Abstract

Reservoir Geomechanics as a reckoned role incorporates with not only performances of reservoir flows but its leakage-proof security. In order to take near and far-field Geomechanics into account during modelling CO2 injection process into deep saline aquifer, it is studied how to sufficiently control a compatible numerical discretization between reservoir flow and reservoir Geomechanics. Application of gridding technique is implemented through source code programming. It is introduced self-developed realization in commercial software package from unstructured meshes for the sake of efficiently extending overlying strata up to ground surface, to a regularly refined focus capable of stepwise capturing CO2 plume, surrounding by circumference divided into cells gradually scaling up along radiant directions. The resultant distributions of ground surface deformation simulated using different gridding algorithms are compared and some principles counter-intuitive to a usual work flow in analyzing ground surface heave induced by deep gas injection are studied. In addition, as a foundation to facilitate the decoupling strategy, an tractable interface is programmed to provide completely identical models as well as their discretization between reservoir flow simulation software CMG’s GEM and reservoir Geomechanics modelling package Itasca’s FLAC-3D.

1. INTRODUCTION

Geomechanics is playing a significant role in assessing the leakage-proof security related near-field integrity and far-field manifestation, along with subsurface volume perturbation induced by pressure transients from its reservoir initial state to distribution driven by injected flows, particularly, e.g., in case of geological sequestration of carbon dioxide into deep saline aquifer [1]. Among the geomechanical responses to gas injection, an obvious mechanics is the inflation of reservoir. It is well known that effective stress acting on porous rocks can be simplified as the difference of total stress applied by regional tectonic stress and overburden weight subtracting the pore pressure [2], which means any increase in pore pressure caused by injection will reduce the effective stress acting on porous rocks [3], hence leading to reservoir inflation. In many cases, this change in depth transfers all the way and can reach the surface being manifested as extremely slight ground heave or sag. Though ground surface deformation falls into hardly detectable magnitudes, geodetic methods, such as tiltmeter array, Interferometric Synthetic Aperture Radae (In SAR), GPS surveying [4] as well as certain combination of these approaches, are usually feasible to monitor and reflect the reservoir pressure growth caused by CO2 plume spreading at reservoir depth.

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