Drilling optimization in Achimoc

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SPE-175815-MS

Drilling Optimization in Achimov Horizontal Wells by IntegratingGeomechanics and Drilling Practices

S. Dymov,V. Kretsul, and P. Dobrokhleb, Schlumberger

Copyright 2015, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE North Africa Technical Conference and Exhibition held in Cairo, Egypt, 14–16 September 2015.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents

of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect

any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written

consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may

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Abstract

A workflow was developed for effectively handling the development challenges in the Yamal oil and gas

province of Western Siberia. The workflow integrates geomechanics with efficient drilling practices and

leads to the implementation of an engineered drilling system (EDS).

The workflow for development in the Yamal region encompasses phases from modeling to drilling

practices, incorporating an understanding of challenges faced and lessons learned from previous drilling.

The majority of Achimov operations in the Yamal region have been conducted in the Urengoy gas

condensate field, which is divided into different blocks. The Achimov deposits are deep (true vertical

depth is about 3,750 m), exhibit abnormally high formation pressure (over 600 atm), and containmultiphase hydrocarbons with the presence of heavy paraffins. Unstable shale formations must be drilled

prior to penetrating the Achimov itself. These challenges affect drilling efficiency and interfere with the

well construction schedule. The Achimov formations present two main challenges to horizontal drilling:

wellbore instability leading to stuck pipe incidents in the build section or inability to run liner/casing to

total depth and loss circulation, potentially compromising well delivery and project objectives. Thus,

previous drilling has shown that developing and producing from these enigmatic resources requires more

than just horizontal wells and hydraulic fracturing.

Companies are aggressively pursuing Achimov deposits, hoping to extract additional gas and gas

condensate volumes from the declining fields and to implement a strategy to raise natural gas production.

Successful operations require an integrated approach, using multiple data sources, to determine the key

parameters needed to understand the Achimov formations and extract the hydrocarbons. Among thesuccesses, an operator company for Urengoy field demonstrated how developing a mechanical earth

model (MEM) of the reservoir and continually improving processes paid big dividends.

By implementing the EDS approach, unique results have been achieved in drilling and completion of

Achimov horizontal wells. Outstanding drilling performance in Achimov horizontal wells has led to a

revision of the field development plan.

Introduction

Urengoy oil and gas field is the largest field on the volume of hydrocarbon reserves in Russian. The

structure of the field includes four productive deposits - Cenomanian, Neocomian, Achimov and Middle



Jurassic. The Achimov formation of the Urengoy field extended to a number of satellite fields (East

Urengoy, Novy-Urengoy, Samburgskoye). The Achimov formation is one of the most promising objects

for further development of the Urengoy field group. They lie at a depth of 3600-3900 meters and have a

much more complex geological structure compared to the Cenomanian (located at a depth of 1100-1700

m) and Valanginian (1700-3200 m) deposits. In addition to these, the Achimov deposits lie at abnormally

high formation pressure of 600 atmospheres and complicated by tectonic and lithological screens, the

formation fluid is characterized by multi-phase state. Nevertheless, development of the hardly accessibleAchimov deposits allows to recover additional volumes of gas and gas condensate in fields with declining

production, as well as to execute a strategy of operating companies to increase production of natural gas.

Achimov deposits of the Urengoy field group is distributed on the area of more than 12 thousand km2.

There are 11 operators holding a license for development of the field, the major of which are “Gazprom

dobycha Urengoy”, JSC “Achimgaz”, JSC “Arcticgaz”. In 2011, project design institute “TyumenNII-

giprogaz” has finalized “Unified development plan for hydrocarbon Achimov deposits”. A general project

document allowed to create a balanced strategy for development of complex geological object, unified

enforcement of different operators on the basis of rational hydrocarbons recovery, taking into account the

economic interests of all producers [4].

