The extreme heterogeneity of carbonate in the form of fracture corridors and super-permeability thief 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. In-depth conformance control in high temperature reservoirs is still a challenge, due to severe gel syneresis and the associated significant reduction in gelation time.
In this work, a laboratory study was conducted to evaluate a polyacrylamide/chromium gel system for application in a high salinity and high temperature carbonate reservoir. Gel formulation was evaluated using different lab experiments including gelation time, gel strength, and long-term stability. Gelation time was determined by bottle tests and viscosity measurements. Long-term stability was evaluated both in bulk and in a core sample. For long-term gel stability in cores, a single-phase displacement experiment was performed to evaluate the ability of the selected gel system to effectively block high permeability zones. Multiphase displacement experiments were then conducted to evaluate the effectiveness of the gel treatment in oil recovery improvement. In these tests, the gel solution was injected into specially prepared heterogeneous carbonate core samples, in which high permeability channels were created by drilling centrally and mid-way through two core plugs that were brought together to constitute each composite core system.
Single-phase displacement experiments showed significant reduction in brine permeability after gel treatment. Even when the bulk gel volume was reduced by 50%, the original brine permeability is lowered by more than 80%. Multiphase displacement experiments demonstrated significant incremental recoveries post gel treatment. For example, a surfactant-polymer (SP) flood conducted after gel treatment resulted in an ultimate oil recovery of 83% original oil in core (OOIC), while without any gel treatment the SP flood attained an ultimate oil recovery of 67% OOIC. Results in this study demonstrate the potential of the studied gel system and its favorable impact on sweep efficiency for both water and chemical flooding applications in high temperature and high salinity carbonates.