Summary This paper describes a material derived from natural sources that can be used to crosslink a variety of acrylamide-based polymers over a broad temperature range to produce gels for conformance applications. Delayed crosslinked polymer systems have been used for many years in conformance applications. For the past decade, the most widely used system has been based on chromium (3+) crosslinked polyacrylamide. Organic crosslinkers, such as phenol/formaldehyde and polyethyleneimine (PEI) have also been used with a variety of polymers. However, these systems are being scrutinized by governmental agencies and have been scheduled for phase out in some countries. Because of these issues, a single, environmentally friendly crosslinker that could be used with a variety of polymers over a broad temperature range was the focus of this study. This paper details the laboratory development of an environmentally friendly, natural polyamine crosslinker system. This crosslinker can be used with a variety of polymers, such as polyacrylamide, AMPS/acrylamide, or alkylacrylate polymers. Gels ranging from stiff and "ringing" type to "lipping" gels have been obtained. The data illustrate a simple, commercially available system that can be applied to field operations. Potential crosslinking mechanism(s) of the system will be discussed. Introduction Water production in oil-producing wells becomes a more serious problem as the wells mature. Remediation techniques for conformance control are selected on the basis of the water source and the method of entry into the wellbore. Treatment options include sealant treatments and relative permeability modifiers (also referred to as disproportionate permeability modifiers). This paper primarily discusses water control with water-based gels for applications in wells in which the oil- and water-producing zones are clearly separated and can be mechanically isolated. Chromium(III) crosslinked polyacrylamide gels can be choice materials for matrix-fluid shut-off systems.1–4 The crosslinking reactions in these gel systems take place by the complexation of Cr(III) oligomers with carboxylate groups on the polymer chains (Fig. 1). Because of the nature of the chemical bond between Cr(III) and the pendant carboxylate groups, formation of insoluble chromium species can occur at high pH levels. Other problems with these systems include thermal instability, unpredictable gel times, and gel instability in the presence of chemical species that are potential ligands. The gel times are controlled by the addition of materials that chelate with chromium in competition with the polymer-bound carboxylate groups.5,6 Another popular water-based gel system for water-control applications is based on a phenol/formaldehyde crosslinker system for homo-, co-, and ter- polymer systems containing acrylamide.7–11 Depending on the polymer composition, these gels are thermally stable, and the gel times are controllable over a wide temperature range. The crosslinking mechanism involves hydroxymethylation of the amide nitrogen, with the subsequent propagation of crosslinking by multiple alkylation on the phenolic ring (Fig. 2).12,13 Several variations of the same technology were created to overcome the toxicity issues associated with formaldehyde and phenol. These processes generally involve replacing formaldehyde and phenol with less toxic derivatives that generate phenol and formaldehyde in situ, or are themselves active components of the crosslinking system. For example, formaldehyde can be replaced with hexamethylene tetramine (HMTA), glyoxal, or 1, 3, 5-trioxane. Substitutions for phenol included phenyl acetate, phenyl salicylate, or hydroquinone, among others.12,13 Extensive patent literature for this technology exists.14–22 Recently, a less toxic crosslinker was tested extensively in field trials worldwide and enjoyed a very high success rate.23–27 This system is based on PEI crosslinker and a copolymer of acrylamide and t-butyl acrylate (PA-t-BA). PEI is a low-toxicity material that is approved in the United States for food contact.28–31 PA-t-BA is a relatively low molecular-weight polymer. The low molecular weight is expected to provide rigid "ringing gels." The crosslinking is believed to take place in situ by amidation of the pendant ester groups on the base polymer (Fig. 3). Recent test results indicate that a variety of polymers containing acrylamide pendant groups react with PEI nitrogens through a transamidation reaction pathway to provide gels (Fig. 4).32 Because of recent changes in European environmental regulations, PEI is targeted for phase-out from the Norwegian section of the North Sea within the next few years. A search for biopolymers containing amino groups suggested that chitosan (Fig. 5) may react with acrylamide-based polymers in a manner similar to PEI. Chitosan is a polysaccharide obtained by de-acetylating chitin, a homopolymer containing ß-(1-4)-2-acetamido-2-deoxy-D-glucose (Fig. 6) that occurs in the shell or skin of anthropods or crustaceous water animals. Chitosan is also present in the environment, although in lesser amounts than the chitin. The degree of deacetylation in the commercially-available chitosan materials is usually in the 70 to 78% range. The chitosan solubility in acidified water, for example in acetic or hydrochloric acid, is in the 1 to 2% range. The viscosity of the solutions depends on the molecular weight of the polymer. If the pH of the solution is increased above 6.0, polymer precipitation occurs. This paper presents results using chitosan as an environmentally preferable crosslinker for use in combination with acrylamide- based polymers. Gel treatments using this material should contain a biocide. The advantage here is that if inadvertently discharged, the chitosan will biodegrade. Experimental Methods Preparation of Chitosan Solutions. Commercial solid chitosan samples were dissolved in fresh water solutions containing 1% acetic acid to make 1.0 to 1.5% polymer solutions. Chitosan lactate salt, which is also commercially available, can be dissolved directly in fresh water to prepare solutions with similar polymer concentration. The viscosities and clarity of the solutions depended on the polymer molecular weight and the degree of de-acetylation. Aqueous solutions of chitosan salts are also available commercially, which can be used directly for crosslinking base polymers. The preformed chitosan salts are insoluble in salt water or seawater.