CALIBRATING CORE, LOG AND SEISMIC DATA TO ASSESS EFFECTIVE STRESS AND HYDROCARBON SATURATION, COOPER BASIN, SOUTH AUSTRALIA

Abstract

Laboratory measurements of acoustic properties of representative rock samples, simulating in-situ effective stress and fluid saturation, provide useful guides for calibrating and interpreting seismic and sonic log data. This paper addresses some of the major implications arising from a petro-acoustic study for the evaluation of reservoir depletion of Cooper Basin gas reservoirs using logs and seismic. Measurement of P- and S-wave velocities on cores under varying pressure conditions reveals that the stress dependency of Cooper Basin rocks is very large, while core porosity remains effectively unchanged.

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 P-wave velocity 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.

The velocity dispersion due to frequency difference between ultrasonic laboratory measurements on cores and theoretical low (seismic) frequency is about 1%, and thus laboratory-measured velocities are comparable with sonic log and seismic data in the Cooper Basin. The potential of the velocity ratio (Vp/Vs) for detection of fluid type and the saturation status at in-situ reservoir effective stress, and prediction of Vs from Vp, are demonstrated for the Cooper Basin rocks. Acoustic measurements on cores, wireline data and seismic modelling are used to predict the expected change in seismic response as the reservoir depletes. Synthetic seismic profiles indicate that the zero-offset reflectivity of a shale to reservoir interface decreases by 28% for a 30 MPa pressure depletion in a typical gas expansion drive reservoir. Such changes should be easily measurable between repeated surveys, suggesting that time-lapse seismic for the monitoring of in-situ effective stress and saturation may have application in Cooper Basin reservoirs. Although these findings refer specifically to the Cooper Basin, the methods used and results of this study may be applicable elsewhere.