Rheological investigations of water based drilling fluid …Rheological investigations of water...

© 2016 The Korean Society of Rheology and Springer 55

Korea-Australia Rheology Journal, 28(1), 55-65 (February 2016)DOI: 10.1007/s13367-016-0006-7

www.springer.com/13367

pISSN 1226-119X eISSN 2093-7660

Rheological investigations of water based drilling fluid system developed using

synthesized nanocomposite

Rajat Jain1, Triveni K. Mahto

2 and Vikas Mahto

1,*1Department of Petroleum Engineering, Indian School of Mines, Dhanbad 826004, India

2Department of Applied Chemistry, Indian School of Mines, Dhanbad 826004, India

(Received July 16, 2015; final revision received January 16, 2016; accepted January 28, 2016)

In the present study, polyacrylamide grafted xanthan gum/multiwalled carbon nanotubes (PA-g-XG/MWCNT) nanocomposite was synthesized by free radical polymerization technique using potassium per-sulfate as an initiator. The polyacrylamide was grafted on xanthan gum backbone in the presence ofMWCNT. The synthesized nanocomposite was characterized by X-ray diffraction technique (XRD), andFourier transform infrared spectroscopy analysis (FT-IR). The morphological characteristics of the nano-composite were analyzed by field emission scanning electron microscopy (FESEM) and atomic forcemicroscopy (AFM) analyses. Also, its temperature resistance property was observed with Thermogravi-metric analysis (TGA). The effect of nanocomposite on the rheological properties of the developed drillingfluid system was analyzed with a strain controlled rheometer and Fann viscometer. Flow curves were drawnfor the developed water based drilling fluid system at elevated temperatures. The experimental data werefitted to Bingham, power-law, and Herschel Bulkley flow models. It was observed that the Herschel Bulkleyflow model predict the flow behavior of the developed system more accurately. Further, nanocompositeexhibited non-Newtonian shear thinning flow behavior in the developed drilling fluid system. Nanocom-posite showed high temperature stability and had a significant effect on the rheological properties of thedeveloped drilling fluid system as compared to conventionally used partially hydrolyzed polyacrylamide(PHPA) polymer.

Keywords: nanocomposite, carbon nanotubes, rheological properties, flow model, drilling fluid

1. Introduction

Various natural polymers like xanthan gum, starch, guargum, cellulose; synthetic polymers like polyacrylamide,polyethylene glycol, polyacrylates; and semi-syntheticpolymers like carboxymethyl cellulose, polyanionic cellu-lose etc are used to develop a drilling fluid system. Basi-cally, a drilling fluid is a fluid which is used for thedrilling of oil and gas wells to bring out hydrocarbons tothe surface. These polymeric additives are added to thedrilling fluid to impart rheological properties (plastic vis-cosity, apparent viscosity, yield point, and gel strength), toimprove filtration characteristics, and to mitigate wellboreinstability problems. The proper analysis of rheologicalproperties will help in determining the dynamic perfor-mance of the drilling fluid during hole cleaning, cuttingssuspension, hydraulic calculations, etc. A drilling fluidhaving good rheological properties like low plastic vis-cosity value, good gel strength, and low yield point willprovide a better rate of penetration (ROP) value. Thehigher value of ROP is desired as this will result in lowcost and less drilling time (Homed and Belhadri, 2009;Caenn and Chillingar, 1996). The fluids are classified intoNewtonian and non-Newtonian fluids based on their

observed behavior between shear stress and shear rate onflow curves or rheograms. Newtonian fluids show directproportionality relationship between shear stress and shearrate, whereas in case of non-Newtonian fluids this rela-tionship is non-linear. The drilling fluids are non-Newto-nian ones as their viscosity is strongly related to thevelocity at which drilling fluid flows through the hydrau-lic system. Rheological models can describe mathemati-cally the shear stress-shear rate relationship of a fluid.Important models like Bingham plastic model, power-lawmodel, and Herschel–Bulkley model describe the flowbehavior of the drilling fluids. Most of the drilling fluidsfollow Herschel-Buckley rheological model more closelyas it accommodates the existence of yield point along withshear stress to shear rate non-linear relationship. Thismodel considers the effects of Bingham plastic model andpower-law model (Hamad et al., 2011; Nasiri et al., 2009;Jung et al., 2011; Mahto et al., 2013).

