Rock stratification plays an important role in the outcome of hydraulic fracture treatments. Recently we have proposed a hydraulic fracture model in 3D poroelasic layered media that is based on Griffith-type global fracture criterion with anisotropic specific fracture energy, in addition to conventional mass and energy balances. A numerical realization of the model reveals the importance of fracture mechanisms in the vicinity of the stratified rock interface for fracture containment. In this work we present stress and energy analyses of a growing fracture interacting with an interface between rigid and soft layers of rocks, as well as experimental observations of fracture behavior in the vicinity of the interface on two small scale models. Stress and energy analyses show that when a fracture propagates from a rigid layer toward a softer layer, the fracture will break through the interface. However, when a fracture propagates from a soft layer to a rigid or stiffer layer, crack arrest can occur and other fracture mechanisms such as the formation of secondary fractures across the interface (again leading to fracture breakthrough), delamination along the interface, or crack kinking resulting in fracture containment, are demonstrated both numerically and experimentally. A fracture mechanisms map (FMM) in the vicinity of an interface is proposed to assist in hydraulic fracture treatment design.
1. INTRODUCTION
Hydraulic fracturing can be critical to the economics of hydrocarbon production in many reservoirs, such as those requiring fracture stimulation, frac&pack completions, waterflood injection above fracture pressure, or downhole solids disposal. Typical of many reservoirs is a system of hydrocarbon bearing sandstone layers inter-spaced with shales. To economically develop and produce such layered reservoirs, it is essential to understand the mechanisms of fracture height growth (fracture containment) so that we can design and predict fracture dimensions during our hydraulic fracturing treatments. For example, knowing when a shale layer would act as a barrier to fracture growth impacts the total fluid volume needed for the treatment and the optimum number of individual treatments to cover a producing interval with multiple sand-shale sequences.
A significant amount of previous work in this area has pointed to stress contrasts as a primary mechanism for fracture containment in many cases [e.g., 1,2]. However, there remain many field examples of containment where no stress contrast exists [3]. Also existing 3-D and pseudo 3-D geometry models, based primarily on stress contrasts, have not been shown effective in predicting the fracture height growth based on recent field tilt-meter and micro-seismic data. An improved understanding of fracture containment mechanisms and the development of a more accurate containment model can greatly impact the economics of hydraulic fracturing.
In our previous work we address the problem of fracture propagation in layered elastic media [4,5]. The purpose of that work was the analysis of crack stability in the vicinity of an interface in a media with anisotropic fracture toughness (different fracture resistances for directions perpendicular and parallel to the bedding planes in layered rocks).
