SPE 165338

Drilling Challenge of Unstable Hole

Condition in Kilo Field: A Case Study in Southeast Asia
Jerry Tobing and Erwindo Tanjung, Pertamina Hulu Energi ONWJ

Copyright 2013, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE Western Regional & AAPG Pacific Section Meeting, 2013 Joint Technical Conference held in Monterey, California, USA, 19−25 April 2013.

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
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Abstract

One of the drilling challenge in Kilo Field of Offshore North West Java Indonesia is unstable hole condition. The critical
formation is named Main Massive with the dominant clay-rich shale lithology. The unstable hole had caused stuck pipe,
casing delays, loss of expensive tools in the hole, and undesirable sidetracks.

Recently, there were 2 wells was drilled in Kilo Area. Based on the offset wells data drilled from this platform, there were
some wells running in normal operation and the others were having unstable hole problems. All of the offset wells were using
dispersed water based mud system in which generally, the drilling operation of those offset wells could be managed until
casing run to the bottom. The recent 2 wells encountered unstable hole problem are KKNA-5 and KKNA-3ST development
wells.

KKNA-5 was designed to drill from the new slot using the same mud system as the offset wells from surface until
intermediate section. In production section, mud system was changed to inhibited polymer referred to laboratory test result of
cutting samples from surrounding area. However the unstable hole problem was still happened causing stuck pipe and BHA
lost in hole.

KKNA-3ST as reentry well also encountered same problem. The production section was drilled using inhibited mud system;
however the unstable hole was uncontrolled while running the casing. The casing was parted and left in hole. The sidetrack
drilling was decided using previous dispersed mud system, but again the casing was set shallower due to unable to run until
casing point.

Preliminary root-caused analysis led to the mud material quality. However, following other drilling wells in other fields was
drilled without similar drilling problem in Kilo area, which mean the uncertainty or material quality was diminished. The
deeper engineering analysis was also conducted without carrying out unstable hole problems.

This field case study presents a new drilling challenge in Kilo Field. Further drilling study especially related to time-
dependent wellbore instability shall be conducted, considering drilling operation of some offset wells was running smoothly.
The integrating geomechanic study with drilling fluid design shall mitigate unstable hole problems in this area.

Introduction

Offshore North West Java Area was located in Indonesia developed since 1966. This production sharing contract area was
initially signed between PERTAMINA as Indonesia Government representative and Independent Indonesian American
Petroleum Company (IIAPCO). The contract shares from IIAPCO were divested to Indonesia Inc. (ARCO) in 1978. After
that, ARCO and BP were merged in May 2000. The company’s name was officially changed to BP West Java Ltd. on 16
February 2001. Eight (8) years later, BP was sold the shares in this field to PERTAMINA on July 1st, 2009 named as
Pertamina Hulu Energi Offshore North West Java (PHE ONWJ).
2 SPE 165338

Kilo Field as part of ONWJ Area was initially developed since 1986. KKNA platform located in this Kilo Field were used to
drill some wells since 1998 (Figure 1 shows the field location). Some of them were drilled in normal operation and the others
were having some drilling problems, such as pack off, tight spot and also casing delay. And nowadays, drilling operation in
Kilo Field becomes a serious challenge for the company related to unstable hole problem especially in Main Massive
Formation.

Main massive formation as part of Upper Cibulakan Formation consists of clay stone interbedded with very fine to medium
grained, glauconitic sandstones. Thin to thick limestones are present in the upper part. In the offshore area, this sequence is
divided into three units: ‘Massive’, ‘Main’, and ‘Post-Main’. Since 1998, there were several times earthquake happened in
the region which suspected creating fault and weaken the formation strength. In the latest well drilled 2012, based on visual
looked on the drilling cutting; it has found the troublesome shale formation has been extensively fractured, and showed
heterogenous tipe and dimension. The presence of the fissure network becomes so significant that it dominates rock
mechanical behavious and wellbore stability. The LOT has also reduced compare to well drilled in 90s.

