Optimization and application of hydraulic fracture propagation in carbonaceous and siliceous shales of marine-continental transition facies

Aiming at the differences in mechanical properties of carbonaceous and siliceous shale rocks in transitional facies, a hydraulic fracturing model was established using the finite element ABAQUS numerical simulation software. The influence of lithological differences and fracturing parameters on the characteristics of hydraulic fracture propagation was analyzed, and the optimal number and spacing of carbonaceous and siliceous shale hydraulic fracturing clusters were optimized by introducing fracturing index. The results show that: the hydraulic fracture length of carbonaceous shale is shorter than that of siliceous shale, while the fracture width is wider than that of siliceous shale. The siliceous shale has strong brittleness and strong resistance to deformation, forming long and narrow hydraulic fractures with a large sweeping range; the initiation pressure of hydraulic fractures in carbonaceous shale is small, and the propagation pressure is higher than the initiation pressure. On the other hand, the hydraulic fracture initiation pressure of siliceous shale is higher, but there will be a significant pressure drop after the initiation of the fracture, which proves that siliceous shale is easier to fracturing and requires less fracturing fluid volume; the optimal number of clusters and the cluster spacing of carbonaceous shale are optimized by fracturing index as 12 clusters and 11 m cluster spacing, and the optimal inter-cluster and cluster spacing of siliceous shale are 13 clusters and 9 m cluster spacing. It is suggested that “small number of clusters with large cluster spacing” is easy to be used for on-site hydraulic fracturing of carbonaceous shale, and “multiple clusters with small cluster spacing” for siliceous shale fracturing; under the fracturing parameters with a larger fracturing index, the daily gas production is also larger, and the daily gas production of the optimal fracturing parameters coincides with the daily gas production trend of the same fracturing parameters on site at 97%.