SPE-172251-MS

Cation Drilling Mud for Longhole Drilling

Gaidarov Mitalim Magomed-Rasulovich, Khubbatov Andrey Atlassovich and Gaidarov Azamat Mitalimovich,Gazprom VNIIGAZ LLC; Baizakov Marat Korkitovich, KazNIPImunaygas JSC

Copyright 2014, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE Annual Caspian Technical Conference and Exhibition held in Astana, Kazakhstan, 12–14 November 2014.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contentsof 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 reflectany position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the writtenconsent 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 maynot be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Summary

Introduction

Cation drilling mud for longhole drillingIncrease of geological prospecting works and target well depth requires improvement of drilling tech-nology as well as scientific and technical level for well construction upgrading. One of the significanttrends in longhole construction upgrading is application of effective drilling mid and makeup technologyallowing to prevent operating problems and drilling costs minimization.

The following main problems with any of the operating fluid are expected in course of longholeconstruction:

– poor condition of clay argillaceous rock. In spite of that considerable advances have been made,problem of the rock poor condition still exists and subject to be solved.

– low thermal stability of drilling mud. Depending on geotechnical conditions low thermal stabilityof drilling mud may result in major complications up to well abandonment. Thermostable waterbase mud rated for 160 °C and above and up to 130-140 °C in aggressive salt scenario is practicallynon-existent.

– problems with scaling and development of abnormally pressured zones.

Longhole construction process is considerably compounded by aggressive conditions. Increase of geo-logical prospecting works including up to 15000-25000 m depth wells construction is planned as part ofdevelopment of Kazakhstani Sector of the Caspian Sea. A distinguishing feature of the wells in thePeri-Caspian Depression is deep producing depth, temperature aggressiveness, presence of abnormallypressured zones, poor condition of clay argillaceous rock, different mineralization of formation water andinterstitial water as well as noncompatible complexes - post-salt, salt and pre-salt. Although wide fieldexperience in drilling of the Peri-Caspian Depression wells is available and theoretical work aimed todrilling mud perfection has been done, it must be admitted that basic problems of well constructionupgrading are caused by incorrect fluid specification for drilling conditions.

These problems may be solved by means of newly developed methods and drilling mud compositionthat meet longhole drilling requirements.

For historical reasons, standard water drilling mud applied abroad and in our country presents claysuspension stabilized with anionic-nonionic high-molecular polymer compositions. The following aremostly used as stabilizing polymers: water soluble cellulose ester (anionic and nonionic), starch (non-ionic), acrylic reagents (anionic), lignosulfonates (anionic) and lignitic material (anionic).

A wealth of accumulated experience in oil and gas well drilling gives evidence of material weaknessesof anionic-nonionic drilling mud.

Major weaknesses of the standard anionic-nonionic drilling mud, including faults due to the Peri-Caspian Depression conditions, are listed below:

1. Low inhibiting properties that cause increase of structure rheological coefficient (“jumps” ofprocess parameters) and excess volume of drilling mud;

2. Low wellbore stability in clay conditions;3. Biodegradation of anionic and nonionic polymers;4. Poor resistance to polysalt and temperature aggressiveness as well as to pH environmental change;5. Noncompatibility of nonsaline and saline water that causes deterioration of mud properties and

increase of stabilizer demand etc.6. Complexity of mud properties control and regulation in course of well drilling etc.

Repeated attempts to remove these shortcomings of the standard anionic-nonionic drilling mud had noeffect. As a rule, perfection of the traditional anionic-nonionic drilling mud is aimed to replacement of anyreagent by another one or quantity change of equivalence ratio. Such methods do not allow to createeffective drilling mud and eliminate the above defects. But the indicated weaknesses may easily beremoved if the system is stabilized with cationic polymer. One of the cationic-stabilized mud systems(“Katburr” [k�tbur] cationic stabilized mud) upgrade has been tested in well No.939 in trough of theAstrakhan gas condensate field. Major faults of the anionic-nonionic drilling mud and ways to eliminatethem are detailed below.