Instability formations, which form the wall of a bore hole is very serious and common problem encountered in drilling
practice. Hole instability is not as dangerous as abnormal pressure, but nevertheless it slows down drilling process and
increases drilling cost. Commonly, there are two (2) types of formations caused hole stability problems; unconsolidated
formations and shale formations.

When a formation losses its stability, it enters the well. The consequences of this phenomenon are as follow:
a. Accumulation of various type and size of caving and cuttings in the hole, increase of solids content in the mud,
followed by related shortcomings. Figure 2 shows the cutting sample from KKNA-5 well.
b. Bridging and filling up the bore holes which necessitate drilling the plugs of caving to reach the bottom. In its turn
drilling the caving results in bit wear, shorter footage per bit, slower drilling, and longer rig time.
c. Drill pipe sticking
d. Hole enlargement, which results in an increase of amounts of cement, mud making materials, poor cementing, and
poor removal of cutting from the bore hole.

Especially in Kilo Field, unstable hole problems were suspected due to shale formation. Typical of shale as early suspicion
refers to below phenomenon are:
- The shale showed time dependent wellbore instability in its reactivity
- The shale showed amount and character of any fracturing or weak planes in the formation as the cause weaken its
integrity significantly.
- The shale could be heterogeneous and its in-situ stress is anisotropic.

Fundamental of Hole Stability

A rock in the earth curst experiences three dimensional compression due to the effect of the overburden pressure. Besides the
compression stresses caused by the overburden pressure a formation may experience some tectonic stresses resulting from
diastrophic movements of the earth crust.

In any point of the earth crust an element of rock is in the state of equilibrium. Such state of equilibrium exists until the rock
is penetrated by a bore hole. An element of rock on the wall of the wall of the bore does not have any longer the support of
the rock massive – the side pressure, which existed before and provided stability. Instead of the formation side pressure the
formation on the wall of the hole experiences hydrostatic pressure of drilling fluid. This pressure is lower than the original
side pressure of the rock massive. Due to this, stresses appear on the wall of the bore hole.

Consolidated formations of high strength experiences some elastic deformations, which do not affect hole stability.
Unconsolidated formations as well as shale which have may microscopic fractures and bedding planes are not strong enough
to resist stresses. They start falling into the bore hole. Some natural and induced fractures usually contribute to the loss of
formation stability.

Gravity is the contributing factor to instability of unconsolidated formations. Fortunately, instability of unconsolidated
formations is not a very series problem as it can be rather easily overcome if a good filter cake is formed on the wall of the
bore hole and care is taken to avoid any mechanical damage of such formation in the process of drilling.
SPE 165338 3

Sloughing Shale

Shale is a sedimentary rock formed by the deposition and compaction of sediments over periods of geological time. It is
primarily composed of clays, silt, water, and small quantities of quartz and feldspar depending on water content, shale may
be a highly compacted rock or a soft, unconsolidated rock, normally described as mud or clay shale. Shale may also exist in a
metamorphic form such as slate, phyllite and mica schist.

In oil well drilling, two types of sedimentary shale are normally encountered unconsolidated shale and compacted shale.
Drilling of both types results in sloughing or caving of the shale section. Drillers normally refer to the type of hole instability
resulting from drilling shale sections as sloughing shale. Research has shown that the severity of shale sloughing is related to
the percentage content of montmorillonite (or active clay content) and the age of the rock.

Factors influencing shale sloughing can be conveniently divided into three groups; mechanical factors, hydration factors, and
miscellaneous factors.

1) Mechanical Factors
Mechanical factors affecting shale sloughing are attributes largely to the erosion effects caused by the annular flow of
mud. Erosion of shale is directly related to the degree of turbulence in the annulus and mud viscosity. Most hydraulic
programs are designed with the object of providing laminar annular flow.
Other mechanical effects include breakage of shale due to impaction by the drill string and caving due to horizontal
movement of the shale section. The lateral effect is due to the fact that creating a hole in the earth disturbs the local stress
system, which leads to dynamic movement within the shale section. This movement leads to breakage of the shale bed
adjacent to the well into small fragments which fall into the hole.

