Synthesis and Investigation of Polyacrylamide Chromium Gel Polymer for Reducing the Water Cut in Oil Producing Wells via Polymer Flooding- Static Test

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1 Synthesis and Investigation of Polyacrylamide Chromium Gel Polymer for Reducing the Water Cut in Oil Producing Wells via Polymer Flooding- Static Test Bahram Mokhtari *1, Hamid Simjoo 2, Masoud Enayati 3, Mahdi Kazempour 4, *1- Iranian Elite Academy, Aghdasieh, Tehran, Iran, Tel: +98 (912) Energy Recovery Laboratory, Tarbiat Modares University, Tehran, Iran 3- Lavan Island Oil Laboratory, Iranian Offshore Oil Co., Tehran, Iran 4- Chemical Engineering, Iran University of Science and Technology, Tehran, Iran Abstract Water production in oil reservoirs is one of the most serious problems in the oil industry. Water soluble polymers with a suitable crosslinker have been successfully used to reduce water production in the field. This research was conducted to experimentally synthesis and evaluation of Partially Hydrolyzed Polyacrylamide (PHPA)/Chromium (III) gel polymer used as sealants to shut-off water production in one of the Iranian oil reservoir. The use of gelling systems as diverting agents or blocking agent is widely practiced today to improve production of oil and gas. These systems, which are typically composed of a water soluble polymer and crosslinker are dissolved in water and injected into the target zones. The polymer may be naturally such as xanthan biopolymer or synthetically such as polyacrylamide. The crosslinker may be metal ions or metallic complexes that bond ionically or by chelating to the polymer conditions. In this research, laboratory studies were conducted to synthesis and prepare optimum gel polymer using crosslinking reaction between PHPA as a water soluble polymer and chromium (III) acetate as a metallic crosslinking agent. Keywords: Crosslinking Agent, Gel polymer, Partially Hydrolyzed Polyacrylamide. 1. Introduction Unwanted water production in associated with crude oil is increasing worldwide. Remediation techniques need to be applied to control the unwanted water production and improve the reservoir s economic and productive life. These remediation techniques are generally referred to as Conformance Control. Reservoir conformance is the process of applying various methods and technologies to a reservoir or wellbore to reduce or control unwanted water or gas production so that recovery efforts are 1

2 effectively enhanced and operator profitability improved. Conformance control is not the treatment of a single well but the analysis of the reservoir performance as a whole. A better understanding of how and why water is produced will help to identify the best conformance solution [1]. Geochemical studies have indicated that the brines became salty as a result of their interaction with various rocks over long time intervals [2]. From the late 1980s there has been increasing awareness of water control issues in major oil and gas reservoirs in the Middle East and elsewhere [2]. However, the conventional methods have been ineffective against problems that do not originate in the wellbore (fractures, coning, crossflow). Also from economical point of view, Mechanical methods require a workover rig and are therefore expensive [3]. Chemical means can be applied without a rig on location, making them more convenient for the operator and less expensive. Unlike cement and bridge plugs, chemicals including gelling polymers, can be placed deep into the formation. They form a gel that acts as a physical barrier that hinders the flow of water for a long period of time. The length of this period depends on the characteristics of the reservoir, the gel, and water movement in the treated reservoir [4]. 2. Partially Hydrolysis Polyacrylamide/Chromium (ІІІ) Gel Polymer The most common polymers used for gel treatment of petroleum reservoirs are polyacrylamides with varying degrees of hydrolysis charge densities and molecular weights. These polymers are typically inexpensive and can be crosslinked with metallic and organic cross-linkers to produce suitable gels. Commercially available acrylamide polymers come in three forms: anionic (contain negative charges on polymer backbone), neutral and cationic (contain positive groups on the polymer backbone). In general, both anionic and cationic polyacrylamides contain a significant amount of neutral acrylamide groups. Anionic polyacrylamides can be cross-linked with multivalent cations such as aluminum, chromium and zirconium to produce gels. The rate of crosslinking and gel formation strongly depends on the charge density of the polymer as well as the source of tri or tetravalent cations. Other factors affecting the rate include temperature, ph, salinity and the concentration of polymer and cross-linker. Complex forms of metallic cations are the best sources for these cross-linkers. This is done to reduce the rate of reaction between the cation and the polymer. For example, chromium chloride cannot be used directly to form suitable gels with anionic polyacrylamides due to its fast reaction [5-8]. However, chromium acetate is used to produce good gels with plenty of time to place the gelant in desired zones in the formation before setting. This process also is reported to produce gels at temperature exceeding 200 º F [6]. At elevated temperatures (90º C) the rate of gelation of high molecular weight polymers with chromium acetate is too fast for placing the gelant in the target zones. Furthermore, thermal hydrolysis of the acrylamide groups on the polymer produce additional crosslinking sites for interaction with Calcium and Magnesium ions, resulting in syneresis of the gels [9]. It is believed that syneresis is the result of increasing the crosslink density with time. It seems that syneresis is a function of brine salinity, ph, temperature, and gel composition. Although syneresis could become a hindrance for the long-term effectiveness of the gel placement in the fracture, in pore scale it does not have significant effect [8]. It suggests that gel syneresis in treated matrix might not create a substantial problem. This study indicates that even gels, which syneresis extensively in porous media, might still be effective in permeability reduction to a large extent. 2

