Abstract
Moment Tensor Inversion (MTI) of microseismic data recorded during hydraulic fracturing shows that a large number of events have been found to have moment tensors consistent with either vertical or bedding plane slip. We are endeavoring to verify the occurrence of bedding plane slip events via analog experiments and numerical modeling. Though extensive experimental and numerical studies have been carried out to obtain a better understanding of the potential for crossing and arrest of a hydraulic fracture (HF) intersecting a natural fracture (NF), the actual mechanical interaction between a hydraulic fracture near a natural fracture or a bedding plane discontinuity has not been studied, particularly under triaxial stress and hydraulic fracturing conditions. In this paper we present the results of lab-scale experimental work with shales and numerical modeling to demonstrate HF/NF interaction with emphasis on slippage of a discontinuity surface. The tested samples have a near critical stressed incline natural fracture before fracturing. Hydraulic fracture is induced by injecting oil into a horizontal well with a vertical notch. Injection pressure, stresses, and acoustic emission (AE) are monitored during the test. In addition, strain gauges are used to measure the slippage on the natural fracture. Furthermore, a coupled distinct numerical model is built to simulate the analog tests. The fluid flow in the hydraulic fracture, fluid leak-off (for rock sample), induced deformation in the matrix and displacement in fracture are considered in this coupled model. At the breakdown point during hydraulic fracturing, a strain jump is recorded, which is accompanied by increased AE activity. Analysis of the data clearly shows the occurrence of slippage on the joint in response to an approaching hydraulic fracture. Expectedly, the degree of shear slip varies with natural fracture dip, and friction angle. The numerical results help reveal the complex stress and pore pressure distributions in the test assembly and show shear displacement on the natural fracture after pressurization of hydraulic fracture. The value of the maximum induced shear displacement on the natural fracture increased by orders of magnitude as the hydraulic fracture propagated in the sample.