2) Hydration Factors
A number of factors are involved in the hydration of shale. For practical purposes, shale hydration force and osmotic
hydration are recognized and are quantifiable. Shale hydration force is related to the relief of compaction on the shale
section. Osmotic hydration is related to the difference in salinity between the drilling mud and formation water in the
shale.
During sedimentation, the shale section is progressively compacted by weight of the overburden. The force of
compaction squeezed out large percentage of the adsorbed water and water from the pores of the shale. The compaction
force is equal to the matrix stress. The drilling of shale section relieves the compaction force on the borehole face and as
a result a shale hydration force is developed. The shale hydration force is approximately equal to the matrix stress.
Osmotic hydration occurs when the salinity of the formation water of shale is greater than that of the drilling mud. In
water based muds, the shale surface acts as a semi permeable membrane, across which osmotic hydration takes place.
Since osmotic hydration is dependent on the difference in salinity between the formation water in shale and drilling mud,
the process can result in either an adsorption or desorption force. An adsorption force is developed when the formation
water of shale is more saline than the drilling mud. A desorption force is produced when the salinity of drilling mud is
greater than that of the formation water of the shale.
Adsorption of water by shale usually leads to dispersion and swelling. Dispersion occurs when the shale subdivides into
small particles and enters the drilling mud as drill solids. Swelling occurs due to increase in size of the silicate minerals
making up the clay structure, and if the developed swelling pressure increases the hoop stress around the borehole above
the yield strength of the shale, hole destabilization takes place. Hole destabilization manifests itself by means of caving
or sloughing shale.

3) Miscellaneous Factors
Shale sloughing has been correlated with a number of factors which were found accelerated the rate of shale heaving into
the well bore.
Dipping shale was found to slough more than horizontally laid shale. This is because during adsorption of water, shale
expansion takes place in direction perpendicular to its bedding planes, which results in greater shale heaving when the
section is highly dipping.
The process of heaving in brittle shale containing no active clays is explained by the penetration of water between the
bedding planes and micro-fissures of the break the cohesive forces between the fractures surfaces causing the shale to
fall apart.

In abnormally – ore geo-pressured shale – the water content of the rock is much higher compared with that of normally
pressured shale. In addition, the plasticity of the shale is abnormally high relative to the overburden load. Hence, when a hole
is drilled through a shale section containing abnormal pressure, the shale will be squeezed into the hole owing to the
difference between formation pore pressure and mud hydrostatic pressure. It follows that if such abnormally high pressure
were catered for prior to drilling the shale section, sloughing of shale could be reduced.
4 SPE 165338

KKNA-5 Drilling Operation

KKNA-5 well is development well that was started to be drilled on 18 May 2012 and completed on 26 August 2012 within
55 operation days. The architecture of this well is 30” conductor pipe x 13-3/8” surface casing x 9-5/8” intermediate casing x
7” production casing liner.

The operation sequences of this well as follow:

