SENSITIVITY OF METHODS AND PARAMETERS FOR SUBSURFACE PRESSURE PREDICTION IN SANDSTONE AND CARBONATE RESERVOIRS

Abstract

The understanding of subsurface pressure regimes is vital for the safe exploration and extraction of hydrocarbons. Interdependence of effective pressure (Pe) and geophysical properties (resistivity, compressional velocity Vp and shear velocity Vs) in granular media is often exploited as one of the main rock physical attribute for reservoir characterization. This is particularly relevant for 4D feasibility investigations experiencing simultaneous pressure and saturation changes. Effective pressure changes in carbonate reservoir, however could be trickier because of the change in effective stress coefficient (α) as a result of rock and fluid interaction. It is therefore important to understand the sensitivity of the methods and parameters for the prediction of subsurface pressures. Using the different methods available for the prediction of effective and pore pressures, the response of effective and pore pressures to geophysical properties in siliciclastic and non-clastic (carbonate) reservoir were compared. In the sandstone reservoir, it was found that the compressional wave velocities have the most definitive correlation to effective pressure. In the carbonate reservoir, the response of effective pressure to P and S wave velocities was compared, using Biot's coefficient (β), and the experimentally determined effective stress coefficient (α). It was found that the calculated effective pressure was markedly different for the two methods, with Pe using α exhibit values higher than the one using β. Also, in the clean reservoir, the effective pressure values obtained using Vs is higher than the values obtained using Vp. This information crucial because it exhibits that correlations between acoustic velocities and effective pressure could change drastically leading to very different interpretations of the changes due to pressure and saturation. It also signifies that experimentally determined effective stress coefficient is crucial to prediction of effective pressure in carbonate reservoirs that generally do not follow compaction trends. Finally the relationship between bulk rock properties in sandstone and carbonate rocks was established. It was found that the dry rock bulk modulus and the Biot’s coefficient possess a linear relationship with the saturated bulk modulus, for both sandstone and carbonte rocks.