A New Approach to Modeling Fracture Growth in Unconsolidated Sands

Abstract

Abstract Fracture growth in soft rocks and unconsolidated sands is currently modeled assuming linear elastic, brittle behavior. Observations from past experimental work show that fracture propagation in unconsolidated sands is a strong function of fluid rheology and leak off and is accompanied by large inelastic deformation and shear failure leading to high net fracturing pressures. This paper presents a new approach to modeling fracture propagation in unconsolidated sands. It is shown that the classical approach to fracture modeling that uses the stress intensity factor at the fracture tip is not suitable for unconsolidated sands. The fracture propagation criterion in our model is not based on the conventional stress intensity factor approach. Both shear and tensile failure are modeled, and both play an important role in controlling the fracture growth. Fluid loss is modeled accurately using a filtration model that accounts for a reduction in porosity and permeability induced by particles in the injected fluids as well as the possibility of filter cake formation. The model predicts considerably higher net fracturing pressures due to plastic yielding in shear around the fracture. In general there is a zone of shear failure ahead of the tip which subsequently fails in tensile mode at higher net fracturing pressures resulting in fracture propagation. Shear failure is observed to be the dominant failure mechanism in case of low efficiency fluids, with very little fracture growth in tensile mode. However, tensile fractures of considerable length surrounded by a zone of shear failure are obtained in the case of high efficiency fluids or in the case of fracture face plugging by particles present in the injected fluid. The model thus clearly shows for the first time that fractures in sands can behave quite differently under different conditions. The results of the sensitivity study conducted on the important parameters are consistent with the observations reported in experiments. The model significantly improves our understanding of the effect of key parameters such as the injection rate, fracturing fluid properties and mechanical properties of the sand on the fracturing process. It is expected that the model will find application in designing frac-pack treatments and solid waste disposal in unconsolidated formations.