1. Low inhibiting properties. Clay, especially expansive and dispersive, ingresses into drilling mud andincreases colloidal and solid content, maximizes structure rheological coefficient, deteriorates unctuous-ness that to adversely affects drilling process. Presence of the expansive and dispersive clay requiresperiodical mud diluting as treating facilities are inefficient. Periodical diluting leads to overspend ofchemicals and excess mud volumes that need to be utilized etc. For example, excess mud volume subjectto utilization during drilling in trough of the Astrakhan gas condensate field was from 2000 m3 to 2500m3.

Various inhibiting drilling muds were developed for drilling and retention of stability of the plastic clayintervals. Complications related to wellbore stability loss are of frequent occurrence in oversaline depositsof the Astrakhan field trough where loose sticky clay is interlaid with poorly consolidated sandstone anduncemented sand. Use of inhibiting drilling mud is focused on minimization of clay expansion anddispersion as well as sludge impact on structure rheological and filtration coefficient of the drilling mud.Wells drilling in thick clay masses is always accompanied with excess drilling mud volumes regardlessapplication of highly inhibiting systems. Today none of the inhibiting drilling muds used in trough of theAstrakhan gas condensate field may be selected and used for drilling in oversaline deposits withoutleading to excess volumes. Use of highly inhibiting muds in oversline deposits of the Astrakhan fieldtrough is ineffective both for retention of wellbore stability and mud volume reduction. We are of theopinion that inhibiting properties of the drilling mud shall be assessed in field conditions based on mudvolume accumulated during interval drilling process, i.e. the more excess mid volume, the worst inhibitingproperties and conversely. If the used drilling mud is not accumulated in excess volumes during drillingprocess, this system is prefect for the drilled interval.

Principle of clay inhibition by cationic polymers is rather differs from inhibition by anionic polymers.Clay nature is that «�» and «�» defect zones are present on particles surface and actively engage with

2 SPE-172251-MS

water molecules. Cations and/or anions are adsorbed on the active zones. This leads to charge neutral-ization and minimization of clay-water interaction. Contact of clay rocks with operating fluid based onanionic polymer leads to electrostatic interaction of cationic clay zone with polyanion resulting in pooradsorption of the latter on the charged surface. Inhibition of clay rock by anionic polymer consists inability of the latter to encapsulate or cover the exposed surface creating structured water layer and thusslowing water penetration down. As anionic areas are prevail on the clay surface, water is activelypenetrated into the rock through these areas. Injection of electrolytes leads to partial destruction of thestructured water layer and better water penetration. Better inhibiting effect may be achieved by retainingthe structured water layer owing to increased anionic polymers.

Cationic polymers are better adsorbed on the anionic clay surface. Owing to anionic predominance onthe clay surface, cationic polymer is distributed and adsorbed more evenly and stable structured waterphase is generated in near surface layer. Cationic polymer adsorption neutralizes the rock charge andreduces its electrostatic interaction with water. The formed stable structured water layer creates monitor,which prevent water penetration into the clay. Injection of different electrolytes does not destruct thestructured water layer, quite the opposite, adds on it, therefore better inhibiting effect is observed.

Preliminary analyses of inhibiting properties of the cationic and anionic- nonionic muds have shownthat cationic system is more preferred. Inhibiting properties were evaluated based on sample weightchange. The sample was held in the tested cationic solution and relative sample weight increase (%) wasobserved (fig.1).

Weight of the sample is increased depending on liquid quantity.Relative increase (%) was calculated by the following formula:

where m2 – weight of the sample held in the tested mud for 7 days;m1 – weight of the initial sample.2. Low wellbore stability in clay conditions. Today, regardless achieved success in sphere of drilling

muds, there is no shared vision on what operating fluid shall be used for injection and retaining clay rockstability.

