Abstract:In this paper, we investigate production induced microseismicity based on modelling material failure from coupled fluid-flow and geomechanical simulation. The model is a graben style reservoir characterised by two normal faults subdividing a sandstone reservoir into three compartments. The results are analysed in terms of spatial and temporal variations in distribution of material failure. We observe that material failure and hence potentially microseismicity is sensitive to not only fault movement, but also fluid movement across faults. For sealing faults, failure is confined to the volume in and around the well compartment, with shear failure localised along the boundaries of the compartment and shear-enhanced compaction failure widespread throughout the reservoir compartment. For non-sealing faults, failure is observed within and surrounding all three reservoir compartments as well as a significant distribution located near the surface of the overburden. All shear-enhanced compaction failures are localised within the reservoir compartments. Fault movement leads to an increase in shear-enhanced compaction events within the reservoir as well as shear events located within the side-burden adjacent to the fault. We also evaluate the associated moment tensor mechanisms to estimate the pseudo scalar seismic moment of failure based on the assumption that failure is not aseismic. The shear-enhanced compaction events display a relatively normal and tight pseudo scalar seismic moment distribution centered about 10 6 Pa, whereas the shear events have pseudo scalar seismic moments that vary over 3 orders of magnitude. Overall, the results from the study indicate that it may be possible to identify compartment boundaries based on the results of microseismic monitoring.

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.

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