本研究利用台灣電力公司在彰濱地區鑽井之井下電測資料及鑽取之岩心來推估彰濱地區的地下現地應力場，以提供二氧化碳注儲及安全性評估的基礎資料。利用地層密度電測及聲波電測資料，可分別求出鉛直應力(Sv)及地層孔隙液壓(Pp)。另依據World Stress Map (WSM)對井孔崩落(Borehole breakout) 的定義，由井壁電阻影像(Formation Micro-Imager, FMI)可量測井孔崩落發生的方位、長度及寬度，再收集由單軸試驗所測得之單軸抗壓強度(C0)及水破試驗可推估之最小水平應力(Shmin)。最後利用Zoback(2007)所提出之公式可求出最大水平應力值 (SHmax)。最大水平應力方向則可結合四臂井徑電測分析和井壁電阻影像判釋獲得。 由聲波電測推測地層的壓密作用，顯示在深度0到3000公尺之間處於正常壓密 (Normal compaction) 狀態；換言之，此井的地層孔隙液壓梯度相當於靜水壓梯度。利用井徑分析結果顯示此區的最大水平應力方位以西北東南向為主，但在深度1960公尺以下，由於井孔崩落個數不多，改以橢圓形的坍孔(elliptic washout)輔助判斷，但最大水平應力方向由西北東南向轉變成東西向。統整井下電測及岩心所得之各應力梯度值如下：鉛直應力梯度約為21.9 MPa/km，最小水平應力梯度約為14.1 MPa/km，最大水平應力梯度約為20 MPa/km，得到Sv > SHmax > Shmin，顯示目前彰濱地區的現地應力狀態為以正斷層為主的應力系統。 ;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).
