Acoustic velocities as a function of effective pressure in low to moderate porosity shaly sandstones, Part 2- Implications for hydrocarbon exploration

Abstract

Hydrocarbon production may cause changes in dynamic reservoir properties including pressure and fluid saturation. Understanding the magnitude of such variations is essential for the exploration of new reserves and optimising the performance of existing fields. Laboratory measurements of acoustic properties of representative rock samples, simulating in situ pressure and fluid saturation, provide a useful guide for calibrating and interpreting seismic and sonic log data. A petro-acoustic study was carried out to investigate factors influencing acoustic properties of Cooper Basin sandstones. This study integrated field data and petrography with laboratory measurements of acoustic properties of representative rock samples. In a previous paper we highlighted experimental data; here we elaborate on implications of these results in predicting rock property and pore fluid type from sonic logs and seismic reservoir monitoring. Analysis of data from acoustic measurements at ultrasonic frequencies on samples from the Cooper Basin reveals that the pressure dependency of the Cooper Basin rocks is very large. The velocity-pressure 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 method is proposed to correct the sonic log reading for pressure variation in the study area. At in-situ reservoir pressure Vs and Vp, are strongly correlated and dry and water-saturated samples show significantly different velocity ratios (Vp/Vs). The VpVs ratio is not affected by porosity and clay content and therefore has potential as a gas indicator in the region. 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.