Numerical modeling of time-lapse seismic data from fractured reservoirs including fluid flow and geochemical processes

摘要

Fractured reservoirs, especially in low permeable carbonate rocks, are importanttarget for hydrocarbon exploration and production because fractures can controlfluid flow inside the reservoir. Hence, quantitative knowledge of fracture attributes isimportant for optimal hydrocarbon production. However, in some cases fractures cancause leakage of injected CO2 during enhanced oil recovery (EOR) or CO2 sequestration.Furthermore, CO2 can geochemically interact with reservoir fluids and hostrock. Hence, time-lapse monitoring of the progress of CO2 in fractured reservoirs isalso very important.In order to address these challenges, I have developed an integrated approach forstudying fluid flow and seismic wave propagation in fractured media using DiscreteFracture Network (DFN) models. My seismic simulation study suggests that CO2saturated reservoir shows approximately ten times more attenuation than brine saturatedreservoir. Similarly, large P-wave velocity variation in CO2 saturated reservoirand amplitude variation with offset (AVO) results for our example model predictsthat CO2 is easier to detect than brine in the fractured reservoirs.The effects of geochemical processes on seismics are simulated by time-lapse modelingfor t = 1000 years. My modeling study suggests that intra-aqueous reactions aremore significant during injection of CO2 for t = 6 years, while slower mineral reactionsdominate after pressure equilibrium is achieved that is from t = 6 to 1000 years.Overall both types of geochemical reactions cause change in reflection coefficient of 2to 5%, which may be difficult to detect in some cases. However, the significant changein the seismic properties at the boundary of the CO2 front can be used to detect theflow path of CO2 inside the reservoirs. Finally, a method for generating stochasticfracture models was extended and improved to more realistic field model for seismicand fluid modeling. My detail analysis suggests that fractures generated by isotropicstress field favor orthogonal sets of fractures in most subsurface rocks that can be converted to seismic model, similar to DFN study. The quality and validity of themodels is assessed by comparisons to DFN models, including calculations of fractaldimension measures that can help to characterize fractured reservoirs.