Indian Journal of Chemical TechnologyVol. 4, March 1997, pp. 83-88

Synthesis and evaluation of partially hydrolysed polyacrylamide (PHPA)as viscosifier in water based drilling fluids

A Borthakur", S R Dutra Choudhury'" P Sengupta', K V Rao' & M C Nihalani""Regional Research Laboratory, Jorhat 785 006, India

"Oil India Limited, Duliajan, Assam, IndiaReceived 30 April 1996; accepted 16 September 1996

High molecular weight (106) partially hydrolysed polyacrylamide (PHPA) has been prepared andits efficacy as viscosifier in water based drilling mud has been evaluated. When PHPA is added tobentonite mud system, it causes a viscosity hump indicating encapsulating effect. The breakdownpoint of the hump varies with the concentration of the solid mud and also with the molecular weightof the polymer. The polymer mud system is non-Newtonian in nature. The formulated polymer mudcontaining PHPA as viscosifier, carboxy methyl cellulose(CMC)/poly anionic cellulose(PAC) as fluidloss reducing agent, Na salt of sulphonated asphaltene as shell stabilizer, bentonite as builder andbarite as high density material were evaluated at room temperature and after aging at 80-120°C. Po-lymer mud system can be used in reservoirs having temperature below 120°C. PAC shows better ef-ficacy as fluid loss reducing agent with PHPA polymer mud than CMC.

Drilling fluid popularly known as drilling mud isused during the drilling of oil and gas wells. Thepurpose of the drilling fluid is to cool and lubric-ate the drill bit, suspend formation cuttings andlift them to the surface. It is also used to preventloss of water and drilling fluids into the formationthrough which the bore hole is being drilled andcontrols the entry of liquids into the bore holefrom the formation being penetrated during thedrillingl-3. The drilling fluid is designed to bethixotropic, i.e., having high viscosity under lowshear conditions while moving up the well-borecarrying suspended solids and have low viscosityunder the high shear conditions near the drill bitwhere rapid fluid movement is necessary to coolthe drill bit. In designing a suitable drilling mud,considerable importance is given of late in apply-ing environmentally acceptable polymer mudtechnology'>. Both water based and oil basedmuds can be used. But water based muds havesome advantages over oil based muds", Waterbased muds are environmentally cleaner, havebetter penetration rates and usually require lesspressure to circulate.

A single polymer does not meet all the propert-ies of the mud necessary for the drilling of thewell. A drilling mud usually consists of a viscosifi-er, a deflocculant/thinner, a filtrate loss reducingagent and a shale stabilizer. Partially hydrolysed

·Author to whom correspondence should he addressed.

polyacrylamide is used as viscosifier in fresh wa-ter with bentonite as the builder and barite ashigh density material.

In the present communication, studies on thepreparation of partially hydrolysed polyacryla-mide and its application as viscosifier using ben-tonite as builder are reported. Rheological behav-iour of the polymer mud and its temperature sta-bility are studied at different temperatures. Thecompatibility of this viscosifier in presence of fullyformulated mud is also studied.

Experimental ProcedurePartially hydrolysed polyacrylamide (PHPA)

was prepared by polymerization of acrylamidefollowed by partial hydrolysis. Solution polymeri-zation of acrylamide monomer using redox initia-tors in aqueous medium are reported":". Recrys-tallised acrylamide monomer (0.28 m) was dis-solved in degassed distilled water. Potassium per-sulphate (0.074 m mol) and sodium metabisulfite(0.074 m mol) were added while stirring the solu-tion with a mechanical ·stirrer under nitrogen at-mosphere at 35°C for 5 h. The pH of the solutionis maintained at acidic pH of 5 by adding dil.H2S04 if required. The final solution was perfect-ly clear and highly viscous. Polyacrylamide so ob-tained was isolated by precipitating in methanol.dried in vacuum oven for 5-6 h at 60°C. About 80-85% polymer was obtained. Alkaline hydrolysisof the polymer was then performed using 2% so-

84 INDIAN 1. CHEM. TECHNOL., MARCH 1997

Table I-Physical characteristicsof PHPA polymer

PHPA Degree of hydrolysis,%

Molecular weight,Mnx 106

I 32.15IT 34.23III 35.6:n : = 6.8 x 10-4 Mfil·66

1.31.97.0

Table 2-Molecular weight distribution of polyacrylamide

Poly Fractionation of polyacrylamideacrylamide

n III IVY,% M Y,% M Y,% M Y, % M

I 30 3.10 33 2.10 35 3.10 2 11.10II 30 2.10 20 2.10 40 2.10 10 11.10III 10 6.10 20 5.10 25 4.10 45 13.10

Y = yield., M = molecular weight.

