Local asymmetric fracturing to construct complex fracture network in tight porous reservoirs during subsurface coal mining: An experimental study

Hydraulic fracturing to enhance permeability of tight reservoirs is meaningful but difficult in constructing of complex fracture network. This study aimed to examine the effectiveness of tight porous reservoir stimulation in the context of hydraulic fracturing to increase the fracture complexity as much as possible. The simulation experiments were performed to reasonably establish the relation between the laboratory results and the field fracturing operations. Large size stratified and intact samples were casted to resist the size and boundary effects by mixing river sand and/or pulverized coal with concrete cement and freshwater in specific proportions. The samples with artificial slot orienting were casted to understand the fracture initiation, orientation, and deflection mechanisms considering the influence of caprock structure and re-fracturing without temporary plugging. The samples were tested under the scaled geostress conditions of corresponding in-situ stress with a true triaxial loading system, and the reasonable dimensionless groups derived from the literature. Consequently, this study proposed an asymmetric fracturing method based on the experimental results considering the scaling structure of overlying-reservoir-underlying in porous reservoirs. The local asymmetric fracturing is a type of reservoir stimulation method with small scale high-pressure fluids such as carbon dioxide，liquid nitrogen, water, etc., jet cutting to create slot and perforation for the aid of fracture initiation and orientation. Hydraulic fractures were influenced by the fracture-induced stress shadow and ratio of the maximum to minimum horizontal stress. The validation and comparative results showed that, compared with the re-fracturing, the asymmetric fracturing significantly increased the complexity of fracture network. Although the applicability of the asymmetric fracturing depending on the corresponding engineering pilots, the derived method could hopefully address the properties of the strata structure of the reservoirs efficiently. Therefore, it has a high potential for application in the fracturing of local underground coal seam and special-trap type oil gas reservoirs to construct fracture network.