Velocities of compressional and shear waves in limestones

Abstract

Carbonate rocks are important hydrocarbon reservoir rocks with complex textures and petrophysical properties (porosity and permeability) mainly resulting from various diagenetic processes (compaction, dissolution, precipitation, cementation, etc.). These complexities make prediction of reservoir characteristics (e.g. porosity and permeability) from their seismic properties very difficult. To explore the relationship between the seismic, petrophysical and geological properties, ultrasonic compressional‐ and shear‐wave velocity measurements were made under a simulated in situ condition of pressure (50 MPa hydrostatic effective pressure) at frequencies of approximately 0.85 MHz and 0.7 MHz, respectively, using a pulse‐echo method. The measurements were made both in vacuum‐dry and fully saturated conditions in oolitic limestones of the Great Oolite Formation of southern England. Some of the rocks were fully saturated with oil. The acoustic measurements were supplemented by porosity and permeability measurements, petrological and pore geometry studies of resin‐impregnated polished thin sections, X‐ray diffraction analyses and scanning electron microscope studies to investigate submicroscopic textures and micropores. It is shown that the compressional‐ and shear‐wave velocities (Vp and Vs, respectively) decrease with increasing porosity and that Vp decreases approximately twice as fast as Vs. The systematic differences in pore structures (e.g. the aspect ratio) of the limestones produce large residuals in the velocity versus porosity relationship. It is demonstrated that the velocity versus porosity relationship can be improved by removing the pore‐structure‐dependent variations from the residuals. The introduction of water into the pore space decreases the shear moduli of the rocks by about 2 GPa, suggesting that there exists a fluid/matrix interaction at grain contacts, which reduces the rigidity. The predicted Biot–Gassmann velocity values are greater than the measured velocity values due to the rock–fluid interaction. This is not accounted for in the Biot–Gassmann velocity models and velocity dispersion due to a local flow mechanism. The velocities predicted by the Raymer and time‐average relationships overestimated the measured velocities even more than the Biot model.

Prediction of Porosity and Density of Calcarenite Rocks from P-Wave Velocity Measurements

“…Furthermore, the P-wave velocity of the dry and water saturated calcarenite rocks were reducing as the porosity is increasing (Figures 3 and 4). This observation is in consistent with the findings of Han et al [28] and Klimentos [29], for sandstone and limestone by Oghenero [26] and Assefa et al [7]. In spite of the fact that the samples investigated composed of high proportion of calcite mineral, there was variation in the ranges of V P .…”

“…Various researchers [7,8] demonstrated that the prediction of petrophysical properties in rocks is often difficult due to the possession of complex textures and petrophysical properties, porosity and permeability, which are dependable and are influenced by the diagenetic processes they may have undergone right from the time of deposition to the late diagenesis set in. Certain studies that combined seismic, petrophysical and petrological data to established useful relationship between velocity and petrophysical of rocks were carried out by Kahraman et al [5] and Yasar et al [9].…”

On the applicability of Gassmann model in carbonates

“…Rafavich et al (1984), Wilkens et al (1984) concluded that porosity is the major factor influencing velocity and that pore-fluid type has no statistically relevant influence. In contrast, Japsen et al (2000) and Assefa et al (2003) measured consistently lower shear modulus for water-saturated samples of low porosity chalk and oolitic grain-packstone, respectively. Their data implies that in carbonate rocks, the assumption of constant shear modulus in Gassmann's theory might not always be valid.…”

Application of Gassmann’s equation for laboratory data from carbonates from Austria

“…Gomez et al also (2007) published data where Gassmann underpredicts the saturated bulk modulus. Approaches for using Gassmann's equations are, for example, discussed by Assefa et al (2003), who published compressional and shear wave velocities for dry and saturated limestone. They carried out measurements of vp and vs under in situ conditions for dry and saturated samples and analyzed the data linked to porosity and pore structure.…”