Xanthan gum is a water soluble biopolymer and it hasvarious applications in different industries like pharma-ceutical industry, food industry, cosmetic industry and oiland gas industry due to its emulsion stabilization property,compatibility with food gradients, and pseudoplastic rhe-ological properties. In oil and gas industry, it is widelyused as a viscosifier and impart viscosity to the drillingfluid. It helps in maintaining shear thinning behavior of*Corresponding author; E-mail: [email protected]

Rajat Jain, Triveni K. Mahto and Vikas Mahto

56 Korea-Australia Rheology J., 28(1), 2016

the drilling fluid which is an essential property for theoptimum performance of the drilling fluid subjected tovariable shear rate conditions during various drilling oper-ations. A polyacrylamide synthetic polymer is also used todevelop a water based drilling fluid system. It maintainsrheological and filtration characteristics of the drillingfluid system and helps in stabilizing reactive clay bearingshale rock formations during the drilling of an oil and gaswell. Polyacrylamide polymer has low shear resistanceand mechanical stability as compared to natural polymers.Natural polymers are more shear resistant, less sensitive tothe pH, and have good salt tolerance. Hence, the proper-ties of polyacrylamide polymer need to be improved forits application in the development of water based drillingfluid.

The inception of nanotechnology in the various indus-tries like food industry, textile industry, oil and gas indus-try, etc. had brought many fruitful results (Jain et al.,2015; Abdo and Haneef, 2012; Pal et al., 2012; Sen et al.,2009; Abdo and Haneef, 2013; Jimenez, 2002). Also, thistechnology had helped in resolving many problems asso-ciated to oilfield operations in the petroleum industry. Pas-sade-Boupat et al. (2010) has filed a patent on theapplication of carbon nanotubes as excellent thickeningand suspension agent in high temperature drilling fluids.They have also reported about the thermal stability of thecarbon nanotubes at temperatures ranging as high as325°C or even higher. Recently, Li et al. (2015) hadreported the application of cellulose nanoparticles as rhe-ology modifier and fluid loss control additives in waterbased drilling fluid system. Barry et al. (2015) had studiedthe effect of nanoparticles and nanoparticles embeddedclay hybrids (iron-oxide clay hybrid and Al2O3-SiO2 clayhybrid) on the rheological and filtration properties of thelow solid content bentonite fluids under low pressure-lowtemperature and high pressure-high temperature condi-tions. They found that the addition of clay hybrids resultedin low fluid loss volume due to a restructured mode ofclay platelet interaction attributed to surface charge mod-ification. Sadeghalvaad and Sabbaghi (2015) had success-fully synthesized TiO2/polyacrylamide nanocompositeusing solution polymerization and found that this nano-composite could be used as an effective rheology modifierand fluid loss control additive in the nano-enhanced waterbased drilling fluid system.

Carbon nanotubes (CNT) are ideal fillers for polymercomposites due to their high Young’s modulus combinedwith good electrical and thermal conductivity (Luo et al.,2015). The addition of a small amount of CNTs stronglyimproves thermal, electrical, and mechanical properties ofthe polymer matrix. Also, the interlayer distance in mul-tiwalled carbon nanotubes (MWCNT) is close to the dis-tance between graphene layers in graphite, hence theinclusion of these in the polymer matrix may enhance var-

ious properties of the polymers. The partially hydrolyzedpolyacrylamide (PHPA) is used as a shale inhibitor in var-ious drilling fluid formulations and it also aids in the rhe-ological properties of the drilling fluids. However, it haslow strength and low thermal stability. Hence, it is nec-essary to improve mechanical and thermal characteristicsof polyacrylamide (PA) by inserting MWCNT into thepolymer matrix during grafting on the xanthan gum back-bone for its application as rheology modifier. The com-patibility of the polymers such as polypropylene with theCNTs has been improved with matrix modification bygrafting it with reactive moieties like acrylic acid, acrylicesters, etc. (Kelarakis et al., 2006). The effective perfor-mance of the MWCNT in the polymer matrix stronglydepends on their homogenous distribution throughout thepolymer matrix showing good compatibility with the poly-mer. Good interfacial bonding and interactions betweenMWCNT and polymer matrix may improve the propertiesof the composites. The inclusion of MWCNT in the poly-mer matrix during the polymerization will produce a nano-composite having enhanced properties like good shear sta-bility, high mechanical stability, and excellent thermal sta-bility. The nanocomposite may have ability to improverheological and fluid loss control properties of the devel-oped drilling fluid system. Thus, the incorporation ofMWCNT into the polymer matrix during the polymeriza-

Fig. 1. (Color online) (a) Schematic diagram for the preparation

process of nanocomposite and (b) chemical structure of the syn-

thesized nanocomposite.

Rheological investigations of water based drilling fluid system developed using synthesized nanocomposite

Korea-Australia Rheology J., 28(1), 2016 57

tion process may form a nanocomposite having more ver-satile properties.

