The Enhanced Oil Recovery Mechanisms in Heavy Oil Reservoirs by Chemical Compound Flooding after Multiple Cycles of Huff-n-Puff

Due to the serious gas channeling, high water cut, and poor thermal recovery efficiency in heavy oil reservoirs after multiple cycles of huff-n-puff, it is a challenging process to recover oil from heavy oil reservoirs after multiple cycles of huff-n-puff. Chemical compound flooding (CCF) systems are being employed for oil recovery. To clarify the optimal injection mode and slug volume of CCF and to investigate the synergistic mechanism of enhanced oil recovery (EOR), parallel sand-filled pipes were used to compare different oil displacement methods in this work. After evaluating the properties of the plugging agent, oil displacement agent, and viscosity reducer used in the experiment, parallel sand-filled pipe experiments were carried out. Then, the optimal displacement method was obtained by analyzing the key performance indexes, and the EOR mechanism was proposed. The results showed that the optimal injection mode for heavy oil reservoirs after multiple cycles of huff-n-puff involves plugging agent flooding (0.1PV) → oil displacement agent flooding (0.2PV, 2000 mg/L) → viscosity reducer flooding (0.2PV, 0.50%) → subsequent water flooding (until no oil is produced). This injection sequence results in a 35.75% increase in recovery rate. In this way of injection, the gel formed by the plugging agent effectively blocks the dominant channel of the water phase in high-permeability layers. Before the oil displacement agent broke through the plugging layer, it spread into the low-permeability layer, increasing sweep volume. When the plugging layer was broken through, more viscosity reducer entered the high-permeability layer, further enhancing displacement efficiency. During the chemical injection process when the volume of the oil displacement agent was too high and the volume of the viscosity reducer was too small, after the plugging agent broke some injections into the high-permeability layer could be ineffective and the viscosity reduction process is not sufficient. When the oil displacement agent was less and the viscosity reducer was too much, the utilization degree of low-permeability pipe would be lower. The study can serve as an important basis for improving the oil recovery of heavy oil reservoirs after multiple cycles of huff-n-puff.