Experimental investigation of fracture initiation position and fluid viscosity effect in multi-layered coal strata

Abstract The multi-layered coal strata in Lin-Xing block of China is rich in tight sandstone gas and the coalbed methane (CBM). The feasibility of multi-gas production and the optimal layer for fracturing in multi-layers requires a better understanding of fracture vertical extension mechanism through sandstone coal interbedding. To analyze the problems, six true tri-axial experiments were conducted on the multi-layered specimens of sandstone and coal outcrops. The transition zone was simulated by the cement with certain strength and thickness. In the experiments, the perforation position, fluid viscosity, and lithological difference were discussed. The results showed that the effect of viscosity exhibited a stepped manner as a function of fracture complexity. High-viscosity fluid significantly improved the fracture penetration ability when fracturing in coal layer. The results also show that when indirectly fracturing in sandstone, low-viscosity fluid was under the risk of natural fracture activation near wellbore to cause fracturing failure. When both layers were perforated, complex fracture network was easily formed as multi-layers were fractured. Perforation in both sandstone and coal layers were the best method based on the fracture penetration ability and the overall fracture complexity. T-shaped and step-wise fractures were found in the interface. The transition zone made it easier for fracture vertical extension through layers. The field production data was in good agreement with the experimental results. These results provide a guideline for the optimal perforation position selection and fracturing parameters in coal strata.