In this study, polyacrylamide-g-xanthan gum/MWCNT(PA-g-XG/MWCNT) nanocomposite was synthesized byfree radical polymerization technique. The schematic dia-gram of the preparation process and chemical structure ofthe nanocomposite is shown in Fig. 1. The synthesizednanocomposite was characterized by X-ray diffractiontechnique (XRD) and Fourier transform infrared (FT-IR)analysis. The thermal and morphological properties of thenanocomposite were studied using conventional techniqueslike thermogravimetric analysis (TGA), atomic force micro-scopy (AFM), and field emission scanning electron micro-scopy (FESEM) analyses. Further, nanocomposite wasadded to the fresh water solution of polyanionic celluloseand pregelatinized starch polymers to develop a waterbased drilling fluid system. The flow behavior of thedeveloped drilling fluid system was studied by analyzingflow curves. The effect of nanocomposite on the rheolog-ical properties of the developed drilling fluid system wasalso observed. Flow models were fitted to the experimen-tal data obtained from the rheometer to study the flowcharacteristics of the developed drilling fluid system atelevated temperatures.

2. Experimental

2.1. MaterialsMultiwalled carbon nanotubes (MWCNT) were purchased

from Nanobeach, New Delhi, India. Synthesis grade acryl-amide, potassium persulfate, and potassium chloride saltwere purchased from the Merck Pvt. Ltd., Mumbai, India.Methanol, and acetone were obtained from the CDH,Chemicals Ltd. Xanthan gum (XG), low viscosity gradepolyanionic cellulose (PAC-LVG), pregelatinized starch(PGS) and partially hydrolyzed polyacrylamide (PHPA)were procured from Oil & Natural Gas Corporation Ltd.(ONGC), India.

2.2. Synthesis of polyacrylamide-g-xanthan gum/

MWCNT nanocompositeThe free radical polymerization was carried out in an

inert nitrogen gas atmosphere to synthesize PA-g-XG/MWCNT nanocomposite. Prior to polymerization, disper-sion of MWCNT is required to achieve the best combi-nation of matrix-nanoparticle properties. This was achievedwith the help of ultrasonic bath (Fisherband model FB15101, UK). Further, the reaction took place in a roundbottom (RB) flask kept on a magnetic stirrer (Tarsons,Model-Spinot Digital). The constant temperature (70°C)was maintained by an oil bath. 0.35 g of xanthan gum wasadded to the well dispersed MWCNT solution. Then, 20ml 7.03 M acrylamide solution was poured into the homo-geneous solution. After complete mixing of all the com-

ponents 1.48 × 10−4 mol of potassium persulfate (KPS)dissolved in 5 ml of water was slowly added to the homo-geneous solution and the reaction continued for another2.0 h at the same temperature and constant stirring speed.Then, the reaction mixture was cooled to room tempera-ture and the product was precipitated with acetone. Theproduct was washed several times with the methanol andwater solution (85:15) to remove any impurities and homo-polymer (Pandey and Mishra, 2011). Afterwards, it wasdried in a hot air oven for 24 h, pulverized by mortal-pes-tle and sieved through 100 mesh screen. Then, a specificamount of the synthesized nanocomposite was hydrolyzedas per the method mentioned by Cai et al. (2013).

2.3. Characterization 2.3.1. X-ray diffraction analysis

The powdered samples were dried in a hot air oven at50°C and then processed for the XRD measurements.XRD patterns for xanthan gum, MWCNT, and nanocom-posite were obtained by D8 FOCUS X-ray diffractometer,Bruker, (USA) equipped with Cu Kα radiation source at λ= 1.5406 Å, at 40 kV, and 40 mA. The reflection peaksbetween 2θ = 10° and 90° and the relative intensities wereobtained.

2.3.2. Fourier transform infrared spectroscopy (FT-IR)

The samples were dried in oven at 50°C and stored inair tight microcentrifuge tubes before FT-IR measure-ments. Further, around 1 mg of the sample was mixedwith a very small amount of potassium bromide (KBr) andpellet was prepared using a hydraulic press by applying apressure of 100 psi for 60 seconds. The IR spectrum of theKBr pellet was recorded with Perkin-Elmer SpectrumTwo, FT-IR instrument (USA) in the range of 450-4000cm−1.

2.3.3. Field emission scanning electron microscopy

(FESEM)

FESEM analysis was carried out to study morphology ofthe samples by FE-SEM Supra 55 model, Carl Zeiss (Ger-many) with air lock chamber after platinum coating to getclear images.

2.3.4. Atomic force microscopy (AFM)

Nanocomposite (0.01% w/v) was dispersed in water andthe sample for analysis was prepared by casting a drop ofwell dispersed nanocomposite solution on a glass sub-strate. Prior to analysis the glass substrate was dried. Fur-ther, dried glass substrate was characterized by the dimen-sion fast scan model, BRUKER (USA) powered by Peakforce tapping technique. This tip-scanning system pro-vides measurements on both large and small size samplesin air or fluids. The sample scanning at > 125 Hz was car-ried out to find the region of interest.



