Stress measurements using anelastic strain recovery (ASR) method were carried out at the first offshore gas production test site in the eastern Nankai Trough, Japan. Since the ASR method was applied to unconsolidated samples for the first time, laboratory experiments were executed to verify this method. The shore-based verification and calibration experiments were carried out on the same three core samples used for the onsite measurements. Calibration experiments consist of two steps:
K0 consolidation using a triaxial compression test system, and then
ASR measurement using the same equipment as the onsite ASR measurements just after the K0 stress was unloaded.
From raw anelastic strain data just after K0 stress unloading in nine directions of a sample, three principal strains were calculated. All curves varied smoothly and similarly with increasing time. Strain in the dummy sample that does not deform except when it undergoes temperature change showed almost no variation with time, and the measurement system drift was very small relative to the anelastic strains in the samples. This indicates that strains in the samples were anelastic strains induced by K0 stress unloading. These results suggest that the ASR method is suitable not only for hard rocks but also for soft and unconsolidated formations. The orientations of the three principal anelastic strains must be the same as the orientations of the three principal stresses applied to the sample. In these experiments, the maximum principal stress must be the vertical stress. Thus, the deviation of the maximum principle stress orientation from the vertical direction represents the error in the measurements. According to the results of the calibration experiments, the error in orientation in this method was less than approximately 20°.
