Comparative Study of Gravel Suspension Properties of Hydroxyethyl Cellulose and Xanthan Gravel Pack Fluids

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1 Volume-6, Issue-5, September-October 2016 International Journal of Engineering and Management Research Page Number: Comparative Study of Gravel Suspension Properties of Hydroxyethyl Cellulose and Gravel Pack Fluids John A.O. 1, Joel O.F. 2, Chukwuma F.O. 3 1 World Bank African Centre of Excellence, Institute of Petroleum Studies, University of Port Harcourt, NIGERIA 2 Centre for Petroleum Research and Training, University of Port Harcourt, NIGERIA 3 Department of Chemical Engineering, University of Port Harcourt, NIGERIA ABSTRACT Transportation of proppant from surface to the desired location downhole during gravel packing operations in the petroleum industry is challenging. Various fluid systems have been used to do this. Depending on the type of polymer used as viscosifier, the fluid systems have different properties. Part of the criteria used to qualify gravel carrier fluids in hydraulic fracturing and gravel pack operations is their ability to suspend and transport solids. Almost all fluids can transport solids if the velocity is high enough. However, at low velocity or in a static state, the tendency of the solids to settle out is high for many fluids. Therefore, it is essential to formulate fluid system that is capable of proppant suspension and transport, especially at the low velocity areas of the wellbore during job execution. The fluid property and gel structure play a very important role in this regard. In this paper, the sand suspension properties of Hydroxyethyl cellulose (HEC) and xanthan polymer fluids were investigated at various temperatures for different polymer loadings. The study showed that settling rate is affected by thermal stability of the polymer fluid system and by the rate of degradation of the polymer by chemical breakers. HEC exhibited higher apparent viscosity than xanthan for the same polymer concentration, but xanthan showed superior sand suspension capability than HEC at the same condition. The result obtained can help in engineering designs to determine optimum pump rate without fear of proppant settling out of the suspension. Keywords Gravel suspension, hydroxyethyl cellulose, proppant, rheology, xanthan. I. INTRODUCTION The goal of sand control in oilwell completions is to produce hydrocarbon without formation sand. Gravel packing is a reliable and often preferred sand control technique [1]. Gravel packing is becoming very common in long horizontal intervals and challenging environments with the advent of enhanced drilling and fluid technology which has made possible extended reach and long horizontal drilling. Intervals in excess of 2500ft in length are common and 6,938ft have been recorded [2]. The annulus and the perforations must be completely packed with proppant for a gravel pack to be deemed successful, as incomplete packing may lead to low well productivity and /or sand production [3]. Selecting an appropriate gravel carrier fluid is very important in achieving a successful gravel pack job. Gravel pack fluid must have good gravel suspension capacity, rheology, adequate fluid loss or leak off, break at a controlled rate and no or minimal formation damage. Premature sandouts may occur if adequate proppant suspension is not provided by the carrier fluid during placement. A variety of carrier fluids have been used and this includes diesel, crude oil, brine, foams, crosslinked polymer fluids and viscous linear gels [4]. Using brine has carrier fluid, usually called water pack, uses the velocity of the fluid to transport gravel of low concentration, usually 0.5 to 1ppa; while Slurry or viscous Pack relies more on the viscosity of the fluid to transport high gravel concentration up to 6ppa or more [5]. Water pack does not contain polymer, thereby eliminating the fear of potential residue formation damage. Water pack provides tight annular packs but could have high leak off rate in permeable zones resulting to bridging in screen or casing annulus and this can cause premature screen out [6]. Slurry packs permits pumping at low rate and transports more gravel deeper into the perforations, reducing pumping time and hence reducing operational cost. Increased gravel concentration would also reduce gravel and formation sand intermixing, phenomena associated with water pack, which can lead to low productivity [1][7]. Formation damage potential by the 427 Copyright Vandana Publications. All Rights Reserved.

