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Abstract Gelled polymer treatments applied in injection and production wells are used to alter fluid flow in oil reservoirs so that oil production is increased and water production is decreased. A common gel used in field treatments is made by crosslinking partially hydrolyzed polyacrylamide (HPAM) with chromium acetate. The initial step of the crosslink is the uptake of chromium acetate by a carboxyl group on the HPAM. The kinetics of the uptake reaction was studied as functions of the concentrations of chromium, HPAM and hydrogen ion. The data were regressed to derive a rate expression for the disappearance of unreacted Cr(III) in the gelant. Introduction The application of crosslinked-polymers gels for permeability modification of petroleum reservoirs has been effective to improve displacement efficiency, increase crude oil production and to reduce water production. The treatment of an injection well consists of injecting an aqueous solution containing polymer and a crosslinker into the high permeability zones or fractures of the reservoir where the polymer and crosslinker react to form a three dimensional gel network, reducing the effective permeability of these zones. Displacing fluids injected after the treatment are diverted into the previously unswept, low permeability zones resulting in additional oil recovery and less water production. Gel treatments applied in production wells can also reduce water production and increase oil production although the mechanisms are not as well defined. One widely used gel system is an aqueous solution of partially hydrolyzed polyacrylamide and a chromium(III) salt. Chromium(III) forms a complex ion in solution and reacts by a ligand-exchange reaction with the carboxylate, or hydrolyzed, groups on the polymer molecules to form crosslinks resulting in a network or gel. These reactions are described by Equations 1 and 2 where L represents a ligand in the chromium complex. The first reaction of a polymer, P1, with the chromium complex is called the uptake reaction. The reaction of a second polymer, P2, with the chromium complex creates a crosslink between the polymers and is termed the crosslink reaction. Partially hydrolyzed polyacrylamide (HPAM) is a linear polymer with amide and carboxylate side groups. The reactivity of HPAM is dependent on the fraction of carboxylate side groups that is described as the degree of hydrolysis, usually stated as a percentage value. The degree of hydrolysis for HPAMs used to form gels ranges between 2 and 20%. Gel times are shorter (faster reaction rates) with increased carboxylate content of the HPAM. The reactivity of the chromium(III) complex ion is dependent on its structure and the types of ligands. Generally, inorganic salts are present as chromium monomers and have relatively short gel times. Chromium acetate is a preferred crosslinker presently due to its relative inertness to reservoir conditions and relatively long gel times that are due to the stronger affinity of acetate as a ligand as compared to inorganic ligands. The structure of chromium acetate complex ion is affected by pH and age Tackett1 found that at pH values below 4.5 and after sufficient time a green cyclic chromium trimer was the dominant species in a solution. As the pH of the solution is increased, hydroxyl groups replace the bridging acetate group. The cyclic structure is retained up to pH 5.5. Further increase in pH converts the cyclic trimer to a linear trimer. Depending on pH, there exists an equilibrium between the cyclic trimer and linear trimer with different degrees of hydroxyl substitution. Additional increase in pH causes the complex to precipitate. The trimer species were the dominant structures. Other structures were indicated including chromium oxylate in the commercial product from McGean-Rohco, the source of chromium acetate used in this work.
