Rock physics modeling
The degree of changes in seismic response to production within the reservoir are highly dependant on the physical properties of the reservoir rocks. Accurate modeling of these properties from existing data will allow consideration of their effect on the 4D response. Modeled acoustic/elastic properties include Vp, Vs, bulk density, bulk Poisson's ratio, and reflectivity, both at normal incidence and non-vertical incidence. The rock physics model will allow variation in fluid saturation, fluid properties, and reservoir pressure, allowing fluid substitution modeling to represent realistic reservoir changes. Because many of the input parameters to rock physics modeling are often not well known, this task should include a sensitivity analysis, thus giving guidance toward what additional petrophysical data acquisition may be desirable prior to 4D data analysis.
Fluid substitution modeling
Fluid substitution models can be used to assess the effect of different reservoir scenarios on the 4D response. Scenarios modeled will determine expectations of reservoir production changes. Examples include, but are not limited to:
Saturation change representing water influx from injection well or a natural aquifer
Pressure change in areas where reservoir pressure is not maintained by injection and/or an aquifer
Fluid property changes representing the release of solution gas as reservoir pressure falls below the bubble point
A combination of saturation, pressure, and fluid property changes as predicted by simulation of reservoir production
Changes in the overburden or reservoir physical rock properties brought about by production induced geomechanical changes.
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