Stress Path, Pore Pressure and Microstructural Influences on Q in Carnarvon Basin Sandstones

Abstract We have carried out triaxial loading combined with ultrasonic testing on two Carnarvon Basin reservoir sandstones from the Australian Northwest Shelf. These sandstones comprise a quartzitic granular matrix interspersed with kaolinitic clays and glaucony. Helium porosities are 21% and 24% with permeabilities of 203 and 21 mD. Sandstones were tested dry and oil saturated under ambient conditions and stepwise in a dry condition in a triaxial cell to 60 MPa effective confining pressure. Oil saturated samples were tested along three different stress paths:constant pore pressure (5 MPa) reaching effective confining pressures of 60 MPa, then unloading to 5 MPa,constant effective confining pressure (10 MPa) with confining and pore pressures rising incrementally to 65MPa and 55 MPa respectively then decreasing, andconstant confining pressure of 65 MPa with pore pressure rising incrementally from 5 to 60 MPa, then returning to 5 MPa. Ultrasonic measurements comprised full waveform recording of both transmitted P-waves and S-waves at nominal centre frequencies of 800 kHz and 400 kHz respectively. Waveforms were recorded after pore pressure equilibration at each pressure increment. Velocities and the intrinsic attenuation quality factors, Qp and Qs, were calculated from the waveform data using spectral ratio methods. Effective stress coefficients (?) were also calculated. Subtle dependencies of both velocity and Q were found with stress path and particularly with pore pressure. We have explained these results in terms of integrated Biot-Squirt flow models using microstructural evidence to estimate local flow-path lengths and geometries. Preliminary investigations into the validity of effective stress laws in sandstones showed that ??is significantly different from unity. Introduction Understanding the dependence of velocity and attenuation on confining pressure and pore pressure in reservoir sandstones is vital for both extracting subsurface dynamic elastic parameters from seismic data and also to relate laboratory measurements to larger scale, lower frequency data. The influence of pore pressure on velocity and attenuation is of obvious interest to the oil industry as the ability to predict overpressure and lithology pre-drill can have significant impact on well costing. Although the influence of effective stress on acoustic velocity in sandstones has been intensively researched, the direct influence of pore pressure has been investigated to a lesser degree and very little attention has been paid to attenuation or quality factor (Q). As such, the effect of pore pressure conditions on Q is largely unknown, and this would seem to be an oversight considering that Q can be used to evaluate prospective reservoir properties such as liquid/gas content (Klimentos, 1995), pore geometry (Green and Wang, 1986; Shatilo et al., 1998) and permeability (Akbar et al., 1993). Theory suggests that Q will decrease under overpressured conditions as effective stresses are lower. Khaksar et al., (1999) documented P- and S-wave velocities and attenuation in Cooper Basin sandstones and noted that Vp, Vs, Qp andQS all increase with increasing effective confining pressure (P'c) at constant pore pressure (Pf).