lution at 60°C for 8-10 h till the evolution of am-monia gas ceases. Calculated quantity of NaOHsolution was used. The hydrolyzed product(PHPA) was then recovered by precipitating inmethanol and dried in vacuum oven for 5 h at60°C. Molecular weight of polyacrylamide wasdetermined by viscometric method!" where :1]: =6.8 x 10-4 MnO.66• The intrinsic viscosity :1]: wasdetermined at high shear rates after attaining theNewtonian behaviour. Brookfield viscometermodel LVlDII equipped with a UL adapter wasused to determine the solution viscosity. Degreeof hydrolysis .of the PHPA was determined by es-timating the nitrogen content by Kjeldahl method.The characteristics of PHPA are presented inTable 1.

Molecular weight distribution of polyacrylamidewas studied by fractionation method. About 10 gof polyacrylamide was dissolved in 500 mL dis-tilled water to make a 2% solution. It was thentaken in a three-necked round bottomed flask fit-ted with a mechanical stirrer and a temperaturecontrol unit. Temperature of the unit was main-tained at 25°C. Methyl alcohol was added drop-wise form a burret at the top of the reaction unitwhile the polyacrylamide was stirred continuouslywith the mechanical stirrer. When the solution be-comes turbid, addition of methanol is stopped.The temperature of the unit was raised so that thesolution becomes clear. It was then cooled to25°C and allowed to stay for 1/2 h. The precipit-ate was filtered off, dried in a vacuum oven andprocessed for the determination of molecularweight. The solution left in the flask was againtreated with dropwise addition of methyl alcohol

~r-------------------------------,-- 4 I Benlollill+ 6 I BenlOnl.*' 8 I Benlollill-G 101 IIInlolllll

30

~...~§20..,...!o!

jQ.

Fig. l+-Concemration dependence on plastic viscosity ofPHPA I in bentonite mud at 30T

and processed as before. Altogether four fractionswere separated. The molecular weight of eachfraction and their yield (%) are shown in Table 2.

Apparent viscosity (AV), plastic viscosity (PV)and yield value (YV) of PHPA - bentonite mudsystem were evaluated using Fann viscometer type35 .SA. The thermal stability of PHPA solutionwas evaluatged in a Parr pressure reactor at ap-propriate temperature.

The completely formulated muds were pre-pared using the synthesized PHPA as viscosifier,polyanionic cellulose (PAC) or carboxy methylcellulose (CMC) as fluid loss controlling agent,and sodium salt of sulphonated asphaltene asshale stabilizer. Barite was used to increase thespecific gravity of the muds. Drilling mud gradebentonite clay conforming to American PetroleumInstitute Specifications, were used as the basicmud building material after hydrating fully. Theprepared mud contained about 4% bentonite,0.2% PHPA, 1.5% sulphonated asphaltene andsufficient amount of barites to make the mud spe-cific gravity 1.2. The concentration of PAC/CMCwere varied to evaluate its efficacy as fluid losscontrolling agent. The fluid loss characteristics ofthe muds were determined at 100 psi differentialpressure using an API fluid loss apparatus (NLBaroid model 44000). The rheological character-istics, fluid loss property and free water release

BORTIlAKUR et aL: PARTIALLY HYDROLYSED POLYACRYLAMIDE 85

16r-------------------------~14

12

10

4

2-- PHA\(+- PHPAII•..•.. PHPAlIi

°o~~--~--~~--~--~~--~0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16pt9 CcIIICetltntloll,lIdL

Fig. 2-Concentration dependence on plastic viscosity ofPHPA having different mol. wt, in bentonite mud at 30·C.

time of the prepared mud were determined atroom temperature and after aging at high temper-ature under rolling condition in a roller oven.

Results and DiscussionConcentration dependence of PHPA polymer

on plastic viscosity of the mud system is present-ed in Fig. 1. Viscosity increased rapidly with in-creasing polymer concentration, reached a maxi-mum value and then decreased producing ahump, however, subsequent increase in viscositywith increasing concentration was slow. The vis-cosity hump with a break over point was ob-served in the low solid muds studied up to 10%bentonite concentration. The break over point wasobserved to shift with the concentration of thesolid (Fig. 1) and also with the molecular weightof PHPA (Fig. 2). On increasing the solid mudconcentration.lthe break-over point shifted to thehigher concentration of PHPA. Similar effect wasobserved with increasing molecular weight ofPHPA polymer. It is well established that benton-ite particles present in polymer mud system arehighly anisodimensional. It can build a structureat very low solid concentration because of inter-action between attractive and repulsive surfaceforces between their electrical double layer. Dueto this phenomena PHPA polymer can encapsul-

l00r---------------------------------,- 4 1II1IIII1II1.+ 6 1 BlllIIIIIIII* 8 1 Bentoni.-& 101Blnlllllill

80

_60~-!!

o~--~--~--~--~--~--~--~--~o ~ ~ ~ ~ u ~ ~ uptR CIIIt •••••• tI•.