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractConformance polymer systems have been successfully applied for many years to control undesired water production from hydrocarbon wells. However, currently available polymer systems present a number of limitations for high-temperature conformance applications (> 300 o F). Based on laboratory research, this paper documents the results of the development and evaluation of polymer gel systems used as sealants to shut off water production in high-temperature environments. The polymer systems were evaluated by their effectiveness to: (a) provide adequate gel time for placement (up to 400 o F), (b) limit permeability to water at temperatures up to 375 o F in sandpack flow experiments, and (c) provide long-term thermal stability in sandpack flow experiments at elevated temperature (up to one-year study).A commercially available polymer system that has been successfully used in field applications (up to 275 o F) has been modified to extend its applicability up to 375 o F. Recently developed base polymer, crosslinker, and retarder were tested successfully to extend the temperature range of applicability of this polymer system. Discussed are: (1) methodology used for gelation time measurement of polymer systems at elevated temperatures, and (2) laboratory results regarding gelation time of crosslinked polymer systems when varying temperature, base polymer concentration, crosslinker concentration, retarder concentration, salinity of mixing brine, and/or pH of solution.Additionally, this paper discusses and describes the dynamic flow through porous media experiments performed to simulate high-temperature / high-pressure reservoir conditions to evaluate the performance of polymer systems at elevated temperatures (up to 375 o F). Specifically, this paper details:(1) the physical laboratory equipment and test conditions used for dynamic flow studies, (2) experimental procedure regarding short-term and long-term testing, and (3) the effect of temperature versus permeability reduction over time.

surfaces and sensitivity of the metal-based crosslinker to the chemical environment can reduce gel-system effectiveness. fax 01-972-952-9435.References at the end of the paper.

This paper presents the results of laboratory evaluation regarding the effectiveness of novel, organically crosslinked, high-temperature, conformance polymer gel systems as sealants. Effectiveness of these sealant gels is evaluated by attempting water flow through high-permeability cores under residual oil conditions. The effectiveness of the sealants toblock water permeability at temperatures up to 350°F,provide long-term sealant properties at these temperatures, andprovide adequate gel time for placement is measured. The ultimate goal is to determine whether the selected crosslinked polymer systems provide useable extended gel times and maintain thermal stability to 350°F. Discussed arethe physical laboratory model and test conditions used to perform dynamic core flow studies over extended periods in determining the impact of sealant exposure to elevated temperatures and subsequent required modifications,experimental procedure used for dynamic core flow studies,the effect of temperature on permeability reduction over time,the impact of threshold pressure (differential pressure required before fluid flow begins through a treated core) on permeability reduction over time,laboratory methodology used for gelation time measurement of a new, novel, organically crosslinked, high-temperature, conformance polymer gel system, andlaboratory results regarding gelation time of the polymer system as a function of temperature up to 350°F. Introduction Excessive water production from hydrocarbon reservoirs is one of the most serious problems in the oil industry. Remediation techniques for controlling water production, generally referred to as conformance control, include the use of polymer systems to reduce or plug permeability to water. This paper mainly discusses water control in high-temperature environments for treating hydrocarbon-producing wells to reduce water production for applications in which water and hydrocarbon zones are clearly separated. The principle of operation of this technique is to pump the polymer system into the formation around the wellbore and then propagate through the rock matrix. In-situ gelation takes place, plugging pore spaces and channels, thereby limiting undesired water flow. Then, a permanent barrier strategically placed only in the water zone is formed because the oil- and water-producing zones can be mechanically isolated. Literature Review A variety of techniques for controlling water production have been attempted by the oil industry. Earlier attempts to reduce water production included mechanical isolation, squeeze cementing, solid slurry (clay) injection, and oil/water emulsion and silicate injection. More successful results have been obtained with in-situ polymerized systems, crosslinked polymeric solutions, and silicate-based gels.1 Polymer gel systems have emerged over the last decade as one of the most effective tools for controlling water production. One of the most widely used polymer systems employs polyacrylamides (PAMs) or acrylamide co-polymer and chromium [Cr(III)] as a crosslinker.2 Cr(III) has been extensively used because of its high success rate and relatively low cost. However, the short gelation times of this system at elevated temperature limit their application to lower temperature reservoirs.3 Other problems with this system include thermal stability, unpredictable gel times, gel instability in the presence of chemical species that are potential ligands, toxicity concerns, and limited propagation into the target pore volume.4 Another polymer system widely used is a water-based gel based on phenol/formaldehyde crosslinker for homo-, co-, and ter-polymer systems containing acrylamide. The loss of phenol by partitioning into the crude oil that it contacts has been identified as an important issue for this polymer system.5 Toxicity issues associated with formaldehyde and phenol need to be overcome as well.

