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A Petrophysical Study On The Influence Of Effective Stress And Fluid Saturation On Acoustiv Velocities In Sandstones

Khaksar, Abbas

Doctor of Philosophy, 1999

University of Adelaide

Abstract

Hydrocarbon production may cause changes in dynamic reservoir properties including pressure and fluid saturation. Laboratory measurements of acoustic properties of representative rock samples, simulating in situ effective stress and fluid saturation, prove useful for the calibration and interpretation of seismic and sonic log data. The aim of this study was to investigate the relationship between the acoustic velocities and petrographical and petrophysical character of low to medium porosity sandstone cores from the Cooper Basin, South Australia, identified as medium to low porosity gas-bearing shaly sandstones.

Systematic laboratory measurements of the velocity of compressional and shear elastic waves were undertaken on 22 Cooper Basin core samples under dry (air-saturated), partial and fully water-saturated conditions. The measurements were conducted at ultrasonic frequencies under controlled pore pressure and confining stress conditions using a pulse-echo technique. Petrographic information and rock properties including porosity, permeability and capillary pressure curves were necessary to interpret the experimental results. The velocity-effective stress and velocity-saturation relationships for the studied samples were investigated. The role of other rock properties on these relationships was examined and the results compared to other studies. The implications of the experimental results for hydrocarbon exploration and development have been discussed with examples from the Cooper Basin reservoir rocks.

Velocities and quality factors (Vp, Vs, Qp and Qs) in the Cooper Basin sandstones depend on effective stress, defined as Pe = Pc - nPp, with n =/< 1. The value of the effective stress coefficient, n, for Vp and Vs approaches unity at high differential pressures Pd, Pd = Pc - Pp, whereas the pore pressure dependency of Qp and Qs, increases as the differential pressure, Pd increases. The deviation of effective stress coefficient from unity for velocities and quality factors is attributed to the heterogeneity in elastic properties of the main rock components (quartz and clay).

Experimental results show that acoustic velocities in dry and water-saturated samples increase non-linearly with effective stress over the stress range from 5 to 60 MPa. A regression equation in the form of V = A - Be-DP describes empirically the velocity-effective stress relationship for the sampled Cooper Basin rocks under both dry and water-saturated conditions. In this equation; V is the wave velocity and P is effective stress, A is the crack free velocity, D shows the rate of crack closure, and the difference between A and B is the velocity at zero effective stress. The change of velocity with stress is attributed mainly to the closure of microcracks which hardly affects the total porosity but significantly increases the elastic moduli of the rocks. The stress sensitivity of Cooper Basin sandstones is relatively large in comparison with data reported in the literature.

There is no significant correlation between porosity, clay content and stress sensitivity of acoustic velocities at high pressures for the studied samples. The difference in stress sensitivity of the samples may primarily be explained by variations in pore geometry, grain contacts conditions and the distribution and location of clay particles within the sample skeleton (quartz grains). At elevated effective stresses, porosity is the rock property best correlated with velocity. Although considering a clay content term in velocity-porosity transforms in shaly sandstones is essential, the effect of clay minerals on velocities may not adequately be explained if only the volume fraction of clay is taken into account. The insignificant correlation between clay content and velocity, at elevated effective stresses, in the Cooper Basin sandstones is preliminary related to the textural characteristics and the type of clay distribution within the studied samples.

The saturation heterogeneity at pore-scale, which is shown in capillary pressure data, controls the velocity-saturation in partially water-saturated samples. The steady decrease of Vp as saturation decreases from the high saturation range to near irreducible conditions suggests a simultaneous drainage of water from pores with a variety of high to moderate aspect ratios, while microcracks (low aspect ratio pores) retain water. Closure and degree of saturation of the low aspect ratio pores control the velocity-effective stress and velocity-saturation relationships at low saturation and stress conditions.

Comparison of measured velocities with theoretical low frequency predictions by the Biot-Gassmann equation suggests that velocity dispersion is negligible in the studied samples. Thus laboratory-measured velocities may actually be compared with sonic log and seismic data in the Cooper Basin.

The velocity-effective stress relationship obtained from laboratory data is consistent with the sonic log anomaly observed in partially pressure-depleted reservoirs in the Cooper Basin. Neglecting the pressure effect on velocity results in the overestimation of rock porosity by the sonic log in overpressured formations, and underestimation of porosity in pressure depleted zones. A calibration function (dV/dP = 79.35e-0.05P, V in m/s and P in MPa) is proposed to correct the sonic log reading for pressure variation in the study area.

At in-situ reservoir effective stresses, Vs and Vp are strongly correlated and dry and water-saturated samples show significantly different velocity ratios (Vp/Vs). The Vp/Vs ratio is not affected by porosity and clay content and therefore has potential as a gas indicator in the study area. The strong stress sensitivity and the distinct Vp/Vs values for dry and water saturated Cooper Basin cores suggest that the dynamic changes in pressure and saturation of the reservoir rocks may also be detectable from acoustic impedance or travel time at seismic and sonic log frequencies.

Future studies should focus on the velocity-stress relationships for different rock types under different saturation conditions. A quantitative approach is needed to link the acoustic and petrographical properties in partially saturated rocks. The use of capillary pressure curves and wettability data in describing the scale of saturation heterogeneity and their relationship to acoustic velocities may provide useful information on the saturation status of reservoir rocks from seismically derived interval velocities.


Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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