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Old 05-15-2009, 07:34 PM
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Default Mitigation of drilling problems and enhancement of

S.K.Mishra and M.M.Samal
Oil India Limited, Duliajan, Assam.
Drilling through depleted reservoirs is as difficult as drilling wildcat wells. Presence of both high and low pressure formation in the same drilling section makes the operation more complicated. Hydrostatic pressure maintained to overcome hole stability problem often leads to Loss circulation, differential pipe stuck in the depleted permeable zone. The higher mud weight also damages the reservoir by invading the producing formation defeating the core purpose of drilling. These problems can be addressed to some extent by using an additional casing string wherein the depleted reservoir can be drilled separately. An additional casing job is associated with
increase in cost, time overrun and more importantly, subsequent drilling of slim holes is also never trouble free.
Invasion of any foreign material, solids and / or liquids from drilling fluids results in formation damage which impairs productivity besides causing drilling related problems. Use of appropriate reservoir specific bridging materials in drilling fluids protect against invasion. Bridging technique is one of the solutions to such problems. This not only gives an operating margin between pore pressure and formation break down pressure but also saves the producing reservoir from invasion by drilling fluid.
Bridging products are assorted chemicals / materials of various particles sizes and functions.
These are designed based on formation pressure, porosity and pore throat size etc. These materials are so chosen that they are completely acid soluble and or bio degradable so that they do not pose problem during flowing back and well completion.
Oil India Limited (OIL) adopted various bridging techniques in drilling and work over operations in depleted reservoirs. This paper discusses in brief the technology adopted and benefits derived from it with few case studies.
Oil India Limited (OIL), a national oil company, has its existence for over last five decade in logistically difficult and geologically complex areas in North East of India. Most of its reservoirs of Naharkatia and Moran oil fields in Assam, discovered in early sixties are in their advanced stage of depletion
. Few reservoirs are much below the hydrostatic pressure.