Figure 1—Clay sample weight change (%) depending on inhibiting mud type

SPE-172251-MS 3

When drilling mud contacts with clay watering is produced – water phase is penetrated into clay. Watermolecules are penetrated into interstructure space of the clay particles. Interaction of the penetrated watermolecules with clay minerals governs clay behavior. If the penetrated filtrate interacts with clay structuralelements and retains stable bond, clay rock in the wellbore attains stability. At that, wellbore stability ispotential at clay expansion when water molecules penetrate into interpack space and are structured as wellas at no clay expansion case when water molecules can not be structured in the interpack space. Inpractice, interpack expansion during clay deposit drilling shall be minimized by using inhibition drillingmud. Owing to inhibition properties of the mud clay expansion and drilled clay sludge dispersion isminimized that allows to maintain stable structure rheological coefficient and to reduce mud excessvolume, treating costs, chemicals and utilization. Noninhibiting drilling mud may also provide wellboreclay stability, but the drilled clay sludge is expanded, dispersed and leads to destabilization of the systemswith all consequences. However, if the penetrated filtrate interacts with clay structural elements butweaken structural bond, clay loses its stability. In this case drilling mud is always conferred with bondingproperties allowing for stable bond and wellbore stability.

Potential to provide wellbore stability in clay conditions using traditional anionic-nonionic drilling mudis high, but gives place to cationic mud. It appears that this is due to insignificant understanding of claydeformation and destruction mechanism. Many scientists and specialists are of the opinion that increaseof the mud inhibiting properties directly results in wellbore stability increase. In practice, in case of claystability loss, wellbore stabilization is provided by inhibiting mud not depending on destruction behavior.It is known that clay rock deformation and destruction may be in two ways: plastic (clay) or friable (shale,argillite).

Bond between structural elements shall be maintained in order to ensure wellbore stability in clayconditions. Water and water electrolytes penetrated into the structural elements separate them and weakenbond: the produced hydrogen bond is very weak and therefore clay integrity is lost. Water molecules ofthe water-based polymers generate stable hydrogen bond between clay structural elements, and clayretains stability. Polymers with water molecules create water mesh and bond clay particles. But expansionprocess is running on and clay plasticity is increased. Nevertheless, wellbore stability is achieved.

Polymers slow down clay expansion rate and reduce expansion index, whereas clay expansioninhibitors increase expansion rate and reduce expansion index of the clay rock.

Dipole-dipole interaction is observed between water dipole molecules and polymers if the polymer ifnonionic, and/or ion-dipole interaction is provided if the polymer is ionic. Hydrophobic effect is alwaysobserved between the polymer and water molecules: between polar chains of polymer and watermolecules. Interparticle interaction energy increases with molecular weight increase. As a result, watersoluble polymers as against electrolyte solution, produce regulating effect on water molecules thatconsiderably increases water viscosity.

Thus, injecting the polymers has regulating effect on water molecular structure that allows to changepolymerized and monomeric water ratio by forming superstructures with hydrogen bond. Owing to thechanged polymerized and monomeric water ratio, changes in dispersed environment and drilling mudproperties are also observed.

Low-molecular polymers are distributed evenly but structure the water by “islands”. High-molecularpolymers better structure the water, therefore solid polyhedral molecular skeleton is produced.

Based on the above it may be concluded that minimal index of clay expansion is provided by expansioninhibitors and maximal – by polymers. Maximal clay stability may be attained by stable interparticlehydrogen bond provided that distance between the particles is minimal. Combining polymers andinhibitors allows to achieve optimal results in clay stability. But it must be kept in mind that the increasedclay stability is accompanied with partial loss of inhibiting properties by drilling mud.

Stability of fractured shale is attained by using high-structure drilling mud containing hydrophilic andhydrophobic adhesive colmatant that provide “roughcast” of wellbore [A.A. Khubbatov (2012)].

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In this case nonionic polymers and colmatant compatible with cationic polymers play a decisive rolein wellbore stability.

3. Biodegradation of anionic and nonionic polymers. The most active bacterial cells leading to polymerbiodegradation are mainly anionic. Such anionic-nonionic polymers as starch, water soluble celluloseester, biopolymer and lignosulfonates in drilling mud present bacteria growth-supporting environment.Use of even small quantity of cationic polymers neutralizes bacteria charge. Bacteria being neutralizedmay not induce biodegradation of polymers used in the well construction.

