pH-Sensitive Polymers for Novel Conformance-Control and Polymer-Flood Applications

Novel conformance-control and polymer-flood applications that exploit the pH sensitivity of partially hydrolyzed polyacrylamide (HPAM) are proposed. The key feature of this process is the injection of the HPAM solution under acidic conditions. The low pH makes polymer molecules coil tightly, resulting in a very low polymer-solution viscosity. This allows the polymer solution to be injected into the reservoir at a substantially reduced injection pressure. Once injected, the acid reacts with the formation minerals to cause a spontaneous pH increase, uncoiling the polymer chains and causing a large increase in solution viscosity. Such a viscosity-control scheme can be exploited for placement of a concentrated polymer solution in high-permeability zones, where it later viscosifies to divert subsequently injected fluids (in-depth conformance control), or to reduce the high pressure drop near the wellbore during polymer injection (injectivity improvement). Extensive laboratory experiments were systematically performed and interpreted to evaluate the novel applications of pH-sensitive HPAM. The evaluations require (a) quantification of steady-shear viscosities, (b) characterization of geochemical reactions with acids, and (c) transport evaluation of HPAM solutions in cores. Rheological measurements show that shear viscosities of HPAM solution have a pronounced, but reversible, dependence on pH. The peak pHs observed in several shut-ins guarantee that spontaneous geochemical reactions can return the polymer solution to its original high viscosity. The use of a weak acid is the key. Coreflood results show that the HPAM solution under acidic conditions can be propagated through cores with much higher mobility than at neutral pH. However, low-pH conditions increase adsorption (polymer loss) and require additional chemical cost (for acid). The optimum injection formulation (polymer concentration, injection pH) will depend on the specific reservoir mineralogy, permeability, salinity, and injection conditions.