| Summary: | 碩士 === 國立中央大學 === 應用地質研究所 === 102 === This study uses downhole geophysical logs and cores from a borehole drilled by Taiwan Power Company (TPC) in Changhua Coastal area to estimate in-situ stresses that provide a basic data for geologic sequestration of carbon dioxide and safety assessment. The gradient of vertical stress (Sv) and pore pressure (Pp) are derived from formation density and sonic logs, respectively. Formation Micro-Imager (FMI) log is used to determine orientation, length and width of borehole breakouts following the criteria of World Stress Map (WSM) project. Combined with unconfined compressive strength (C0) from rock experiments and minimum horizontal stress (Shmin) measured from leak-off tests, the maximum horizontal stress (SHmax) can be estimated. Orientations of maximum horizontal stresses were determined from both four-arm calipers and Formation Micro-Imager (FMI).
Formation pressures estimated from shale transit time (STT) indicate a hydrostatic pressure or normal compaction in the sedimentary strata from surface to 3000 m deep. Borehole calipers and breakouts show that orientation of the maximum horizontal stress trends from NW-SE at shallow to EW at depth. Elliptic washouts were added in interpretation below the depth of 1960 m because of relatively few breakouts. Calculated stress gradients include vertical stress, 21.9 MPa/km, minimum horizontal stress, 14.1 MPa/km, and maximum horizontal stress, 20.3 MPa/km. The result shows that subsurface in-situ stress state in the Changhua Coastal area is mainly in normal fault regime (Sv > SHmax > Shmin).
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