2.3.5. Thermogravimetric analysis (TGA)

Thermal stability of the nanocomposite was examinedusing thermogravimetric analysis (TGA) in Netzsch-STA449 Jupiter (Germany). A total of 15 mg sample sealed inaluminium pans was scanned from 30°C to 500°C at a rateof 10°C/min in the protective nitrogen gas environment.

2.4. Development of water based drilling fluid systemVarious amounts of nanocomposite were added to the

aqueous solution containing 0.4 wt/v% low viscosity gradepolyanionic cellulose (PAC-LVG), and 2.0 wt/v% prege-latinized starch (PGS) polymers at high speed for thedevelopment of water based drilling fluid system.

2.4.1. Rheological studies and filtration characteris-

tics of the developed drilling fluid system

The flow curves for the developed drilling fluid systemwere obtained with a shear rate controlled rheometer. Themeasurements were conducted with a coaxial cylindersystem combination in Physica Rheolab MC1 rheometerat atmospheric pressure. The flow curves of shear stress(τ (Pa)) vs. shear rate (γ (s−1)) for various solutions wereobtained by varying the shear rates from 10 to 1000 s−1 atdifferent temperature conditions (40°C, 50°C, 60°C, 70°C,and 80°C). Also, the rheological properties like plastic vis-cosity, apparent viscosity, yield point, gel strength of thedeveloped drilling fluid system were obtained by FannVG viscometer, model 35, Fann Instrument Company(Houston, Texas) as per American Petroleum Institute(API) recommended procedures for field testing of drillingfluids. The Fann VG viscometer is widely used for thefield testing of drilling fluids at atmospheric pressure androom temperature conditions (30°C). In addition, fluidloss control property of the developed drilling fluid sys-tem was measured with the Fann API filter press, FannInstrument Company (Houston, Texas) by applying 100psi of differential pressure to the developed drilling fluidformulations at room temperature conditions.

The following formulas were used to obtain the rheo-logical parameters as per API recommended practice ofstandard procedure for field testing of drilling fluids:

Apparent viscosity (µa) = Ø600/2 [mPa·s], (1)

Plastic viscosity (µp) = Ø600 – Ø300 [mPa·s], (2)

Yield point (yp) = (Ø300 − µp)*(0.5) [Pa] (3)

where Ø600 was the viscometer reading at the stirring speedof 600 rpm and Ø300 was at 300 rpm.

2.4.2. Thermal stability of the developed drilling fluid

formulation

Thermal stability of the developed water based drillingfluid formulation was determined under dynamic condi-tions by aging it in a roller oven for 16 h at 80oC tem-

perature. The aging of the developed water based drillingfluid formulation was done in a closed stainless steelaging cell (Fig. 2) which consists of an outer cap and aninner cap equipped with safety valve. The drilling fluidformulation was poured inside this aging cell and the capswere fitted at the top end. After aging the drilling fluid inthe roller oven, the aging cell was cooled down to roomtemperature and the caps of the aging cell were removed.Drilling fluid presented inside the cell was mixed in theHamilton Beach mixer and its rheological and filtrationproperties were again measured. The water inside theaging cell didn’t evaporate and the concentration of thePA-g-XG/MWCNT remained constant. The rheologicaland filtration properties of the drilling fluid formulation,before and after aging, were measured as per API recom-mended procedures.

3. Results and Discussions

3.1. Synthesis of nanocompositeThe PA-g-XG/MWCNT nanocomposite is synthesized

using potassium persulfate (KPS) as a free radical initia-tor, in a nitrogen gas atmosphere at 70°C. DifferentMWCNT to monomer ratios (0.005 to 0.02) are used toget completely dispersed polymer matrix with MWCNT.However, at higher MWCNT to monomer ratio the higheramount of carbon nanotubes is hindering the polymeriza-tion process. Hence, it couldn’t polymerize properly whenthe initiator is added to the homogeneous solution. Also,an agglomeration of nanotubes is observed at higher con-centrations. The higher amount of xanthan gum is notused due to its high viscosity imparting tendency to thesolution leading to improper polymerization process. Thefinal synthesized product with MWCNT to monomer ratio

Fig. 2. (Color online) Stainless steel aging cell for thermal sta-

bility test of the developed drilling fluid formulation.



(0.015) during the synthesis is processed for the hydroly-sis. Then, the synthesized nanocomposite with a degree ofhydrolysis value (D.H.) 30 is tested in the development ofthe water based drilling fluid system (Cai et al., 2013; Linand Luo, 2015).

