Abstract

Production-induced depletion of hydrocarbon and water reservoirs leads to deformation, compaction, displacements, and stress changes in the reservoir as well as in the surrounding rock. Shell's geomechanical finite-element computer program was used to simulate this for the depletion of stacked oil-saturated reservoir sands from the Mars basin in the Deepwater Gulf of Mexico (GoM), USA. 3D Quantitative insight was obtained in the response and mechanical interaction of depleting sands and surrounding mudstones as a function of production. The calculated deformation and stress change were used to model changes in two-way travel time of vertically-propagating seismic P-waves. Comparison of our 3D-model results with repeat-seismic 2D field timeshift data shows reasonable agreement. This confirms the value of integrating geomechanics in forward models of seismic timeshifts related to depletion, and may find application in reservoir fluid-flow monitoring with time-lapse seismic. Systematic variation of the depletion pattern in stacked sands of idealised lenticular shapes helped to understand some basic aspects of compaction-induced timeshifts, and revealed their potential and limitations in deep stacked-reservoir settings. Our Mars model of compaction-induced timeshifts needs further calibration to field data, and addition of the salt-sediment geomechanical interaction. However, already at this stage, our study suggests that forward models of compaction-induced seismic timeshifts can help to better interpret field observations of timeshifts, with a promise to detect undepleted compartments and fluid-flow barriers in producing reservoirs.

Introduction

The most well known compaction-induced effect in time-lapse (4D) seismic is the increase in acoustic impedance (AI) brought about by the densification and acoustic-velocity-increase in the producing/depleting sands (compared to that in the bounding mudstones). Modeling studies, field cases and operational impact of this technique have been published [see e.g. 1–4] yet some important rock physics questions remain, notably on the magnitude of acoustic velocity changes during production-induced depletion and fluid injection (see Appendix). The focus of most AI-change models is on the compacting reservoirs, and rock property changes in the bounding non-depleting formations are often not considered.

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