1. Moved in. soft pinned, jacked up, preloaded, positioned rig to KKNA-5.
2. Rig up Hammer, run 30" conductor to seabed, drove 30" for refusal.
3. Nipple up diverter and pressure tested 29-1/2" diverter.
4. Made up 17-1/2" BHA, cleaned out conductor, drilled/ logged/ surveyed 17-1/2" hole using 7% KCl Polymer mud
system, wiper tripped, conditioned hole, and pulled out of hole.
5. Run and cemented 13-3/8" casing.
6. Nipple down diverter, cut 13-3/8 casing, installed wellhead, nipple up BOP and pressure tested.
7. Made up 12-1/4" BHA, cleaned out conductor, drilled/ logged/ surveyed 12-1/4" hole using Low pH Desco mud
system, wiper tripped, conditioned hole, and pulled out of hole.
8. Run and cemented 9-5/8" casing.
9. Made up and function tested 8-1/2" directional BHA, drilled/ logged/ surveyed 8-1/2" to 6280’ MD using Low pH
Desco mud system with slow penetration, then changed to 9% KCl Polymer mud system with better performance.
Circulated hole clean, pulled out of hole, but unfortunately BHA got stuck. Retrieved some of BHA part, attempted
to fish out remaining BHA with negative result.
10. Plugged and abandoned the original well.
11. Run whipstock assembly and milled casing window.
12. Made up and function tested 8-1/2" directional BHA, sidetrack drilled/ surveyed 8-1/2" hole to 8890’ MD TD using
7% KCl Polymer mud system, pulled out of hole, and laid down BHA.
13. Run and cemented 7"Liner. Some of the 7” liner hanger assembly dropped and became fish
14. Cement squeezed top liner packer and performed milling cement job.
15. Performed fracturing job including perforation job.
16. Run 3-1/2" tubing and accessories.
17. Nipple down BOP and nipple up Christmast Tree.
18. Rig down, skidded in, secured cantilever, jacked down, jet legs, and moved off

KKNA-3ST Drilling Operation

KKNA-3ST well is reentry well that was started to be drilled on 24 June 2012 and completed on 08 August 2012 within 45
operation days. The architecture of this well is 30” conductor pipe x 13-3/8” surface casing x 9-5/8” intermediate casing x 7”
production casing x 3-1/2” monobore liner.

The operation sequences of this well as follow:

1. Skidded cantilever over slot #3.
2. Secured and killed the well, nipple down Christmast Tree, nipple up and tested BOP.
3. Plugged and abandoned KKNA-3 well and recovered single 2-7/8” tubing.
4. Performed window mill and bit then scraper run.
5. Run 9-5/8” whipstock and MWD to ± 2116’ MD. Set and weight tested whipstock.
6. Milled window exit through single casing of 9-5/8” 53.5 ppf L-80 casing at 2116’ MD sidetrack point.
7. Cleaned window area to ensure the 8-1/2” BHA will pass.
8. Run 8-1/2” BHA and directionally drilled / logged/ surveyed 8-1/2" hole to section TD at ± 6665' MD / 4660' TVD
using 7% KCl Polymer mud system. Conditioned hole, performed wiper trip, and pulled out of hole.
9. Run 7” casing with washed and reamed, observed 7” casing parted with TOF at 2107’ MD. Recovered remaining 7”
casing with success.
10. Set 9-5/8” cement retainer and cement squeezed operation.
11. Run 9-5/8” whipstock and MWD to ± 1785’ MD. Set and weight tested whipstock.
12. Run 8-1/2” BHA and sidetrack directionally drilled/ logged/ surveyed 8-1/2" hole to section TD at 6680’MD using
Low PH Desco mud system. Conditioned hole, pulled out of hole.
13. Run and cemented the 7" casing but casing was unable to go to bottom
14. Nipple down BOP, installed tubing head, and nipple up BOP.
15. Set Whipstock at 6115’MD and milled casing window.
16. Run 6-1/8” BHA and drilled/ logged/ surveyed 6-1/8” hole to well TD at 8850’MD Low PH Desco mud system,
conditioned hole, and pulled out of hole.
SPE 165338 5

17. Displaced hole with completion fluid and POOH.

18. Picked up and run 3-1/2” lower completion and set hanger.
19. Performed fracturing job.
20. Run 7” scraper. Reciprocated down through perforation to packer setting depth. Perforated intended production
zones.
21. Run 3-1/2" upper completion with production packer.
22. Spaced out tubing hangers. Sting into snap latch locator. Land tubing hangers and set production packers.
23. Install BPV. Nipple up BOP and nipple up Single Christmast Tree.
24. Performed unloading well.
25. Secured the well and moved the rig off the location.

