ABSTRACT

For an intended application of stress measurement at great depth in the case of scientific drilling, a core-based, anelastic strain recovery (ASR) method was described in this paper. Then, an example of the application of the ASR method to an active fault drilling project was shown. The anelastic strain of a drilled core specimen in nine directions, including six independent directions, was measured after its in-situ stress was released. The core specimen used was sandstone taken from a depth of 592m. Acquired anelastic strains reached several hundred microstrains that could be used for a three-dimensional analysis resulting in the determination of orientations and the estimation of magnitudes of the principal in-situ stresses. The obtained principal orientations and magnitudes can be considered to be valid. Consequently, it can be concluded that the anelastic strain recovery measurement is well suited for the task of directly determining the orientations of principal in-situ stresses and to estimate the magnitude of the stresses at great depth.

INTRODUCTION

Determination of both orientation and magnitude of three-dimensional in-situ rock stress at great depth is very important in various geoscience and geoengineering problems. For example, in order to improve our understanding of the mechanism of earthquake generation and propagation, it is of critical importance to determine the spatial distribution of in-situ stress and, in particular, to determine the shear stress and normal stress on the fault planes which unstably slipped and caused the earthquake. Unfortunately, there is no foolproof method by which the magnitudes and orientations of the three-dimensional in-situ stress can be reliably measured at great depth, although various field and laboratory measurement techniques have. been proposed. In case of a scientific deep drilling project, we suggest that combined applications of borehole methods and core-based methods should e employed. A simple and inexpensive method to determine in-situ stress from anelastic strain recovery (ASR) measurement of oriented cores, which is Considered as having a relatively explicit theoretical basis in comparison to other core-based methods, was proposed by Voight (1968) and Teufel (1983). Originally, vertical stress was assumed to be one of the three principal stresses, and only two-dimensional measurement was conducted. Subsequently, this method was theoretically extended to a three-dimensional version by Matsuki (1991). However, the three-dimensional method has found little practical application (Matsuki and Takeuchi, 1993; Lin et al., 2003). This is why the present study was undertaken and carried out in order to verify whether the ASR method in three dimensions can be applied for determining the in-situ stresses at great depth in a scientific drilling project and to acquire skills in sample preparation and application of appropriate measurement techniques.

In this paper, the theory of anelastic strain recovery method and its experimental techniques for determining the three-dimensional in-situ stress will be described. Next, the results of a case study where the ASR method was applied to a sandstone core sample retrieved from a depth of about 600m to determine the orientations and to estimate the magnitude of its three-dimensional in-situ stress will be presented.

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