Abstract:
Hydrocarbon producing reservoirs undergo physical conditions over the life of the
reservoir. Such changes are dominantly manifested in properties such as velocity,
density, etc. due to the interchanges of the hydrocarbon and the replacing fluids. To
ensure optimum and maximum possible recovery of hydrocarbons from these reservoirs,
time-lapse 4D seismic technology is routinely used as a supportive tool to manage the
reservoirs. The reservoir parameters such as lithofacies, porosity, pore fluid type,
saturation and pore pressure are studied with the help of rock physics measurements and
models. It is primarily because all these parameters are directly or indirectly related to the
seismic velocity of the subsurface formation, and can be modeled for different reservoir
conditions.
Thus, for the study of 4D feasibility analysis, fluid substitution modeling is done by
applying the rock-physics models to analyze the seismic attributes namely AVO in terms
of amplitude and its phase. For this, various plots for density, P-wave velocity, S-wave
velocity with fluid variation; crossplots for impedance/ reflectivity coefficient; and
intercept-gradient AVO were plotted. Rock physics models for the crossplot of AVO were
analyzed for multiple scenarios against the base (initial) case using different combinations
of fluids under different substitution conditions to tap the sensitivity for characterizing
these reservoirs. In this study, I modeled AVO plot sensitivity with respect to varying input
parameters (such as mineralogy, fluid type, pore structure, fluid saturation, etc.), which
could then be used to characterize different reservoir conditions important for 4D
feasibility analysis. In the modeled results, I found that changing the pore fluid in the
base case for a brine-filled sandstone reservoir for oil records a decrease in reflection
coefficient value for near and far offsets. Gas substitution further decreases this value.
Carbonate formations taken for this study consist of mud supported carbonates and have
a strength comparable to sandstones, thus show fluid sensitivity. Reflection coefficient
value for carbonates change corresponding to texture, which could easily be detected and
analyzed for identification of a potential prospect.
