Guerin, G. (2); Mello, Ulisses T.(1); He, Wei (2); Anderson, Roger N.(2); Boulanger, Albert (2); Teng, Yu-Chiungand (2); Xu, Liqing (2); Neal, Randall (3) and Meadow, Billy (3)
(1) IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598
(2) Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, 10964, U.S.A.
(3) 4D Technology, Inc. Dallas, Texas.
The ultimate way to validate the interpretation of the 4-D seismic analysis and inversion described in the previous parts of this series is the "ground truth": drill and recover the bypassed hydrocarbons detected by the 4-D seismic analysis. But in order to lower the risk that bypassed hydrocarbons might be missed, reservoir simulation techniques can be used to validate and constrain 4-D seismic interpretation before spudding the well. Reservoir simulation is a low-cost solution to validate independently the 4-D analysis that uses legacy seismic datasets, where different orientation, processing and quality can potentially introduce noise in the 4-D seismic interpretation. The main objectives of a reservoir simulation are to reproduce the drainage pattern of the reservoir fluids during the production, and provide guidance and financial assessment about the future production. The added mission of reservoir simulation in 4-D analysis is to relate this drainage interpretation to changes in seismic attributes, especially seismic impedance. Seismic impedance changes are directly related to both the changes in density and velocity of wave propagation within the reservoir. During reservoir simulation, the parameters monitored and replicated by the multiphase fluid flow equations are pore pressure and the oil, gas and water saturations within the pore spaces as they change over time, during production. Since the changes in impedance mapped by 4-D seismic interpretation are directly related to the changes in the above mentioned parameters, the changes in fluid saturations and pressure over time predicted by the reservoir simulator can be qualitatively compared to the observed 4-D seismic impedance change. A quantitative comparison requires the estimation of the difference between a seismic impedance map obtained from the reservoir simulation and the seismic impedance map obtained from the 4-D seismic analysis. A reservoir simulation derived seismic impedance map can be estimated by using some form of the Biot-Gassman equations to compute the seismic attribute changes associated with pressure and fluid saturation variations with production.