P-wave velocities have been measured under varying stress conditions in shales with porosities ranging from 3 % to above 50 %. The velocities show a decreasing trend with increasing porosity. Stress sensitivity is linked to porosity, so that the velocities in the lower porosity shales exhibit stress dependence, while the high porosity shales are almost stress insensitive. Pwave anisotropy ranges from 0 - 20 %, increasing with decreasing porosity. This anisotropy is found to be predominantly of lithological origin.
Based on these observations, a rock physics model was established. The model treats the shale as consisting of solid mineral grains surrounded by a mixture of bound and free water. Mathematically, the approach builds on the classical work of Hashin-Shtrikman. Physically, the bound water is confined to the clay mineral surfaces and to intragranular space. Bound water elastic properties are at this stage adjustable parameters of the model, that may be calibrated through further experimental observations. By implementing complex elastic parameters for the bound water in the model, observed frequency dependence of velocities in shale can be properly described.
Shales play an important but often neglected role for seismic interpretation. Obviously, since seismic waves travel most of their time in the overburden, which consists largely of shale, the reflected signals from the reservoir also depend strongly on the overburden. On one side, for the purpose of reservoir characterization, this is a nuisance, which calls for procedures for correcting the seismic response. An example would be to improve AVO (Amplitude Vs Offset) interpretation by correcting for anisotropy in the shale layers. Also, significant changes may occur in the overburden during reservoir depletion , which may affect time-lapse (4D) seismics. Such changes are important to understand and potentially correct for in order to improve the picture of what is happening inside the reservoir. On the other hand, the seismic signals carry valuable information about the overburden. Such information may be utilized for instance in detection of high pore pressures or in estimation of mechanical properties for improving borehole stability during drilling .
A necessary step in order to improve seismic interpretation is to improve the rock physics knowledge. In this Paper, we address wave velocities in shale by presenting a set of laboratory experiments with well preserved shale samples from oil fields, and by introducing and applying a theoretical model to describe the observed features.
2. EXPERIMENTAL OBSERVATIONS
2.1. P-wave velocity measurements in a triaxial set-up
P-wave velocities have been measured under varying stress conditions in shales with porosities rang-ing from 3 % to above 50 %. Table 1 lists the clay content and porosity (from weight analysis of water loss during drying) of the shale samples tested. The clay content is thought to be high enough that all the shales have a continuous and hence load-bearing framework of clay minerals. In all the shales tested, XRD analysis shows that a substantial part of the constituent clay is in the form of swelling (mixed layer or smectite) minerals.