On the territory of the Urengoy group of fields, productive deposits with commercial reserves of

hydrocarbons are spread almost all over the horizons of the Achimov deposits and comprise oil and gascondensate formation included Ach

3, Ach

4, Ach

5

1, Ach5

2-3, Ach6

1 and oil formations Ach6

0-1, Ach6

0-2 [3,

4]. The Achimov formation is considered to be one of the most complex objects for development in the

region due to the following factors [1, 4, 7]:

1. Abnormally high initial reservoir pressure – 59-61 MPa at a depth of 3800 m;

2. Reservoir temperature 105-115°C;

3. Low permeability formations – 0.1 – 10 mD;

4. High content of condensate in the gas – 275-320 g/m3;

5. Simultaneous occurrence in strata condensate content gas and oil;

6. Tectonic fragmentation of individual sections of deposits;

7. Administrative division of the common hydrocarbon reservoir in accordance with the licensed area.

Previous experience

Complicated geological conditions led to the search for new technological and technical solutions. The

need of the well productivity improvement has justified horizontal drilling into Achimov formation with

requirement to design and drill wells with a horizontal section in the reservoir over 200-300 meters [4, 8].

During the pilot development of the Achimov deposits it was assumed that the production from the deposit

by horizontal wells allows to create a larger surface of drainage and to reduce depression and increase

productivity in comparison to vertical wells [22].

While drilling the transitional zone above the Achimov deposits, often occurred wellbore breakouts

leading to attempts to keep wellbore stable by increasing mud density but also causing losses of mud into

the top depleted formations [4]. Also, the practice of horizontal drilling showed issues related to a high

sensitivity of a safe window of pressure gradients to collapse and fracture wellbore to well trajectory,

presence of a transitional zone of a top of reservoir with high pore pressure imposed conditions of accurate

selection of the depth for a production casing shoe to isolate the Achimov. All these factors led to serious

accidents, including formation fluid influxes, fluid loss, wellbore collapse, lost in hole bottomhole

assemblies (BHA) and remedial sidetracking [14].

Drilling of the first horizontal wells was associated with number of incidents, as a result drilling in

productive formations, running of completion equipment and putting well into operation took a long

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period of time. With all this going on, productive formation was in contract with drilling fluids for a long

time, which greatly reduced productive characteristics of the wells [22].

Analysis has proved that wellbore productivity was induced by formation damage in the horizontal

Achimov wells. Complications arising during construction of horizontal wells leaded to significant and

irreversible contamination of productive formations [21, 22]. Actual production rates of horizontal wells,

including those with multi stage hydraulic fracturing (MSHF), were significantly lower when the

production rates from S-shaped wells after fracturing [21].

Due to the fact that the construction of horizontal wells was accompanied by a large number of

accidents and significant non-productive time combined with lower than expected well productivity [21],

horizontal drilling was considered as not economically feasible; operators have continued with drilling of

S-shaped wells only [4, 5, 9, 10].

In the field development project, designed by a leading oil and gas institute “TyumenNIIgiprogaz” was

proposed drilling of several hundreds of directional wells, followed by two-stage hydraulic fracturing in

the interval of Ach3-4

Ach5

on each of the wells. This project provides optimum technical and economic

indicators of development and provides maximum oil recovery factor with minimal risks while drilling

and conducting geological and technical operations. Potential annual gas extraction of gas may achieve

60 billion cubic meters and 18 million tons of condensate by 2020-2022. There was also planned to runinto operation oil deposits. The projected maximum levels of oil production are more than 11 million tons

per year [1, 15, 16, 17].

For further project economics improvement, a number of different scenarios for field development plan

were simulated. Calculations results proved the necessity of horizontal drilling with longer horizontal

section in the reservoir (600-1200 m and more), and multi-stage hydraulic fracturing [5, 19, 20, 41, 42,

43]. In this relation, there was taken an objective to re-assess possibility of sub-horizontal wells drilling

taking into account new technical solutions.

Analysis of Achimov well construction experience identified a need to find technological solutions in

two main areas:

1. Providing high-quality well construction with horizontal section of 600-1200 m or more (withfollowed MSHF in horizontal section).

2. Minimizing formation damage thereby preserving potential well productivity in a formation with

a high clay content in a wide range of temperature and pressure conditions.

The first horizontal wells to the Achimov deposits drilled earlier in the Urengoy field, were constantly

exceeding drilling schedule even though these wells had lateral section length less than 800 meters. Such

poor performance was accounted to the technology and geology challenges resulted in high accident rate.

There was risk of hydraulic fracturing while drilling in the Achimov reservoirs. At the same time, there

was a risk of wellbore collapse in the intervals of clay interlayers with high pore pressure. [14]. Thus, to

meet the objectives it was necessary to provide wellbore stability of Achimov horizontal wells.

