Study on water flooding sweep characteristics of horizontal-vertical combined well pattern in offshore heavy oil reservoirs

In order to solve the problems of poor water flooding effect and unclear sweep characteristics of offshore heavy oil reservoirs, a three-dimensional plate flooding experiment was carried out to clarify the sweep characteristics of water flooding in different heterogeneous reservoirs under the combined well pattern of horizontal and vertical. Furthermore, numerical simulation methods are employed to reveal the influences of injection rate, well pattern switching timing, horizontal well length, and perforation density on waterflooding sweep efficiency under field-scale conditions. The results indicate that planar heterogeneity induces injected water to form preferential flow paths along near-well high-permeability zones, while vertical heterogeneity leads to significant water flooding in bottom high-permeability layers. After the completion of development with the inverted nine-spot well pattern, the waterflooding oil recovery factor reaches 39.86%, with remaining oil concentrated in low-permeability layers and inter-well areas of production wells across all layers. Upon switching to the five-spot combined well pattern, horizontal wells can effectively mobilize the remaining oil in top low-permeability layers, improving waterflooding sweep performance and increasing the recovery factor to 57.87%. Under the same development duration, waterflooding sweep extent and oil recovery maintain a positive correlation with the injection rate and perforation density, the optimal well pattern switching timing is 70% water cut, and the optimal horizontal well length is 200 m. This research provides technical support for the efficient waterflooding development of offshore heavy oil reservoirs.