Summary Application of gelled polymer treatments to change the flow characteristics of a reservoir is a viable improved oil recovery technique. Many gel systems are affected by the solution pH in that gel time is pH dependent. The treatment of carbonate reservoir rock is of particular concern because (1) fluid-rock interaction can alter the pH of the injected solution from the design value and (2) dissolution of carbonate can increase the divalent ion concentration, which can also affect gelation behavior. In this study, the interaction between injected potassium chloride brine and dolomite was investigated through displacement experiments in dolomite cores and mathematical modeling based on equilibrium thermodynamics. Parameters varied were pH of the injected solution, flow rate, and the common ion effect through variation of the calcium and magnesium ion concentrations in the injected solution. Core effluent values of pH and concentrations were measured. Experiments at different flow rates established conditions in which equilibrium was achieved in the core. Equilibrium values of pH were almost constant at a value of 10 when the injected pH was varied between 4 and 10. Results indicate that equilibrium conditions exist for most field conditions. A geochemical model was used to predict equilibrium pH and concentrations, as well as the amount of dolomite dissolved. The model accurately predicted effluent pH of experimental displacement data for the conditions wherein equilibrium was achieved. All model parameters were obtained from literature equilibrium data and were not dependent on curve fitting of the experimental data. Introduction Systems utilizing Cr(III) as the polymer crosslinker are probably the most frequently studied and used gelled polymer systems for water conformance treatments. Both xanthan and partially hydrolyzed polyacylamide form gels with Cr(III). Both of these gel systems are affected by the pH of the solution in at least two ways. One is that the gelation time decreases significantly with increasing pH. The other is that Cr(III) is subject to precipitation in solutions with pH over 5.5, and the precipitation is aggravated with increasing pH. The treatment of carbonate reservoir rock is of particular concern because of the fluid-rock interaction, which can alter the pH of the injected solution. Previous studies show that change of pH of the injected solution inhibits the propagation of Cr(III) in carbonate cores or sandstone cores containing carbonate as the solution flows through porous rock. Seright studied the propagation of chromium acetate or chromium chloride through Indiana limestone cores. Cr(III) concentration in the effluent never reached the injected concentration after injecting about 10 pore volumes of chromium solution for any case studied. Chromium propagated more rapidly when the counterion was acetate as opposed to chloride. No chromium was detected in the effluent after injecting 10 pore volumes of chromium chloride solution through a limestone core. Stavland et al. studied the retention of chromium(III) in Brent and Berea sandstone cores (with about 2% carbonate content). The authors found precipitation was the most important reason for chromium retention in cores. Precipitation was caused by the dissolution of carbonate minerals that increased the pH of the injected solution. Their experiments also revealed that the retention rate of Cr(III) was lower with less carbonate present in the cores. McCool et al. studied the interaction between a dolomite core and a xanthan-Cr(III) gel system. Significant amounts of Cr(III) precipitated because the pH in the injected solution increased due to the dissolution of dolomite. Equilibrium relations and the dissolution kinetics in dolomite-carbonic acid-water systems have been studied for such purposes as soil science, the study of secondary changes in sedimentary deposits, the neutralization of acid mine drainage, and the acidizing of petroleum wells. Most previous studies were conducted in agitated batch reactors, rotating disk, or fluidized bed reactor systems in the laboratory by using relatively pure dolomitic rock or synthetic dolomite. A few investigators have studied the dissolution reaction using flow through packed-bed reactors or consolidated rock cores. Many investigators recognize the complexity of the dissolution of dolomite. Most have attempted to simplify the modeling by delineating the rate-limiting steps. It is apparent that in making assumptions and interpretations of data most investigators have often been limited by the type of apparatus, the size and origin of dolomite used, and the experimental conditions. Consequently, the kinetic equations and mechanisms proposed by different investigators explain their experimental data, but they are not easily generalized. The rate equations derived by Plummer and Busenberg and those from Chou et al. are often used to predict the dolomite dissolution rates within a 2- to 100-fold difference, depending on the reaction conditions. It is believed that the dissolution rate is controlled by surface reaction, reactant or product diffusion, or a combination depending on reaction conditions. Also, the dissolution of dolomite reaches equilibrium much faster in a closed-to-atmosphere system than in an open system in which the transport of carbon dioxide from the atmosphere is involved in the reactions. The dissolution rate is affected by the rock lithology such as impurity content, crystal size, rock texture, and Ca/Mg ratio in the rock and the injected solution chemistry such as the pH and composition. The effect of the rock formation on dolomite dissolution was the dominant interest in many previous investigations. Rauch and White investigated the effect of lithology on carbonate dissolution rate. The authors found that the dissolution rate decreased as the percentage of dolomite and disseminated insolubles increased. The general chemistry of dolomite dissolution has been studied extensively, though some dissolution mechanisms are still under debate. As a salt of a weak acid, dolomite dissolves in strong acid, carbonated water, and water by different mechanisms. In water, the general chemistry is When carbon dioxide is present, the dissolution has the reaction When in strong acidic solution, the following reaction occurs:
Summary Conformance control for carbon dioxide miscible flooding using gel has not been widely attempted. Laboratory research efforts at the University of Kansas have produced promising in-situ gelation techniques aimed at this application. Three in-situ gel systems were developed and tested in laboratory cores. Two systems are based on a new biopolymer, termed KUSP1, and the third gel system uses the reaction of sulfomethylated resorcinol and formaldehyde to form a gel. KUSP1 gel systems were studied using two different methods of inducing in-situ gelation. In the first method, gelation was accomplished by injecting CO2 at low pressure into the Berea sandstone core saturated by alkaline polymer solution. Permeability reduction to the brine and CO2 in the range of 80% was achieved. Stability of the gel was tested in the presence of supercritical CO2 When supercritical CO2 was used to induce in-situ gelation, the same degree of permeability reduction was achieved. The gel remained stable after the injection of many pore volumes of supercritical CO2The second method of initiating in-situ gelation involved the use of an ester. Hydrolysis of the ester, monoethylphthalate, in the alkaline polymer solution caused the pH to drop to levels where in-situ gelation occurred. The permeability of the treated core to supercritical carbon dioxide was about 1 md which was equivalent to a permeability reduction of 95%-97% of the initial brine permeability. The third gel system, based on the reaction of sulfomethylated resorcinol and formaldehyde (SMRF), was gelled in situ and contacted with both brine and supercritical CO2. Permeabilities to carbon dioxide on the order of 1 md or less were observed. This permeability is equivalent to a reduction of about 99% in the initial brine permeability. Reduced permeabilities were maintained after injecting many pore volumes of supercritical CO2 and brine. Introduction Carbon dioxide miscible flooding is one of the most important tertiary oil recovery techniques employed in the United States. However, the process experiences major difficulties in field application because of reservoir heterogeneity due to high permeability contrast. CO2 tends to finger through the high permeability zones and bypass the oil. Early CO2 production occurs with increased recycling and other operating costs. Different methods have been investigated for improving the overall efficiency of the CO2 flooding process. In almost all these methods, attempts have been made to achieve a favorable mobility ratio by affecting the CO2 relative permeability. Examples of these methods are:water alternating gas (WAG) process, carbon dioxide-foam process, and viscosified carbon dioxide process. Another technology which is under study is permeability reduction by in-depth placement of polymer gels. The objective of this research is to reduce the permeability in permeable zones of the reservoir. Reduction of matrix permeability in the CO2 process has been studied by other investigators. No systems were found that gave satisfactory permeability reduction when exposed to prolonged injection of CO2. Three new in-situ gel systems developed and tested in our laboratory are described in this paper. Two of these systems are based on a biopolymer termed KUSP1. The third system is based on a modification of a previously reported organic crosslinking system. Experiment The experimental program consisted of gelling each polymer system in a 1 ft Berea core which was mounted in a core holder and determining the permeability of the treated rock to brine and carbon dioxide at supercritical conditions. Five separate tests were conducted. Dispersion tests were run in some tests to estimate the pore volume contacted by the injected fluids after treatment with a gelled polymer system. Equipment and Materials Experimental Apparatus. Fig. 1 is a schematic presentation of the experimental apparatus used in this work. An ISCO syringe pump was used for injecting CO2 brine, and gel solutions into the core. All the experiments were conducted at constant rate. The effluent of the core was collected by a fraction sample collector for further analysis. A TEMCO high-pressure core holder equipped with pressure ports was used. The rubber sleeve was filled with water and the injection pressure was kept at 500 psi below the sleeve pressure because higher sleeve pressures caused the rubber sleeve around the pressure taps to deform and seal off the pressure ports. One ft Berea cores, 2 in. in diameter, were used in all experiments. Pressure ports were located such that the core was divided into four sections. The first and fourth sections were 5 cm in length and sections two and three were 10 cm long. The pressure difference for each section and the overall pressure difference were measured by pressure transducers and recorded via a computer-based data gathering system. The apparatus was placed in an air bath in which the temperature of the core and the injected fluids was kept constant. The pressure of the core was maintained by a TEMCO back-pressure regulator connected to a cylinder containing nitrogen at high pressure. The back pressure was maintained at 1200 psi. Details of the experimental setup are presented elsewhere. Gels Produced from KUSP1. KUSP1 is an acronym for a biopolymer developed at the University of Kansas. The polymer is a ?-1,3-polyglucan and is produced by fermentation of a bacterium known as Alcaligenes faecalis and certain species of Agrobacterium. The polymer grows on the surface of the bacteria. During the fermentation process, the polymer laden bacteria aggregate and settle out from the growth medium. Polymer is extracted from the bacteria by suspension in dilute alkali. Neutralization of the alkaline polymer solution produces a hydrogel. The gelation process is reversible and the hydrogels are stable at high temperatures in neutral solutions. The polymer degrades in alkaline solution with time and at elevated temperatures.