Wellbore stability issues

Wellbore instability may be caused by radial change in both the mechanical stress and chemical and

physical environments when a hole is drilled, exposing the formation to drilling mud [44, 45]. Practically,

the first cause for instability may be related to selection of inadequate mud density and hydraulics

program. The second cause is related to selection of inappropriate type of drilling fluid.

Table. 1 shows functions and limitations of well circulation process for horizontal wells to ensure

stability of the wellbore.

SPE-175815-MS 3



Experience of Achimov well construction indicates that geology of Urengoy field is characterized byhigh sensitivity of safe window of collapse and fracture pressure gradients to well trajectory. In some

wells increasing density by 0.2 g/cm3 (in some cases less than 0.2 g / cm3) fluid loss initiated, while

reducing by the same value, there were observed drag increase, resulting in long reaming time and often

stuck pipe. Also was observed an effect of wellbore ballooning, a phenomenon in which fluids are lost to

the formation during over-pressured operations, such as found in increased pressures from equivalent

circulating density operations, and then flow back when pressure is reduced. This may be confused with

a kick [58].

Fluctuations of pressure led to wellbore collapse. Photo 1 shows pieces cavings from Achimov

formaion. As it appears cavings happened due to fractures. A large number of fractures were observed in

Achimov cores by many researchers [59]. Thus, a question about selection of mud density for wells drilled

to Achimov that doesn’t lead to negative consequences is very important.

Physical and mechanical factors providing wellbore stability

According to the results of domestic and foreign studies [14, 49, 53, 54, 55, and others.], increasing of a

well trajectory inclination is leading to reduction of a safe range of mud densities, so keeping a wellbore

stable requires higher mud density and, at the same time, mud losses starts at lower density (Figure 2).

Table 1—Functions and limitations of well circulation process

Function Limitations

Prevent wellbore sloughing and caving Formation damage

Compensate formation pressure Fluid loss and hydraulic fracturing

Hole cleaning Wellbore erosion

Figure 1—The pieces of caving from the Achimov deposits

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The rule applied in the industry state the need to increase mud density by 60 kg/m3 for every 30°

inclination angle to stabilize borehole does not account for lithology of drilled rocks. Therefore, there is

a need to have an approach, taking into account geological characteristics of the area. Since in the

beginning of the 1980s to determine a safe operational range of mud density was used charts of Bradley

[51, 52, 55, etc.]. New technologies are continually being developed and applied and earlier technologies

refined.

Many researchers have shown that, at the equal state, the highest stress is located on the walls of

borehole [45, 53]. According to [50, 53, and others.] instead of solving the space problem of determining

a stress state of the wellbore it is enough to define a function of stress in the cross-section of a borehole.

For well situated in an elastic transversely isotropic rock massive, by N.R. Rabinovich was resolved a task

in stresses. Several algorithms and software applications [44, 46, 47, 48] were developed extrapolating

these solutions for a horizontal well, situated in an isotropic massive, determining an operating range of

densities for drilling fluid which do not lead to undesirable consequences (collapses wellbore, fracturing).

The widespread of information technologies in the oil and gas industry and further development of

geomechanics for oil and gas wells allowed to create more accurate models of wellbore stability for

inclined and horizontal wells, taking into account anisotropy of rock strength and rock deformation

properties [57].

Currently, there are many oilfield service companies designed and apply a variety of tools for rock

sample testing and use software, allowing to do geomechanical research and to do calculations of a stable

wellbore conditions. However, to achieve convergence of a model, results should be calibrated with the

use of leak-off tests and updating of the model in real time based on the logging while drilling with the

tools in the BHA and mud logging.

Figure 2 shows a combined graph of pressure gradients on the basis of geomechanical calculation for

one of the Achimov wells [14]. This graph shows changes of pressure gradients with depth. It can be seen

Figure 2—Stable mud weight range

SPE-175815-MS 5



how in one of the points vary pressure gradients of wellbore fracturing and collapse depending on the

azimuth and inclination angles. The largest mud weight window is when the zenith angle is zero degrees,

but with the angle increase, the safe window between the gradients of collapse and fracture of the borehole

narrows down and at the 90 degrees it almost disappears. This explains a significantly higher accident rate

while drilling horizontally in comparison to S-shaped wells. Also there is an influence of the azimuth

angle of the trajectory position in space to the value of safe window between fracture and collapse

gradients.