2 residue of polymeric fluids and non-uniform packing are some shortcomings associated with slurry pack [6][8][9]. Viscous fluids employed some thickeners to obtain high viscosity. Depending on the means of viscosification, the fluid system can be grouped into polymeric fluids and polymer-free fluids. Polymeric fluids contain polymers which serve as the viscosifying agent. The random-coil polymers are subjected to severe viscosity loss at high temperatures and this include guar, hydroxypropyl guar (HPG) and hydroxyethyl cellulose (HEC); helical polymers are thermally stable and include diutan, welan gum, xanthan and scleroglucan [10]. Viscoelastic surfactant (VES) is polymer-free. The most commonly used carrier fluids are HEC, and Viscoelastic surfactant [1][11]. HEC, the most commonly used polymer for preparing gravel carrier fluids, is one the fluids with the least potential of causing formation damage; it has a good proppant transport property, good rheological properties, cost effective, easy to break or flow back and insensitive to salinity [7]. Some modified grades of HEC are stable up to 250 o F [1]. HEC is made from a synthetic source; produced from water insoluble cellulose by reaction with ethylene oxide which adds to the backbone the hydroxyethyl group, which makes it soluble in water soluble [12]. Mixing procedure, shearing and filtration processes introduced in HEC fluid preparation has helped to eliminate fish eye or micro fish eyes usually formed as a result of incomplete polymer hydration [13][14]. HEC system has no gel strength to aid gravel transport in situations such as highly deviated wells or in long intervals and this may result to premature sandouts [7]. Gravel carrier fluid for packing highly deviated wells and long intervals, ideally, must show little or no gravel settling to enable high proppant concentration to be transported to the desired interval. The fluid must exhibit adequate fluid loss to ensure compact packing of gravel, must completely break at the required time and leave no residual solids in the formation to minimize formation damage. More so, the fluid must be compatibility with formation and wellbore fluids and show low frictional pressure to avoid fracturing the formation when there is narrow margin between pore pressure and fracture gradient [11]. gum is a high-molecular weight natural hateropolysaccharide, which is produced by the bacteria fermentation of the micro-organism Xanthomonas Campestris [12][15]. gum has some excellent qualities such as good proppant suspension capacity at relative low polymer loading, good rheology, insensitive to salinity, good thermal stability, mechanical shearing resistance, excellent fluid loss/leak off rate and hydrates in most ph range [16][17]. Some xanthan grades are stable o up to 350 F [1]. gum possesses inherent gel strength, making it suitable for gravel packing highly deviated wells or long intervals. More so, improvement in the manufacturing process to produce clarified xanthan and availability of equipment to mix, shear and filter the polymer has greatly reduced its formation damage potentials [17]. VES is polymer-free, therefore the least damaging of all carrier fluid. It generates viscosity as a result of association of surfactant molecules to form aggregates called micelles [18][19]. VES temperature stability depends on the surfactant used to create the emulsion, o stability up to 300 F available but the fluid breaks down when it comes in contact with liquid hydrocarbon [1]. In other to determine if gravel can be placed at low rates, rheology and proppant settling needs to be evaluated. The performance of gravel-carrier fluid ability to suspend and transport gravel performance is best measured by the proppant suspension property. In this study, the rheology and proppant suspension properties of the most commonly used polymers, HEC and xanthan gum, were evaluated for different polymer loadings and at different temperatures. More so, the rate of polymer degradation by gel breakers also investigated in relation to gravel suspension. This is important the fluids engineering designs, provides a criterion for field quality control/quality assurance test and aid decision making for selecting gravel pack fluid systems for a various applications. II. METHODOLOGY 2.1 Fluid recipe and mixing Table 1 and Table 2 show the chemical formulations used for HEC and gels respectively. Fluids with 40lbs/1000gal (40ppt) and 60lbs/1000gal (60ppt) polymer loadings were prepared for both polymers and the additives were added in the order listed in the Tables, using a constant speed Waring Blender with rheostat. TABLE 1 HEC FLUID RECIPE S/N Chemical Concentration (per 1000 gals) 1 Fresh water 1000 gals 2 KCl 2% bwow 3 Biocide lbs 4 Biocide lbs 5 Iron Chelating Agent 10 lbs 6 Surfactant 5 lbs 7 HEC 40 lbs 8 ph Adjuster As required (ph = 8 to 9) 9 Gel breaker 10 lbs 428 Copyright Vandana Publications. All Rights Reserved.