3 There are, however, cautions against the use of the gels vulnerable to syneresis for treatment of fracture zones [6, 9]. 3. Measurement of Gel Polymer Property One of the most important characteristics of a gel polymer system is gelation time which could be considered as a fundamental parameter in oil field applications. The gelation time will determine the injection period and gelant how deep into the formation can be placed. Also, the amount of time available to the operator before the pumping pressure of the polymer system significantly increases when applied in the field could be calculated from the gelation time. If the estimated gelation time is longer than the actual gelation time, the gelant system cannot be placed as deep as predicted, and the well is shut in for a longer period than necessary [1, 10]. Bottle testing method as an experimental method provided a semi-quantitative measurement of gelation rate and gel strength. In this test experiment, which was defined by Sydansk, gel strength was expressed in alphabetic code of A through J which is shown in Table 1 The gel strength codes ranged from high flowing gels ( A) with barely any gel structure visibly detectable to rigid rubbery gels ( I ). Although Bottle testing represents a faster and inexpensive method to study gelation kinetic, the experimental observations obtained from this technique have to be considered with caution because of its subjective and qualitative nature [7]. In field applications, where large volumes of gel were injected, the time required to inject the gel was typically one week, but could be up to one month in some cases [11]. The gelant solution could be converted to partially or full gel polymer during this time. Therefore, it is important to know how gel properties change with time. Table 1: Gel strength rating and description Code Gel Description Code Gel Description A No detectable gel formed F High deformable non flowing gel B Highly flowing gel G C Flowing gel Moderately deformable non flowing gel D Moderately flowing gel H Slightly deformable non flowing gel E Barely flowing gel I Rigid gel 4. Static Test This test describes the tests conducted to provide a gel polymer system based on crosslinking reaction between PHPA as a water soluble polymer and chromium acetate as a metallic crosslinking agent Materials Partially hydrolyzed polyacrylamide was used as completely water soluble polymer. The physical properties of the polymer are described in Table 2. Applying shear on the polymer solutions can result in deforming or breakage of molecular bonds and therefore result in decrease in viscosity. Table 2: Physical properties of PHPA Molecular weight (g/mol) ph 8. 3 Solid polymer content in weighted sample 10 % Degree of hydrolysis 30 % 3

