Assessment and optimization of fracture-karst cave connectivity in horizontal well hydraulic fracturing of carbonate reservoirs

The karst cave serves as the primary storage space in carbonate reservoirs. Simultaneously connecting multiple karst caves through hydraulic fracturing is key to the efficient development of carbonate reservoirs. However, there is lack of systematic research on the mechanisms and influencing factors of fracture propagation in carbonate rocks. This paper established models including karst cave models, single natural fracture-cave models, and multiple natural fracture-cave models based on the discrete lattice method. It thoroughly studied how geological and operational factors affect the fracture propagation and the connectivity of karst caves. The final step involved establishing a prototype well model and optimizing operation parameters. The research indicates that an increase in the Young's modulus and pore pressure of karst cave could facilitate hydraulic fracture connecting with caves. When the pore pressure is lower than that in the matrix, it will generate a repulsive effect on hydraulic fractures. The natural fracture along the hydraulic fracture path significantly facilitates the connection with caves. When the wellbore azimuth is less than 60°, the fracture's diversion radius is small, and hydraulic fractures primarily connect with karst cave through natural fractures. When the wellbore azimuth exceeds 60°, the fracture's diversion radius increases. Under the combined action of hydraulic fractures and natural fractures, the stimulated volume of the karst cave noticeably increases. Under the same liquid volume, increasing the injection rate could enhance the cave stimulated volume. Combining the findings from numerical simulation studies resulted in the development of a diagram that depicts the connectivity of karst caves, providing valuable insight for hydraulic fracturing operations in carbonate reservoirs.