ABSTRACT

INTRODUCTION

The presence of a free-moving fluid in rock modifies its mechanical behavior. In particular, it transforms an otherwise time-independent problem to a time-dependent one through fluid diffusion. The material behaves as a stiffer one under fast than for slow loading. The induced fluid pressure generally brings stress relieve or volume dilation to the solid material. The presence of fluid pressure can also modify the Terzaghi effective stress, which is keenly tied to the onset of some failure mechanisms. The purpose of this paper is to review through examples the manifestation of the pore pressure effect (poroelasticity) in a number practical occasions. A good grasp of these physical mechanisms can equip engineers with the necessary insight to perform prognosis of poroelastic effect for their problems on hand.

POROELASTIC COEFFICIENTS

Based on the continuum mechanics theory it is well established that an isotropic elastic solid can be characterized by two independent material coefficients. For poroelastic material, a continuum mechanics analysis reveals that there exist four independent constitutive coefficients. Two of the poroelastic coefficients, the shear modulus G and the drained bulk modulus K, are tied to elastic properties of the solid skeleton. The third coefficient, known as the Blot effective stress coefficient a, is also a property of the solid constituent. Only the last coefficient, the Biot modulus M, incorporates the fluid elastic property.

For general anisotropic poroelasticity, there are 28 coefficients. Twenty one of them are elastic constants of the drained skeleton. Six are Biot effective stress coefficients in a symmetric tensor form aij. The last coefficient M is again the only one which contains fluid property.

Biot effective stress coefficient: It is established that the volumetric deformation of a fluid saturated rock is proportional not to the total compressive stress AP, but to the 'Blot effective stress' Äp -aÄp. It is associated with the bulk modulus of the skeleton, K, and that for the solid constituent, Ks, as follows:

[Equation available in full paper]

Hence a measurement of Ks, which can be accomplished by an unjacketed test in the laboratory, gives an indirect estimate of a. This statement is also true for anisotropic material: a measurement of Ks, combined with the drained anisotropic elastic moduli, defines the whole tensor. This fact is quite convenient for the laboratory measurement of anisotropic poroelastic properties.

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