Experimental Investigation of Hydraulic Fracturing for Multi-type Unconventional Gas Co-exploitation in Ordos Basin

The accumulated gas production and production cycle would be significantly increased by exploiting the unconventional gas resources of coalbed methane and tight gas jointly in coal measure strata (tight gases–coal interbedding reservoir) in Ordos Basin in China. However, whether hydraulic fractures could connect multiple gas zones effectively in the vertical plane is a key problem. To study hydraulic fracture geometries and vertical propagation rules for coal measure strata, six groups of true triaxial experiments in natural outcrops collected from the eastern margin of Ordos Basin in China were carried out. Results show that unlike conventional symmetric fracture geometry in layered media, the principle of fracture vertical propagation in coal measure strata follows the least resistance and most preferential propagation path. The natural fractures and cleats in coal seam make the overall fracture morphology asymmetric. A moderate stress difference between vertical stress and maximum horizontal stress (4 MPa in this paper) is the key for connection of multiple layers as well as SRV enhancement. Developed natural fractures and bedding planes in interlayer could alter fracture extension path and impede hydraulic fracture crossing layer interface. In the process of hydraulic fracture propagating from interlayer to layer interface and then to the coal seam, the fluid pressure in hydraulic fractures is characterized by an apparent fluctuation of decreasing first and increasing later. Furthermore, the greater the elastic modulus of the interlayer, the greater the pump pressure fluctuations.