Drilling and work over operations in depleted formation are often associated with down hole problems such as mud loss, differentially stuck pipe, well stability and formation damage. Mud losses in permeable depleted reservoir damages the reservoir due to fluid invasion defeating the core purpose of drilling / work over. OIL is using a polymeric viscousifier imparted by Xanvis to combat loss circulation in depleted reservoir to certain extent of success during work over operations. However, for highly depleted reservoirs OIL experienced much difficulties to carry out
work over jobs till a micro bubble based Invasion Control System (MB-ICS) was introduced.
In a field, where a high pressure zone is inter-bedded with normal or low pressure zones, as in case of OIL’s Eocene wells, mud weight maintained to control the high pressure zone results in loss circulation and differentially stuck pipe in the low pressure zones. These necessitate to isolate the pressured layer with a casing or to drill the entire section in one casing stage by strengthening the well bore with some bridging materials. OIL experienced the later to be more economical and time saving.
Bridging materials are selected based on lithology, pressure profile, permeability, porosity, throat size etc of the section to be drilled. The well completion programme is also an important criterion for selecting the bridging materials. These materials generally contain assorted chemicals of various sizes and functions.
OIL adopted various bridging techniques in drilling and work over operations in depleted reservoirs. This paper discusses in brief the benefits derived from using this technology with few case studies.
Bridging technology
Different bridging products are available in the market as a proprietary item of the supplier. Basically, bridging materials are customized based on reservoir pressure, permeability, porosity, pore throat size as well as type of formation. Bridging and well strengthening technique involves both chemical and / or mechanical methods. Generally, all bridging additives contain a deformable inert polymer and finely graded deformable and d egradable materials of various sizes. While the polymer molds the pore throats and micro fractures, the smaller size materials bridge the pores of large fractures of sandstone or lime stone formation. Depending upon the formation, the bridging materials are also incorporated with shale stabilizing chemicals such as
sulphonated asphalt, solid lubricants such as graphite, micronised calcium carbonate for bridging in highly porous formation The bridging materials aggregate to form a thin but firm film at the face of the permeable zone. As the over balance increases, the film gets compressed thereby the permeability drops; thus strengthens the well bore and saves the depleted formation from fluid invasion and stuck pipe providing a window.
OIL has so far used the following mud system for drilling and work over operations in depleted reservoir adopting bridging technology.
1. Carbo- bridge system to drill drain hole of horizontal wells.
2. Ultra Low Invasive Fluid (ULIF) system to drill deep Eocene wells
3. Sized salt system to drill fractured sand stone formation below the salt dome.
4. Micro-bubble system for work over operations in highly depleted producing wells
Carbo- bridge system
OIL drills horizontal wells having a drain hole of 350m to 500m in highly permeable and near hydrostatic sand stone formation. Before drilling 6 inch drain hole, the mud is changed over to Carb-bridge system inside 7 inch liner. OIL selected this system for drain hole drilling primarily to protect the permeable formation from invasion. The constituents of this system were so chosen that the chemicals are highly degradable and hence environmental friendly. As the drain hole does not contain any clay / shale no clay stabilizers are also required. The main constituents of Carbo bridge system are the following.
1. XC polymer for viscosity
2. Pre Gelatinised Starch (PGS) for fluid loss control
3. Sized calcium carbonate ( 0 and 325grade) for bridging
4. Biocide
OIL has completed ten horizontal wells (TVD @2675M ) Approximately,400m of 6 inch drain hole in each of the well have been drilled using this system. The reservoir section drilling in these wells were event free and were completed in slotted liners. Flow back after completion was very fast and did not require any acid wash or well cleaning job. The drilling fluid is incorporated with oil soluble demulsifier prior to running slotted liner to wash the probable emulsion near the sand face. Once the hydrostatic head was lowered by nitrogen shots, the wells unloaded of their own.(Table-1) The production rate from these wells was about three times the normal vertical wells from the same horizon. The production behaviors in these wells indicated carbo-bridge system to be highly effective in invasion control and at the same time facilitates in well activation
with out any additional effort.
The production behaviors of few of the wells are given below.
Q 02 DAYS <4 hrs 240KLPD NIL
S 04 DAYS <4 hrs 200 KLPD NIL
R 02 DAYS <4 hrs 196 KLPD NIL
P 01 DAY <4 hrs 170 KLPD NIL
T 05 DAYS <6hrs 140 KLPD NIL
R 04 DAYS <4 hrs 240 KLPD NIL
Z 03 DAYS <6hrs 180 KLPD NIL
A 02 DAYS <6hrs 210 KLPD NIL
Out of ten wells, one well did not produce due to reservoir uncertainty and one well produced after unloading water for a long period due to probable isolation failure.
ULIF system
OIL is producing about 60% of its total production from Paleocene - Eocene prospects. The reservoirs are 150 to 200m thick at a depth of about 3600 to 4800m and are hydrostatic (Equivalent mud weight EMW 9ppg). Formation immediately above the pay zone comprises 600 to 800m thick splintery shale and silt stones with minor sand stone and lime stones. The argillaceous formations appear to be predominantly non–hydratable with pore pressure above hydrostatic (EMW 13 ppg) and probably tectonically disturbed. Static BHT in these wells are in between 230 to 260 deg. F.
Drilling through this thick shale formation (Kopili) at deeper depths in OILs northern fields in Assam is very troublesome. Kopili shales are non reactive and probably tectonically disturbed. While drilling these shale sections OIL faced lot of problems such as excessive caving leading to well washouts, frequent held up, stuck pipes, high torque and drag and fill in almost all round trips. These problems led to poor hole condition, logging problem, bad cementation job, time overrun and more importantly formation damage arising out of long exposure time. Out of the six
wells drilled in Moran field with normal mud system, only two wells could be completed to target depth while the rest four were terminated at Langpar formation. OIL adopted many measures such as change in casing policy, mud system and mud weight without any success until it used
an Ultra Low Invasive Fluid (ULIF) with high mud weight to drill this section.
this system first in location O of Moran field. Loc O could be drilled successfully
without much hole instability problems. Following this success, the subsequent deep Eocene wells in Moran area were drilled using this system. OIL has drilled five deep Eocene wells so far namely O, P, Q, C. G with this system in its Moran and Shalmari fields. After setting the 9.5/8 inch casing about 100-200m inside Kopili shale, the 8.1/2 inch oil string stage was drilled with 12 to 13.5 ppg mud to contain the pressured Kopili shale.ULIF additive was added to the drilling fluid system in Prang formation which is about 100m above the reservoir sand Lakadang Theria. This
entire section of 500 to 700m could be drilled without any differential pipe sticking, mud loss and borehole instability problems through a positive differential pressure of 1550 to 3000psi approx. in four wells. In one well (Q) while drilling through the low pressure zone, severe mud loss was encountered probably due to presence of natural fracture. However, despite losing about 4500 bbls ( @900KL) of mud, the hole was quite stable and could be logged. All the wells could be drilled to target depth without a single case of differential sticking and within one casing string. In
one location, D in Dikom area, while drilling through a deviated angle of about 26-29 deg (J-bend well ) with CP Glycol-KCl mud system, the pipe got stuck resulting fish in hole. The hole was plugged back and the B hole was drilled with ULIF and completed successfully.
Sized Salt Drill in fluid
Sized salt system is a clay free inhibitive non-damaging reservoir drill-in fluid. It is marketed by various mud service companies in their trade mark system. It is a saturated salt system with sized salt of various sizes as the bridging agent. This system has a very narrow density range limited to 10 to 12ppg. The advantage with this system is that it protects the formation from damage by providing a sleek and firm filter cake. This cake can be removed by unsaturation of brine by dilution or by steam injection.
The main constituents of Sized salt system are 1. Viscosifier,2. Pre Gelatinised Starch (PGS) for fluid loss control ,3. Sized salt ( 0 and 325grade) for bridging 4. Saturated brine and 5. a buffer to control pH.
OIL used this system to drill one of its well through anhydrites in Bagewala field of Bikaner and Nagur basin in Rajasthan. The reservoir lies below the salt dome in this field. The prime concern was to maintain a proper hole size for good well cementation job.
Sized salt system was used to drill 12 ¼ inch hole from about 155m to 563m. While drilling at 546m, complete mud loss was encountered just above the carbonate formation. As sized salt along with calcium carbonate could not arrest loss, conventional loss circulation materials were used. The mud weight was reduced from 10.4ppg to 9.2 ppg and drilling was continued without
further loss. The 8 ½ inch hole was drilled with 10.2 ppg saturated salt mud to 610m. Complete mud loss was encountered at this depth which was cured by placing two successive cement plugs. Later the
well was drilled with 9.2ppg salt mud with calcium carbo nate as the bridging agent to a depth of 856m. Due to restriction in mud weight, (Inflow observed at @9.0 ppg and lost circulation at above 9.2
ppg ) saturated salt brine could not be used below with size salt in this well. Hence, drilling was
continued with carbo bridge system in 9.2ppg brine mud. The hole size of this well was almost
true gauge when logged after drilling to the target depth with carbo bridge system.
Micro-bubble system
Use of brines as work over fluid in depleted reservoirs invariably leads to high fluid loss into the
pay zone resulting in prolonged flow back periods for the wells to come back to their full potential.
Formulating suitable water based, easy to use, work over fluid system for wells where the
reservoir pressures have gone down much below hydrostatic pressures had been a problem for
last several years.
Micro bubble Invasion control system (MB-ICS
) is based on micro-bubble technology, where air
bubbles are generated using specific surfactants. Micro bubbles in conjunction with low shear
rate viscosity (LSRV) fluid have the ability to stop or reduce losses into formation fractures. These
micro-bubbles, which differ significantly from aerated fluids and foams, do not coalesce and upon
entering a lower pressure region within the depleted reservoir agglomerate, thus creating a strong
internal seal. This seal is minimally invasive in nature and readily cleans up with reservoir flow
back as production is initiated.
Surface density of this system is low due to presence of micro-bubbles in the system. These
reductions in density disappear down hole due to the hydrostatic pressure. The fluid can be
formulated with freshwater, seawater or saltwater.
MB-ICS doesn’t require any of the elaborate equipment / infrastructure used in air or foam drilling.
The system uses conventional fluid mixing equipment to form tough and flexible micro-bubbles.