Problem Analysis

Mechanical Factor
Stuck pipe problem on KKNA-5 and casing delays on KKNA-3ST due to unstable hole condition might be related to several
factors. High probably, the formation type caused hole stability problem in Kilo Field is shale formations. The mechanical
factor caused by excessive turbulence flow on the annulus and local stress disturbance by creating the hole might be
happened. The used of optimum (not as high as possible) flow rate without ignoring hole cleaning based on hydraulic
program, and also optimum drilling parameter based on vibration simulation have to be applied to eliminate unstable hole
problem.

Hydration Factor
The hydration factor might be also caused the problem in Kilo Field. The offset wells data on KKNA-3 and KKNA-4 showed
no significant problem during drilling operation compared to KKNA-3ST and KKNA-5 (Figure 3). These wells were using
drilling mud containing 15,000 – 19,500 mg/L of chloride content (Cl-), whilst the formation salinity in Kilo Area is around
20,000 – 30,000 mg/L. Even though adsorption force might be happened on these two wells, it seems the salinity discrepancy
is not too high.

A desorption force was occurred on KKNA-3ST and KKNA-5 well when the salinity of drilling mud (40,000 – 60,000 mg/L)
is greater than that of the formation water (20,000 – 30,000 mg/L). The flow of water from low-salinity fluid to high-salinity
fluid has to be considered. As long as osmotic outflow from the formation is able to counter-act the mud pressure penetration,
the chemical potential difference across the semi-permeable barrier will prevent elevated pore pressure and maintain the
effectiveness of mud support. The salinity content chart of KKNA-3ST and KKNA-5 is shown in Figure 4.

Mud Pressure Penetration Factor

Based on the explanation above, the mud pressure penetration has important role to avoid unstable hole problem. Because
shale formation exhibit low permeability and filtration rates, when a drilling fluid comes in contact with a shale formation, an
inadequate amount of drilling fluid solids will deposit on the wall of the wellbore to create an effective flow barrier. Due to
the first mechanism – mud pressure penetration – small amounts of mud filtrate begin to penetrate the shale, increasing pore
pressure near the well bore over the next few days. As pore pressure increases, it causes a decrease in the effectiveness of
mud support. It will gradually lead to unstable hole problem and failure of the wellbore wall, which can lead subsequent
drilling problem.

ONWJ Area, especially Kilo Field is a mature oil and gas field. The formation pressure on KKNA wells are less than 8.5 ppg.
The offset well data of KKNA-3 and KKNA-4 was using 9.2 – 9.6 ppg mud density. But the unstable hole problems on
KKNA-3ST and KKNA-5 well were recently handled with 9.4 – 10.2 ppg mud density to accommodate high inclination well
profile, caused the stuck pipe and casing delays. Figure 5 shows mud density used on those wells.

Clay Material Factor

The severity of unstable hole due to shale problem is related to the percentage content of montmorillonite (or active clay
content). The used of inhibitive mud system might be eliminated the problem. However based on X-Ray Diffraction Analysis
of cutting sample from KKNA-3ST, the clay type is not reactive material, which is mostly Smectite and Kaolinite as shown
in Table 1. That is why the drilling operation on KKNA-3ST and KKNA-5 did not see significant differences between the
used of inhibitive mud system and dispersed mud system.
6 SPE 165338

Time Dependent Factor

Based on the offset wells data drilled from this platform since 1998, there were some wells running in normal operation and
the others were having some drilling problems, such as pack off, tight spot and also casing delay. All of the offset wells were
penetrating Main Massive Formation using dispersed water based mud system in which generally, the drilling operation of
those offset wells could be managed until casing run to the bottom. The performance chart as shown in Figure 3 describes
each open hole section of KKNA-3 and KKNA-4 was left uncased not more than 4 days. However, KKNA-3ST and KKNA-
5 wells that were encountered unstable hole problem were open more than 4 days for each section. Table 2 shows the
operational time summary of these wells.

Others Factor
If we compared with some offset wells in Kilo Field drilled in the past, drilling operation could be managed until casing run
to the bottom. Several different things on recent drilling operation on KKNA-3ST and KKNA-5 compared with offset wells
are high inclination well and lots of earthquake factor happened recently in Indonesia. Maximum inclination of KKNA-3ST
is 75 degrees while KKNA-5 is 59 degrees. Compared with offset wells of KKNA-3 and KKNA-4, the maximum incliniation
is within 42 to 45 degrees.

