A rock physics core study was successfully conducted to investigate the effects different fluids and reservoir pressure changes have on the velocity of carbonate rocks. Eighty vertical core plugs representing all reservoir and non-reservoir facies over a Cretaceous interval of interest were selected for use in this study. Dense non-reservoir facies with low porosity and permeability showed minor to no velocity changes to pressure changes. 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, saturation changes will produce larger velocity changes than pressure changes in the reservoir.
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. 4D anomalies can be produced by reservoir pressure changes but should be lower magnitude than those related to saturation changes. Large 4D anomalies linked to pressure are more likely due to a significant pressure increase, related to high pressure injection.
Reservoir monitoring with time-lapse seismic images has the potential to identify places in the reservoir layer where changes have occurred between the times when two 3D seismic volumes were collected. Saturation changes as a result of water replacing moderate GOR oil have been shown to produce 4D anomalies in Middle East carbonate reservoirs (Soroka et al, 2005), when there is sufficient porosity and saturation change. Some 4D anomalies, which are difficult to attribute to saturation changes, are suspected to be due to pressure changes related to production and injection.
The reservoir interval of interest consists of a variety of different carbonate facies that are related to depositional environment and diagenetic effects (Strohmenger, et al, 2006). Samples representing all facies from dense homogeneous samples to coarse grained heterogeneous samples were selected for analysis. Figure 1 shows CT-scans of representative core plug samples. The CT-scans highlight the dramatic changes in rock facies that can occur in a carbonate interval. These changes include fine to coarse grain material, interganular to vuggy porosity to dense low porosity material. Samples with styolites and web fractures (early diagenitic, 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. Ultimately the results will be used to develop a better understanding of how pressure changes can affect the velocity of a carbonate reservoir and impact 4D time-lapse responses.
The velocity of a rock changes with compaction or consolidation, which depends upon the overburden and pore pressures. When the pore pressure of a carbonate rock changes due to production or injection of fluids the velocity of the rock can be affected. This laboratory study measures ultrasonic velocities of core samples in an attempt to quantify the amount a carbonate reservoir velocity can change over a range of effective pressures. Effective pressure is the difference between the over-burden pressure and the pore-pressure. The effective pressure range, used in the study covered the lowest to highest pressure conditions observed over the life of the reservoir.