If biodegradation already takes place in the well construction process, chemical treatment of theoperating fluid by injecting the cationic polymer, the system may be revivified (table 1). During drillingprocess in Peschanaya (Krasnodar Territory) bottoms up with high content of bacteria leading to mudbiodegradation were periodically observed. Due to this new mud portions have to be made up instead ofbottoms up subject to utilization. Add of even large quantity of bactericides appeared ineffective. Rig testsaimed to bottoms up revival using cationic polymer have shown high efficiency of such treatment (table1).

Thus, cationic polymer acts not only as stabilizer and inhibitor, but also as bactericide (antiseptic).4. Poor resistance to polysalt and temperature aggressiveness as well as to pH environmental change.

All known anionic-nonionic stabilizing polymers have poor salt and thermal resistance. Acrylic agents aremore resistant to temperature aggressiveness but lose stabilizing properties in salt environment andprecipitate in the presence of calcium and magnesium cations.

Drilling muds based on cationic polymers demonstrate resistance to high temperatures up to 220 °C andpolysalt aggressiveness in any concentration.

Moreover, polymers in the drilling mud, which are thermolabile and exposed to polysalt aggressivenessmay be revivified and attain high resistance.

All currently used anionic-nonionic stabilizing polymers depend on pH of the drilling mud. Optimal pHcontent in the mud for all anionic-nonionic polymers is in the range of 8 – 9 that affects clay rock stability.Maintaining optimal pH level in the drilling mud at high temperatures and salt aggressiveness is verydifficult.

Change of pH level in the range of 3 - 10 has no effect on cationic drilling mud properties, i.e. thereis no need in regulating pH level in the cationic drilling mud.

In this case cationic polymer shows high resistance to pH change in the range of 3-10.5. Noncompatibility of nonsaline and saline water. Prior to opening the salt deposition up, salinization

with technological salt up to saturation with simultaneous stabilizer injection is provided. Depending on

Table 1—Drilling mud revival using cationic polymer in Peschanaya

Content

Process parameters

�, kg/m3Fluid loss

indicator, cm3Gel strength,

daPA Viscosity, mPa*s Yield point, Pa pH

Init. (mud fromwell)

1160 37 110/150 15 18.6 7.5

Init.�0.2%bactericide

1160 37 110/150 15 18.6 7.5

Init.�5% cationicpolymer

1270 12 10/20 34 7.8 7

Init.�5% cationicpolymer �1%starch

1270 5.5 10/15 42 8.3 7

in 45 days of holding at 25-35 °C

N2 1040 41 80/130 11 10 6.5

N4 1270 6 5/5 28 3 7

Note: Muds N1 and N3 were held for a period of 45 days at temperature 25-35 °C provided periodical barbotage.

SPE-172251-MS 5

clay phase in the mud, salinization may lead to either coagulative thinning or coagulative thickening. Asclay phase in the drilling mud is almost always above admissible values, salinization usually leads tocoagulative thickening. Due to this prior to salinization, clay concentration is always reduced by dilutingwith water stabilizing solutions. Quantity and type of stabilizer depend on required fluid loss indicator. Allthe standard anionic-nonionic drilling muds being salinized are exposed to destabilization degradingprocess properties and parameters, e.g. fluid loss indicator grows up in tens of times. Quantity ofstabilizers (fluid loss reducing agent) is increased by 5-10 times as compared to nonsaline systems.Cationic drilling muds, as compared to all known water drilling muds, keep stable process parameters andif they are changed then only for the better.

6. Complexity of mud properties control and regulation in course of well drilling. All currently useddrilling muds are multicomponent that creates some difficulties in mud properties control and regulation.

Cationic systems are low-compound as basic components of the mud are multi-purpose. Therefore lessproperties and parameters are controlled provided cationic drilling mud application.

Let us analysis efficiency of the cationic drilling mud evidence from one of the cationic-stabilized mudsystems (“Katburr” [k�tbur] cationic stabilized mud) upgrade being tested in well No.939 in trough of theAstrakhan gas condensate field during drilling of post-salt and salt deposits.

