THE SHUT-IN IMBIBITION MECHANISM OF HYDRAULIC FRACTURING-ASSISTED OIL DISPLACEMENT IN TIGHT RESERVOIRS BASED ON MOLECULAR SIMULATION

Aiming at the problems of insufficient effective sweep volume and rapid energy depletion of formations during the development process of tight reservoirs, it is considered that the technology of hydraulic fracturing-assisted oil displacement (HFAD) combining shut-in can be applied in the development of tight reservoirs. The molecular dynamics method is applied to analyze the imbibition mechanism of tight reservoir under HFAD condition from the perspective of microscopic forces, the dynamic migration of solutes under HFAD conditions in tight reservoir was described, and the imbibition stages were divided from the molecular scale. A three-phase system of tight reservoir wall-oil-displacement phase is constructed by molecular simulation method. The characteristic differences of conventional development condition and HFAD condition were analyzed from the aspects of relaxation characteristics, concentration distribution, diffusion capacity, and interaction energy between media. The imbibition mechanism of tight reservoir under HFAD condition was explained from the molecular level. The result shows that: compared with conventional development condition, the molecular diffusion coefficient of displacement phase under HFAD condition increases by 20.06%, the interaction energy with the wall increases by 2.3 times. The number of adsorption layers of displacement phase molecules increases, and the desorption effect of oil phase becomes more obvious, the imbibition displacement efficiency increases by 38.73%. In addition, the imbibition efficiency has a peak value as the reservoir temperature changes, which first increases and then decreases. Moreover, the imbibition efficiency is influenced by the wall wettability, the stronger the wall hydrophilicity, the higher the imbibition efficiency. The imbibition process can be divided into three stages from the molecular scale: water molecules preferentially approach the wall surface; The displacing phase fluid competes with the oil phase molecules for adsorption, stripping the adsorbed oil phase into a free state and keeping it away from the wall; The solute molecules of HFAD liquid enter the initial oil phase range to further displace the oil phase and improve the imbibition efficiency. High-pressure injection of HFAD agent in HFAD technology can quickly replenish formation energy, expand the displacement phase fluid sweep volume and improve oil washing efficiency. With the synergistic effect of the two, the imbibition displacement efficiency can be greatly improved. This study can provide a theoretical reference for efficient development of tight reservoirs.