Effect of interlayer mechanical properties on initiation and propagation of hydraulic fracturing in laminated coal reservoirs

Coal macrolithotypes control the heterogeneity of coal physical properties in coalbed methane (CBM) reservoirs and have a significant effect on the success of hydraulic fracturing stimulation. However, only a few studies have focused on this heterogeneity, and the propagation mechanism of hydraulic fractures related to the coal macrolithotype is not well understood. In this study, we considered the Hancheng area, Ordos Basin, China, as an example to understand macrolithotype differences, establish finite element numerical models of the cohesive zone, and evaluated the hydraulic fracture initiation and vertical propagation behavior of laminated coal reservoirs. We used physical experiments, such as direct shear test and Digital Image Correlation (DIC), along with finite element simulation and a finite element model; the results show that the tensile strength of bright coal is the lowest and that of dull coal is the greatest; the average cohesion and shear strength of the bright-dull coal interface are 0.418 MPa and 1.3778 MPa (sigma = 3 MPa), respectively. Notably, behavioral differences are likely to impact the geometric evolution of hydraulic fractures. In the numerical models of hydraulic fracturing for a laminated coal reservoir, the hydraulic fracture propagates predominantly vertically as the interlayer has a lower elastic modulus and higher tensile strength. Because the interfacial shear strength is weak, the fractures can easily penetrate and propagate into the bedded interface between layers, and often, the distance of lateral slip increases into the interlayer interface. Thus, when dull coal is fractured, the geometry of the hydraulic fracture is often characterized as an isolated fracture distribution. The fracture then, rapidly propagates into the interlayer, activating the natural fractures in the bright coal reservoir, thus improving the fracture scale in the interlayers (model 2). However, as bright coal is a productive strata, the geometry of the hydraulic fracture is also dominated by crisscross network structures, and the fractures preferentially propagate along the interface (model 1).