Thus, to reduce the risks associated to wellbore instability of horizontal wells drilling in Achimov

formations it is necessary to perform a pre-drill and real-time geomechanics. Geomechanical model at the

planning stage allow to identify problematic zones and safe limits of equaling circulation density (ECD),

on the base of which technical solutions and technologies may be chosen. To obtain the most accurate

values of safe limits of ECD geomechanical model should be updated in real time on the base of logging

while drilling (LWD) data from BHA. On the base of ECD measurements by annular pressure while

Figure 3—Pre-Drill wellbore stability calculation

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drilling (APWD) measurements were used for drilling regimes selection to ensure compliance with the

calculated safe limits.

Pre-Drill Geomechanics

For successful control and prevention of complications associated with abnormally high formation

pressure, weak formations that may be a cause for fluid loss, as well as the definition of an optimalcompletion and production strategies, it is necessary a full understanding of the rock mechanical

properties, reservoir pressure and stresses in the formations.

The main purposes of wellbore stability calculations for a planned trajectory are to define ECD limits,

the knowledge of which allows you to avoid problems with the stability of the borehole and rock

fracturing.

The study evaluates formation pressure, fracture gradient, rock collapse gradient, calculation and

calibration of the elastic properties of rocks in the near-well environment and calculation of the wellbore

stability in order to determine a safe mud weight window and risks associated with the wellbore

instability.

Logging data allow to do a detailed calculation of wellbore stability. These data not only allow to do

a calculation and calibration of the elastic and strengthen properties of the rock, but also to do check of the constructed model. The check was done by comparing the calculated results with the caliper records.

For geomechanical calculation, the customer provided the following data on offset wells:

Logging: gamma-ray, gamma-gamma density, acoustic, caliper, reservoir micro-imager.

1. Trajectories of drilled wells.

2. Reservoir pressure measurements

3. Well design

4. Drilling reports

5. The main stratigraphic spacing

6. Surfaces of the main productive horizons

7. Data conducted hydraulic fracturingTo accomplish these objectives the following tasks were resolved:

1. Audit of the original data

2. Calculation and calibration of mechanical properties and the strength of rocks surrounding a well

3. Calculation and calibration of rocks reservoir pressure profile with neighboring wells

4. Calculation and calibration of the stress of adjacent wells

5. Calculation of wellbore stability of one of already drilled wells

6. Transfer the elastic-strength properties of the planned well trajectory

7. Planned well trajectory optimization

8. Final conclusions and recommendations for drilling

Outputs of performed calculations are as follow:

1. The top of over-pressurized zone and the transition zone location were determined

2. The mechanical properties of rocks were obtained

3. The values and directions of the main stresses were determined

4. The safe mud weight window for offset wells was calculated

5. The optimum depth of casing shoe and mud density values were recommended

Mechanical Earth model (MEM) is the main input information to calculate the stability of wellbore.

This is numerical representation of the rock and reservoir pressures, tectonic stress, rock mechanical and

SPE-175815-MS 7



strength properties, including deformation and expansion. The calculation is based on data provided by

drilling, logging and geological data and calibrated by tests, measurements and drilling events.

According to the data obtained from MEM, safe mud weight windows were calculated for the reference

wells in the area. The results of safe mud weight window calculations presented as a summary graph of

reservoir pressure and collapse gradients, mud loss and fracturing gradients. At the specified pressure

gradient in the hole (effective weight of the mud), for each point of the borehole, both in depth and in a

circle wall were calculated stresses and checked ability to destruct wellbore under the influence of thesestresses.

The rock failure was assessed by two criteria:

1. Breakout criterion, the shear failure by the large difference of applied stresses (often occurs due

to insufficient pressure gradient in the well);

2. Fracturing criteria or breaking crack occurrence due to high pressure in the well.

Thus, the model allows for each depth of a well to determine safe limits of mud density, that not

contradict to any of the borehole instabilities criteria. That is determined the safe limits of pressure

gradient of drilling fluid in the well, which would provide stability of a wellbore, both collapse and

fracturing. Also, it is important to prevent oil and gas kicks of a formation fluid.

Check of criteria for each point of the well allows to construct a synthetic image of wellbore destructionalong the well trajectory, the image is a scan pattern of destruction depending on the azimuth of each point

of the well.

