risk is the biggest issue in an eco-nomic evaluation of whether to drill awell or not.

Determining if a well is a good candidatecenters directly around what pore pres-sure ranges and rock strengths havebeen exposed below the last casing shoein the open-hole section, and whether ornot that variance in pressure can besustained. A rigorous hydraulic model isalso required to guide the drilling engi-neer through the UBCD-MP process.

The technique can be hazardous with-out a complete understanding of the for-mation makeup. By utilizing such amodel, operators looking to utilize theUBCD-MP blend can effectively forecastthe significant economic benefits bymodeling the entire downhole procedurefrom start to completion. The UBCD-MPblend has the potential to significantlyreduce operators’ costs when appliedcorrectly and managed properly withsoftware that can simulate all areas of awellbore. Software that simply monitorsthe surface and bottom-hole pressuresis not acceptable.

As stated, in conventional drilling, porepressures in the exposed permeablerock must be addressed with a higher or“overbalanced” mud weight, therebylimiting the window of pressure man-agement. The existence of a weak rockor depleted pressure zone may furthercomplicate the driller’s ability toachieve the next casing seat. TheUBCD-MP technology blend may beused to increase the window of pressuremanagement in such cases.

Increasing the window of pressure man-agement in an exposed section of holemay be the difference in attaining totaldepth in a very deep well or finishing awell with three casing strings instead offour.

The potential for cost savings in eco-nomically justified wells is as impres-sive as the newfound ability to drill wellsthat otherwise would not be chosen forinvestment of drilling capital. n

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