Laboratory Experiments and Field Applications of a Gel Sealing System for Improving the Sweep Efficiency of CO2 Flooding

CO₂ flooding is considered an important technical means by which the recovery rate of tight conglomerate reservoirs can be enhanced. Due to the reservoir characteristics of tight conglomerate reservoirs, it is difficult for the fracture direction to be precisely controlled after fracturing, and premature gas channeling in production wells is likely to be caused. Based on the characteristics of tight conglomerate reservoirs, to meet the requirements of injectability, sealing channeling strength, and long-term stability of the gel sealing system, three concentrations each of low-molecular-weight polymers (0.6%, 0.7%, 0.8%) and high-molecular-weight polymers (0.4%, 0.45%, 0.5%) were selected in the experiment. Subsequently, concentration screening, dynamic gel-forming performance testing, and aging time testing were respectively carried out after they were prepared under the conditions of tap water and oilfield injection water. Afterward, the compounded gel sealing system recipes are subjected to core displacement experiments with different permeabilities and aging experiments of the gel sealing system. Finally, the channel-plugging system is carried out for field tests. It is obtained from laboratory experiments that the viscosity of the high-molecular-weight polymer solution prepared with injection water is relatively low, ranging between 380–740 mPa·s, which can meet the requirements for the safe pumping of the gel solution. For the gel sealing system prepared with injection water, the optimal concentration of the high-molecular-weight polymer is determined to be 0.4%. After being aged by introducing carbon dioxide, the gel strength ranges from F to H-, indicating a high gel strength. In the core displacement experiments, under the conditions of cores with different permeabilities, the carbon dioxide gel sealing system (injection water + 0.4% polymer + 0.25% chromium stabilizer + 0.3% chromium) is shown to have good plugging ability and channeling control effect. The aging stability of the gel sealing system under reservoir conditions is found to be within one month. To extend the effective period of the gel sealing system, the carbon dioxide-responsive thickening system R-A previously studied by our project team is combined and used as the pre-slug and sealing slug. In the field test well group, it is shown that the daily oil production per well is increased by 2.9 tons, the daily gas production is decreased by 748 cubic meters, and the cumulative oil production is increased by 348 tons (40 days after channel-plugging). The gel sealing system of this study has achieved initial success in the field test, and it is considered to have certain guiding significance for subsequent chemical channel-plugging in gas flooding.