Effects of Fluids, Frequency and Heterogeneity on Elastic Properties of Carbonate Rocks

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Conference Proceeding

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Understanding factors controlling the elastic behavior of saturated rocks is critical for characterizing carbonate reservoirs. These elastic properties are often mapped using seismic data. Elastic properties, often derived from inversion of seismic data via velocities, form important inputs for reservoir engineering calculations and simulation studies. Normally, in absence of site-specific measurements at reservoir temperature and pressure, empirical models or data from analog formations are used. However, elastic properties of rocks are controlled by variation in saturant type and its dependencies on pressure and temperature, in addition to rock heterogeneity. The characterization of heterogeneities is important for obtaining rock physical parameters and for understanding flow mechanics in carbonate reservoirs using upscaled geological model for reservoir simulation. Broad objective of this study are, 1) Measure the elastic moduli of rocks as they change with saturant and amount of saturations. 2) Calibrate these changes in rock strength, to understand the difference between the static and the dynamic elastic moduli. 3) Investigate effects of frequency-dependence of elastic moduli and effects of its variation on reservoir engineering calculations and simulation studies. Present paper discusses the effects of fluids, frequency and heterogeneity on the elastic properties of carbonate rocks. Their effects on reservoir engineering calculations will be discussed in future communications.

In this study, comparisons are drawn between a set of homogeneous and heterogeneous samples for their varied response to varying saturant, amount of saturation (Sw), frequencies and differential pressures. At ultrasonic frequencies, bulk modulus in the heterogeneous sample varies by 22% from dry to partial (water saturation, Sw = 56%) saturation stage, and by an additional 7% from partial to full saturation, whereas in the homogeneous sample, bulk modulus varies by 27% from dry to partial saturation (Sw = 90%) and by 29% from partial to full saturation. Similarly, the shear modulus in both samples varies by -2% and -12%, respectively from dry to partial saturation, and by 3% and 10%, respectively from partial to full saturation. For another set of homogeneous and heterogeneous samples we measured the bulk and shear modulus as a function of frequency, saturant and differential pressure. We observe that most samples show an increase in bulk and shear modulus with frequency. We also observe weakening and strengthening of the shear modulus from dry or with a light hydrocarbon to fully-brine saturated pore spaces. Investigations were also carried out for finding any possible correlation between the measured permeability and measured velocity in carbonate rocks. We found no definite correlation between permeability and velocity for both homogeneous and heterogeneous samples.