3 TABLE 2 XANTHAN FLUID RECIPE S/N Chemical Concentration (per 1000 gals) 1 Fresh water 1000 gals 2 KCl 2% bwow 3 Biocide lbs 4 Biocide lbs 5 Iron Chelating Agent 10 lbs 6 Surfactant 5 lbs 7 40 lbs 8 ph Adjuster As required (ph = 7 to 8) 9 Gel breaker 10 lbs For the HEC, all the additives were added at low blender speed that will not create air entrainment. The fluid was further mixed for 30 minutes at low speed after adding all the additives. For the gel, before adding the gelling agent, the speed of blender was increased to the maximum point that will not permit air entrainment. Mores so, after adding all the additives, the fluid was mixed for about 10 minutes and allowed to stand static for about 30 minutes for further hydration. The rheology, viscosity, sand suspension and break tests were determined at various temperatures. 2.2 Fluid ph The ph of the fluid was adjusted to the desired value and measured using a digital ph meter. HEC hydrates in alkaline ph, therefore the ph after adding the gelling agent was raised to between 8 to 9 for good gel hydration. hydrates in most ph range but the final hydrated gel ph was kept to between 7 and Rheology and Viscosity The rheological readings of the fluid systems were determined, using Fann viscometer model 35, equipped with F1 spring, B1 bob and R1 rotor. The readings were taken at different temperatures of 80 o, 120 o, 140 o, 160 o and 180 o F, after conditioning the fluid for 30 minutes in a water bath, pre-set to the desired temperature. The viscometer dial readings were converted to the viscosity reading by multiplying by the appropriate speed factor [20]. 2.4 Temperature Viscosity and ph values depend on the temperature the measurements were done. Temperature was measured using a certified digital thermometer. 2.5 Proppant suspension test The proppant suspension capabilities of the both HEC and fluids were determined by mixing the hydrated gel with the required amount of U.S. mesh 20/40 resin coated proppant to form a 10ppg slurry, which was poured into a 100cc glass measuring cylinder, placed in a water bath pre-set to the desired temperature; the volume of clear free fluid formed at the top of the slurry bed was measured at regular time interval. The test was conducted at 80 o, 120 o, 140 o, 160 o and 180 o F. 2.6 Gel Break Test The rate of HEC and polymers degradation was evaluated for 140 o F and 180 o F, by introducing equivalent of 10 lbs/1000gal (10ppt) Sodium Persulfate (SP) breaker into 200cc of the fluid in glass jar while stirring, placed it in a preheated water bath at the required temperature and rheological readings were taken at regular intervals until the viscosity at 511 1/s shear` rate is 10cP, when it is considered broken. III. RESULTS AND DISCUSSION 3.1 Viscosity and Rheology During the preparation of the HEC polymer fluids, the ph of the solution was lowered to about 2 before adding the HEC powder to facilitate good dissolution of powder polymer, slow down its solubility rate and prevent or minimize the formation of fish eyes, formed as a result of undissolved powder. can hydrates in most ph range. The final ph of the hydrated gel was kept between 7 and 8. TABLE 3 VISCOSITY OF HYDRATED GEL Polymer Viscosity (cp) at 511 s -1 Concentration Shear rate at 80 o F 40 lbs/1000 HEC lbs/1000 gal 28 60lbs/1000 gal HEC lbs/1000 gal 42 The viscosities of the hydrated gels immediately after mix are shown in Table 3. The viscosity of the HEC fluid is higher than that of for the same polymer loading. The viscosity of the 40ppt gel is lower than that of the 60ppt gel of the same polymer at the same temperature for all shear rates for both HEC and. The viscosity of both polymer fluids reduces with increase in temperature, with HEC showing drastic reduction as seen from the steepness of the graphs in Figure 1. This shows that has far more thermal resistance and thermal stability. Increasing the polymer loading improved the stability of both fluids, again with xanthan showing better improvement. HEC and xanthan polymer fluids are non- Newtonian as their viscosity reduces with increase in shear rate as shown in Figures 2 and 3. HEC has a higher viscosity than xanthan at high shear rates for the same polymer concentration, while xanthan has a far higher Viscosity than HEC at low shear rates. At low shear rate 40ppt xanthan gel has just a little lower viscosity than 429 Copyright Vandana Publications. All Rights Reserved.

