Abstract
Waterflooding in fractured reservoirs is a challenging task due to the presence of high conductive flow pathways such as fractures. Much of the injected water passes through fractures without sweeping the oil in the low permeable area, which results in an early breakthrough. Implementing deep conformance control techniques can be a remedy for this early water breakthrough. pH-sensitive microgel injection is a conformance control method in which the dependency of microgel viscosity to pH guarantees easy injection of these microgels into formations at low pH environments. Because of the geochemical reactions among rock minerals, microgels, and a pre-flushing acid, the microgel pH increases; therefore, these microgels swell and block high conductive fractures. In this study, a designed visual cell containing rock samples is implemented to observe rock–microgel interactions during a pH-sensitive microgel flooding into a fractured carbonate medium. First, the dependency of fracture aperture changes to the acid pre-flush flow rate is examined. Then, we investigate the effect of pH-sensitive microgel concentration on its resistance to block fractures during post-water flooding by studying the gel failure mechanisms (e.g., adhesive separation, cohesive failure). Finally, the effect of an initial aperture of fracture is examined on microgel washout when water injection is resumed. The results showed that both decreasing the acid flow rate and lowering the initial aperture could increase the rate of aperture changes. Moreover, the microgel solution with a concentration of 1 wt% showed the highest resistance (98.2 psi/ft) against post-water injection. Additionally, this microgel concentration had the highest permeability reduction factor. Meanwhile, the smaller initial aperture of fracture contributed to a higher microgel resistance.
Highlights
The complexity of fluid flow and heterogeneity of fractured reservoirs provide both opportunities and operational problems during oil production from these reservoirs
The fracture aperture changes were calculated by analyzing the images taken during the injection process
The purpose of this research was to investigate the process of deep conformance control of pH-sensitive microgels in fractured carbonate reservoirs
Summary
The complexity of fluid flow and heterogeneity of fractured reservoirs provide both opportunities and operational problems during oil production from these reservoirs. The injected water rapidly flows through fractures and high permeable strata and breaks through in production wells This early water breakthrough leaves a high residual oil saturation in low permeable areas (rock matrix) and reduces ultimate oil recovery [1,2,3]. By injecting pH-sensitive microgels into the reservoir and blocking the fractures (Fig. 1b), the post injected water shifts toward lower permeable matrix areas. The oil in these areas is swept, and eventually, oil production increases.
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