A Plasticity Mechanism for Hydraulic Fracture Height Containment

Abstract

ABSTRACT: In this paper we present a new mechanism that explains hydraulic fracture containment in the target zone. The study is based on the modelling of hydraulic fracture propagation in weak rocks and comparison of the results of propagation in the vertical direction with propagation in the horizontal direction. The modelling is based on both analytical modelling with a Mohr-Coulomb dislocation theory and on FEM modelling of hydraulic fracture propagation in weak formations described by Mohr-Coulomb elastoplastic material and a cohesive-type fracture propagation criterion. It was found that the resistance to fracturing, expressed in the dislocation model by the required force and in the FEM by an effective fracture toughness that increases with fracture extension tending to asymptotic value, depends strongly on the fracture propagation direction. A dominant parameter that quantifies these effects is a new brittleness index (BI) that depends both on rock strength and insitu stresses. This BI defines a) if a hydraulic fracture will be created and b) if the fracture will remain contained in the target zone and c) what deviation is expected from LEFM modelling of hydraulic fracturing. 1. INTRODUCTION An important design objective in hydraulic fracturing is the fracture to remain vertically contained in order to achieve a long penetration in the pay zone. A contained fracture will eliminate the risk of mixing hydrocarbon fluids with water or natural gas with oil if the surrounding layers contain different fluids. The overarching objective is to maximize the flow of hydrocarbon in the wellbore through a long conductive path formed by a propped fracture. Clearly, the mechanical properties of reservoir and surrounding layers affect the fracture height. An obvious explanation for the often-observed fracture containment in the reservoir layer, is the higher horizontal stress, called closure stress, in the barrier layers, usually shales. In order to restrict fracture height growth, the commercial hydraulic fracturing simulators assume that the barrier shale layers are under higher closure stress or they have higher resistance to fracturing, expressed by the fracture toughness. However, though there were cases that such conditions do not exist in the barrier layer, the hydraulic fractures still remained contained. Often, hydraulic fractures do not even reach the shale-reservoir interface. Nevertheless, in practice a cased-cemented well is perforated in the lower part of the reservoir to minimize the risk of hydraulic fracturing reaching the upper barrier layer.