Modeling Hydraulic Fracture Entering Stress Barrier: Theory and Practical Recommendations

Abstract

Numerical modeling of hydraulic fracturing is complicated when a fracture reaches a stress barrier. For high barriers, it may require changing of a computational scheme. Despite there are examples of modeling propagation through barriers, there is no general theory clarifying when and why conventional schemes may become inefficient, and how to overcome computational difficulties. The paper presents the theory and practical recommendations following from it. We start from the definition of the barrier intensity, which exposes that the barrier strength may change from zero for contrast-free propagation to infinity for channelized propagation. The analysis reveals two types of computational difficulties caused by spatial discretization: (i) general arising for fine grids and aggravated by a barrier; and (ii) specific, caused entirely by a strong barrier. The asymptotic approach which avoids spatial discretization is suggested. It is illustrated by solving bench-mark problems for barriers of arbitrary intensity. The analysis distinguishes three typical stages of the fracture penetration into a barrier, and provides theoretical values of the Nolte-Smith slope parameter and arrest time as functions of the barrier intensity. Special analysis establishes the accuracy and bounds of the asymptotic approach. It appears that the approach provides physically significant and accurate results for fracture penetration into high, intermediate and even weak stress barriers. On this basis, simple practical recommendations are given for modeling hydraulic fractures in rocks with stress barriers. The recommendations may be promptly implemented in any program using spatial discretization to model fracture propagation.