Fig. 3-Concentration dependence on yield point of PHPA Iin bentonite mud at 30·C

ate the bentonite producing slippage in the drilledformation. It appears that this effect is a clear in-dication of uniform distribution of the polymer inthe bentonite mud. As expected, similar humpwas also observed in the plots of yield point (YP)vs polymer concentration (Fig. 3) and for appar-ent viscosity (AV) vs polymer concentration.

Bentonite concentration is one of the main cri-teria for preparing a polymer mud. It can be ob-served (Fig. 1) that viscosity of the polymer mudincreases as the concentration of bentonite is in-creased. Bentonite clay is composed of flat, platelike particles with two different surfaces. The faceor planar surface is negatively charged, and theedge surface is positively charged. It has also ca-tion surrounding the clay molecules. But it hasmore planar surface exposed and thus more nega-tive charges. So the electrical field surroundingthe clay molecules is. predominantly negative pro-ducing a net particles-repelling effect.

PHPA in water is ionized and the negativecharges carried by the acrylate functions of PHPAmolecules repel each other. So, there is a strongrepelling effect between the acrylate function ofthe polymers and the face or planar surface ofbentonite mud. As a result there is a extendedform of polymer chain in bentonite mud produc-ing maximum hydrodynamic volume and hence

86 INDIAN J. CHEM. TECHNOL., MARCH 1997

Table 3-Rheological parameters of PHPA II solution in 4%bentonite (Test temperature = 30°C)

PHPAI AV,cP PV,cP YP, Gel 0, Gel 10,conc., Ib/100ft2 Ib/100ft2 Ib/l00ft2g/dL0.02 20 11 18 7 80.04 26 8 36 10 170.06 15 9 12 4 210.08 15 9 12 3 200.10 16 11 10 3 160.15 17.5 12 11 3 150.20 18 12 14 3 130.30 20 13 18 3 180040 22.S 14 20 3 20

the viscosity. This effect will go on increasing withthe increase of bentonite concentration upto asaturation point, i.e., the critical concentration ofbentonite beyond which the increase of viscosityis negligible. From Fig. 1 it appears that 8% ben-tonite is the critical concentration of PHPA I.Above this concentration the increase of viscosityis due to the increase of free bentonite.

Distribution of molecular weight of PHPA playsan important role in developing a viscous polymermud. It is observed (Fig. 2)' that the change in vis-cosity for PHPA I to PHPA II is more than thatfor PHPA II to PHPA III. This is due to the pres-ence of large amount of low molecular weight po-lymer « 106). The data in Table 2 indicates thatPHPA I contains 63% low molecular weight po-lymer, but this value reduces to 50% for PHPA IIand 30% belongs to PHPA III. So about half thepolymer has high molecular weight (> 106) forPHPA II. This is the reason for such a differencein viscosity for PHPA II and PHPA I. After ac-quiring 50% high molecular weight polymer therise of viscosity is not so significant with 70% po-lymer present in PHPA III. Since PHPA shouldhave high molecular weight (= 3 X 106) for betterefficacy as drilling mud additive II.

PHPA polymer mud exhibited thixotropy be-haviour. Several authors characterized the rheo-logical behaviour of polymer mud by their YP/PVratioI2,13. Low PV and high YP is desirable forbetter hole cleaning without excessive friction lossin the drill string and annular fluid erosion of thewell bore. Ideal mud system should have YP/PV> 1. Rheological parameters such as YP, PV, AV,initial gel strength (gel 0) and gel strength after 10min time (gel 10) are presented in Table 3 as afunction of concentration of polymer. Althoughthe initial gel strength is low but the progressivegel strength after 10 min, i.e., gel 10 is high en-ough to suspend the solid mud and drill cuttings

at static condition. From Table 3 it appears that atlow concentration YP/ AV ratio can be kept closeto one particularly at hump forrning concentrationor at higher concentration of about 0.3-0.4 per-cent polymer. However, at low concentration(200 ppm) the break over point of hump forma-tion begins. Therefore, it is better to use higherpolymer concentration of 0.3 or 0.4 per cent in 4per cent bentonite mud.