Conclusions and Recommendations

Hydraulic calculation and vibration simulation shall be conducted properly to minimize the risk of unstable hole problem due
to mechanical factor.

The problem of unstable hole condition might be directly related to osmotic process between drilling mud and shale
formation. The salt concentration in mud system should be matched with shale section. In this case, the osmotic (or
hydration) force is equal to the shale hydration force and the pressure causing water to flow between mud and shale is zero.

Mud pressure penetration might be increasing pore pressure near the well bore over the next few days causes a decrease in
the effectiveness of mud support. The analysis of mud density plan shall be conducted and also the used of bridging and
plastering mud materials to reduce fluid invasion into formation.

Time dependent factor might be the cause of unstable hole problem. To prevent this factor, it is better to minimize the time
for which an open hole containing a shale section is left uncased. Further drilling study especially related to time-dependent
wellbore instability shall be conducted.

Trajectory design with high inclination might be the factor of unstable hole problem. Dipping shale was found to slough more
than horizontally laid shale especially in heterogeneous shale with anisotropic stresses. That is why the hole deviation should
be kept to a minimum number to minimize the wellbore failure.

The integrating geomechanic study such as in-situ overpressure, weak bedding plane, chemical activity, insufficient mud
weight, fractured shale formation, and also sufficient drilling practice combined with drilling fluid design shall be conducted
to mitigate unstable hole problems in Main Massive Formation of Kilo Field.

Acknowledgements
The authors express their appreciation to the management of Pertamina Hulu Energy Offshore Northwest Java (PHE ONWJ)
for the permission to publish this paper.

Reference
Ewy, R. T., Morton, E. K. (2008). Wellbore Stability Performance of Water Base Mud Additives. SPE Annual Technical
Conference, Denver, Colorado, USA.
Farahat, M. S. Drilling Problem Handbook. pp 35 – 38.
Heidug, W.K. and Wong, S.-W. (1966). Hydration Swelling of Water-absorbing Rocks: A Constitutive Model. Int. J.
Numerical and Analytical Methods in Geomechanics, 20: 403-430.
Hemphill, T. (2005). Management of the Safe-Operating Window: It's More Than Just Density. SPE Latin American and
Caribbean Petroleum Engineering Conference, Rio de Janeiro, Brazil
Mody, F.K. and Hale, A.H. (1993). A Borehole Stability Model to Couple the Mechanics and Chemistry of Drilling Fluid
Interaction. Proc. SPE/IADC Drilling Conf.
Tan, C. P., Qadmani, M. A., Povstyanova, M., Mohiuddin, M. A., Rahim, M. H. A. (2009). Successful Mitigation of Time-
Dependent Shale Instability in Khafji Field through Drilling Fluid Design Optimization. SPE Saudi Arabia Section Technical
Symposium and Exhibition, Alkobar, Saudi Arabia.
SPE 165338 7

KKNA

Figure 1: ONWJ Field Location

Figure 2: Cutting Sample of KKNA-5

8 SPE 165338

Figure 3: Performance Chart of KKNA Wells

Figure 4: Salinity Content Chart

SPE 165338 9

Figure 5: Mud Density Chart

Table 1: X-Ray Diffraction Analysis

Clay Minerals Carbonate Minerals Other Minerals Total
% % % %

Depth
K Feldspar

No.
Plagioclase

Carbonate
feet
Kaolinite

Dolomite
Smectite

Chlorite

Siderite

Quartz
Calcite

Pyrite

Other
Clay
Illite

Well KKNA – 3ST

1 2300 16 5 15 2 7 - 5 40 1 2 7 38 12 50

2 5300, 5600, 5900 - 10 18 12 4 - 3 46 1 2 4 40 7 53

10 SPE 165338

Table 2: Operational Time Summary