Reservoir stress path characterization and its implications for fluid-flow production simulations

Abstract

The reduction of fluid pressure during reservoir production promotes changes in the effective and total stress distribution within the reservoir and the surrounding strata. This stress evolution is responsible for many problems encountered during production (e.g. fault reactivation, casing deformation). This work presents the results of an extensive series of 3D numerical hydro-mechanical coupled analyses that study the influence of reservoir geometry and material properties on the reservoir stress path. The stress path is defined in terms of parameters that quantify the amount of stress arching and stress anisotropy that occur during reservoir production. The coupled simulations are run using an explicit coupling code between Elfen (Rockfield Software Ltd) and Tempest (Roxar). It is shown that the stress arching effect is important in small or thin reservoirs that are soft compared to the bounding material. In such cases, the stresses will not significantly evolve in the reservoir, and stress evolution occurs in the over and side-burden. Stiff reservoirs do not show stress arching regardless of the geometry. Stress anisotropy reduces with the bounding material Young's modulus, especially for small reservoirs, but as the reservoir extends in one or the two horizontal directions, the reservoir deforms uniaxially and the horizontal stress evolution is governed by the reservoir Poisson's ratio. Furthermore, the effect of the stress path parameters is introduced in the calculation of pore volume multiplier tables to improve non-coupled simulations, which otherwise overestimate the average reservoir pore pressure drawdown when stress arching is taking place.

Reservoir stress path and induced seismic anisotropy: results from linking coupled fluid-flow/geomechanical simulation with seismic modelling

“…As this would involve a constant shift, not performing a static-to-dynamic elasticity conversion will not effect the main conclusions of this paper. Segura et al (2011) model the influence of reservoir geometry and material properties on stress path using a more extensive suite of models considered in this paper. Using poroelastic constitutive material behaviour, Segura et al (2011) observed that the stress arching effect is significant in small, thin reservoirs that are soft compared to the surrounding rock.…”

“…Segura et al (2011) model the influence of reservoir geometry and material properties on stress path using a more extensive suite of models considered in this paper. Using poroelastic constitutive material behaviour, Segura et al (2011) observed that the stress arching effect is significant in small, thin reservoirs that are soft compared to the surrounding rock. Under such circumstances, the stresses will not evolve within the reservoir and so stress evolution occurs primarily in the overburden and sideburden.…”

“…However, changes in pore pressure do not necessarily lead to a hydrostatic change in effective stress. For instance, a reduction in fluid pressure within a reservoir is often accompanied by a slower increase in the minimum effective horizontal stress with respect to the vertical effective stress change (e.g., Segura et al 2011). This asymmetry can result in the development of stress anisotropy that may promote elastic failure within the rock, such as fault reactivation and borehole deformation.…”

“…The stress path of a reservoir during production is sensitive to the geometry of the reservoir system, pore pressure and the material properties of the reservoir and surrounding rock mass (e.g., Segura et al 2011). In the field, stress path can be measured from borehole pressure tests, and so maps of reservoir stress path are extrapolated out into the reservoir via limited discrete measurements.…”

“…This work follows the coupled fluid-flow and geomechanical characterization of reservoir stress path of Segura et al (2011), who explore the influence of reservoir geometry and material property contrast on stress path for poroelastic media. We present results from coupled fluidflow and geomechanical simulations for the same geometries to investigate stress-induced seismic anisotropy.…”