Discrete particle models, which simulate inter-particle mechanics explicitly and can be coupled with fluid flow mechanics, often provide a more realistic simulation of granular deformation and fracture than continuum models. We apply such models to investigate fracture processes in weakly cemented media,. providing insights on material parameters which influence a change from discrete brittle fracturing (as occurs in stiff and strong geomaterial) to general dilation and parting (as occurs in very soft and weak geomaterials). The primary influence on parting behavior is shear bonding at the granular scale. We also investigate slurry injection processes in granular media by coupling fluid flow simulators with particle models for several near wellbore assemblies. Although there are clear challenges remaining with scaling issues and practical model size, we conclude that coupled particle and fluid flow codes can simulate slurry injection processes well, reproducing dilation and parting patterns consistent with laboratory observations and pressure response consistent with field observations.
There are several important petroleum, mining, and environmental engineering applications that involve large-scale deformation, failure, and fluid flow processes in weakly consolidated media. These include gravel injection and "frae-pack" operations to both stimulate a well and provide sanding control, grout injection to create barriers for contaminant flow in porous media, and slurry waste injection in deep wells. Unfortunately, the geomechanical aspects and controls on such operations remain poorly understood. Continuum models have difficulty capturing the basic physical processes of microcracking, disaggregation, and grain movement that occur during fracture and slurry injection in weakly consolidated media. These are inherently "discontinuous" failure processes. Traditional fracture mechanics approaches are particularly ill suited for modeling such phenomena because they are fundamentally based on stress singularities and strain energy dissipation processes at an advancing fracture tip. Fracture or "parting" of weakly consolidated media with near zero shear strength, however, is dominated by energy dissipatedeforming, shearing, and dilating material over a large area; fracture toughness and traditional tip mechanics are relatively inconsequential. The objective of our research, funded in part by the U.S. Department of Energy and the Alberta Department of Energy, has been to develop an improved understanding of such processes by developing alternative modeling techniques. One component of our effort has involved coupled particle and fluid flow modeling.
In this paper we first present an overview of traditional linear elastic fracture mechanics, starting from first energy principles and extending to the stress intensity factor approach common to most hydraulic fracture models. We describe the limitations of such models when considering distributed damage proc-esses involved in fracture and parting of weakly consolidated media, and suggest an alternative approach using discrete particle modeling techniques. We investigate and conclude that particle models can capture observed physical processes in weakly cemented media, providing insights on material parameters which influence a change from discrete brittle fracturing (as occurs in stiff and strong geomaterial) to general dilation and parting (as occurs in very soft and weak geomaterials). The primary influence on parting behavior is shear bonding at the granular scale. Tensile bond properties have much less influence.
Next we investigate slurry injection processes in granular media by coupling fluid flow simulators with part
