Coupling Study of Fracture Propagation–Filtration–Seepage during Hydraulic Fracturing Assisted Oil Displacement in Offshore Low-Permeability Reservoirs

Offshore low-permeability reservoirs are mainly composed of complex fault-block structures with poor physical properties, which makes establishing an effective displacement relationship particularly challenging. Hydraulic fracturing assisted oil displacement (HFAD) can effectively increase the oil production of a single well by creating fractures to replenish the producing energy. In this study, the Khristianovich-Geertsma-de Klerk (KGD) model is used to calculate the propagation of vertical fractures, and the flow tube method is used to calculate the two-phase oil-water flow in filtration and seepage. The semianalytical mathematical model for coupling fracture propagation, filtration, and seepage of the HFAD fluid is established, followed by conducting the sensitivity analysis and the main controlling factors analysis. In contrast to conventional methods, the filtration and seepage of the HFAD fluid during the fracturing process are taken into account. Based on actual well data, the fracture half-length and the wellhead pressure are verified. The mean calculation accuracy of the fracture half-length is 88.5%, and the mean calculation accuracy of the wellhead pressure is 90.7%. The research results indicate that the fracture propagation and the filtration and seepage of the HFAD fluid occur simultaneously, including rapid propagation in the early stage, discontinuous propagation in the middle stage, and only infiltration without propagation in the later stage. Infiltration is considered a key factor in the fracture propagation of HFAD, with a higher infiltration rate leading to weaker energy storage at the fracture tip. The viscosity of the HFAD fluid significantly impacts fracture half-length and vertical infiltration distance, contributing 35% and 27%, respectively. To increase fracture half-length, select a thinner reservoir, increase HFAD fluid viscosity, and boost the injection rate. To increase the vertical infiltration distance, choose higher permeability reservoirs, reduce HFAD fluid viscosity, and increase cumulative injection volume. The results of the research study provide a valuable basis for the design of an HFAD construction scheme for offshore low-permeability reservoirs.