Drilling applying Katburr was planned in the post-salt deposits (interval 0-2900 m).Post-salt deposits (0-2900 m). Application of Katburr takes place at the depth of 60m providing

additional treatment of the circulating fluid. Drilling mud before treatment has the following parameters:density 1140 kg/m3, relative viscosity 70 s, fluid loss indicator 6 ml, gel strength/10 15/25 daPA, pH 10.Changing for Katburr is provided by injecting cationic polymer, which contains up to 5% of quaternaryammonium compounds (Q.A.C.) in marketable condition, up to 3% of potash chloride and up to 3-4% offloatation reagent “Oksal” T-92, after that the mud had the following parameters: density 1140 kg/m3,relative viscosity 28 s, fluid loss indicator 3 ml, gel strength1/10 0/5 daPA, viscosity 30 mPa*s, yield point2 Pa, pH 10. Further pH level was 7. Q.A.C. cationic polymer content in the mud was brought to andmaintained at 7-8%. Process parameters of cationic mud during conductor drilling have not been changed.

Conductor drilling was carried out using bore bit of diameter 508mm and completed by lowering thecasing of diameter 426 mm for the depth 350 m.

Further, up to the depth 1849 m drilling was successful. Structure rheological coefficient was regulatedupwards, i.e. by injecting biopolymer and aluminum hydro-gel as well as downwards – by injectingQ.A.C. cationic polymer. At the depth of 1849 m it was decided to trip out hole for geophysicalmeasurements. Process parameters of the drilling mud before hoisting are given in table 2.

Table 2—Process parameters of cationic mud during drilling

Interval, m

Process parameters

�, kg/m3 Relative viscosity, s Fluid loss indicator, ml Gel strength1/10, daPA Viscosity, mPa*s Yield point, Pa

350-732 1240 37 2 0/5 35 4

732-1200 1250 50 3 5/10 38 6

1200-1494 1260 56 2 7/10 43 7

1494-1727 1280 56 4 46/67 37 11

1727-1849 1280 80 2 34/76 40 14

Hoisting, redressing (new borehole drilling)

500-791 1250 90 5 25/48 40 12

791-1849 1220-1260 50-80 2-5 20/55 36 13

�� 2900 1280-1310 70-100 0.5-1.5 15-25/30-50 35-45 11-16

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Following aborted attempt to run geophysical instrument in the hole (“stop” at depth 357m), it wasdecided redress borehole by 2 drill collars. Borehole redressing up to depth 1849 m was performed onload. Process parameters after redressing have almost not been changed (table 2).

Analogue geological complication due to neogene and paleogene lithology presented by interlaying ofloose plastic clay, poorly consolidated sandstone and uncemented sand takes place in the earlier drilledwells No. 723 and No. 2 Devonskaya. Conductor lowering to well No. 2 Devonskaya to block off instableneogene deposits and part of paleogene deposits was provided to the depth of 1350 m. Due to geologicalcomplication presented by wellbore instability the conductor was lowered to the depth 850 m.

After the first string of casing set in a well No.723 geological complication takes place (loss of boreholestability). Duration of the hole redressing at drilled depth 800-900 m is 16-20 days.

The indicated reasons and subsequent process fault during hole redressing generated a need for drillingof a new hole, i.e. in course of wellbore redressing, which probably has been performed with proceduralviolations, side tracking was performed, which was interpreted as “redressing” that affected makingdecision on mitigation of the complication.

During further “redressing” of the wellbore rock instability in nearwellbore zone was observed,especially in neogene deposits. At that, sand content was increasing followed by rockslide. It was decidedto upgrade cationic drilling mud to improve process parameter of the operating fluid staying within designestimated cost. Increase of starch concentration up to 1-1.5% was provided. Such system, providedstandard structure rheological coefficient and fluid loss indicator, is more effective for loose plastic clay,poorly consolidated sandstone and instable argillite scenario.

Laboratory researches of the cationic mud inhibiting properties before and after starch injection haveshown that inhibiting properties are partially lost, but clay stability in the wellbore is increased owing tobond properties of polymer and structure rheological coefficient is changed. Application of this technol-ogy in the complicated wellbore allowed to achieve wellbore stability, but mud inhibiting properties werepartially lost (figure 2).

