In-situ stress field and fault reactivation
Speaker: Pei-Chi Yen
Abstract
We want to evaluate the in-situ stress field and the potential risk of fault reactivation in one place, take the 1999 Chi-Chi earthquake in Taiwan and the Mutineer and Exeter fields, Australian North West Shelf for examples. First, the physical properties we should know is obtained from wire-line logs from some wells including P- and S-wave sonic velocity, gamma ray, electrical resistivity, density and temperature. And we can use the physical properties to infer physical mechanisms involved in the displacement during the earthquakes. Furthermore, we can get the average dip of bedding from both cores and FMI (or FMS) logs. Second, after getting the basic geology and geophysical data, we can assess the in-situ stress at the drill sites. Want to know the in-situ stress, we can inferred from leak-off tests (LOTs), borehole breakouts and drilling-induced tensile fractures (DTFs) from borehole FMS/FMI logs, and shear seismic wave anisotropy from DSI logs. And then know the rock strength, the stresses magnitudes and directions that made the breakouts and the fractures.So, we can know the in-situ stress in one place from the tests and logs mentioned above. And the principal stresses are assumed to be the vertical stress (Sv), and a maximum and minimum horizontal stress, SHmax and Shmin respectively. Third, we get the fault geometry from the seismic cross sections to construct the 3D structures of the faults. And we use all the information mentioned above with Trap Tester, a soft ware to draw the 3D Mohr Circles and the Mohr-Coulomb Failure Envelopes, to model the well bore stability and the safe and successful production of the fields. In the end, we can know that if we inject the CO2 or other materials to the wells is safe or not.
Reference
Adrian White, Richard Hills (2004) In-situ stress field and fault reactivation in the Mutineer and Exeter Fields, Australian North West Shelf. Exploration Geophysics 35, 217-223.
J. H. Hung, K. F. Ma, C. Y. Wang, Hisao Ito, Weiren Lin, E. C. Yeh (2009) Subsurface structure, physical properties, fault-zone characteristics and stress state in scientific drill holes of Taiwan Chelungpu Fault Drilling Project. Tectonophysics 466, 307-321.