Temperature-Stable Acid-Gelling Polymers: Laboratory Evaluation and Field Results

Summary A polymer acid-gelling agent effective in various temperatures up to 400 degrees F [204 degrees C] has been developed. By selecting the appropriate polymer concentration the user can vary viscosities desired, depending on formation temperature. This article details the evaluation process leading to the final selection of the polymer from many available and gives tabular data and curves on molecular weights, viscosities, temperature thinning, and other information on a variety of polymers. Field results illustrating the benefits of retaining high viscosity of gelled acid in high- temperature formations are summarized. Introduction The benefits of using gelled acid for stimulation of oil and gas wells are well known, and field results support the use of gelled acid. Three polymer systems found to be useful as viscosity- building agents in stimulation and acidizing applications up to 200 degrees F [93 degrees C] are (1) xanthan polymer, (2) a liquid gelling agent, and (3) a metal crosslinked polymer system. Patent literature describes several polymer systems having utility as acid gelling agents up to 200 degrees F [93 degrees C], but none of the examples has rheology data to support its use above 200 degrees F [93 degrees C]. Examples are (1) acrylamide and acrylamide polymers crosslinked with various aldehydes, (2) ethylene oxide/propylene oxide block polymers used at high concentrations, and (3) branched or emulsion polymers of diallyldimethyl amine, cellulose derivatives crosslinked with chromium salts, and branched polymers prepared by reacting ethylene oxide with acrylamidomethylpropane sulfonic acid (AMPS). In many of these systems, a lack of chemical stability to strong acid prevents their use at elevated temperatures. To be effective as a gelling agent above 200 degrees F [93 degrees C, a polymer must (1) be chemically stable, (2) have high intrinsic viscosity at application temperatures, (3) be compatible with all other materials involved in the treatment, and (4) be in a physically usable form for field mixing. To find gelling agents to meet these requirements, a systematic evaluation of the hydrolitic stability of various types of polymers was made and rheology data were obtained to evaluate the polymers' temperature thinning properties. The ability of several systems to produce viscosity (as a function of molecular weight) was studied. Available Polymers Commercially available water-soluble polymers of potential use for well treatments are discussed in two general categories.Of the polysaccharide polymers, xanthan polymer has been shown to have the best acid stability; guar gum and cellulose derivatives have no application in hot, strong acid.Fig. 1 shows the general structure of synthetic polymers prepared from olefinic monomers and lists some derivatives currently available from this structure. Also in this category are synthetic polymers composed of condensation products of ethylene oxide (PEO) or propylene oxide (PPO). These materials are inherently stable to acid hydrolysis and can be stabilized in nonacid systems with antioxidants. Polymer Hydrolytic Stability Hydrolytic stability of various polymer types was studied to help select acid thickening agents potentially useful in excess of 200 degrees F [93 degrees C]. At this point, the prime consideration was what chemical structures currently available in polymers could survive contact with hot, strong acid without appreciable degradation of the polymer. The polymers' ability to produce viscosity or other properties was not considered. For the polymer types listed in Fig. 1, the olefin-derived backbone materials would not be expected to undergo acid hydrolysis; however, pendant groups attached to the backbone, which imparts water solubility to the polymer, must remain intact to avoid polymer insolubility or formation of undesirable chemical products. By determining the loss of nitrogen from refluxing 5 % and 20% hydrochloric acid, hydrolysis of polymers from the following compounds was monitored:acrylamide,AMPS,dimethylamino ethylacrylate quarterized with methyl chloride (DMAEA-Q), anda proprietary Halliburton cationic polymer designated "Polymer A" for the purposes of this article. Results are shown in Table 1. JPT P. 2011^