Abstract Chromium acetate - polyacrylamide gel systems have been used primarily to treat fracture systems, casing leaks and near wellbore regions in matrix rock where short gel times are acceptable. In-depth treatment of matrix rock is limited by gelation time for this system. This paper describes a study of a method to increase gelation time of chromium polyacrylamide systems by using excess acetate ion to reduce the rate of crosslinking between chromium and the polymer. Data are presented to show the effect of excess acetate ions (provided as sodium acetate) on the rate of gelation in bottle tests. Displacement experiments were conducted in sandpacks of various lengths to confirm effects anticipated from bottle tests. Gel samples were prepared at concentrations of 5000 ppm polyacrylamide, 109 ppm chromium acetate and a pH of 5, in sample bottles maintained at 25 C. The effect of excess sodium acetate concentration on the gel times of partially hydrolyzed polyacrylamide-chromium acetate gel systems was determined by varying the mole ratio of acetate to chromium from 3.00 (stoichiometric) to 600. Gelation delays on the order of several days were achieved by increasing the acetate/chromium ratio from 3.00 to 91. Gelation was prevented (for the observation period of one year) when the acetate/Cr ratio was greater than 260. The delay in gelation time was related to a critical acetate/Cr ratio that was characteristic of the concentration and source of the polymer and Cr(III). Aged chromium stock gave significantly shorter gel times than freshly prepared stocks. Displacement experiments were conducted in sandpacks using gel solutions with selected acetate/Cr ratios to determine flow resistance during placement and during brine flow after shut-in. At low ratios of acetate/Cr, buildup of flow resistance in the sandpacks due to formation and filtration of aggregates limited the penetration of the gelant and caused termination of injection. In most of the experiments the time for development of flow resistance was on the order of 10% of the bulk gel times. However, when the acetate/Cr ratio was 600, no buildup of flow resistance was observed when 4 PV of gelant was injected into a one ft sandpack and this system had not gel led in situ at four months after placement. In situ gelation of chromium acetate-polyacrylamide systems at 25 C can be controlled by varying the acetate/Cr ratio. P. 407
- Research Report > Experimental Study (0.48)
- Research Report > New Finding (0.46)
- Well Drilling > Drilling Fluids and Materials > Drilling fluid selection and formulation (chemistry, properties) (1.00)
- Reservoir Description and Dynamics > Improved and Enhanced Recovery (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Downhole chemical treatments and fluid compatibility (1.00)
Abstract Injecting gelant to change the flow characteristics of a reservoir is a viable improved oil recovery technique. Many gel systems are affected by the solution pH in that gel time is pH dependent. The treatment of carbonate reservoir rock is of particular concern because 1) fluid-rock interaction can alter the pH of the injected solution, moving the pH away from the design value and 2) dissolution of carbonate can increase the divalent ion concentration which can affect the gel behavior. In this study, the interaction between injected potassium chloride brine and dolomite was investigated through displacement experiments in dolomite cores and mathematical modeling based on equilibrium thermodynamics. Parameters varied were pH of the injected solution, flow rate, and the common ion effect through variation of the calcium and magnesium ion concentrations in the injected solution. Core effluent values of pH and concentrations were measured. A geochemical model was used to predict equilibrium pH and concentrations, as well as the amount of dolomite dissolved. The model was compared to the experimental displacement data for the conditions wherein equilibrium was achieved. All model parameters were obtained from literature equilibrium data and were not dependent on curve fitting of the experimental data. Experiments at different flow rates established conditions in which equilibrium was achieved in the core. The injected solution pH was varied between values of 2.6 and 11. The corresponding measured effluent pH values and simulated results could be divided into three stages. When the injected solution pH was less than 4, the effluent pH was much higher than the injected pH and increased with increasing injected pH. For injected pH values between 4 and 10, the effluent pH was almost constant at a value of 10. At injected pH values above 10, the effluent values were nearly equal to the injected values. At the lowest pH stage, effluent concentrations of Ca and Mg decreased exponentially as pH was increased. In the middle stage, the divalention concentrations were almost constant and not a strong function of pH. When Ca and Mg were present in the injected solution, the dissolution of dolomite was depressed and effluent pH was lower compared to the cases where Ca and Mg were not in the injected solution. For a specified total divalent ion concentration in the injected solution, the presence of both Ca and Mg had a larger effect on equilibrium pH than did either species individually. Pressure measured along the core during the displacement showed that dissolution of dolomite had an insignificant effect on rock permeability for the conditions of these experiments. P. 397
- North America > United States > West Virginia > Appalachian Basin > Berea Sandstone Formation (0.89)
- North America > United States > Pennsylvania > Appalachian Basin > Berea Sandstone Formation (0.89)
- North America > United States > Ohio > Appalachian Basin > Berea Sandstone Formation (0.89)
- (2 more...)