PHPA polymer molecule can attach or adsorbon the exposed clay particles forming bridges.As the degree of hydrolysis Increases, bridging al-so increases resulting the increase of mud viscos-ity. But polymeric bridges between the clay parti-cles increases upto a certain degree of hydrolysisof PHPA. The adsorption of PHPA on the claysurfaces will go on increasing producing saturated,aggregates. It was established that!" the optimumdegree of hydrolysis for PHPA is 30%. In thepresent work, PHPA with 30-60 % hydrolysiswere also studied. But it was observed that thedifference in . rheological value obtained with a60 % PHPA and a 30 % PHPA is negligible.

Thermal stability of PHPA mud was investigat-ed by keeping the solution at test temperature for16 h in the cell. As presented in Table 4 at 100and 120°C the rheological parameters such as AV,PV, YP are stable enough with only minor varia-tion. But at 140°C AV, PV and YP are lower indi-cating the decomposition of the polymer. The var-iation in the fluid loss for this system is only mar-ginal with increasing temperature. These studiessuggest that PHPA polymer can be applied as vis-cosifier in drilling mud at and below 120°C. Butthe high value of fluid loss indicates that PHPApolymer requires fluid loss additive for successfulapplication.

As stated above PHPA is not the only compo-nent used in the polymer mud systems. It alsocontains shale stabilizers, fluid loss reducingagent, thinners, barite and bentonite besides vis-cosifier. Hence the rheological characteristics ofthe polymer mud is highly influenced by the in-clusion of these materials. As presented in Table5, the efficacy of PAC as fluid loss additive wasfound to be better than that with CMC in the for-mulated mud systems. It appears that 0.2% PACis sufficient to produce ideal rheological behav-iour in the PHPA I and PHPA II treated polymermud both at room temperature and after aging at80°C. But the rheological parameters deteriorat-ed at 120°C. In fact a polymer mud containing0.2% PHPA IIPHPA II, 0.2% PAC, 1.5% sulpho-nated asphaltene and 4 % bentonite, with re-quired amount of barite resulted in YP/PV > 1

BORTHAKUR et al.:PARTIALLY HYDROLYSED POLYACRYLAMIDE 87

Table 4-Thermal stability of PHPA solution in 4% bentonite (Test temperature = 30·C)

Initial After aging

PHPA AV,cP PV,cP YP,lbll00ft2 FL,mL Test AV,cP PV,cP YP,lb/100ft2 FL,mLconc. % temp.X'

1/0.3 24.5 14 21 13 100 28 17 22 16120 24.5 16 17 14140 11.5 10 3 14.5

110.4 34 23 22 14 100 33 19 30 18120 31.5 18 28 15.5140 12 9 4 18

II/O.4 35.5 20 37 13 100 37 22 39 14120 35.5 21 38 14140 5 5 15.5

Table 5-Rheology of formulated PHPA mud. (Test temperature = 30·C)

PHPA, PAC, CMC, Ag.temp., AV, rv, YP, FL, Gel 0, Gel 15,% 0/0 0/0 ·C cP cP Ib/100ft2 mL Ib/100ft2 Ib/100ft2

PHPAI,0.2 0.2 30 46 24 44 6.0 12 60

80 40 25 30 5.5 8 380.2 0.5 30 72 30 85 5.5 20 92

80 67 28 70 5.5 11 70120 32 27 12 6.0 3 5

PHPAII0.2 0.2 30 40 32 36 6.5 18 57

80 27 18 24 7.5 5 32

PHPAI0.2 0.5 30 52.5 26 53 18.0 18 30

120 19 16 6 18.5 1 18

PHPAII0.2 0.5 30 35 19 32 17.0 17 33

80 28 16 24 16.0 11 21120 13 9 4 16.5 1 I9

PHPAIII0.2 0.5 30 55 38 34 20.5 4 46

80 40 40 30 18 3 4

at room temperature and also after aging at 80°e.The gel 0 and gel 10 are high enough with toler-able fluid loss 6.0-S.SmL. With PHPA polymerCMC as fluid loss produces ideal rheological va-lue but it fails to limit the fluid loss. The mud sys-tem also flocculated, showing the incompatibilityof CMC as fluid loss additive.

ConclusionHigh molecular weight (106) PHPA can be used

as viscosifier in water based drilling mud upto areservoir temperature below 120°e. When PHPAis added to bentonite mud, it causes a viscosityhump indicating encapsulating effect. The encap-sulating effect is influenced by the concentration

of bentonite and molecular weight of PHPA. Inthe formulated mud PAC is found to be morecompatible with PHPA mud system than CMC asfluid loss reducing agent.

AcknowledgementThe authors acknowledge Dr Anil C Ghosh,

Director, RRL, Jorhat, for providing facilities.

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