Simulation of thermally active and pH-sensitive polymers for conformance control
A waterflood has been used as a secondary recovery process to maintain the reservoir pressure and displace the oil towards the producer. However, the existence of high-permeability zones (thief zones) can cause early water breakthrough and excessive water production, thus, leaving a significant amount of oil bypassed in heterogeneous reservoirs. In this work, thermally active (Bright Water®) and pH-sensitive polymers have been proposed as an in-depth conformance tool with detailed simulation studies. Thermally active polymers are triggered by temperature change, whereas pH-sensitive polymers are triggered by pH change. Upon activation, polymers provide high resistance to subsequent fluid flow and divert the flow into adjacent unswept zones. As a result, this leads to improved sweep efficiency, low oil-water-ratio, and incremental oil recovery. The modeling of a pH-sensitive polymer was based on the principles of the microgel modeling procedure developed by Huh et al. (2005). In addition, a modified model was developed to calculate equilibrium swelling ratio explicitly in terms of pH and ionic strength of solution instead of using a root-finding algorithm. Thermal active polymers were modeled in terms of gelation reaction, gel viscosity, gel adsorption, and permeability reduction factor. Thermally active and pH-sensitive polymers were coupled with UTGEL reservoir simulator in an attempt to assess applicability of these gels as a conformance tool. Sensitivity analysis studies were conducted through 3D synthetic models to investigate technical feasibility of thermally active and pH-sensitive polymers as an in-depth conformance tool. Results indicated that incremental oil recovery and conformance control depend on the polymer concentration, slug size, permeability contrast between matrix and thief zone, vertical to horizontal permeability ratio (kv/kh), treatment location, oil-to-water viscosity ratio, and adsorption level, among others. It is concluded in this study that the permeability contrast between matrix and thief zones appears to be one of the most important parameters that impacts treatment performance. Therefore, a high permeability contrast is a prerequisite to achieve technically and economically successful treatment.