Figure 2—Clay sample weight change (%)

SPE-172251-MS 7

Tool breaking has occurred at the depth 1717 m in course of “redressing”. Fishing operations weresuccessful. Full complex of well log survey was carried out at depth 1850 m.

Thus, high-inhibiting solutions providing fixed low structure rheological coefficient, no excess mudvolume and nominal diameter of wellbore are optimal for drilling intervals of expanded plastic clay; thesame solutions but added with polymers to increase rheological coefficient and bonding properties areeffective for drilling expanded clay intervals interlaid with loose poorly consolidated sandstone and sand.

Instable argillite intervals are laid below 1200-1300 m. Argillite present small interlayers abovepermeable rocks. Thickness of the argillite deposit is in the range of some centimeters to some meters. Dueto salt movement in trough excess tension is accumulated in the argillites and dispersed over a particulargeological period of time. Formation period and structure of the Astrakhan gas condensate field is thattoday friable argillites in trough are in tension that leads to generation of microfractures and results inargillite instability. Experience in application of acetate-potash mud in well No. 2062 of the Astrakhan gascondensate field has shown that it is enough to keep sylvic oil concentration at level not less 10% forwellbore stabilization in the interval of instable argillites [A.A. Khubbatov (2012)]. At that, the mud shallhave minimal fluid loss indicator, and structure rheological coefficient shall provide for structural regimeof liquid flow in annular space.

During the process of drilling in the instable argillite intervals sludge quantity shall be constantlycontrolled. If too much sludge and friable shale is lifted out, it is required to increase structure rheologicalcoefficient of the mud, ensure structural regime of liquid flow in the annular space and to increaseconcentration of solid and liquid colmatants (hydrophobic as well as hydrophylic). Colmatants with highadhesion effect are more effective. Adhesion colmatant being ingressed into the microfracture bonds openlayers and thus increases wellbore stability [U. S. Karaballin (1985)].

During further drilling for surface casing the cationic mud has stabilized structure rheologicalcoefficient and fluid loss indicator provided stable condition of the wellbore.

Drilling for surface casing was completed at the depth of 2900m due to Early Cretaceous depositsderoofing at depth 2858 m. Once Lower Permian salt deposits are deroofed, the surface casing of 324mmdiameter is lowered for the depth 2896 and successfully cemented.

Some chemicals appeared unuseful for drilling for the surface casing. As Q.A.C. cationic polymer wasalso used as bactericide, there was no need in Remacid (table 3). Moreover, pH level control was not

Table 3—Chemicals used for drilling of well No.939 in post-salt deposit (interval 350-2900m)

Chemical Design consumption rate, kg/m

Actual consumption rate, kg/m

Incl. 2 bore Excl. 2 bore

Bentonite 7.2 - -

Q.A.C. cationic polymer 27.5 18.3 11.5

Biopolymer 1.2 1.2 0.9

Remacid 1.2 - -

Sodium acetate 3.0 2.7 -

Starch 6.1 3.9 2.6

Polyanionic cellulose 6.1 - -

Potash chloride 19.96 14.8 12.4

T92 54.5 57 23.6

Sylvic oil (distilled) 72.6 63.6 41.7

Polyecol 1.8 1.3 1.1

Barite 193.57 2.8 2.8

Trietanolamine 3.03 - -

Graphite 1.53 1.2

Aluminium sulphate - 6.5 6.5

Sodium hydrate - 2.6 2.6

8 SPE-172251-MS

carried out, therefore trietanolamine was not required.Needless chemicals, which will not be used in optimized mud, were identified during drilling of well

No.939. The needless chemicals are: sodium acetate, polyacrylic acid sodium salt, polyanionic celluloseas well as Remacid and pH regulators. It should be noted that use of aluminium sulphate and sodiumhydrate was additionally agreed for mud completion and structure rheological coefficient control.

Actual flow rate of basic components (Q.A.C. cationic polymer and starch), even including one morebore, is considerably less than the design consumption rate, and biopolymer flow rate is in accordancewith the design rate (table 3).

