Dominant Influencing Factors and Field Application of CO2 “Pseudo-horizontal Well” Gas Assisted Gravity Drainage (GAGD) in Complex Fault-Block Low Permeability Reservoirs

The gas assisted gravity drainage (GAGD) in low-permeability conditions and its Technical limitations was of significant scientific significance. The optimal well pattern after artificial fracturing also needs to be urgently explored. Firstly, the production characteristics of GAGD in low-permeability reservoirs and the dominant influencing factors were revealed through laboratory experiments, and the technical limitations were determined in combination with numerical simulation. Then, the “pseudo-horizontal well” GAGD method was proposed for fractured reservoirs, clarifying the differences in streamline distribution and sweep efficiency compared with conventional gas injection, as well as the potential of enhanced oil recovery. Finally, good results in enhanced oil recovery and gas channeling inhibition were achieved through field pilot tests. The research shows that compared with high-permeability reservoirs, GAGD in low-permeability reservoirs is more sensitive to the dip angle. At high dip angles, its production characteristics are mainly manifested as slow initial oil production, a long period of high-speed oil production, and a fast oil production rate, with a small amount of oil still recoverable in the high gas-oil ratio stage. The technical limitations of GAGD in low-permeability reservoirs are: permeability above 12 × 10−3 μm2 and dip angle above 10°. The “pseudo-horizontal well” GAGD technology can significantly uniform streamlines, increase the swept efficiency by 27.3% compared with conventional gas injection, and increase the recovery factor by 6.87%. A successful pilot field test was carried out, with a stage increase in the recovery factor of 2.93%, a exchange rate of 0.68 t (Oil)/t (CO2), and a significant delay in gas channeling. The research results can provide technical options and beneficial references for enhanced oil recovery by CO2 injection in fractured low-permeability reservoirs.