In vertical hole breakout’s direction is parallel to minimum horizontal stresses. In inclined borehole

breakouts are associated with sidewalls of the hole. If hydrostatic pressure of mud column is not enough

for compensation of the stresses around the borehole then such destructions as walls shear can occur. This

kind of failures trigger large cavings in borehole which cannot be transported with mud flow and

eventually can result in drill string overpulls, stuck pipe situations, lost BHAs and new borehole re-drills.

Borehole stability calculations for planned well trajectory were done with translation of the mechanical

properties of the medium from offset wells using existing stratigraphic benchmarks.

Drilling plan, selected for particular geological model, needs to be also correlated and conform to all

trajectory requirements for overall process optimization. For example, preferred hole inclination for

certain Achimov well is seventy nine degrees on one of the intervals, although hydraulics analysis can

reveal that due to hole cleaning such conditions can pose potential problems. Division of tangent interval

into two or more different inclination zones in some places will require longer drilling times but will be

much safer from borehole stability standpoint. Another possible way to simplify situation could be

changing drilling azimuth of the borehole.

For particular case, planned direction of the well was not optimum for borehole stability and thus

required additional methods and solutions for safe drilling process allowing acceptable equivalent

circulating densities.

Real time wellbore stability controlRelatively narrow safe mud density window and additional wellbore integrity risks determined the need

for real time wellbore stability calculations, updates to pre-drill geomechanical model, monitoring and

control for drilling parameters. Advanced logging and measurements while drilling technologies, includ-

ing multipole sonic-while-drilling, high-speed telemetry, neutron porosity, density measurements, were

used to control drilling regimes in real time and provide actual datum for geomechanics updates.

During drilling operations geomechanics team performed monitoring and control of the following main

parameters:

● Updating wellbore stability model in a real time;

8 SPE-175815-MS



● Optimization mud weight and rheology using geomechanics calculation results;

● Monitoring and analysis of mechanical drilling parameters;

● Monitoring and giving recommendations about optimization of drilling and tripping operations;

● Monitoring of the wellbore state.

Reliable information received in real time, allowed to have maximum control of the wellbore and

provided the most efficient process to select the drill regimes and other operations while drilling. Wellborestability control allowed skipping several pre-planned wiper trip procedures and increase the penetration

rate by 30% comparing to the similar previously drilled wells in this region.

Mud chemistry solutions for wellbore stability

Build MEM model combined petrophysical and geomechanical data for productive collectors and

overlaying layers and also provided conformal data interpretation for all intervals. Several MMS

statements were developed after research in Urengoyskoye oilfield. One of them says that geomechanical

model development requires full existing data set. Herewith any analysis should consider time-factor for

mechanical properties changes of the drilled rocks due to interaction with drilling fluids and downhole

pressure fluctuations. In cases when drilling fluid is not relevant to downhole conditions walls of the borehole become unstable, leading to differential sticking of the drill string and BHA elements, drag

increase, inability to transfer required weight to the bit. Also horizontal part of the borehole intersects

productive formations and mentioned factors can negatively affect production in the future. Situation can

lead not only to construction time and cost increases but also to the unplanned termination for horizontal

drilling and compromise for well objectives. Thus borehole stability assurance, selection of fit for purpose

drilling fluid and its properties promotes second objective - maximizing level of preserved natural

properties of terrgenous type reservoirs with high volumes of clay materials and broad temperature-

pressure ranges.

A number of laboratory tests are available to try and quantify chemical interaction between various

drilling fluids and a particular formation. These tests include:

● Classification of formations (cationic exchange capacity and clay content);

● Visual immersion testing;

● Hydration (yield) tests;

● Cuttings hardness tests;

● Capillary suction tests;

● Linear swelling tests;

● Dispersion tests;

● Confined-pressure testing;

● Triaxial testing

● Estimation of fluid behavior in modelled downhole environments [44, 45]

According to empirical law, speed of chemical reactions proportionally increases with temperature(approximately 2 times every 10° C) and pressure.

As a result, inactive in normal surface conditions clay mineral (such as kaolinite, mica) can intensively

interact with drilling fluid and fluid filtrate in downhole conditions [60].

Wellbore instability is one of the major challenges when drilling a well. Drilling fluid type for stable

borehole depended on particular geology and as experience indicates - no single drilling fluid can be

effective on all areas. Many researchers tried to base fluid selection on classification of clay slates using

mineral compositions and structures. Difficulty of such approach is in many variable factors that

determine properties of clay slates and complexity in combing these factors into separate simple

SPE-175815-MS 9



categories. Borehole stability is also influenced by such factors as tectonic stresses, formation pressures,

and clay formations structure and compaction levels. [44, 60]

First step in selection of fluid composition is collection of all available information about geology,

history of rock stresses development and faults distribution in the region.

