A Study of Gel-Based Conformance Control Within Fractured Carbonate Cores Using Low-Field Nuclear-Magnetic-Resonance Techniques

Abstract

Summary The extreme heterogeneity of carbonate in the form of fracture corridors and superpermeability zones challenges the efficient sweep of oil in both secondary- and tertiary-recovery operations. In such reservoirs, conformance control is crucial to ensure injected water and any enhanced-oil-recovery (EOR) chemicals optimally contact the remaining oil with minimal throughput. Gel-based conformance control has been successfully applied on both sandstone and carbonate reservoirs. Achieving effective deep conformance control in high-temperature reservoirs, however, remains a challenge because of severe gel syneresis and significant reduction in gelation time. The first step to improve the performance of gel in these challenging environments is the accurate understanding of the gel-conformance-control mechanism inside reservoir rocks. In this work, a laboratory study was conducted to evaluate a polyacrylamide/chromium gel system for application in a high-temperature/high-salinity carbonate reservoir. Oil-displacement experiments on carbonate-core samples, combined with nuclear-magnetic-resonance (NMR) measurements, were performed to demonstrate oil-recovery improvement using gel treatment and to illustrate the mechanisms of the improvement. In these tests, the gel solution was injected into specially prepared heterogeneous carbonate-composite-core samples, in which different configurations of high-permeability channels were created. Gel treatment was conducted after waterflooding and was followed by chase waterflooding. Oil-recovery improvement by gel treatment was 18% of original oil in core (OOIC) in the composite core with high-permeability channels extending midway through the composite, whereas the improvement was 38% of OOIC in the composite core with channels extending all the way through the composite. Detailed spatial fluid variations inside the core samples before and after gel treatment were closely monitored using low-field NMR techniques. Heavy water (D2O) was used in place of water to enhance the contrast between oil and brine for NMR by eliminating the protons in the aqueous phase. NMR measurements indicated that the bypassed oil during waterflooding was effectively recovered after gel treatment. Results in this study demonstrate the potential of the studied gel system and its favorable effect on the sweep-efficiency-improvement application in high-temperature/high-salinity carbonate reservoirs. The NMR study enhances our understanding of how gel helps improve the sweep efficiency of subsequent floods through blocking/reducing the permeability of highly conductive zones.