Abstract Carbon dioxide miscible flooding is widely used in carbonate reservoirs in West Texas and has potential application to carbonate reservoirs in other areas of the country including Oklahoma, central, and southwest Kansas. One of the major difficulties of this process is the CO2 early breakthrough because of its unfavorable mobility ratio and because of the heterogeneous nature of this type of reservoir. Injection of alternating slugs of water and carbon dioxide, known as the WAG process, is the remedy most widely applied to control the early breakthrough, but its success has not been universal. Conformance control using gels has not been widely attempted. Laboratory research efforts at the University of Kansas have produced promising in situ gelation techniques aimed at this application. Three in situ gel systems have been developed and tested in laboratory cores to control the movement of supercritical CO2 in matrix rock. Two systems are based on a new biopolymer, termed KUSP1, which is soluble in alkaline solutions above pH 10.8, but forms firm gels when the pH is reduced to 10.8 or below. The third gel system uses the reaction of sulfomethylated resorcinol and formaldehyde to form a gel. This paper describes the behavior of these gel systems at 32.2-41 C in Berea sandstone cores (initial permeability 70 - 700 md) when exposed to supercritical carbon dioxide and brine after in situ gelation. KUSP1 gel systems were studied using two different methods of inducing in situ gelation. In the first method, gelation was accomplished by injecting CO2 at low pressure into the Berea sandstone core saturated by alkaline polymer solution. Permeability reduction to the brine and CO2 in the range of 80 % was achieved. Stability of the gel was tested in the presence of supercritical CO2. When supercritical CO2 was used to induce in situ gelation, the same degree of permeability reduction was achieved. The gel remained stable after the injection of many pore volumes of supercritical CO2. The second method of initiating in situ gelation involved the use of an ester. Hydrolysis of the ester, monoethylphthalate, in the alkaline polymer solution caused the pH to drop to levels where in situ gelation occurred. The permeability of the treated core to supercritical carbon dioxide was about 1 md which was equivalent to a permeability reduction of 95-97 % of the initial brine permeability. The third gel system, based on the reaction of sulfomethylated resorcinol and formaldehyde, termed SMRF, was gelled in situ and contacted with both brine and supercritical CO2. Permeabilities to carbon dioxide on the order of 1 md or less were observed. This permeability is equivalent to a reduction of about 99 % in the initial brine permeability. Reduced permeabilities were maintained after injecting many pore volumes of supercritical CO2 and brine. Introduction Carbon dioxide miscible flooding is one of the most important tertiary oil recovery techniques employed in the United States. However, the process experiences major difficulties in field application because of reservoir heterogeneity due to high permeability contrast. CO2 tends to finger through the high permeability zones and bypass the oil. Early CO2 production occurs with increased recycling and other operating costs. Different methods have been investigated for improving the overall efficiency of the CO2 flooding process. In almost all these methods, attempts have been made to achieve a favorable mobility ratio by affecting the CO2 relative permeability. Examples of these methods are:water alternating gas (WAG) process carbon dioxide-foam process, and viscosified carbon dioxide process. P. 325
- North America > United States > Texas (0.86)
- North America > United States > Kansas (0.70)
Summary In observations of the long-term properties of a series of Cr(III)/polyacrylamide (PAAM) gels, the gels either underwent syneresis upon aging or swelled in contact with excess brine. Both syneresis and swelling can substantially change the volume and properties of a gel placed in a formation and therefore influence the properties of a gel placed in a formation and therefore influence the effectiveness of a crosslinked-polymer treatment. Efforts were made to develop an understanding of these phenomena and to describe them in terms of the gel's physical and chemical states. A long-term gel's physical and chemical states are characterized by two parameters, effective crosslinking density and chromium density, determined by equilibrium swelling and equilibrium dialysis, respectively. Swelling and syneresis properties were correlated to the effective cross-linking density described in polymer network theory. A model based on Flory and Hermans' swelling equations was developed to calculate the effective crosslinking densities of gels prepared from solution. Attempts were made to relate the swelling and syneresis properties to the compositions of gel systems to allow prediction of properties to the compositions of gel systems to allow prediction of long-term stability of a gel based on its composition. In analysis of the amount of chromium that reacted with the PAAM was made by successive equilibrium dialyses of