Abstract

This paper presents numerical results for a hydraulic fracture interacting with a frictional geological discontinuity in layered sedimentary rocks. A 2D hydraulic fracture model has been developed that can consider the details of the interaction and includes elastic rock deformation coupled to both fluid flow and frictional slippage. The interaction can result in fracture blunting, crossing or entering the interface, depending on modulus and stress contrasts, as well as the shear strength of the discontinuity. The numerical results are compared to experimental and field observations. When hydraulic fractures interact with long geological discontinuities, fracture growth can be stopped or retarded by the process. Treating pressure is increased by the large pressure gradients generated at offsets in the fracture path. Proppant transport can be impeded by such offsets because of the associated narrowing of fracture width. In some cases, the hydraulic fracture cannot easily enter frictional discontinuities when the interfaces have low frictional strength. In addition, pre-existing flaws are likely to serve as sites for re-initiation, allowing further hydraulic fracture growth into the intact side of the discontinuity. A higher closure stress acting in the intact rock favors arrest of the hydraulic fracture at the discontinuity, e.g., when a fracture attempts to cross a soft-to-stiff bedding plane. As a part of the interaction, the main hydraulic fracture will typically also lose fluid into cross-cutting frictional discontinuities.

Introduction

Hydraulic fractures are often placed in sedimentary formations that contain natural fractures and bedding contacts. This paper presents a numerical study of the interaction of a hydraulic fracture with frictional geologic discontinuities, including bedding planes that separate rock layers with different elastic properties. The work presented here is restricted to interaction of a hydraulic fracture with an orthogonal interface as would most often be the case for a vertical fracture growing towards or into horizontally bedded rock layers. The interaction that occurs can lead to the hydraulic fracture blunting, entering into, or crossing (with or without an offset developing) the interface. In particular, the widely-used modelling assumption that fractures remain planar as they grow through interfaces is not often the case. A number of laboratory and field measurements1–5 have shown offsetting of the fracture path and development of non-planar fracture geometries. As an example, Fig. 1 shows a sand-propped hydraulic fracture in a coal seam mapped by Jeffrey et al.5 In this case, the vertical fracture was deflected into an interface between the coal and roof-rock, producing a T-shaped fracture geometry.

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