Virtual time-lapse seismic monitoring using fully coupled flow and geomechanical simulations

Abstract

SEAM Time Lapse, a collaborative technical project of SEG and the Society of Petroleum Engineers, created integrated geologic, reservoir, and geophysical models to simulate temporal changes in the geometry and physical properties of a complex reservoir with the detail and accuracy needed to explain the subtle effects seen in time-lapse surveys of real oil fields. The geologic model consisted of 2 billion grid cells, representing a region 12.5 × 12.5 km in horizontal extent and 5 km in depth and including a 420 m thick reservoir with upper and lower units separated by an impermeable shale layer and offset by faults. Deepwater clastic turbidite channels and lobes were used to create a typical shallow Gulf of Mexico reservoir that also can serve as an analogue of other turbidite fields around the world. Stratigraphic detail within the reservoir was retained in the simulation model through careful finite-element meshing. The reservoir simulation computed the fully coupled three-phase fluid flow and linear geomechanical response in a production scenario involving 11 production wells and six water-injection wells penetrating the reservoir's three compartments. Seismic surveys were simulated with isotropic elastic-wave modeling before the start of production and after 27.5 months of production at a simulated rate of about 67,500 barrels per day, with about 32,500 barrels per day of water injection. Rock properties were updated by petrophysical models calibrated to turbidite systems in the Gulf of Mexico. Analysis of the model and simulations improves understanding of the complex interaction of fluid effects, pressure changes, and rock deformation. For example, compaction in the reservoir may cancel time-lapse fluid effects, and compaction or dilation in the surrounding shales may override the observed reservoir signal in the seismic bandwidth. Strain-induced velocity changes in the shales have a much larger effect on estimated time-lapse time shifts than do strain-induced changes in path lengths. Geomechanical effects impact the interpretation of 4D seismic difference attributes and require careful consideration. The integrated, full-physics SEAM approach used for this model provides a unique data set to explore these complex production effects and the value of 4D seismic surveys. The models and data, which also include simulations of time-lapse gravity and electromagnetic surveys, are publicly available through SEAM.