4 60ppt HEC as shown in Table 4. The shear thinning viscosity is a very important property; an indication that when static or at low velocity areas of the wellbore the viscous fluid should have adequate viscosity to suspend the proppant. The shear thinning nature will also contribute to lowering the drag or frictional pressure as a result of lower viscosity at higher shear rate. Viscosity (cp) at 511 1/s Temperature ( o F) 40ppt HEC 40ppt 60ppt HEC 60ppt Figure 2: Viscosity of 40ppt and 60ppt HEC Fluid at Different Shear Rates at 80 o F Figure 1: Effect of Temperature on Viscosity of HEC and TABLE 4 VISCOSITY OF HEC AND XANTHAN POLYMERS AT DIFFERENT SHEAR RATES Shear Rate (1/s) 40ppt HEC Viscosity (cp) 40ppt 60ppt HEC 60ppt Figure 3: Viscosity of 40ppt and 60ppt Fluid at Different Shear Rates at 80 o F TABLE 5 RHEOLOGY READINGS FOR HEC AND XANTHAN POLYMER FLUID AT DIFFERENT SHEAR RATES 40ppt HEC Temp Shear Rate (1/s) n' K' ( o F) Dial Readings Copyright Vandana Publications. All Rights Reserved.

5 ppt HEC Temp Shear Rate (1/s) n' K' ( o F) Dial Readings ppt Temp Shear Rate (1/s) n' K' ( o F) Dial Readings ppt Temp Shear Rate (1/s) n' K' However, n values for HEC are higher than that for of the same polymer concentration. Consistency Index, K, describes the pumpability of the fluids; pressure loss increases as K increases. with concentration of the polymers. fluid showed lower values of K than HEC fluid of the same polymer concentration. These values are essential to select the optimum pump rate as regards the fluid s velocity, viscosity and proppant settling rate. 3.2 Proppant Suspension Proppant suspension property of HEC and polymer fluids depends on temperature. Generally, the rate of gravel settling increase with temperature for both polymers as seen in Figures 4, 5, 6 and 7, showing a plot the percent of free fluid, which is a measure of gravel settling, formed at the top of the gelsand slurry with time. At static condition, gravel settling in HEC began almost immediately, event at surface temperature, and increases with temperature. Increasing the polymer concentration of HEC improved the sand suspension property at low temperature, but almost making no difference at high temperature. The thermal instability and the poor gravel suspension at high temperature create a major concern about the limitations of application of HEC at high temperature wells, deviated wells and long interval packing. showed an excellent gravel suspension capacity at the test temperatures, despite having a lower viscosity than HEC. 40ppt, at all the tested temperatures, showed a superior gravel suspension than 60ppt HEC. Increasing polymer concentration from 40ppt to 60ppt greatly improves the gravel suspension capacity of xanthan fluid both at low and higher temperature. ( o F) Dial Readings The rheological readings and fluid viscosity increases with increase in polymer concentration at the same temperature for both polymers as shown in Figures 2 and Table 5. The Flow Behaviour Index, n, and Consistency Index, K, calculated in Figure 5, are very essential rheological parameters for fluid designs and optimization. At low values on n, the flow profile is flat and tends towards plug flow; as n decreases, the more shear thinning the fluid. 40ppt HEC and 40ppt showed lower values of n than the equivalent 60ppt gels. Figure 4: Percent free fluid formed with time at the top of the slurry at 80 o F. 431 Copyright Vandana Publications. All Rights Reserved.