4 Chromium acetate was used as crosslinking agent that was provided by Aldrich. Sodium chloride, Potassium Chloride, Calcium chloride and Magnesium chloride were used as different additives which were provided by Merck. Also, water formation which was received from one of the Iranian south oil reservoir, used as in situ fluid to evaluate gel polymer performance in a real circumstance. Wash water was used as solvent to prepare gelant solution. Generally, gelant was formed by mixing an acrylamide polymer and crosslinking agent at the room temperature. Polymer solution was prepared by adding a preweighed amount of partially hydrolyzed polyacrylamide powder into wash water. Then the solution was continuously stirred on a magnetic stirrer until uniform viscous solution was obtained. Chromium acetate solution was prepared by adding a preweighed amount of chromic acetate powder into wash water. Afterward, polymer and crosslinking solution were mixed at a specific ratio. Once the mixing was done, solution stirred to take a homogeneous gelant The Effect of Polymer Concentration The polymer concentration is critical for the structure and properties of the gel polymer network. Any type of gel system needs a minimum concentration of polymer which called critical overlap concentration (COC). As indicated by the Bottle testing results in Table 3 the COC value is about 5000 ppm for the synthetic polymer. The gel strength code which is an indication about gel network formation is A, i.e., No detectable gel formed for 5000 ppm. The gel appears to have the same viscosity (fluidity) as the original polymer solution and no gel is visually detectable. Therefore, any gel was not formed at blew of the COC. Table 3: Gel codes for experimental condition: weight ratio PHPA/Cr is 40:1, solvent is tap water, T= 90 ºC Time (hr) Polymer concentration (ppm) A B C D D 6 A C D E E 24 A D E F G 150 A E G H I Also, for a good explanation of the polymer concentration effect during development of gelation reaction, some pictures were taken. Samples were investigated approximately after 40 hours aging at the room temperature. Figure 1 shows the critical overlap concentration effects. Figure 1: Critical overlap concentration effects. From left side: a) ppm, b) ppm, c) 5000 ppm PHPA. Experimental condition: weight ratio PHPA/Cr is 40:1, solvent is tap water, in ambient temperature 4

5 As indicated by the figure 1, gel network was not formed in sample a because the polymer concentration is about the COC, but polymer concentration in other samples is higher than the COC; thus, gel polymer network could be formed in these samples. Afterward, gel polymer properties were investigated at the concentration more than the COC. Some samples were prepared which the properties is illustrated in Table 4. Table 4: Gel codes for experimental conditions: weight ratio PHPA:Cr is 40:1, solvent is tap water, T= 90 ºC Time (hr) Polymer concentration (ppm) E E F G G 45 F G H H H 70 G H I I I 100 H I I I I The results in Table 4 show that gel polymer can be formulated over a broad range of polymer concentrations above the COC. As can be seen from bottle testing result, gel strength increases with increasing polymer concentration. After 100 hours aging at 90 º C, The final gel strength code range is about H (i.e., gel surface only slightly deforms upon inversion which is considered as slightly deformable non flowing gel) to I (i.e., there is no gel-surface deformation upon inversion which is considered as rigid gel). Also, polymer concentration has a significantly effect on the gelation rate The Effect of Crosslinker Concentration After determination of critical overlap concentration of polymer, several samples were prepared over a broad range of polymer and crosslinker concentrations to investigate the effect of crosslinking agent on the gelation kinetic. Bottle testing results in Table 5 indicate that, as crosslinker concentration increases from 1:80 to 1:40, gelation rate increases so that gel strength promote from C (i.e., most of the obviously detectable gel flows to the bottle cap upon inversion which is considered as Flowing gel) to G (i.e., gel does not flow to the bottle cap upon inversion which is considered as moderately deformable non flowing gel) after 45 hours aging at 90 º C. Table 5: Gel codes for experimental condition: PHPA is ppm, solvent is tap water, T= 90 º C Time (hr) Weight Ratio PHPA:Cr 20:1 40:1 60:1 80:1 5 F E C B 45 - G E C 70 - H F D H G E This effect up to a point affects gel strength because when level of crosslinker increases from 1:40 to 1:20, final gel strength dramatically decreased during gel formation at 90 º C. Although, gel network with the ratio 1:20 had a significant Polymer solution without crosslinking gel with a low strength crosslinking density is low gel with a high strength crosslinking density ishigh strength after 5 hours (gel strength code is F i.e., the gel flows about half way down the bottle upon inversion which is considered as highly deformable non flowing gel) and the gelation rate was faster than other samples, but the gel network was subsequently destroyed during 5