Summary This article describes the use of associative polymer technology (APT) to achieve fluid diversion during an acid stimulation treatment. APT involves the use of a very-low-viscosity aqueous-polymer solution. It reacts immediately with the formation surface to significantly reduce the ability of subsequent aqueous fluids to flow into high-permeability portions of the rock. The first stage containing the APT predominately will enter the most permeable area, diverting the following acid stage(s) to less-permeable sections of the rock. APT has little or no effect on the flow of subsequent hydrocarbon production. Furthermore, in rock containing significant proportions of sandstone-type lithology, the water permeability of the treated zone is decreased permanently, resulting in post-treatment reduced water production from the treated zone. A general description of associating polymers and their properties, as well as a detailed description of the laboratory development of the current system, are both discussed. Laboratory data will show the effectiveness of APT in reducing the ability of aqueous fluids to flow through porous media. Parallel flow studies using water-saturated and oil-saturated cores are presented that show the ability of APT to divert acid in both sandstone and carbonate cores. These tests also show the ability of APT to decrease water permeability in the water-saturated core while the diverted acid increases the oil permeability of the oil-saturated core. Introduction Most intervals contain sections of varying permeability. In matrix-acidizing treatments, the acid tends to predominantly enter the highest permeability portions and bypass the most damaged (lower permeability) layers. In some cases, high-permeability layers are also predominantly water-bearing; thus, acid also mainly enters those zones. In some cases, the acid may also break into a nearby water-bearing zone. In attempts to achieve uniform placement of acid across all layers, various placement techniques have been used.1 The most reliable method uses mechanical isolation devices (such as straddle packers) that allow injection into individual zones one at a time until the entire interval is treated. However, this technique is often not practical, cost-effective, or feasible. Without a packer, some type of diverting agent must be used. Typical diverting agents include ball sealers, degradable particulates, viscous fluids, and foams. Although these agents have been used successfully, all have potential disadvantages and none address the problem of increased water production that often follows acid treatments. Therefore, it would be a major advantage to have a material that could inherently decrease the formation permeability to water while also providing diversion. One method of controlling water production uses dilute polymer solutions to decrease the effective permeability to water more than to oil. These treatments may be referred to as relative permeability modifiers (RPMs), disproportionate permeability modifiers, or simply, bullhead treatments. The latter name is so called because these treatments can be bullheaded into the formation without the need for zonal isolation. RPM systems are thought to perform by adsorption onto the pore walls of the formation flow paths.2–4 A laboratory study was initiated to develop an RPM based on a hydrophobically modified (HM) water-soluble polymer.5 This group of polymers was selected for study because their properties can be altered in ways that render them valuable for oilfield applications.