6 % Free Fluid # HEC 60# HEC Figure 5: Percent free fluid formed with time at the top of the slurry at 140 o F. % Free Fluid # HEC 60# HEC Figure 6: Percent free fluid formed with time at the top of the slurry at 160 o F. % Free Fluid # HEC 60# HEC 40# XANTHAN 60# XANTHAN Figure 7: Percent free fluid formed with time at the top of the slurry at 180 o F. 3.3 Polymer Degradation Chemical breakers are normally added to the gravel-carrier fluids to facilitate the degradation of the polymer fluid, breaking down the long polymer chains to shorter molecules, thereby reducing the molecular weight and viscosity. As seen from Tables 6 and 7, the rate of polymer degradation for both HEC and fluids is a function of temperature. The rate of degradation increases with increase in temperature. The 40ppt and 60ppt HEC fluids were easily degraded by 10ppt SP breaker at 140 o F and 180 o F, breaking into a clean colourless fluid. showed a good break profile at 180 o F with the same concentration of breaker but was difficult to break at 140 o F, especially for the 60ppt gel, which will require a stronger oxidizer breaker or breaker catalysts. The faster the gel fluid breaks, the sooner it will lose its capability to suspend gravel. Therefore, it is very important to check the rate of degradation of polymer fluids so as not to jeopardize gravel pack jobs as a result of the breaker acting too fast and gravel deposited before getting to the target zone. Similarly, it s necessary to put into consideration the adequate degradation of the polymer fluid, especially while trying to improve viscosity, thermal stability and suspension capacity by increasing polymer concentrations at low lower temperature range. TABLE 6 DEGRADATION OF 40PPT AND 60PPT GELS WITH 10PPT SP-BREAKER AT 140 o F Time (minute) Viscosity of 40ppt Fluid with 10ppt SP breaker at 511 1/s Shear Rate (cp) Viscosity of 60ppt Fluid with 10ppt SP breaker at 511 1/s (cp) HEC HEC Copyright Vandana Publications. All Rights Reserved.

7 TABLE 7 DEGRADATION OF 40PPT and 60PPT GELS WITH 10PPT SP-BREAKER AT 180 o F Time (minute) Viscosity of 40ppt gel with 10ppt SP breaker at 511 1/s Shear Rate (cp) Viscosity of 60ppt gel with 10ppt SP breaker at 511 1/s Shear Rate (cp) HEC HEC IV. CONCLUSION HEC and xanthan polymer fluids showed good rheological and shear thinning properties, essential for proppant suspension at low rates. HEC has higher viscosity than xanthan at high shear rate but lower at low shear rates. polymer fluid showed a good thermal stability at the test temperatures; HEC was thermally unstable. showed far superior proppant suspension capacity, even at lower concentrations, than HEC at all temperature ranges, making it suitable for gravel packing deviated wells and long intervals. Proppant settling increase with increase in temperature and reduces with increase in polymer concentration. The rate of gravel settling also depends on polymer type, polymer concentration, and breaker type and breaker concentration. Optimum concentrations of polymers and appropriate breaker must be used to achieve desired result, depending on temperature application and well profile, to ensure gravel pack fluid stability, adequate proppant suspension and transport property, good rheology, good leak off and ensure complete degradation of the polymer to avoid formation damage. ACKNOWLEDGEMENT Authors wish to thank the World Bank and staff of Centre for Petroleum Research and Training, Institute of Petroleum Studies, University of Port Harcourt. REFERENCES [1] F. El-Dhabi and R. Bulgachev, Gravel Packing Depleted Reservoirs. Paper SPE presented at the SPE European Formation Damage Conference held in Noordwijk, The Netherlands, 7-10 June, [2] T. Grigsby and S. Vitthal, Openhole Gravel Packing An Evolving Mainstay Deepwater Completion Method. The paper SPE was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in San Antonio, Texas, 29 Sept 2 Oct, [3] D.W. Bryant and L.G. Jones, Completion and Production form Alternate-Path Gravel Packed Wells. The paper SPE was first presented at the 1994 SPE Formation damage Symposium held in Lafayette, LA, Feb 7-10, [4] L.W. Lake, J.D. Clegg and W.L. Penberthy, Petroleum Engineering Handbook Sand Control, Chapter 5, Volume IV Production Operations Engineering, SPE, , [5] M. Tolan, R.J. Tibbles, J. Alexander, P. Wassouf, L. Schafer and M. Paiar, Gravel Packing Long Openhole Intervals with Viscous Fluids Utilizing High Gravel Concentrations: Toe-to-Heel without the Need for Alternate Flow Paths. Paper SPE presented at the 2009 Asia Pacific Oil and Gas Conference and Exhibition, Jarkata, Indonesia, 4-6 August, [6] M.J. Economides, L.T. Walters and S. Dunn-Norman, Petroleum Well Construction, Halliburton. Released Copyright Vandana Publications. All Rights Reserved.

