Abstract

Subsurface pressure and temperature variations can alter rock properties both near and relatively far from the disturbance, causing detectable changes in seismic travel times. In modelling such phenomena, we employ large-scale geomechanical simulations to translate reservoir depletion values into rock deformations accounting for overburden heterogeneity and heavy faulting patterns derived from seismic interpretation. Subsequent velocity perturbations are then obtained using straindependent stiffness perturbations from nonlinear elasticity theory. By applying this methodology to a channelized turbidite reservoir located in offshore West Africa, we show that appropriate calibrations can generate geomechanically-derived overburden time-shift predictions that reproduce signatures from an actual 4D seismic monitor survey, providing valuable insights into reservoir pressures, rock compressibility and fault stiffness. The results also demonstrate the importance of integrating nonhomogeneous overburden descriptions in successfully explaining many 4D seismic signatures.

1. INTRODUCTION

The advent of reservoir geomechanical models has enabled further levels of subsurface characterization by incorporating the complex interplays between finely characterized rock properties, detailed structural elements and changes in reservoir conditions. The models allow field management strategies to account for the operational risks associated with changes in reservoir pressures and temperatures, and therefore any potential impact this may have in a field’s drilling and production plan. These same pressure and temperature variations can alter rock properties both near and relatively far from the reservoir, causing detectable changes in seismic travel times.

Many authors have demonstrated the use of geomechanical simulations (driven by the pressure and temperature changes) to explain changes in travel times derived from time-lapse seismic surveys over compacting reservoirs (see Hatchell et al., 2003; Sayers, 2004; Hatchell and Bourne, 2005; Herwanger et al. 2007; Janssen et al., 2007; Kosco et al, 2010; Herwanger et al., 2013). Following these ideas, we employ largescale geomechanical simulations to translate reservoir depletion values into rock deformations accounting for overburden heterogeneity and heavy faulting patterns derived from seismic data.

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