Vertical propagation behavior of hydraulic fractures in coal measure strata based on true triaxial experiment

To jointly exploit unconventional gas resources of coalbed methane (CBM), shale gas, and tight gas, instead of coal seam, a horizontal well was drilled in neighboring gas-bearing layer; then hydraulic fracturing was performed to connect coal layers. A series of triaxial fracturing experiments using natural outcrops were conducted to investigate influences of in-situ stress, elastic modulus, and natural fractures on fracture propagation. Results showed that a large in-situ stress difference (4 MPa) between vertical stress and maximum horizontal stress favored connection of wellbore with coal layers. In hydraulic fracture propagation from interlayer to layer interface and then to coal seam, fluid injection pressure was characterized by an apparent downward and then upward fluctuation. Fracture geometry was affected by natural fractures, including beddings and cleats. In coal measure strata, vertical propagation of hydraulic fractures followed the least resistance principle and moved along the most preferential propagation path. Experimental results from the novel method can provide theoretical support for effective field implementation in gas co-exploitation strata.