46th U.S. Rock Mechanics/Geomechanics Symposium,
2012. American Rock Mechanics Association
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The Eshelby like approach is used to successfully model the stress, strain and displacement fields associated with a depleting/inflating reservoir. A modified Cam-Clay material model is implemented as the constitutive equations for the reservoir material. The exterior of the reservoir is modeled as an infinite homogeneous linear elastic material and body forces are not considered. These assumptions allow a semi-analytical approach to be applied. The stresses exterior to the fault are induced by the depleting reservoir. The fault is represented as an elastic-plastic (modified Cam-Clay) high aspect ratio ellipsoidal inclusion loaded by the calculated exterior stress field. Two mechanisms are discussed as criteria for fault activation, runaway instability and negative rate of work of plastic strains. Two initial conditions are considered for the fault material model. First where the parameters for the material model are supplied by the user. Second where the initial effective mean stress and shear stress are assumed to be in the immediate vicinity of the intersection of the critical state line and yield surface. The differing predictions that follow from these two assumption are discussed. We find that fault activation is more likely to occur in the scenario when we are inflating the reservoir rather than depleting the reservoir. This is due to movement of the stress path towards more unstable regions of the MCC material model under inflation scenarios.
Analytical techniques offer a convenient tool for understanding the geomechanical aspects of depleting reservoirs. Applications of these techniques include “quick look” solutions for well failure estimates, evaluating time-lapse seismic candidates, effects of reservoir tilting, CO2 sequestration, estimates of fault activation etc. As will be shown, the region of negative hardening is responsible for most of the fault instabilities that are found.
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