The Impact of Natural Fracture Thickness on Hydraulic Fracture Interaction Mechanics

摘要

Field observations indicate the thickness of natural fractures can vary widely, from a fraction of a millimeter to greater than a centimeter. Natural fracture thickness as well as mechanical properties can influence whether an induced hydraulic fracture may arrest, cross or divert. Commonly used fracture interaction criteria treat natural fractures as frictional interfaces without considering any natural cement filling. The objective of this study is to evaluate the natural fracture thickness effect on hydraulic fracture propagation. Young’s modulus contrast, differential stress, and rock?cement interface properties are also included in the analysis. In our finite element method simulation, hydraulic fracture propagation and interaction with natural fractures is modeled using the cohesive element approach. The simulation results are validated against the analytical fracture crossing criterion proposed by Renshaw and Pollard (1995), which treats the natural fractures as frictional interfaces. Different mechanical properties are assigned to regions that represent natural fractures for the purpose of investigating the effect of natural fracture thickness. Results show that the hydraulic fracture crossing behavior is strongly influenced by the natural fracture thickness and the rock?cement interface friction coefficient. Incorporating natural fracture thickness generates results that deviate from the frictional approach depending on the natural fracture stiffness. For a compliant natural fracture, as the natural fracture thickness increases the likelihood of diversion increases. We attribute this to increased shear stress along the rock cement interface compared to the zero?thickness case. For stiff fractures, crossing becomes more probably as the natural fracture thickness goes up. Higher differential stress also increases the chances for crossing, consistent with previous studies. One application of this work is to investigate the hindrance of hydraulic fracture height growth by bedding?parallel veins, which are abundant in unconventional resource plays such as the Vaca Muerta formation. Field observations indicate the thickness of these bedding parallel veins changes from 1 mm to 15 cm, with an average of 5 cm. For low friction and stress ratio conditions, the frictional criterion predicts crossing, but our simulations suggest that complaint natural fractures greater than 5 mm thick can stop fracture height growth.