Experimental investigation on hydraulic fracture propagation of coal shale reservoirs under multi-gas co-production

The gas-bearing formations, including shale, coal and limestone rock, are alternately and vertically developed in coal shale reservoirs. Multiple unconventional natural gases can be exploited together through the hydraulic fracture longitudinally connecting different production layers. Therefore, the degrees of formation connection and fracture complexity are the two key factors to determine the results of gas co-production. The true tri-axial hydraulic tests are carried out on the samples of artificial coal shale strata, which are comprised of different lithological combinations of layered rock. The vertical propagation geometries of hydraulic fractures are analyzed and the effects of different factors on the fracture patterns are summarized. The results show that the weak planes, such as lithological interfaces, bedding planes and coal cleats, have obviously inhibitive effects on the fracture-height growth, and the fracture height propagation always exhibits asymmetric characteristics. The trajectory of hydraulic fracture is easier to be arrested by the weak planes for the case of low vertical stress difference coefficient, as when the value is equal to 0.1. The high vertical stress difference coefficient and high fluid injection rate are beneficial for the hydraulic fracture to cross the interface vertically. In addition, as the hydraulic fracture penetrates the lithological interfaces, the cleat system can be activated, which improves complexity degree of hydraulic fractures. The results are expected to provide a guideline for understanding the hydraulic fracture morphology of coal strata and designing the field fracturing operation.