Abstract

Interfacial slip is one of the mechanisms that can alter the growth of a hydraulic fracture when it encounters weak planes or natural fractures. In shallow or over-pressured formations, interfacial slip between formation bedding planes is possible when the effective normal stress on the bedding interfaces is low. Fracture height growth could be hindered or stopped by interfacial slip when a vertical hydraulic fracture propagates in such formations.

An interfacial slip model has been developed and implemented in a pseudo-three-dimensional (P3D) hydraulic fracture simulator. In the model, the width deformation of a fracture with interfacial slip is calculated using a displacement discontinuity (DD) method. An interface crossing criterion (Renshaw and Pollard, 1995) is used to determine if the hydraulic fracture crosses a particular bedding plane during height growth. Two interface properties of a coefficient of friction and a shear stiffness, required by the model, are defined, and the application of the Renshaw and Pollard criterion and the DD method is explained.

The direct effect of bedding plane interfacial slip is on fracture height growth and width deformation, but because of the coupling effects in hydraulic fracturing, the entire fracture geometry and fracture pressure will be affected. The simulation results based on the interfacial slip model are compared with the fracture height and pressure measured from a fracturing treatment with tracer logs, as well as with the fracture length inferred from post-fracturing production analysis. In this field case, a T-shaped fracture was considered likely to be generated based on the observed field measurements, and there is a good match in fracture height, length, and pressure between the simulation results using the interfacial slip model and the measured data.

A hydraulic fracturing simulator that includes an interfacial slip model can be used to better simulate fracturing in coalbed methane and other formations where slip between bedding planes is possible, and to improve fracture design and evaluation for such treatments.

Introduction

Interfacial slip is one of the mechanisms that can alter the growth of a hydraulic fracture and can occur when a hydraulic fracture encounters formation bedding planes or natural fractures. For a vertical hydraulic fracture propagating in a formation with horizontal bedding planes, the fracture height is directly related to the interfacial slip between bedding planes. The mechanisms of height containment have been studied since the early times of hydraulic fracturing research, both experimentally and theoretically. It is well known now that in-situ stress contrast is the dominant mechanism that affects fracture height growth. It has also been shown that fracture growth can be arrested by weak interfaces when the effective normal stress across the interface is low. For horizontal bedding planes, the normal stress is the vertical overburden stress. In shallow formations, the normal stress is low and interfacial slip between formation bedding planes is more likely. Many coal bed methane (CBM) formations are at shallow depth, and interfacial slip can have an important effect on fracture propagation in such formations.

In this paper, we present a fracture height growth model that takes into account the bedding plane interfacial slip. The model has been implemented in a hydraulic fracture simulator. Interfacial slip affects not only the fracture height, but also the fracture width and fracturing pressure. The effects and the implications on fracturing treatments are demonstrated through a numerical example and a field case study.

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