6 development of gelation reaction. This phenomenon is related to phase stability, which is the ability of a gel network to resist syneresis. Generally, syneresis is a collapse of the gel network which is characterized by a loss of inherent adhesion of gel system, reduction of gel volume and expulsion of water from gel structure. These states are usually reported as instability of gel polymer network. In this section of static test, the main cause of instability is related to the significant increment of crosslinking density and decrease of molecular weight between two polymer segments in the gel network. Therefore, due to reduce of space between two polymer connection in the network the value of water keeping decrease and water remove from structure and gel shrinkage happen. Gel phase stability could be studied indirectly during bottle testing. Figure 2 demonstrates the instability of gel polymer network due to excess crosslinker content in the gel network which leads to decline final gel strength. Figure 2: Effect of excess crosslinker content on the strength of gel network. Vertical sample with a good stability, horizontal sample with phase instability 4.4. THE Effect of Aging at High Temperature Polymer samples with different content of crosslinker were prepared and placed at high temperature media and aging test was conducted for them. Gelation time, gel strength and gel stability was investigated for each sample using bottle testing method. Also, for providing a good interpenetration about history of gel polymer properties during development of gelation kinetic, some pictures were taken through different period of time. The pictures were related to the samples which their properties are presented in Table 6. Figure 3 shows the samples that were aged for two hours at 90 º C. Table 6: Experimental conditions for gel samples preparation. Solvent is tap water, T= 90 ºC Gel No. PHPA concentration (ppm) Water (w%) Cr HPAM : : : : : : : : :40 6

7 Figure 3: From left side, samples 1 to 3, 1 and 2, 4 to 6, and 7 to 9 As indicated form figure 3 and the bottle testing results, the gel strength code for all gelant samples were initially about A to B i.e., no detectable gel formed. But, gelant was converted to partially gel polymer after two hours aging at 90 ºC. At this time, the gel strength code was about D to F i.e., the polymers have undergone sufficient crosslinking so that they do not flow like a gelant solution. Also, phase instability due to excess crosslinking density was occurred in sample 3 after two hours (Figure 3). Figure 4 shows the pictures that were related to the samples that were aged for 40 hours at 90 º C. Figure 4: From left side, samples 1 and 2, 4 to 6, and 7 to 9 that were aged for 40 hours at 90 º C Following pictures in figure 5 were related to the samples that were aged for 112 hours at 90 º C. Figure 5: From left, samples 1 and 2, 4 to 6 that were aged for 112 hours at 90 º C Also, effect of polymer concentration (at constant crosslinker content) and crosslinking density (at constant polymer content) on the gel feature could be easily recognized from these pictures during aging process. For example, the gel strength in samples 4 to 6 in figure 4, which polymer content is ppm, was significantly more than the samples 1 and 2 in figure 3, which polymer content is ppm. Also, Figure 4 could be used to explain effect of crosslinking density on the gel strength so that, sample 9, which the ratio of polymer to crosslinker is 40:1, has a more viscosity than the sample 48, which the ratio of polymer to crosslinker is 80:1, while polymer content is ppm for them. Thus these pictures confirm that 7

8 polymer and crosslinker have an important effect on the gel strength. One of the main points which is understood from aging process is about increasing of gel strength with increased aging time at 90 º C which is shown in Table 7. It suggests that the increase may have been caused by curing or more complete intermolecular crosslinking reactions with increased time. Table 7: Effect of aging time on the gelation kinetic for sample 4 Figure No Aging Time (hr) Code Strength F G H As a result, according to obtained results from aging test at high and low temperature condition, gel network with polymer concentration about to ppm and polymer to crosslinker ratio about 40:1 is considered as a proper gelant solution which has a good physical and mechanical property. 5. Determination of Optimum Dosage Based on Reservoir Condition Various samples with different content of polymer and crosslinker were prepared. These samples were placed in a water formation. Afterward, gel properties were investigated at 90 ºC by bottle testing method. Also, for providing a good analysis of gel behavior in the harsh condition during development of gelation kinetic, some pictures were taken through different period of time. Following pictures were related to the samples which their properties are presented in Table 17. Table 8: Experimental conditions: weight ratio PHPA:Cr is 40:1, solvent is tap water, T= 90 º C Gel No. PHPA Concentration (ppm) Placed in formation water NaCl (ppm) in solvent Yes Yes Yes No Yes Yes Yes No Yes Yes Yes No Yes Yes Yes No 0 All samples were prepared in tap water. Then water formation as in situ fluid was added to the gelant solution. Afterward, gelation behavior was investigated based on 8