3.2. Characterization

3.2.1. X-ray diffraction (XRD) analysis

The wide angle XRD pattern of MWCNT, XG, and PA-g-XG/MWCNT composite are shown in Fig. 3. The sam-ple MWCNT shows a well resolved and sharp peak at 2θ= 26.5°, which could be indexed as (002) plane of hex-agonal graphite structure. The other characteristic diffrac-tion peaks of MWCNT at 2θ = 43.5° and 53° are associatedwith (100) and (004) planes of graphitic structure inMWCNT, respectively (Sadegh et al., 2015; Liang et al.,2015; Stamatin et al., 2007; Kawasaki et al., 2005;Zhuang et al., 2014). The broad diffraction band between13° to 29° in Fig. 3 can be assigned to the amorphousnature of XG. However, the diffractogram obtained forPA-gXG/MWCNT presents broad diffraction band, adja-cent to (002) plane of hexagonal graphite structure, whichis associated with the amorphous nature of XG. The incor-poration of MWCNT could be observed with a little bit ofreduced intensity, indicating interaction of polymer matrixwith MWCNT.

3.2.2. Fourier transform infrared spectroscopy (FT-

IR) analysis

The FT-IR spectra of MWCNT, PA, XG, and PA-g-XG/MWCNT composite are compared in Fig. 4. In all spectra,a broad band at 3440 cm−1 is seen, which is attributed tothe hydrogen bonded hydroxyl group (O-H) (Jain et al.,2014). For MWCNT (Fig. 4), the spectral peaks at 1750

cm−1 to 1630 cm−1 can be assigned to C=O groups of dif-ferent environments (carboxylic, ketone/quonone) and C=Cin aromatic rings, respectively (Stobinski et al., 2010). Inaddition, the various absorption peaks emerging in thespectral range of 1280-990 cm−1 are associated with C-Obonds in different chemical surroundings. The absorptionpeaks appeared at 2925 and 2855 cm−1 are indicative ofsymmetric and asymmetric stretching of methylene (-CH2-),respectively. While the peak appeared at 1460 cm−1 is asso-ciated with in-plane bending of CH2 (Wu et al., 2011).

The FT-IR spectra of XG and PA show absorption peaksat 2926 cm−1, owing to the CH stretching frequency ofalkyl group and at 1410 cm−1 due to the CH bending ofmethyl group. XG shows the bands of wavenumbers at1037 cm−1, 1628 cm−1, and 1730 cm−1 due to the CO stretch-ing of alcohol, asymmetric stretching of carboxylate ionand CO stretching of alkyl ester, respectively (Kumar etal., 2009). The FT-IR spectrum of PA shows absorptionbands at 1655 cm−1 and 3189 cm−1 indicating the bendingand stretching vibrations of N-H bond, respectively. Whilein the spectrum of nanocomposite, all the characteristicpeaks of XG, PA and MWCNT appeared with slightly lessintensity than the characteristic peaks of pure XG, PA andMWCNT. Hence, it is possible to conclude that the graft-ing of polyacrylamide on the biopolymer backbone tookplace in the presence of MWCNTs.

3.2.3. Field emission scanning electron microscopy

(FESEM) analysis

The FESEM images of the MWCNT, polyacrylamide,xanthan gum and nanocomposite are shown in Fig. 5. Theexistence of curled, and entangled morphology of theMWCNT can be intuitively observed by FESEM. Themicroimage gives a rough value of 1-2 nm (Kumar et al.,2014) for the diameter of MWCNT. Xanthan gum and

Fig. 3. (Color online) XRD patterns for the MWCNT, xanthan

gum, and synthesized nanocomposite.

Fig. 4. (Color online) FT-IR spectra of (a) MWCNT, (b) xanthan

gum, (c) polyacrylamide, and (d) nanocomposite.



polyacrylamide both display the fibrous nature withhomogeneous and continuous surface and no cracks. Inthe case of synthesized nanocomposite, the morphologyhas been altered due to the interactions between MWCNTand the groups present on the surface of biopolymer (Leeet al., 2008; Mundargi et al., 2007). The grafting of poly-acrylamide in the presence of MWCNT has affected the

morphology of the xanthan gum and this has resulted inthe wrinkled morphology of the nanocomposite.

3.2.4. Atomic force microscopy (AFM) analysis

The tapping mode topography image and 3D atomicforce micrograph of the synthesized nanocomposite areshown in Fig. 6. The surface roughness of the nanocom-

Fig. 5. FESEM images of (a) polyacrylamide, (b) MWCNT, (c) xanthan gum, and (d) synthesized nanocomposite.

