ABSTRACT

The propagation of elastic waves in fractured rock is investigated, both theoretically and experimentally. However mechanical properties of fractures rock can be estimated from the computation of seismic velocities. Fractured rock parameter as the unconfining compressive stress c s related to shear stiffness and normal stiffness of fractures is initiated experimentally under uniaxial stress condition using FRAME STAND model 37-5570(CT-713A). An additional coefficient of fitting parameter n has also been introduced to model the experimental data. The developed models are then compared with the empirical nonlinear Goodman’s hyperbolic model, to determine how good the results will fit. However the parameter n provided a best fit for the existing experimental data at certain values of the fractured rock parameters over the entire range of the compressive stress. The observed variability in the value of n suggested that n may be a function of rock fracture properties as well as the previous stresses loading history of the fracture.

1. INTRODUCTION

Seismic velocity measurements are utilized by a large number of geo-science, geo-engineering and geo-resource disciplines. However the dependence of seismic velocities on effective stress and pore pressure dependent has an important effect for petroleum reservoir and geomechanical applications. The dependence of stress on seismic velocity has been confirmed for different rocks experimentally in the laboratories. The laboratory measurements of compression wave velocity ( v p ) and shear wave velocity ( v s ) were performed for each rock sample under different loads. Compression and shear wave velocities are sensitive to change in shear stress on a fractured intact rock and to a rock that has a preexisting fracture. The two types of waves display a shift in frequency content and a change in the amplitude of the wave as the fracture close or open or a new fracture is formed. However the compression wave velocity identified as the first arrival wave is a longitudinal wave; it has the direction of particle motion coinciding with the wave propagation. The shear wave velocity identified as the lower wave velocity has particle motion in the plane perpendicular to the direction of wave propagation. The shear wave velocity has two basic types; the SH-wave which has particle motion parallel to the boundary and the SV-wave which has particle motion perpendicular to both the wave propagation direction and the particle motion of the SH-wave. Thus when shear wave velocity passes through anisotropic fractured petroleum reservoirs, it will split into fast (qs1) and slower (qs2) polarized components (Martin and Davis, 1987), giving evidence about reservoir fracturing character and principal stress direction.

2. BACK GROUND OVERVIEW

2.1. Stress dependent seismic wave velocity· Eberhart et al. (1989) measured compression velocity v p and shear velocity v sfor 64 fully water saturated sand stones samples, with porosity ranged from 2 to 30 percent over an effective pressure range of 0.02 to 0.49 Kbar using a relationship for a given value of porosity and clay content. They derived a general empirical relationship between velocity v and effective pressure P e for each individual rock sample ignoring the

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