Rock Physics Core Study Quantifies the Effects of Fluid and Pressure Changes from Velocity for a Wide Range of Carbonate Reservoir and Non-Reservoir Lithofacies
A rock physics core study was successfully conducted to investigate the effects different fluids (gas, oil and brine) and reservoir pressure changes have on the velocity of carbonate rocks. Sixty vertical core plugs representing all reservoir and non-reservoir facies over a Cretaceous interval of interest were selected for use in the study. Dense non-reservoir facies with low porosity and permeability showed minor to no velocity changes to different pressures and fluids. The velocity of reservoir facies with significant porosity and permeability were observed to change with fluid type, effective pressure and porosity. Samples with higher porosity and more heterogeneity showed the most velocity change with pressure. Based on the ultrasonic velocity results, significant saturation changes involving gas-charged-oil or gas being replaced by brine, will produce larger velocity changes than even large pressure changes in these carbonate reservoirs.
The interval of interest consists of a variety of different carbonate facies that are related to depositional environment and diagenetic effects. Samples representing all facies from the dense non-porous to coarse grained heterogeneous with porosities as high as 35% were selected for analysis. The different facies included fine to coarse grain material, interganular to vuggy porosity and denser low porosity carbonates. Samples with styolites and web fractures (early diagenetic, cross-cutting, discontinuous hairline fractures) were also sampled for analysis. The different rock facies are being studied to determine if they behave differently to changes in pressure.
The results of this rock physics core study are helping in the interpretation of 4D seismic responses in carbonates. Observed 4D seismic anomalies are more likely the result of saturation changes and less likely to be related to reservoir pressure changes. Larger 4D anomalies linked to pressure are more likely due to very large pressure changes that are related to injection or production, especially in the vicinity of a highly fractured zone. The results of this special rock physics core study are also being used to help build carbonate rock physics models which more accurately account for reservoir fluid and pressure changes and therefore lead to more reliable 4D seismic interpretations.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009