the gel followed by chromium analyses of dialysates by atomic and visible absorptions. The overall fractional conversion of chromium in the gelation was between 0.39 and 0.70. The equilibrium conversion of CR(VI) to CR(III) ranged from 0.65 to 0.85. The effective crosslinking density obtained from the swelling experiment was less than 5 % of the chromium density for all samples studied. A substantial fraction of reacted chromium is suspected to be present in the gel matrix either in the form of intramolecular crosslinks or attached to only one polymer segment. Introduction The application of crosslinked-polymer systems for permeability modification of petroleum reservoirs has received increasing attention in recent years. A crosslinked-polymer treatment generally involves injection of a polymer solution into high-permeability zones or fractures previously swept by the displacing fluid, The polymer solution reacts, either before or after injection, to form a three-dimensional (3D) gel network that reduces the effective permeability of the invaded portions of the reservoir. Fluid subsequently injected is diverted to other, tighter regions of the formation, improving overall volumetric sweep efficiency. One method of applying a crosslinked-polymer system involves crosslinking of PAAM by CR(III) in the formation. In this process, chromium is introduced in the (+6) state as sodium dichromate and is subsequently converted to CR(III) with a reducing agent, such as thiourea. The CR(III) then interacts with PAAM in the formation to form a gel. The process consists of two reactions in sequence: a redox reaction that controls CR(III) release, and a Cr(III)/PAAM crosslinking reaction. By control of the rate of reduction of CR(VI) to Cr(III), gelation can be delayed until the gelling polymer solution reaches the desired portions of the formation. For the design of this process, gelation characteristics of concern include the timing of gelation, rheological properties of the gelling fluid, and long-term properties of the developed gel. Variables that influence the timing of gelation and rheological properties of gelling fluids have been identified and their effects investigated. The long-term properties of developed gels, however, have not been thoroughly investigated. This work was undertaken to study the long-term properties of Cr(III)/PAAM gels and to relate them to the physical and chemical gel structure. Long-term properties of a series of Cr(III)/PAAM gels were observed and summarized in Ref. 6. Gels were found either to undergo syneresis (showing a separation of liquid from the gel as a result of gel shrinkage) upon aging or to swell in contact with excess solvent (brine). While the syneresis phenomenon is known by many involved in permeability modification, gel swelling in brine is nearly ignored at present. Both phenomena, however, have been described in polymer network theories and are believed to be common to many gel systems. Because syneresis and swelling can change the volume and mechanical properties of a gel placed in a formation, these phenomena need to be described quantitatively for design purposes. The development of a method to describe Cr(III)/PAAM gel swelling and syneresis properties in terms of scaling parameters defined in polymer network theory was this study's major objective. A second objective was to examine the chemical nature and to specify the chemical state of Cr(III)/PAAM gels by analyzing their chemical compositions. Such a study was expected to allow a correlation between the physical properties, such as swelling and syneresis properties, and the gel's chemical state. Development of this correlation would make it possible to describe and predict longterm properties in terms of the chemical composition of the gel system. The approach taken to characterize physical and chemical states of long-term Cr(III)/PAAM gels is described below. A gel's physical state can be characterized in terms of its effective crosslinking density, the number of elastically effective crosslinks per unit volume of polymer. According to Flory and others, per unit volume of polymer. According to Flory and others, this quantity is the most descriptive parameter for the physical structure of a gel network and is directly related to swelling and syneresis phenomena. The effective crosslinking density can be determined from an equilibrium-swelling experiment in which the degree of swelling of a gel is quantitatively determined. The degree of swelling is defined as the fractional change in gel volume relative to the sample volume at the gel point, with a negative change indicating syneresis. The degree of swelling is a direct measure of a gel's crosslinking density, the relationship being defined by swelling equations of Flory and Hermans discussed later. Because a gel's structure is determined by its chemical composition, it should also be possible to relate the swelling and syneresis properties to the composition of the gel system through the effective properties to the composition of the gel system through the effective crosslinking density. Such a correlation would allow prediction of the long-term stability of a gel, measured by its