Fig. 6. (Color online) Tapping mode AFM topography image (left) and 3D atomic force micrograph (right) of the synthesized nano-

composite.



posite is explored by AFM as it gives higher resolutiontexture of the surface morphology at nanometer scale(Chafidz et al., 2012; Li et al., 2004). The average rough-ness (Ra) and root mean square roughness (Rq) values fornanocomposite are 9.05 nm and 13.9 nm, respectively.The analysis has revealed that the films are flat andMWCNT nanoparticles are found to be well dispersed inthe polymer matrix. No agglomeration and clustering ofnanoparticles have been seen in the nanocomposite sam-ple during the analysis. Further, it can be said that theMWCNT nanofillers are well dispersed in the polymermatrix.

3.2.5. Thermogravimetric Analysis (TGA)

The TGA curves and corresponding DTG curves of par-tially hydrolyzed polyacrylamide (PHPA) and synthesizednanocomposite are presented in Fig. 7. The incorporationof MWCNT into the polymer matrix is found to enhancethe thermal stability of the nanocomposite (Sahoo et al.,2010). It is evident from the TGA curves that the weightloss percent of the nanocomposite is lower than the PHPA.This may be due to the incorporation of nanometer-scaledispersed MWCNT in the polymer matrix. The initialweight loss is due to the presence of the moisture in thesamples. In the case of PHPA, initial step of degradationin the temperature range of 45°C-125°C corresponding to

TDTG (98°C) is due to water loss in the sample. In the tem-perature range around 125°C-300°C corresponding toTDTG (224°C), the degradation is attributed to the ammonialoss and formation of the imine. The weight loss around224°C is 35 wt% and for nanocomposite this value isfound to be 8 wt%. The last degradation stage at around300°C-500°C corresponding to TDTG value around 335°Cis might be a result of the condensation of residual amidegroups and cyclic amide rings (Baybas and Ulusoy, 2012).In case of nanocomposite, the initial weight loss is due tothe moisture present in the sample. The weight loss is verylow in the temperature range 222°C-330°C correspondingto TDTG value of 255°C. The weight loss in this tempera-ture range is probably caused by self-dehydroxylation of–OH group on the surface of the biopolymer. About 18%weight loss is observed in this temperature range in thecase of nanocomposite, which is lower than the PHPA. Inthe next degradation step, the higher rate of weight lossbetween 345°C-500°C (with corresponding TDTG value of360°C) in the case of nanocomposite is due to degradationof imine group (Sen et al., 2009). At 500°C, the weightloss value for PHPA (86 wt%) is higher than the synthe-sized nanocomposite (62 wt%). On the basis of aboveresults, it can be concluded that synthesized nanocompos-ite degrades slowly at elevated temperatures as comparedto the PHPA polymer.

3.3. Effect of nanocomposite on the rheological behav-ior and filtration characteristics of the developed

drilling fluid systemFlow behavior of the fluid is well described by the flow

curve or rheogram. Based on the shear stress and shearrate relationship, fluids are described as Newtonian andnon-Newtonian in nature. A non-linear relationship bet-ween shear stress and shear rate is observed in case ofnon-Newtonian fluids. This arises due to complex struc-ture and deformation effects exhibited by the materialspresent in these fluids. The quantitative evaluation of thesteady shear flow behavior of developed drilling fluid for-mulation (0.4 wt.% PAC-LVG + 2.0 wt.% PGS + 0.4wt.% Nanocomposite) at elevated temperatures is made byfitting the experimental data obtained on the flow curve toflow models. The rheological models considered for thisstudy are given below:

Bingham plastic model

, (4)

power-law model (Ostwald-de Waele model)

, (5)

Herschel-Bulkley model

(6)

where σy is the yield stress (Pa) of each model, k is con-

σ = σy kγ+

σ = kγn

σ = σy k+ γn

Fig. 7. (Color online) TGA and DTG curves of (a) nanocom-

posite and (b) partially hydrolyzed polyacrylamide (PHPA).