8 [7] D.R. Underdown, A.L. Calvert and D.P. Newhouse, Comparison of HEC and XC Polymer Gravel Pack Fluids. Paper SPE was presented at the SPE Sixth Annual Technical Conference and Exhibition of SPE held in San Antonio, Texas, October 8-11, [8] G.A. Thibodeaux, S.B. Gill, B.M. Richard and C.W. Bowman, Comparative Study of Gravel/Water Packing 12 Gulf Coast Wells. Paper SPE presented at the 66 th Annual Technical Conference and Exhibition of the SPE, Dallas, TX, October 6-9, [9] J.M. McGowen, S. Vitthal, M.A. Parker, A. Rahimi, and W.E. Martch Jr., Fluid Selection for Fracturing Highpermeability Formations. Paper SPE presented at the 68 th Annual Technical Conference and Exhibition of the SPE, Houston, TX, 3-6 October, [10] B.R. Reddy, Viscosification-on-Demand: Chemical Modification of Biopolymer to Control Their Activity by Triggers in Aqueous Solutions. Paper SPE presented at the International Symposium of Oilfield Chemistry, Woodlands, TX, April, [11] S. Jain, B. Gadiyar, B. Stamm, C. Abad, M. Parlar, and S. Shah, Friction Pressure Performance of Commonly used Viscous Gravel Packing Fluids. The paper SPE was approved for presentation at the SPE Annual Technical Conference and Exhibition, Florence, Tuscany, Italy, September, [12] S. Abass, A.W. Sander and J.C. Danovan, Application of Hydroxyethyl cellulose for Chemical EOR. Paper SPE was prepared for presentation at the SPE Enhanced Oil Recovery Conference held in Kuala Lumpur, Malaysia, 2-4 July [13] R.C. Cole and S.A. Ali, A Comparative Study of Succinoglycan Gravel Pack Gel Properties to Those of HEC. Paper SPE presented at SPE Annual Technical Meetings, New Orleans, Louisiana, September, 25-28, [14] D.P. Vollmer and D.J. Alleman, HEC No Longer the Preferred Polymer. Paper SPE presented at the SPE International Symposium on Oilfield Chemistry, Houston, Texas, February 13-16, [15] D. Lipton and D.B. Burnett, Comparisons of Polymers Used in Workover and Completion Fluids. This paper SPE 5872 was prepared for presentation at the 4 th Annual California Regional Meeting of the SPE of AIME, Long Beach, California, April 8-9, [16] S.L. Wellington, Biopolymer Solution Viscosity Stabilization Polymer Degradation and Antioxidant Use. The paper SPE 9296 was first presented at the 1980 SPE Annual Technical Conference and Exhibition, held in Dallas, September 21 24, [17] H.T. Nguyen and J.M. Lafontaine, Effect of Various Additives on the Properties of the Clarified XC Polymer System. Paper SPE MS was presented at the SPE Formation Damage Control Symposium, Louisiana, February 22-23, [18] C. Montgomery, Fracturing Fluids. Paper presented at International Conference for Effective and Sustainable Hydraulic Fractioning. An ISRM Specialized Conference, Brisbane, Australia, May [19] G.A. Al-Muntasheri, A Critical Review of Hydraulic Fracturing Fluids over the Last Decade. Paper SPE presented at the SPE Western North American and Rock Mountain Joint Regional Meeting, Denver, Colorado, U.S.A., April, [20] Fann Instruments (2013): Model 35 Viscometer Instruction manual, manual number , Revision N, February Copyright Vandana Publications. All Rights Reserved.