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46350-AC8
Experimental Investigation of the Effect of Pressure and Fluid Saturation on Carbonate Rocks

Gary Mavko, Stanford University

Carbonate rocks have strong economic significance, accounting for 60% of the world's oil reserves.  Nevertheless, systematic laboratory studies on the geophysical properties of carbonates have been rare, largely due to complexities associated with the range of carbonate microtextures, mineralogies, and chemical reactivities of carbonate minerals with pore fluids.

In this project, a comprehensive laboratory study on carbonate rocks was undertaken to understand how properties such as mineralogy, pore shape, porosity, and fluid type control seismic wave propagation.  Samples span a range of depositional facies, porosities, pore types, and mineral composition. Characterization of the samples included porosity, permeability, mineralogical composition, and P- and S-wave velocities. Since confining pressure was shown to alter sample microstructure of some samples, a bench-top setup was used for ultrasonic velocities, using transducers of 1 MHz for P-waves and 0.7 MHz for S-waves.

XRD and thin section analyses show heterogeneous mineral composition associated with the different depositional facies. Samples range from almost pure calcite (sites FP, MA and GR) to mixed calcite and dolomite (sites FP and MSA). Thin sections show a variable carbonate microfabric, ranging from packstone (MSA and low porosity FP) to grainstone (high porosity FP and GR). MA samples are defined as "chalky" because of their fine granular texture resulting from lime mud and fine turbiditic calcarenite mixing.

The velocity-porosity plot (Figure 1) confirms a strong dependence of P- and S-wave velocities on porosity. Samples dominated by calcite (FP and GR, respectively) follow a fairly smooth and continuous trend over a porosity range from 2% to 52%; those with mixed calcite-dolomite composition (MSA) show a slightly flatter trend over the range of porosity from 2% to 12%. In contrast, MA samples depart significantly from the main trend, which seems to correlate with the increase of micrite and sparite interstitial sediments.

Another key question in rock physics studies is how seismic velocities (elastic moduli) vary with pore fluid content.   This is critical for remote seismic detection of hydrocarbons and monitoring reservoir changes associated with water injection, hydrocarbon production, as well as the movement and trapping of sequestered CO₂.  To address this problem, measurements were made on initially dry samples and again on samples saturated with various aqueous solutions.

Most of core plugs were saturated with water, saturated with CaCO₃ in order to minimize dissolution in calcite-rich samples. Some samples were saturated with degassed, distilled water and others with a slightly acidic, aqueous solution of carbonated water.  The observed changes in velocities between dry and saturated conditions were compared with predictions of the Gassmann fluid-substitution equations, which predict that the rock effective elastic shear moduli should be independent of saturation while the bulk modulus will vary with the bulk modulus of the pore fluid.

Figure 2 plots differences between observed saturated-rock moduli and those predicted using Gassmann's equations. Three general behaviors are observed:  (a) ultrasonically-measured bulk and shear moduli that are both larger than those predicted by Gassmann's equations (1st Quadrant); (b) ultrasonic bulk moduli that are larger than, and ultrasonic shear moduli that are smaller than, the Gassmann-predicted values (2nd Quadrant); and (c) ultrasonic bulk and shear moduli that are both smaller than those predicted by Gassmann's equations (3rd Quadrant);

After saturation, samples were allowed to dry in order to measure whether any change in porosity and permeability had occurred. Samples saturated with CaCO₃ aqueous solution and samples saturated with degassed, distilled water mostly show negligible change in porosity and permeability.  In contrast, samples saturated with carbonated water show a greater variation in both porosity (up to 5%) and permeability (up to 400mD).

These results indicate that seismic fluid substitution in carbonates is likely to be more than a purely mechanical problem. The presence in reservoirs of fluids and lithologies that are susceptible to chemical reactions poses new challenges when modeling the effect of pore fluids on geophysical observations. These challenges arise because physical and chemical processes have traditionally been treated as decoupled problems. As a consequence, purely mechanical models of fluid substitution are not appropriate for quantifying the effects on in-situ seismic velocities when chemical processes are triggered.  Chemical effects such as mineralogical changes, dissolution, and precipitation induce changes in the properties of the rock frame, which alter the baseline of Gassmann's equations.

Figure 1:  P-wave and S-wave velocities versus porosity for dry (open diamonds) and saturated (filled diamonds) carbonate samples. Data are color-coded by formation: Blue – MSA, Green – FP, Yellow – MA, and Red – GR.

Figure 2:  Difference of measured shear moduli (Mu-wet) and predicted shear moduli (Mu-Gass) versus the corresponding difference bulk moduli. Data correspond to core plugs saturated with degassed, distilled water (blue diamonds), CaCO3 aqueous solution (gray diamonds), and carbonated water (red diamonds).

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