Evaluation of a pH-Sensitive Polymer for Gravel-Packing Operations

Abstract

Abstract Anionic polymers such as polyacrylic acid, polymethacrylic acid, and their derivatives are known to show pH sensitive rheology. Rheological data is presented for a polyacrylic acid type polymer as a function of polymer concentration, pH, shear rate, and temperature. The effects of monovalent and divalent ions on the rheology are also presented. Core flow experiments were conducted on Berea sandstone cores. Fluid loss and filter cake stability was investigated at an overbalance pressure of 100 psi. Its effectiveness was compared with that of a conventional carrier fluid (HEC) used for gravel packing. It was found that the viscosity of the polymer is strongly dependent upon the pH of the solution. At higher pH values (>6), the polymer viscosity reached a high value that is sufficient to suspend most gravel. Hydrochloric acid breaks this high viscosity to a low value (<5 cp) at pH values below 2. The thickening and thinning with pH was found to be reversible. The effect of temperature on the rheological properties was not significant. Compatibility tests showed that the polymer was compatible with most brines (NaCl, NH4Cl, and KCl) used for field applications. Core flow results indicated that the filtration rate was low (<9 ml/hr) compared to that of HEC (106 ml/hr), at an overbalance pressure of 100 psi. The retained permeability of the core was found to be 80% and 40% after using the polymer and HEC, respectively. The invasion depth is also shorter than that of HEC due to the formation of a stable filter cake. Our evaluation study shows that this anionic polymer would be an excellent non-damaging carrier fluid for gravel packing. It has excellent rheological and suspension properties and is easily broken down with a mild acid wash before flowback. Introduction Polyelectrolytes dissociate in solution and form macroions which interact with counter ions in the solution. The interaction forces between these ions give polyelectrolytes their characteristic properties. They have been widely used for several applications (drug delivery, adhesion, paints, chemical valves, etc.). The swelling properties of polyelectrolytes are strongly dependent upon pH, ionic strength, as well as polymer concentrations1–3. The influence of these factors on the swelling of polyelectrolytes were summarized in Ref. 4. Weak polyelectrolytes ionize over a limited pH range5. Examples of strong and weak polyelectrolytes are polystyrene sulfonic, polyacrylic acids, respectively. Polyacrylic acid and its copolymers are classified as polyelectrolytes, which stretch and form uncoiled chains when ionized due to the repulsive forces between carboxylic groups. This behavior increases the viscosity of the polymer chains upon ionization6. The objectives of this paper are:to evaluate pH-sensitive polymers as a carrier fluid for gravel packing operations,to investigate the rheological properties of the polymer solution by studying the effect of pH, salts, polymer concentration, and temperature,to assess the effectiveness of the polymer solution through core flow tests and compare it with a conventional carrier fluid used for gravel packing, andto understand interactions of the polymer molecules with ions present in formation fluids.