sistency index (Pa·sn), n is the flow behavior index (dimen-sionless) and γ is the shear rate (1/s). The shear thinningnature of the fluid is related to the flow index value. Thematerial parameters for Bingham and power-law modelsare determined by linear regression analysis. Also, param-eters for Herschel-Bulkley model, which is a hybrid of theBingham plastic model and power-law model are deter-mined by non-linear regression analysis. The flow curveof the developed drilling fluid formulation (0.4 wt.%PAC-LVG + 2.0 wt.% pregelatinized starch + 0.4 wt.%nanocomposite) for various temperatures is shown in Fig.8. The experimental data obtained on the flow curve arefitted to the Bingham, power-law, and Herschel-Bulkleyflow models. But, the Herschel-Bulkley model fits moreaccurately to the experimental data and predicts the flowbehavior of the developed drilling fluid system at differenttemperatures. The Bingham plastic model is not able topredict the flow behavior of nanocomposite solutionsmore accurately as it follows Newtonian behavior aboveyield stress value. The power-law model underestimatesthe viscosity of the developed formulation at low shearrates. The values for the various parameters of these flowmodels are reported in Table 1. It is observed that the con-sistency index (k) of the polymer solutions increased athigher temperatures. The higher value of consistencyindex is favorable for the drilling operations as drillingfluid with high consistency index value shows good holecleaning efficiency. Also, a decrease in the value of theflow behavior index (n) is indicative of higher shear thin-ning characteristic of the developed drilling fluid formu-lation at higher temperatures. This property is alsovalidated by plotting viscosity vs. shear rate graph for thedeveloped drilling fluid formulation (Fig. 9) where a decreasein viscosity at higher shear rates is observed. This may be

due to the presence of flexible segments of the biopolymerin the nanocomposite polymer matrix as pure xanthangum solutions exhibit shear thinning flow behavior. Theshear thinning behavior is most commonly attributed by adiverse fluid system like polymer melts/solutions, suspen-sion, and emulsions (Soares et al., 2013). As the shear rateincreases, complete alignment of the microstructure in theflow direction is achieved which results in a reduction oflocal drag. At lower shear rates polymer entanglement andaggregate formation due to hydrogen bonding take place,resulting in high shear viscosity at low shear rates. Theshear thinning property is desirable for the drilling fluids,

Fig. 8. (Color online) Effect of temperature on the flow behavior

of the developed drilling fluid formulation (0.4 wt.% PAC-

LVG + 2.0 wt.% pregelatinized starch + 0.4 wt.% nanocompos-

ite).

Table 1. Flow model parameters for the developed drilling fluid

formulations (0.4 wt.% PAC-LVG + 2.0 wt.% PGS + 0.4 wt.%

nanocomposite) at different temperatures.

Flow

model

Temperature

(°C)

σy(Pa)

k

(Pa⋅sn)

n

(-)

R2

(-)

Power-

Law

40

50

60

70

80

-

-

-

-

-

0.13

0.14

0.15

0.17

0.19

0.85

0.83

0.80

0.77

0.76

0.916

0.887

0.826

0.881

0.951

Herschel-

Bulkley

40

50

60

70

80

0.12

0.10

0.98

0.96

0.40

0.19

0.20

0.21

0.21

0.22

0.80

0.78

0.75

0.74

0.72

0.991

0.991

0.995

0.986

0.994

Bingham

plastic

40 2.35 - 0.05 0.885

50 1.75 - 0.04 0.925

60 1.41 - 0.03 0.915

70 1.16 - 0.03 0.826

80 1.07 - 0.04 0.815

Fig. 9. (Color online) Effect of temperature on the viscosity of

the developed drilling fluid formulation (0.4 wt.% PAC-LVG +

2.0 wt.% pregelatinized starch + 0.4 wt.% nanocomposite).



subjected to variable shear rate conditions and is veryessential as they have to perform different jobs at differentshear rate conditions (Mahto and Sharma, 2004; Bybee,1999; Kosynkin et al., 2012; Jain and Mahto, 2015). Duringmixing and pumping, drilling fluid should have low vis-cosity and while returning from the hole to the surface, itshould have high viscosity at low shear rates to bring outdrilled out cuttings to the surface. Also, lower value offlow index represents a flat annular velocity profile and itis a desired property of the drilling fluids for proper holecleaning.

The effect of nanocomposite on the rheological param-eters like plastic viscosity, apparent viscosity, yield pointand gel strength, of developed drilling fluid formulationsis shown in Table 2. The rheological parameters are deter-mined with Fann viscometer which is widely used for thefield testing of drilling fluids. It is evident from the resultsthat the nanocomposite has a significant effect on the rhe-ological parameters of the developed drilling fluid formu-lations. The plastic viscosity, apparent viscosity, yield point,and gel strength increased at higher concentrations of thenanocomposite. This is due to the presence of nanofillerand flexible segments of biopolymer in the polymer matrix.The nanocomposite can be used to impart rheologicalproperty to the developed drilling fluid system. The per-formance of nanocomposite as a rheology modifier iscompared with partially hydrolyzed polyacrylamide poly-mer which is widely used as a drilling fluid additive in oiland gas industry. The comparative results are given inTable 3; it is quite evident from the data reported in the

table that synthesized nanocomposite has a relativelyhigher effect on the rheological parameters of the devel-oped drilling fluid system than the PHPA polymer. Also,the fluid loss control property of the nanocomposite ismore than the PHPA polymer. Low fluid loss volume of adrilling fluid is a desirable property for the drilling fluidsystem while drilling clay bearing rock formations.