tendency either to synerese or to swell. The chemical state of a gel can be characterized in terms of a "chromium density," defined as the number of chromium ions bound to the polymer matrix per unit volume of polymer. Because this quantity represents the population of the crosslinker (chromium) in the gel network, it should relate directly to the effective cross-linking density, which represents the population of elastically effective crosslinks. A correlation of the chromium density to the effective crosslinking density may provide information on the chemical nature of the crosslink (i.e., number of chromium ions involved in a crosslink) as well as the crosslinker's effectiveness in developing the gel network. We determined the chromium density by successive equilibrium dialyses of the gel followed by chromium analyses of dialysates. For well-developed gels considered to be at equilibrium (or near-equilibrium) states, such analyses are also expected to provide information on the equilibrium conversions of both redox and crosslinking reactions. SPERE P. 348
- Energy > Oil & Gas > Upstream (1.00)
- Materials > Chemicals > Commodity Chemicals > Petrochemicals (0.91)
Abstract The formation of chromium(III)-polyacrylamigels was followed kinetically using spectrophotometric methods. The rate of chromium(VI)species conversion by thiourea and bisulfite was monitored to find a kinetic model for the oxidation/reduction reaction. The third order model found here shows first-order dependence on chromium(VI) concentration on thiourea and on pH. Additional results of the thiourea study show that a constant amount of chromium(VI) is consumed at gelation regardless of the initial concentration of that species, indicating that the CR(VI) reduction reaction may be the rate-determining step in gelation. No such model or constant conversion for the bisulfite-reduced reaction could be determined. Introduction The use of high viscosity polymeric fluids and gels for water flow control is well established, with the primary application in enhanced oil recovery. Injection of such fluids can result in selective permeability reduction in formations where secondary recovery techniques have become ineffective. Because injected solutions of high molecular weight polymers are not retained after large quantities of water flush the formation, more durable gelled polymers have been developed. The gelling solution typically consists of a sufficient concentration of metal ion and the polymer, in aqueous solution. Gels made with chromium(III) and polyacrylamide were found to be particularly stable and polyacrylamide were found to be particularly stable and resistant to elution. Because the CR(III) ion is a strong complexing agent, the necessary bonding with the polymer is initiated only by controlled increase in the PH of the solution — a process difficult to achieve in-situ. The preferred method of gelling such solutions is to generate CR(III)from the CR(VI) form using a reducing agent. Huang also found that the type of reducing agent used in such a gelling solution radically affected the time required for gelation after mixing. Thus, it was hypothesized that the oxidation/reduction reaction is the rate-determining step in the overall gelation sequence. The Cr(III)-polyacrylamide gelling, system was studied by Terry, et al., and Huang to obtain useful process design data. They found the gelling reaction rate increases when the concentration of any of the three reactants is increased. The time required for gelation was found to be very sensitive to temperature. Jordan found that the gelation time of a specified system decreases as the temperature is increased. Plots of the logarithm of gelation time versus the reciprocal of the absolute reaction temperature were found to be linear for the systems studied. This correlation showed that the system apparently followed the Hurdand Letteron model, which assumed rate dependence on only one species. The usefulness of gelled polymers in reservoir applications is dependent on having strong gel formation with proper timing so gelation occurs in the desired location. Chromium(III)/polyacrylamide solutions with gel times ranging from half an hour to several months can be made by varying the concentrations of the reactants. It is desirable to predict the rate of the redox reaction for any predict the rate of the redox reaction for any combination of reactants with a kinetic model. Such a model was developed, to describe the reaction of CR(VI) with thiourea as reducing agent. All work has been done in aqueous polyacrylamide solutions. BACKGROUND: CHEMICAL SYSTEM, PROCEDURE, AND EQUIPMENT Chemical System The reacting systems consisted of three components in aqueous solution. Solutions of chromium(VI) ion, polyacrylamide, and reducing agent were mixed and observed as the reaction progressed. The chromium ion was added as a solution progressed. The chromium ion was added as a solution of sodium dichromate dihydrate (Na2Cr2O7.2H2O) The exact nature of the entire gelling process is unknown, and probably consists of a number of reaction steps. P. 397
- Asia > Middle East > Jordan (0.24)
- North America > United States > Kansas (0.15)