Summary Water production becomes a major problem as hydrocarbon-producing fields mature. Higher levels of water production result in increased levels of corrosion and scale, increased load on fluid-handling facilities, increased environmental concerns, and eventually well shut-in (with associated workover costs). Consequently, producing zones are often abandoned in an attempt to avoid water contact, even when the intervals still retain large volumes of recoverable hydrocarbons. Many polymer systems have been applied over the years to control undesired water production from hydrocarbon wells, with varying degrees of success. For approximately eight years, a polymer gel system based on an acrylamide/t-butyl acrylate copolymer (PAtBA) crosslinked with polyethyleneimine (PEI) has been used successfully for various water shutoff applications. This article describes results from a sampling of over 200 jobs performed throughout the world, including the average results from more than 90 jobs performed in one geographic location alone. In addition to "standard" matrix treatments, results will be shown for other types of treatment, including a design to plug annular communication and a combination of sealant and temporary gel in an openhole horizontal completion. In addition, laboratory data pertaining to work aimed at increasing the temperature limit of the system will be presented. The upper placement temperature of the system originally was ~260°F. Data presented in this article indicates the development of a retarder system that allows the upper placement temperature to be raised to at least 350°F. Introduction Controlling water production has been an objective of the oil industry almost since its inception. Produced water has a major economic impact on the profitability of a field. Producing 1 bbl of water requires as much or more energy as producing the same volume of oil. Often, each barrel of produced water represents some lesser, but significant, amount of unproduced oil. In addition, water production causes other related problems such as sand production, the need for separators, disposal and handling concerns, and the corrosion of tubulars and surface equipment. Many methods are available to mitigate water-production problems, and perhaps the most widely used chemical system has been chrome-crosslinked polyacrylamide gels. A previous publication described the advantages of the acrylamide- (PAtBA) copolymer/polyethyleneimine system (herein referred to as OCP, or organically crosslinked polymer) (Hardy et al. 1998). A brief discussion of these advantages follows, with presentations of case histories using OCP and data showing expansion of OCP technology. OCP System Description Gel systems for water and gas shutoff have many requirements, including:Low viscosity allowing easy injection deep into a formation matrix.Capability to control gelation time of the fluid.Sufficient strength to resist drawdown in the wellbore.Temperature stability of the gel for extended periods of time. As will be shown in the following discussion, the OCP system meets all these requirements. The viscosity of the system is ~25 cp as mixed. This relatively low viscosity is due to the relatively low molecular weight, ~250,000, of the PAtBA. This polymer is covalently crosslinked with PEI, which results in excellent control over gelation time. Sufficient strength and temperature stability are also obtainable. In addition, the OCP system is insensitive to formation fluids, lithology, and/or heavy metals. Another advantage of the OCP system is its predictable viscosity profile that can be used to improve diversion over long treatment intervals.

Summary For many years, relative-permeability modifiers (RPMs) have received a great deal of attention from the oil- and gas-production industry. Because of the completion techniques used in many wells, it is not always practical or cost-effective to protect the hydrocarbon interval properly during a water-shutoff treatment. RPMs offer the option of bullheading a treatment without zonal isolation, which is designed to decrease water production with little or no decrease in oil or gas production. This paper describes the laboratory development and optimization of a polymeric RPM. The resulting material can be best described as a brush polymer consisting of a polymeric backbone grafted with methoxypolyethylene glycol (MPEG). Various phases of the development will be discussed, such as the optimization of the molecular weight of the backbone polymer and the concentration of grafted MPEG. Details of laboratory evaluations will also be provided, including a discussion of the use of multi pressure-tap flow cells for permeability-reduction tests, the effect of polymer concentration, and the effect of saturations. These test results show that the RPM polymer should be placed with a systematic approach consisting of proper preflushes and postflushes for optimum results. Introduction Controlling water production has been an objective of the oil industry almost since its inception. Produced water has a major economic impact on the profitability of a field. Producing 1 bbl of water requires as much or more energy as producing the same volume of oil. Often, each barrel of produced water represents an equal amount of unproduced oil. In addition, water production causes other related problems such as sand production, the need for separators, disposal and handling concerns, and the corrosion of tubulars and surface equipment. Current literature describes successful RPM treatments that use various types of chemical treatments in essentially all lithologies. If these RPM treatments were successful in all cases, it would follow that RPM technology would be applied more frequently than currently indicated. Despite these claims of success, none of the materials or techniques used in RPM processes have apparently performed consistently well in field operations. Although the literature contains several theories on RPM mechanisms,1-3 none appear to be universally accepted, perhaps because no single factor determines the success of an RPM. Rather, an RPMs success