Quantitative prediction of oil saturation of unconsolidated sandstone reservoir based on time-lapse seismic “relative difference method”: Taking Zeta oil field in West Africa as an example

LU Hongmei,XU Hai,WO Yujin,GU Ning

Table 2 Comparison between time-lapse seismic “relative difference method” and traditional remaining oil prediction methods.

Content Seismic difference evaluation method Basic idea Remaining oil prediction method Nature Solved problem Scope of application Problems Traditional method (absolute difference method) The difference between monitoring seismic and basic seismic, which is absolute difference The difference means that fluid displacement occurs. The greater the difference, the more complete the fluid displacement Select attributes sensitive to the time-lapse seismic absolute difference, or perform elastic wave impedance inversion, or combine forward modeling Qualitative For reservoirs with different thicknesses within the thickness of thin bed, the extent of fluid displacement can only be roughly inferred qualitatively The reservoir thickness is less than the tuning thickness but uniform or the reservoir thickness is greater than the tuning thickness, and the effects of pressure, temperature, pore type and porosity on seismic can be excluded Ignoring the effect of the thickness, it is not suitable for unequal thickness reservoirs smaller than the tuning thickness; it is not easy to identify the remaining oil in the relatively thick layer within the thin bed category, resulting in the omission of remaining oil reserves Relative difference method The ratio of monitoring seismic to basic seismic, which is relative difference The absolute difference of time-lapse seismic is not only related to the change of fluid saturation, but also closely related to the thickness of the reservoir; the relative difference of time-lapse seismic is not affected by thickness, but only related to the change of fluid saturation. The greater the relative difference in time-lapse seismic, the more thorough the fluid displacement Carry out the Gassmann fluid displacement seismic forward modeling according to the actual logging data, and establish the relationship between the relative differences of time-lapse seismic and the increase of water saturation. Calculate the relative difference of the actual time-lapse seismic and then convert it to the change distribution of the reservoir oil saturation. Predict the remaining oil distribution and calculate the remaining oil reserves by combining with the reservoir thickness distribution data etc Quantitative Quantitatively calculate the change of water saturation, predict the distribution of the remaining oil, and quantitatively calculate the remaining oil reserves Applicable to both thick and thin layers of deep-sea unconsolidated sandstone reservoirs, in which the effect of pressure, temperature, pore type and porosity on seismic can be excluded No actual research work has been carried out on non-deep sea unconsolidated sandstone reservoirs, but the relative difference method may be a more correct research direction