Temperature and pressure gradients should be determined from logs from offset wells, as well as water

saturation levels in shale rocks. Certain shale rocks causing complications should be selected and studied

in laboratory environment. Best material for such studies will be comprised from preserved coringsamples. In cases when such materials are not available drilling cuttings could be considered as an option

although they represent altered particles after interaction with drilling fluid [60].

Influence of the drilling fluid properties on wellbore stability was investigated for five different types

of fluid systems used in region:

1. clay-polymer mud (benchmark);

2. KCl-polymer;

3. Silicate fluid (two compositions were used in research - sodium silicate and potassium silicate

based);

4. Fresh polymer mud with polyamines additives;

5. Oil based mud Inhibiting properties of the selected drilling fluids were tested using recently developed methods and

procedures. Materials in research represented core samples and drilling cuttings acquired while drilling

problematic horizontal wells in Urengoyskoye oilfield. Samples included drilling cuttings as well as

cavings with different sizes from 1-2 mm to 7-12 mm and more in some cases (Foto #1)

Test’s results were compared to the results of geomechanical modeling revealing safe fluid density

ranges required for stable state of the horizontal boreholes for a prolonged period of time. Analysis

showed that most negative influence of the drilling fluid for borehole stability in highly inclined holes

appears right after drilling fresh intervals. These results confirm conclusions made by many authors for

data interpretation from vertical wells and laboratory reports [60 – 62], according to these studies most

significant negative influence of the drilling fluid for wellbore stability occurs in the first hours of the

contact with formations.

For circulating fluids with high inhibiting properties (Silicate, OBM) difference between upper limit of

the safe window (maximum density) for contact time at 24, 48 and 72 hours not so significant opposite

to lower limit (minimum density), and besides with increase of the inclination influence of the time factor

decreases.

Use of fluids with low inhibiting properties (fresh-drilling mud) requires higher densities for stable

borehole which leads to high risks of formation fracturing due to high circulating densities (ECD).

Borehole stability also influenced by pressure fluctuations which can be better controlled by imple-

menting low viscosity fluids and including annular pressure sensors in bottom hole assembly (BHA). In

this cases levels of such effects as swab and surge, hydraulic hummer after start of the pumps can be

significantly reduced.

Advance in the time of a contact between circulating fluid and unstable formations accompanied by

increase of fluid density required for borehole stability and decrease in the safe upper densities limits, for

particular conditions intensive narrowing of the working densities range associated with first two days

after drilling new intervals, after that intensity decreases.

High speed drilling requires usually lower borehole pressures for stable conditions comparing to

formation pressures. After a long contact period of drilling fluid with unstable rock safe conditions require

downhole pressures significantly above formation pressures, herewith high overbalance decreases drilling

speeds and can in some intervals cause mud losses. High drilling rates thus can prevent negative impacts

of the unstable boreholes.

10 SPE-175815-MS



At the same time high rates of penetration (ROP) demand higher flow rates for effective hole cleaning

which also can lead to erosion of the upper wall of the hole with danger of the borehole collapse.

Achievement of the high drilling speeds requires optimum flow rates which would not cause excessive

equivalent circulating densities that will trigger mud losses with scour of the borehole walls.

ECD ManagementResults of the conducted research determined use of the oil based mud in new drilling campaign for

minimization of the borehole stability risks and drilling performance increase.

Compositions of the drilling fluid were adjusted for achievement of low acceptable circulating densities

and low friction levels in the borehole.

Such choice driven by required conditions for minimum downhole pressure fluctuations while drilling

and tripping operations, and also by the need for lowest possible friction factors for drilling torque

reduction, successful liner runs till final depths, drilling fluid filtrate inertness and effective hole cleaning.

Similar solutions, previously used in region, would cause circulating densities while drilling two times

higher than minimum limit for formation fracture, according to geomechanical calculations. Existed high

risks for formation damages and uncontrolled mud loses with inabilities to drill well to planned targets.Excessive pressure fluctuations in the borehole while drilling and tripping operations also could cause

kicks, wellbore collapses and related serious incidents.

