Numerical simulation study on fracture penetration behavior in a reservoir with multiple coal seams

Abstract

Coal seams are widely deposited in marine-continent transitional facies shale reservoirs. For this type of reservoir, horizontal wells and hydraulic fracturing technology are the main ways to exploit the oil and gas resources. However, multiple layers with uneven thickness and significant differences in physical and mechanical properties among them lead to insufficient vertical fracture propagation, limiting the application of horizontal well and hydraulic fracturing technology. Therefore, to reveal the influence of in situ stress conditions and injection parameters on the vertical propagation of fractures, numerical models with multiple coal seams based on logging lithology were established and the behavior of fractures crossing coal seams under different conditions was studied based on the extended finite element method. In addition, the fracture width and pressure at the injection point were also studied. The simulation results showed that the uneven thickness of the coal seam limited the vertical penetration of the fractures compared with that of a homogeneous formation as the fractures tend to pass through the thinner coal seam. Due to the physical properties difference and stress shadow effect, multiple clusters of fractures were prone to deflection in the coal seam, which made it difficult for hydraulic fractures to grow in height. However, the stress shadow effect in the process of multiple fractures propagation also has a positive side. The stress shadow becomes stronger as the difference in fluid distribution increases, leading to a difference in the growth rate of the fracture up and down, and ultimately improving the overall penetration effect of the fractures. Based on the work presented in this paper, the perforation spacing can be optimized to either avoid the stress shadow effect or effectively utilize its positive influence. Meanwhile, the results can guide the placement of horizontal well and the selection of injection rate.