| Abstract |
We have proposed a new method to estimate in situ three-dimensional stress fields using data from drillinginduced tensile fractures (DTFs) observed in a single inclined borehole (Okabe et al., 1998). A DTF is a longitudinal crack consisting of many small parallel cracks which are oblique to the borehole axis. A DTF is characterized by its circumferential position (É?mD) along the borehole surface and the inclination (É¡m) of the small cracks with respect to the borehole axis. Based on the variation of É?mD and É¡m as functions of borehole orientation which changes with depth, an inverse problem is formulated to estimate the threedimensional stress field. We have previously applied this method to well TG-2 located in the northern area of Japan and have estimated the stress field. In the case of TG-2, thermal stresses may be significant since TG-2 is located in the rim of the Matsukawa geothermal field. However, because we did not account for the effects of thermal stresses, only relative stress magnitudes were determined. We recently included thermal stresses and have performed the inversion again. In this paper, we discuss the results and study critically stressed shear fractures observed in the FMI log, using the estimated stress field to investigate the feasibility of identifying permeable fractures. The stress field interpreted by the data from borehole TG-2 shows that the averaged directions of the principal axes of stress are N 45.7Åã W Å} 9.0Åã, 13.9Åã NW Å} 6.0Åã for É–1 (the maximum compressive principal stress), N 51.0 ÅãE Å} 11.4Åã, 20.1Åã SW Å} 4.6Åã for É–2 (the intermediate compressive principal stress) and S 14.1Åã W Å} 8.1Åã, 64.5Åã SE Å} 5.2Åã for É–3 (the minimum compressive principal stress) and the magnitudes of É–1, É–2 and É–3 are 22.7 MPa Å} 1.4, 16.4 MPa Å} 0.6 and 12.4 MPa Å} 0.5, respectively. All three principal stresses have been determined with reasonable certainty. The three stress directions from the result agree with the previous result for the case without consideration of thermal stresses and direction of É–1 obtained from the inversion agrees with that obtained by the stress relief method at a nearby field. Interpretation of critically stressed shear fractures based on the stress field derived above shows that there are not many fractures stressed critically. Stresses normal to the bedding planes are mostly orientated to É–3 and it is possible that fluid flows through the bedding planes while injection PTS (Pressure/Temperature/Spinner) logging. Interpreted critically stressed shear fractures are approximately NSÅ`EW strikes and low dip angle. The strikes agree with those of the three faults estimated in the Matsukawa geothermal field. |