depends on several well and reservoir characteristics, including chemistry, lithology, problem type, permeability, saturation, and many others. Because all of these factors affect the outcome of an RPM treatment, developing a single RPM for all well situations is unlikely. Instead, a better solution would be to focus on specific reservoir conditions (problem type, lithology, temperature, etc.), and to design a process to fit those circumstances. Conventional Water-Reduction Systems Two broad categories of chemical systems are available for reducing water production:Nonsealing systems that allow the flow of fluids through a porous medium.Sealing systems that completely block the flow of fluids in a porous medium. Nonsealing Systems. Nonsealing systems are typically dilute solutions of water-soluble polymers. These polymers most likely reduce effective water permeability by means of a "wall effect,"1 wherein the polymer adsorbs onto the formation, creating a layer of hydrated polymer along the pore throat that inhibits water flow. Sealing Systems. Sealing systems are porosity-fill materials that can be valuable when a water-producing zone can be mechanically or chemically isolated. However, in many situations, a target zone cannot be isolated, and the sealing system sometimes penetrates zones that should not be treated. Although there are claims that sealing systems will reduce water permeability more than they reduce oil permeability,4 it is extremely risky to pump such a system without zonal isolation. Although some sealants do reduce the permeability to water more than to hydrocarbons, the pressure required for the hydrocarbons to break through the sealant may be so high that hydrocarbon production after the treatment is unlikely. The lack of nonmechanical methods to selectively place a sealing system and the high costs for gel placement have increased interest in developing chemical systems thatselectively reduce effective water permeability.do not decrease oil permeability.do not require special placement techniques. RPMs RPMs (sometimes called disproportionate permeability reducers or selective plugging systems) should have physical and/or chemical properties that help reduce water flow from the treated area of a water-producing zone, thereby reducing the water inflow to the wellbore. In the treated zone of a hydrocarbon-producing layer, an RPM should result in little or no damage to the flow of hydrocarbon. Polymers can selectively reduce water permeability more than they reduce oil permeability. This property does not depend on the rock type or the type of polymer (provided it is hydrophilic).5 Perhaps the simplest system that has been used as an RPM is polyacrylamide (PAM) or hydrolyzed polyacrylamide (HPAM). The effectiveness of PAM has been demonstrated in laboratory and field test results.5–9 PAMs are believed to be useful at temperatures up to 160°F in reservoirs with low-salinity brines.10 PAMs have been used with hydrolysis ranging from 0 to 50% and molecular weights from several hundred thousand to 17 million daltons.

disposal and handling concerns, and corrosion of tubulars and surface equipment.Current literature describes RPM treatments that use various types of chemical treatments in essentially all lithologies. If these treatments were successful in all cases, RPM technology would be applied more frequently than is currently indicated. Despite claims of success, none of the materials or techniques used in RPM processes have performed consistently well in field operations. Although the literature contains several theories on RPM mechanisms, 1-3 none appear to be universally accepted. This lack of consensus may exist because no single factor determines the success of an RPM. Rather, an RPM's success depends on several well and reservoir characteristics, including chemistry, lithology, problem type, permeability, saturation, and many others.Because all of these factors affect the outcome of an RPM treatment, developing a single RPM for all well situations is unlikely. Instead, a better solution may be to focus on specific reservoir conditions and to design a treatment process that fits those circumstances. A polymeric RPM was developed recently that allows such customization when used with a systematic approach of proper preflushes and post-flushes. 4 Conventional Water-Reduction SystemsTwo categories of chemical systems are available for reducing unwanted water production in a porous medium: (1) nonsealing systems that allow the flow of fluids, and (2) sealing systems that completely block the flow of fluids.Nonsealing Systems. Nonsealing systems are typically diluted solutions of water-soluble polymers that adsorb onto the formation. These polymers most likely reduce effective water permeability through a wall effect, 1 where the polymer adsorbs onto the formation and creates a layer of hydrated polymer along the pore throat, which inhibits water flow.Sealing Systems. Sealing systems are porosity-fill materials, which can be valuable when a water-producing zone can be mechanically or chemically isolated. However, in many situations a target zone cannot be isolated and the sealing system sometimes penetrates zones that should not be treated. Although claims that sealing systems will reduce water permeability more than they reduce oil permeability have been made, pumping such Abstract For many years, relative permeability modifiers (RPM's) have received a great deal of attention from the oil and gas industry. Because of the completion techniques used in many wells, protecting the hydrocarbon interval properly during a watershutoff treatment is not always practical or cost effective. RPM's offer the option of bullheading a treatment without zonal isolation, which is designed to decrease water production with little or no decrease in oil or gas production. This paper describes the laboratory development and optimization of a polymeric RPM. The resulting material can best be described as a brush polymer consisting of a polymeric backbone grafted with methoxypolyethylene glycol (MPEG). Various phases of the development are discu...