Several elements of the drilling system were selected for determination of possible ways to decrease

equivalent circulating density (Figure 6). Particular were chosen following:

1. Drill pipe diameter

2. Drilling fluid rheology

3. Drilling fluid density

4. Drill pipe rotational speed

5. Flow rates

6. Bit (borehole) diameter 7. Penetration rate

SPE-175815-MS 11



Analysis for ECD sensitivity showed that highest impact would have drill pipe diameter, rheology and

drilling fluid density. Selection of the drill pipe diameter was also limited by required string integrity for

sustaining mechanical loads. Density of the drilling fluid is also affected by required balance for possiblekick and borehole breakouts prevention. Rheology is this case was considered as major possible parameter

that could be adjusted for required ECD reduction.

Specialists from engineering center within service company developed new optimized oil based drilling

fluid with low, flat rheology, reduced gel strength for achieving required ECD levels and efficient hole

cleaning.

Drilling optimization

Results of the theoretical modeling, existing drilling experience combined with production history

precisely indicate the need for reduction of contact time between mud and unstable formations. This

objective can be achieved in several ways: 1) increasing drilling, well construction speeds and 2) reduce

nonproductive timeTechnological methods allowing increase in drilling speed include [3, 17, 20]:

● Trajectory, BHA and wellbore geometry optimization. Improvements can be seen as reduction of

casing sections, adjustments for long intervals of rotary drilling

● Use of tuned combination of the driving mechanism such as motor or rotary steerable systems with

fit for purpose drilling bits lead to better trajectory control and increased penetration rates

● Rotary steerable systems can bring additional value in better hole cleaning due to high rotational

speeds, increased penetration rates, sliding intervals elimination and as a result ability to drill

longer wells with complex trajectories [5, 21]

Figure 4—ECD sensitivity plot for different drilling system parameters

12 SPE-175815-MS



● Use of casing drilling technology

● OBM traditionally lead to higher drilling speeds comparing with results of the wells drilled with

water based fluids [8,22 – 24]

Plan for non-productive time reduction included application of reliable surface equipment, control for

downhole conditions in real time and optimization of drill string and BHA components including PDC

bits, rotary steerable systems, logging and measurements while drilling tools. Downhole conditions weremodelled using most accurate methods of Finite Element Analysis (FEA) with recommendations on

operational drilling parameters such as weigh on bit and surface rotational speeds for optimum BHA

stability, reduced torsional, axial and lateral vibrations.

Planning and execution

Summary of performed steps for borehole stability assurance while drilling and other well construction

operations on different project stages included:

● Selection of the drilling fluid type at the planning stage with requirements for high lubricity in the

borehole resulting in smooth rotation, low vibration levels and minimum mechanical stresses

exerted to the drill string, successful liner runs to final planned depth;

● Development of drilling fluid composition for acceptable ECD ranges, at least two times lower

comparing to results seen with previously used mud systems;

● Defining drilling regimes and procedures for safe ECD ranges and effective hole cleaning;

● Real time control and optimization of the hydraulic program during execution stage;

● Hole cleaning estimation, rheology, drilling and tripping regimes selection;

● Fluid density adjustment in accordance to updated geomechanical model;

● Control for fluid sustainability to barite subsidence;

Implemented measures allowed successfully drill horizontal section according to planned trajectory

within safe ranges for static and circulating densities. For torque and drag reduction was used special

lubricant for OBM with 1% concentration by volume, resulting in 25% reduction of friction factors in the

borehole.

Results

Rock mechanics, mud chemistry and drilling practice are being stretched to their limit to solve severe

borehole stability problems in the giant Urengoy field of Russia. The earth scientist and drilling engineers’

main challenge is estimating those most elusive of all earth parameters, subsurface stress and rock

strength, and their behavior affected by mud chemistry and drilling processes. What they find out can

influence the entire development strategy of the Achimov formations in the region.

Implementation of the developed approach with joint efforts from operator company, drilling contrac-

tor and service company allowed for the first time successfully drill horizontal well in Achimov formation

without incidents. Achieved production levels for horizontal well contributed to changes in field devel-

opment plan. Operator company made a decision to switch to horizontal drilling instead of S-shape wellsand decrease total number of planned wells thus significantly reducing field development costs.

Achieved results in drilling horizontal wells supported consideration for multistage fracturing what

significantly improves productivity of the wells and project effectiveness in general.

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Documents

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