Results of comparative analysis of drilling parameters and wellbore condition of the wells No. 939 and1109 are presented in table 4. The results show that efficiency of cationic mud is considerably high in spiteof additional 1300m bore drilling, which was interpreted as “redressing”. Lack of excess mud during claydeposits drilling gives evidence of high inhibiting properties of Katburr. Earlier used standard muds havenot analogue level of clay inhibition.

Salt deposits (2900-3833 m). Salt deposits are presented by solid and impenetrable cap rock, Kunguriansalt-bearing section formed by salts. Bottom part is presented by salts and fractured, often brine-saturated,sulphate-terrigeneous deposits. Brine developing of different intensity with drilling absorption takes placeduring wells construction.

Filippovsky horizon deposits are presented by thin hardpan limestone layers, separated and overlappedby anhydrite in the lower part of the Kungurian stage. As practice shows, these layers are presented bylithologically screened reservoirs with small reserve of formation energy, low reservoir features andlens-shaped mode of occurrence. Artinskian-Sakmarian deposits are presented by argillite bed interlaidwith highly mudded calcium varieties. Temperature of the bottom salt deposits is up to 105-110 °C.

As it was noted, coagulation and destabilization if systems occur at anionic-nonionic mud salinizationthat gives evidence of noncompatibility of nonsaline and saline water.

Cationic mud is proposed as compatible with nonsaline and saline environments. This mud beingpassed from nonsaline to saline environment is not exposed to coagulation and destabilization: fluid lossindicator and structure rheological coefficient stay table and do not require retreatment.

Due to problem with circulation system piping, makeup of salt-saturated weighted cationic mud duringcirculation is not feasible. Therefore it was decided to provide mud salinization and weighting up onsurface in batches, filling the mud mixing system with cationic mud and water in the ratio 1:1 for reducingthe colloidal fraction down to 20-25%. Then the mud is saturated with salt (28%) simultaneously injectingthe chemicals (Q.A.C., starch, bioxane). Process parameters of the mud salinized and chemically treatedare as follows: density 1310 kg/m3; relative viscosity 25 s; fluid loss indicator 1.6 cm3; viscosity 13mPa*s; yield point 5 Pa; gel strength 5/10 daPA, pH 6.5. Thus 380m3 of salt-saturated nonweighted mudwas obtained. Mud being salinized was weighted in batches up to density 1850 kg/m3.

Volume of the weighted salt-saturated cationic mud became 500 m3 and had the following processparameters: density 1850 kg/m3; relative viscosity 55 s; fluid loss indicator 1.5 cm3 viscosity 53 mPa*s;yield point 14 Pa; gel strength 16/21 daPA, pH 6.5.

Table 4—Comparative figures of drilling and wellbore condition of wells No.939 and 1109

Parameter No.939 No.1109

Drilling interval, m 350-2900 350-3313

Mechanical drilling speed, m/h 4.01 3.79

Mud cost (excl. Utilization expenses), RUB 19 491 900 20 029 221

Excess mud volume for utilization, m3 0 2200

Cavernosity ratio 1.28 1.35

Nominal diameter, mm393.7

Average actual diameter, mm 445 461

SPE-172251-MS 9

Considerable froth was observed during salt-saturated weighted mud makeup. This is due to presenceof cement during cementing followed by saponification of sylvic oil, which was included in cationic mudin the previous interval.

In order to prevent analogue problems with froth it was decided to exclude sylvic oil from the mud inpost-salt interval and to use another hydrophobic colmatant (e.g. asphaltic mastic) for stabilization offractured nonstable argillites.

It was found out that salt aggressiveness does not impact on Katburr parameters that allows to pass thewhole interval without additional treatment (table 5). At that, all the parameters are stable, and forexample, fluid loss indicator was reduced down to 0 cm3 that gives evidence of positive effect of halite,anhydrite and gypsum on Katburr parameters as against traditional anionic-nonionic drilling mud. Itshould be noted that Katburr fluid loss indicator measured in bottomhole simulating conditions attemperature 120°C and differential pressure 3.5 MPa does not exceed 5-6 cm3.