9 experimental conditions. Samples 14, 17, 21, and 25 were also considered as the reference. Figure 6 is related to the samples that were aged for 1, 17 and 24 hours at 90 º C. Figure 6: Samples 10 to 13 (from left side) were aged at 1, 17, and 24 hours in left, middle, and right pictures, respectively at 90 º C Figure 7 is related to the samples that were aged at the ambient temperature for 15 hours. Figure 7: From left side: samples 14 to 17, 18 to 21, and 22 to 25 at the ambient temperature for 15 hours Figure 8 is related to the samples that are placed firstly for 15 hours at the ambient temperature and followed by aging at 90 º C for 6 hours. Figure 8: From left side: samples 14 to 17, 18 to 21, and 22 to 25 at the ambient temperature for 15 hours and followed at 90 º C for 6 hours Also figure 9 is related to the samples that are placed firstly for 15 hours at the ambient temperature and followed by aging at 90 º C for 26 hours. 9

10 Figure 9: From left side: samples (14, 15, 17), (18 to 21), and (22, 23, 25) at the ambient temperature for 15 hours and followed at 90 º C for 26 hours 6. Conclusions 1. Partially hydrolysis polyacrylamide/chromium (ІІІ) gel polymer was prepared for one of the Iranian oil reservoirs condition from the view point of temperature, pressure and in-situ fluid using bottle testing method. 2. Gelation time is a controllable parameter from a few minutes to several days which depends on the gel composition and environmental conditions. 3. Any type of gel polymer system needs a minimum concentration of polymer to form a three dimensional gel network. In this research, this value was about 5000 ppm for the synthetic polymer. 4. At the constant of crosslinker content, gel strength increases and gelation time decreases as the weight percent of polymer in the gelant solution increases. 5. At the constant of polymer content, gelation time decreases as the content of crosslinker in the gelant solution increases. Also, increasing of crosslinker level up to a certain ratio which is about PHPA to Cr (ІІІ) 40:1, cause to increase gel strength but at higher value of crosslinker, about 20:1, final gel strength dramatically decreases. 7. References 1. Vasquez, J., Laboratory evaluation of high temperature conformance polyme" Master Thesis, University of Oklahoma (2004). 2. Kuchuk, F., Water in the oil field, Middle East Well Evaluation Review 19 (1997). 3. Seright, R.S., A strategy for attacking excess water production, paper SPE Nasr-El-Din. H.A., Taylor, K.C., Evaluation of sodium silicate/urea gels used for water shut-off treatments, Journal of Petroleum Science and Engineering 48 (2005) Green, D.W., Willhite, G.P., Enhanced oil recovery, SPE text book series (1998). 6. Sydansk, R., A new conformance improvement treatment Chromium (III) gel technology, paper SPE Sydansk, R., Conformance improvement in a subterranean hydrocarbon-bearing formation using a polymer gel, US Patent No (1987). 8. Vossoughi, S., Profile modification using in situ gelation technology- a review, Journal of Petroleum Science and Engineering, 26 (2000) Moradi- Araghi, A., A review of thermally stable gels for fluid diversion in petroleum production, Journal of Petroleum Science and Engineering, 48 (2005) Shaker Shiran, B., An experimental study of filtration mechanisms of pre-gel aggregates during flow of a Polyacrylamide-Chromium(III) gel system in porous media, Master Thesis, University of Kansas (2006). 11. Seright, R.S., Polymer gel dehydration during extraction through fracture, SPE Production & Facilities, 14 (2) (1999). 10