Numerical Simulation on Cross-Layer Propagation of Hydraulic Fracture in Sand–mud Interbedded Layers: Taking the Shahejie Formation in BZ 25–1 Offshore Oilfield as an Example

The low-porosity, ultra-low-permeability tight sandstone oil and gas reservoirs are characterized by extensive reserves and widespread distribution, thus holding significant promise for exploration and development. In the Bohai Bay Basin, the Bozhong 25–1 (BZ25-1) oilfield presents a geological characteristic where sand–mud interbeds vertically develop within the third member of the Shahejie Formation (Es3). However, the previously employed hydraulic fracturing conditions, characterized by 'low injection rates and high fluid viscosity', encountered difficulties in achieving effective cross-layer propagation of hydraulic fractures, consequently yielding suboptimal outcomes in terms of fracturing modifications. At present, the integrated offshore hydraulic fracturing vessels have gradually entered the development of offshore oil fields, which has improved the pumping capacity of offshore fracturing and can significantly improve the fracturing effectiveness. Therefore, this paper has established multiple sets of three-dimensional fluid–solid coupled finite element models, verifies the reliability through physical simulation experiments, and analyzes the rules of fracture cross-layer propagation from both geological and engineering perspectives. The results indicate that: (1) Increasing the injection rate can directly improve the vertical propagation capability of hydraulic fractures while also affect the length of the fractures. (2) The increase of viscosity will make it easier for hydraulic fractures to pass through the barriers. Once a specific viscosity threshold is exceeded, as the viscosity continues to increase, there is no longer a significant change in the injection volume of fracturing fluid required for fracture propagate through interlayers and the change in fracture geometry becomes less pronounced. (3) Variations in the minimum horizontal principal stress differential between layers notably impact the cross-layer propagation capability, leading fractures to preferentially propagate within layers characterized by lower stress. (4) The increase of reservoir thickness results in an expanded fracture area within the target reservoir, subsequently influencing the effect of fracture propagation across layers. The conclusions drawn from this study can provide theoretical guidance for the extensive hydraulic fracturing development of the Es3 in BZ25-1 region.