Abstract
Natural fractures serve as both reservoir space and flow channel for fluids in shale gas reservoirs. In the process of shale gas reservoir stimulation, the interaction between natural fractures and hydraulic fractures is essential for the generation of fracture network. In this paper, a fracture interaction criterion is derived to predict whether hydraulic fracture would cause shear slippage on natural fracture plane or crossing the natural fracture, and a series of Tri-axial tests on natural shale outcrop blocks are conducted to observe the interaction between hydraulic fractures and natural fractures. The study results show that with a certain stress difference and crack occurrence, larger approaching angle, higher pump rate and lower fracturing fluid viscosity would contribute to hydraulic fracture crossing natural fracture and lead to a larger stimulated volume in shale formation. High brittle deformation of shale formation also increases the complexity of fracture network, and the optimal shale target would be high brittleness but low strength and hardness. Based on the experiment results, we propose the optimum condition for the interaction between hydraulic fracture and natural fracture, which could be much more practical for the formation evaluation and the better fracture design.
1. INTRODUCTION
Natural fractures serve as both the reservoir space and the flow channel of fluids in shale gas reservoirs, which require large-scale hydraulic fracturing to build complex fracture system for gas extracting. For the majority of-shale gas reservoirs, the number of fracture discontinuities is large (Gale et al. 2007). However, for some conditions, the fracture may deviate or twist by only a small amount, having a relatively simple propagation path along many pre-existing fractures (Zhang and Jeffrey 2008; Jeffrey et al. 2009). Factors affecting the fracture geometry include in-situ stress, rock brittleness and natural fracture system (bedding planes and natural fractures) (King, 2010; Britt and Schoeffler, 2009; Rickman et al, 2008).
