Pore pressure change affects the stresses distribution and associated strains within the reservoir rocks. Applying poroelastic theory, the coupling of pore pressures and stresses can be represented by the ratio of horizontal stresses over pore pressure changes (i.e. stress path). Likewise, the stress path ratio is a function of Biot's coefficient, dynamic and elastic properties of rocks, and reservoir shape. Stress path ratio can be estimated through analytical, experimental, and numerical approaches. In this study, the dynamic properties of the dry and brine-saturated cubic of Gosford sandstone was measured using a true triaxial stress cell (TTSC). TTSC was equipped with ultrasonic acoustic sensors. Mechanical properties of the sample such as Young's modulus, shear modulus, bulk modulus and Poisson's ratio were obtained. Then the sample was loaded hydrostatically in TTSC with an initial pore pressure. Compressional wave velocities were recorded during any stress increments to estimate the Biot's coefficient based on empirical equations. The study has resulted in various trends for stress paths under different loading conditions.

1. INTRODUCTION

Poroelasticity is very important in the application of geomechanics, especially in the oil and gas industry (Hillis, 2000). Basically, the poroelasticity concept represents the coupling of stresses and strains with pore pressure while the volume of fluids is changing in the pore space of the rock. A reasonable number of studies has been published to determine poroelastic parameters such as Skmepton's and Biot's coefficient (Prasad & Manghnani, 1997, He et al., 2016, Christensen & Wang, 1985, Winkler & Nur, 1982, Todd & Simmons, 1972, Laurent et al., 1993, Chang et al., 2006, Salem, 2001, Franquet & Abass, 1999). Effective stress (σ') and pore pressure (Pp) are related to total stresses (σ) by a factor called Biot (α) (Biot, 1941).

(equation)

This factor can be measured using both static and dynamic approaches. In the static method, matrix and bulk compressibility of the sample must be determined as it is shown in equations 2 and 3. If the matrix has very low porosity, the compressibility of matrix and bulk are equal and the Biot's coefficient approaches to zero (Franquet & Abass, 1999).

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