During the whole process of deeping in salt deposits no changes of fluid loss indicator and structurerheological coefficient were observed: effect of salt – chlorides and sulphates – on mud parameters maybe considered as positive. Owing to this fact no additional treatment is required that minimizes chemicalsdemand. Demand of fluid loss reducing agent and mud thinner was at an average reduced by 5 over times.Moreover, SMEG-5, gel powder and trietanolamine were useless during salt layer drilling.

Mechanical drilling speed in salt layer was 4.51 m/h provided that design speed rate is 3.54 m/h, whichis less by 27%. Cavernosity ratio in salt layer of well No.939 is ���1.05, which is considerably less thanthe design ration (���1.1) and ratios of the previously drilled wells (average ���1.15).

Drilling for the second service casing was completed at depth 3833 m. the second casing of diameter244.5 mm was lowered for depth 3826 m and cemented up head without any complications.

Field Katburr tests in well No.939 have identified the following advantages:

– pH level of cationic environment does not impact on mud parameters and is in the range of 6-7. pHlevel control in cationic system is not required;

– content and concentration of calcium and magnesium cations have a positive effect on propertiesand parameters of cationic environment. No preventive measures for calcium and magnesiumcations precipitation are taken;

– no biodegradation occurs in cationic system containing starch, water soluble cellulose ester andbiopolymer, therefore bactericide is not required;

– high inhibiting properties and high clay phase. A good result has been obtained for the first time– no excess mud required for clay environment;

– low-compound, as Q.A.C. polymer is multifunction – stabilizer (fluid loss reducing agent), mudthinner, bactericide and clay expansion inhibitor;

Table 5—Katburr parameters in interval 2900-3833 m

Treatment Interval, m �, kg/m3 T, s Fluid loss indicator, cm3 Gel strength 1/10. daPA

Was not treated 2900-3033 1720 53 1.4 19/24

Was not treated 3033-3137 1710 60 0.5 19/33

Was not treated 3137-3296 1700 57 0.5 24/34

Barite injection 3296 1960 80 0 24/35

Was not treated 3296-3412 1950 64 0 24/38

Was not treated 3412-3574 1960 74 0-0.2 24/43

Was not treated 3574-3666 1960 78-83 0.8 38/72

Was not treated 3666-3754 1970 77 0.5 38/67

Was not treated 3754-3780 1900-1960 74 0.5 29/53

Was not treated 3780-3833 1960 71 1 34/62

Note: content of cations Ca2� and Mg2� was analyzed every day and equaled to 3-4 g/l and 1.5-2.5 g/l respectively.

10 SPE-172251-MS

– easy control of fluid loss indicator and structure rheological coefficient. Small adds of watersolutions of Q.A.C. cationic polymer provides for stable mud parameters within the long time;

– stable fluid loss indicator and structure rheological coefficient within the whole well constructioncycle;

– no problems in passing from nonsaline to saline environment: no additional stabilizers are required;– high thermal and salt resistance: low fluid loss indicator at high temperature;– compatibility with grout.

And the following disadvantages:

– high plastic viscosity.

The received results give evidence of implementation of cationic systems in various complex geotechnicalconditions. Today, Katburr testing is provided in six wells of the Astrakhan gas condensate field. Basedon testing results optimal composition of Katburr will be selected for further use in production wells ofthe Astrakhan field.

Thus, on the threshold of planned drilling of deep and super-deep wells depth 15000-25000 m inChelkar deposit of the Republic of Kazakhstan, there is no water-based drilling mud except cationicsystem Katburr, which is resistant to temperature up to 200-220 °C and salt and post-salt aggressivenessand aggressive factors as well as provides inhibition and stability of clay rock. One of the options ofKatburr as the most effective one among the existing drilling muds shall be selected for application inChelkar deposit considering geotechnical conditions.

Appreciation

Abbreviations

References1. U. S. Karaballin 1985. Development of salt mud with adhesive colmatant for retention of clay

stability during well drilling. Ph.D. thesis in Engineering Science. Ufa.2. A. A. Khubbatov et al 2012. On application of acetate-potash alcohol mud in well No. 2062 of

the Astrakhan gas condensate field. Onshore and offshore oil & gas well construction: 35–40

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