3.4. Thermal stability of the developed water baseddrilling fluid formulation

The temperature resistance property of the developeddrilling fluid formulation is analyzed under dynamic con-ditions in hot air roller oven. The polymers degrade at ele-vated temperatures, thus, resulting in poor rheological andfiltration control characteristics. The experimental data fordeveloped drilling fluid formulation (0.4 wt.% PAC-LVG+ 2.0 wt.% PGS + 0.4 wt.% nanocomposite) is shown in

Table 3. Effect of partially hydrolyzed polyacrylamide polymer on the rheology and fluid loss control property of developed base fluid

system (0.4 wt.% PAC-LVG + 2.0 wt.% pregelatinized starch).

Concentration

(wt/v%)

Plastic viscosity

(mPa⋅s)

Apparent viscosity

(mPa⋅s)

Yield point

(Pa)

Initial gel strength

(Pa)

10 min gel strength

(Pa)

API, fluid loss

(ml)

0.1 14.5 24.5 10.0 1.0 1.5 16

0.2 15 26.0 11.0 1.5 2.0 15

0.3 17 28.5 11.5 1.5 2.0 14

0.4 19 32.5 13.5 1.5 2.5 13

0.5 21 35.0 14.0 2.0 4.0 12.5

0.6 26 42.0 16.0 3.0 5.5 11.5

Table 4. Rheological and fluid loss control properties of the

developed drilling fluid formulation (0.4 wt.% PAC-LVG + 2.0

wt.% pregelatinized starch + 0.4 wt.% nanocomposite) before and

after thermal aging test at 80oC for 16 hrs.

Properties Units Before aging After aging

Plastic viscosity (mPa⋅s) 21.0 21.0

Apparent viscosity (mPa·s) 34.0 33.0

Yield point (Pa) 13.0 12.0

Initial gel Strength (Pa) 2.5 2.5

10 minutes gel Strength (Pa) 4.0 4.5

API Fluid loss (ml) 10.8 10.9

Table 2. Effect of synthesized nanocomposite on the rheological properties and fluid loss control property of developed base fluid system

(0.4 wt.% PAC-LVG + 2.0 wt.% Pregelatinized starch).

Concentration

(wt/v%)

Plastic viscosity

(mPa⋅s)

Apparent viscosity

(mPa⋅s)

Yield point

(Pa)

Initial gel strength

(Pa)

10 min gel strength

(Pa)

API, fluid loss

(ml)

0.1 15 26.5 11.5 1.0 1.5 13.5

0.2 16 28.0 12.0 1.5 2.0 12.4

0.3 19 31.5 12.5 1.5 2.5 11.2

0.4 21 34.0 13.0 2.5 4.0 10.8

0.5 24 38.5 14.5 2.5 4.5 9.7

0.6 30 49 19 4.5 7.0 9.0



Table 4, it shows the performance parameters before andafter the thermal aging test. It can be observed that rhe-ological and filtration parameters are not affected signifi-cantly up to the temperature value of 80°C.

4. Conclusion

The following conclusions can be drawn from the pres-ent study:

1) The thermogravimetric analysis confirmed about thehigh temperature resistance property of synthesized nano-composite than the PHPA polymer due to the inclusion ofMWCNT in polymer matrix.

2) The morphology of the xanthan gum was found tohave changed after grafting of polyacrylamide in the pres-ence of MWCNT. No agglomeration of nanoparticles wasfound during AFM analysis. The average roughness (Ra)and root mean square roughness (Rq) values for nanocom-posite are 9.05 nm and 13.9 nm, respectively.

3) The experimental data obtained on the flow curveswere fitted to Bingham, power-law, and Herschel-Bulkleyflow models and it was found that Herschel-Bulkley wasable to predict the flow behavior of the developed waterbased drilling fluid system more accurately.

4) Developed drilling fluid system exhibited non-New-tonian shear thinning behavior with a finite magnitude ofthe yield stress. The appearance of yield stress is due tothe hydrogen bonding effect resulting in a stable config-uration that can show a resistance to flow.

5) The synthesized nanocomposite has a significanteffect on the rheological parameters like plastic viscosity,apparent viscosity, yield point, and gel strength of thedeveloped water based drilling fluid formulations as com-pared to conventionally used partially hydrolyzed poly-acrylamide (PHPA) polymer due to the presence ofnanofiller and tangled flexible segments of xanthan gumin the polymer matrix.

Acknowledgement

The authors would like to acknowledge Indian School ofMines, Dhanbad, India for providing financial support andnecessary laboratory facilities to carry out this researchwork.

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