Abstract:

Tight oil reservoirs are typically developed through multi-stage fracturing, forming fracture-matrix dual-medium systems where the mobilization efficiency of matrix oil determines the ultimate recovery factor. CO₂ flooding can mobilize matrix oil via solution gas drive; however, the dissolution-diffusion behavior of CO₂ in matrix oil and its displacement characteristics remain unclear. In this work, constant-pressure dissolution-diffusion experiments and huff-n-puff tests on matrix cores were conducted, along with long-model CO₂ huff-n-puff experiments, to investigate the efficiency and drainage depth of solution gas drive. The results indicated that the CO₂ dissolution-diffusion under constant pressure during displacement exhibited limited effectiveness in mobilizing matrix oil, with recovery factors in the range of 11.8%-48%; lower pressures and longer core lengths would further reduce the efficiency. In contrast, CO₂ huff-n-puff significantly enhanced the recovery by 30.8%-71.3%, with drainage depths exceeding 16 cm; higher pressures and longer core lengths could improve its performance. Additionally, in long-model huff-n-puff tests, the drainage depth within matrix oil increased with the number of cycles and the pressure. Specifically, for a 75 cm model at 20 MPa, the drainage depth exceeded 60 cm after 7 huff-n-puff cycles. Through the research on the mechanisms of CO₂ solution gas drive for mobilizing the matrix oil in fractured tight reservoirs, this work could provide some insights for optimizing the development strategies and CO₂ flooding operational parameters in such reservoirs.

Key words: tight oil reservoir, CO₂ dissolved gas drive, matrix oil, displacement characteristic