ABSTRACT:
We have investigated the mechanical effect of pore fluid by subjecting cores of Castlegate sandstone to several cycles of pore (Pp) and confining pressures (Pc). Acoustic p- and s-velocities and the volumetric deformation of the sample were measured during each test. The results were interpreted according to the formalism of the effective stress principle. The effective stress coefficient for axial and radial p-waves varied from 0.10 to 0.88 and 0.57 to 0.86, respectively. The effective stress parameters for s-waves and bulk volumetric strain ranged from 0.75 to 1.12 and 0.60 to 0.83, respectively. Increasing Pc and net stress caused a reduction in the effective stress parameters for acoustic and elastic properties. Increasing pore pressure caused the p-wave and volumetric strain coefficients to increase. The effect of axial load was to increase the s-wave and volumetric coefficients and to decrease the p-wave coefficient.
1 INTRODUCTION
Any process related to petroleum production requires knowledge of the rock properties under some particular combination of confining pressure (Pc) and pore pressure (Pp) (Warpinski & Teufel 1993). Knowledge about the combined effect of external pressure and pore pressure may therefore be very useful. For example, it may be possible to relate changes in shear and compressional velocities to stress and pore pressure in such a way that Vp,s=f(Pc,Pp). Subsequently, one may use this function for predicting the distribution of pore pressure in the subsurface (Eberhardt-Phillips et al. 1989, Sayers et al. 2002, Siggins & Dewhurst 2003). Experimental studies have demonstrated that the parameter n equals one for soft sediments and hence, the Terzaghi law applies (Terzaghi 1943, Morrow et al. 1992). However, n may be considerably less than one for cemented rocks, such as sandstones (Todd & Simmons 1972, Prasad & Manghani 1997, Siggins & Dewhurst 2003).
