Applicability of pH-triggered polymers to increase sweep efficiency in fractured reservoirs



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Fractures make both opportunities and problems for exploration and production from hydrocarbon reservoirs. It is always difficult to predict how to optimally produce a fractured reservoir due to the complexity and heterogeneity of fluid flow paths. The following behavior is seen in fractured reservoirs: early water breakthroughs, reduced tertiary recovery efficiency due to channeling of injected gas or fluids, dynamic calculations of recoverable hydrocarbons that are much less than static mass balance and dramatic production changes due to changes in reservoir pressure as fractures close down as conduits. These problems often lead to reduced ultimate recoveries or higher production costs. Polymer gels, in particular in-situ gels that can be placed deep into the reservoir, have been widely used for improved conformance control. In this dissertation, we aim to block the high-permeability zones, fractures in particular, with the microgels to increase the sweep efficiency by diverting the waterflood water to the low permeability zones that still contain unswept oil. vii Polyacrylic acid microgels can swell a thousand fold as the pH of the surrounding solution changes, with an accompanying large increase in viscosity. This pH trigger is simpler than chemical cross-linking and thus offers operational advantages. The ability of pH-sensitive polymers to block high permeability fractures is studied by performing several coreflood and batch experiments. The effect of different rock and salt minerals, polymer concentration, polymer salinity, and temperature on polymer performance is studied in this dissertation. Polymer microgels show excellent consistency in the presence of various salt minerals and in contact with different rock minerals. The placement of microgels into the fractures lowered the overall core permeability in all cases. In addition, polymer microgels were stable after being in reservoir for a month with conditions at 58°C. Consequently, using pH-triggered polymers for conformance control and reducing the permeability of high permeability areas in fractured reservoirs merit further investigations. These polymers are inexpensive and are easy to prepare. The polymer concentration, salinity and shut-in time could be set according to the desired PRF value, injectivity, propagation distance and reservoir mineralogy.