Basic components of MB-ICS
MB-ICS requires a range of chemicals for production and stabilization of micro bubbles.
Chemicals such as Soda Ash for hardness (of mix water) control , a biocide, a viscosifier,
filtration controlling agent, pH buffer, a surfactant to produce micro bubbles and other bridging
and shale stabilizing agents are basic ingredients of this system.
Since OIL used this product for work over operation, shale stabilizing agents are not incorporated
in the system.
OIL tried this system in four depleted wells in Naharkatiya fields. The production behavior of
three serviced wells with MB-ICS system is very encouraging. No significant fluid loss was
noticed in these wells.(Table-2). However, one well #C did not produce as this well was much
below the hydrostatic pressure and this system could not arrest loss. It can be concluded that
MB-ICS served its purpose completely so far as invasion of work over fluid into reservoir is
MB ICS field trial history
Barail 3
rd sand

(NHK 17+128D block

Barail 3
rd sand

(NHK 11D + 18 block)
Jaipur Lower
(NHK 393 block)
Barail 3
rd sand

(NHK 17+128D block)
Mid perforation
depth (M) / pool
datum (M)
Below hyd.
EMW= 27.0
EMW= 15.1
EMW= 34.5
C Completed 0 0 0
Completed 06 29 23
kl/week 1.0

(by stop
Completed 0
completed 01 11 10
Post w/o
prod. rate
Pre w/o
Well Status

OIL benefited a lot by using ULIF in deep Eocene wells. This product not only reduced the cost of
an additional casing string but also eliminated differential sticking completely in wells drilled with a
positive differential pressure up to 3000psi.
Carbo-bridge system used to drill the drain hole of horizontal wells protected the formation from
invasion as is evident from production behaviour. The flow back was fast and did not require any
stimulation job.
Micro-bubble based ICS system though normally used as a drilling fluid in depleted reservoirs,
OIL used this fluid for work over operations in highly depleted pay zones successfully with many
fold increase in productivity.
Sized salt drill-in fluid has its limitation in density. As the formations in the wells drilled in
Rajasthan field had a very small window of tolerance as far as mud weight is concerned,
saturated salt could not be used as planned and the well was completed by using sodium
chloride- carbo-bridge system.
Proper selection of Bridging product based on geological condition can mitigate many drilling
related hole problems. The non productive time (NPT) due to hole instability followed by related
hole complexity can be reduced to a greater extent. The bridging products not only strengthen the
well bore but also protect the formation from invasion, thereby enhance productivity. Different
service providers sell these products in different brand names. This system can be custom
designed, if need be. Since the bridging products are custom made, the system is often
expensive. However, the benefits derived from this technology makes the overall drilling/workover
cost competitive and acceptable. These systems should be recycled whenever possible to reduce
the overall cost further.
Authors are thankful to OIL Management for granting permission to publish this paper in
Petrotech, 2009.Contributions by Mr A.K.Gogoi, Chief Chemist and Mr B.C.Dutta, chief chemist
are thankfully incorporated in this paper.
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