Use of a pH Sensitive Polymer for Conformance Control

Abstract

Abstract A new strategy of using pH sensitive polymers for conformance control is proposed. The rheological properties of some polymers such as anionic derivatives of polyacrylic acid are shown to be very pH sensitive. For example, their viscosity varies from 5 cp at a pH of 2.5 to 20,000 cp at a pH above 6.0. This rheological behavior is reversible as the pH is cycled. Rheological data is presented at different polymer concentrations, temperatures and in solutions with different ionic strength. The effect of temperature on the rheological properties of the polymer is shown to be small. Compatibility tests showed that this polymer is compatible with most brines (NaCl, NH4Cl, and KCl). Core flow experiments were conducted on Berea cores. The results showed that the polymer solution was easily propagated through 6 inch long cores due to its low initial viscosity (~5cp). The polymer solution formed a rigid gel inside the pores after a shut-in period of 24 hours as the pH of the fluid increased above 6. The gelled polymer was found to be stable up to the maximum examined pressure gradients of 4000 psi /ft across the core. Experiments were conducted by using acid pre-flushes ahead of the polymer solution. These results indicate that the polymer can be placed deep into a formation by using pre-flushes designed for a specific rock mineralogy. The pH sensitive polymers studied here are excellent candidates for conformance control. It may also be possible to use them for temporary zonal isolation since they can be easily broken down with a mild acid wash and 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 is 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 as described in Fig. 1. This behavior increases the viscosity of the polymer chains upon ionization6. Recently, polyacrylic acid was evaluated as a carrier fluid for gravel packing operations as in Ref. 7. The study showed that this anionic polymer would be an excellent non-damaging carrier fluid for gravel packing. To our knowledge, this is the first paper that reports experimental studies to appraise a pH-sensitive polymer for conformance control. The objectives of this paper were: to evaluate pH-sensitive polymers for possible conformance control application, to investigate the rheological properties of the polymer solution by studying the effect of pH, salts, polymer concentration, and temperature, and to assess the ability of the polymer solution to flow through cores.