PH sensitive polymers for novel conformance control and polymer flooding applications
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Polymer flooding is a commercially proven technology to enhance oil recovery from mature reservoirs. The main mechanism for improving oil recovery is to increase the viscosity of injection water by adding polymer, thereby creating a favorable mobility ratio for improved volumetric sweep efficiency. However, polymer injection brings on several potential problems: a) a high injection pressure with associated pumping cost; b) creation of unwanted injection well fractures; and c) mechanical degradation of polymers due to high shear near wellbore. The high viscosity of polymer solutions and permeability reduction by polymer retention reduce mobility, and simultaneously increase the pressure drop required for the propagation of the polymer bank. The objective of this dissertation is to develop an improved polymer injection process that can minimize the impact of those potential problems in the polymer flooding process, and to extend this application to conformance control. This objective is accomplished by utilizing the pH sensitivity of partially hydrolyzed polyacrylamide (HPAM), which is the most commonly used EOR polymer. The idea of the “low-pH polymer process” is to inject HPAM solution at low-pH conditions into the reservoir. The polymer viscosity is low in that condition, which enables the polymer solution to pass through the near wellbore region with a relatively low pressure drop. This process can save a considerable amount of pump horse power required during injection, and also enables the use of large-molecular-weight polymers without danger of mechanical degradation while injecting below the fracture gradient. Away from the near wellbore region, the polymer solution becomes thickened with an increase in pH, which occurs naturally by a spontaneous reaction between the acid solution and rock minerals. The viscosity increase lowers the brine mobility and increases oil displacement efficiency, as intended. Another potential application of the low-pH polymer injection process is conformance control in a highly heterogeneous reservoir. As a secondary recovery method, water flooding can sweep most oil from the high-permeability zones, but not from the low-permeability zones. The polymer solution under low-pH conditions can be placed deep into such high-permeability sands preferentially, because of its low viscosity. It is then viscosified by a pH increase, caused by geochemical reactions with the rock minerals in the reservoir. With the thickened polymer solution in the high permeability sands, the subsequently injected water is diverted to the low permeability zone, so that the bypassed oil trapped in that zone can be efficiently recovered. To evaluate the low-pH polymer process, extensive laboratory experiments were systematically conducted. As the first step, the rheological properties of HPAM solutions, such as steady-shear viscosity and viscoelastic behavior, were measured as functions of pH. The effects of various process variables, such as polymer concentrations, salinity, polymer molecular weight, and degree of hydrolysis on rheological properties, were investigated for a wide range of pH. A comprehensive rheological model for HPAM solutions was also developed in order to provide polymer viscosity in terms of the above process variables. As the second step, weak acid (citric acid) and strong acid (hydrochloric acid) were evaluated as pH control agents. Citric acid was shown to clearly perform better than hydrochloric acid. A series of acid coreflood experiments for different process variables (injection pH, core length, flow rate, and the presence of shut-ins) were carried out. The effluent pH and five cations (total Ca, Mg, Fe, Al, and K) were measured for qualitative evaluation of the geochemical reactions between the injected acid and the rock minerals; these measurements also provide data for future history matching simulations to accurately characterize these geochemical reactions. Finally, polymer coreflood experiments were carried out with different process variables: injection pH, polymer concentration, polymer molecular weight, salinity, degree of hydrolysis, and flow rate. The transport characteristics of HPAM solutions in Berea sandstone cores were evaluated in terms of permeability reduction and mobility reduction. Adsorption and inaccessible/excluded pore volume were also measured in order to accurately characterize the transport of HPAM solutions under low-pH conditions. The results show that the proposed “low-pH polymer process” can substantially increase injectivity (lower injection pressures) and allow deeper transport of polymer solutions in the reservoir due to the low solution viscosity. The peak pH’s observed in several shut-ins guarantee that spontaneous geochemical reactions can return the polymer solution to its original high viscosity. However, low-pH conditions increase adsorption (polymer-loss) and require additional chemical cost (for citric acid). The optimum injection formulation (polymer concentration, injection pH) will depend on the specific reservoir mineralogy, permeability, salinity and injection conditions.