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.

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.

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.

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.