鐵砧山現地應力場與斷層再活動分析 ; In-Situ Stress Field and Fault Reactivation Analysis in the Tienchanshan Field, West-central Taiwan

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Title:	鐵砧山現地應力場與斷層再活動分析;In-Situ Stress Field and Fault Reactivation Analysis in the Tienchanshan Field, West-central Taiwan
Authors:	汪蘭君;Lan-jyun Wang
Contributors:	應用地質研究所
Keywords:	參數敏感度分析;現地應力量測;斷層重新再活動;Sensitivity analysis;Fault reactivation;In-situ stress measurement
Date:	2010-07-26
Issue Date:	2010-12-09 10:52:00 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究利用鐵砧山地區鑽井之地層密度電測、地層測驗、滲漏及擠壓水泥試驗資料分別求得區域地層垂直應力(Sv)、地層孔隙壓力(Pf)及最小水平應力(Shmin)隨深度的分佈，並利用臨界斷層破壞理論，估算最大水平應力(SHmax)。結果顯示，在鐵砧山地區打鹿頁岩內之打鹿砂層(TT-1A層)平均深度處(-2750公尺)，垂直應力梯度為23.60 MPa/km(1.04 psi/ft)，最小水平應力梯度為18.70MPa/km(0.83 psi/ft)，其地層孔隙壓力為靜水壓的90%，為靜岩壓的40%；而在打鹿頁岩層以下(約-3000公尺)，地層孔隙壓力則約為靜水壓的135%，呈現過壓狀態。再者，由前述計算的地層孔隙壓力、最小水平應力以及地層垂直應力的資料，假設臨界斷層滑移的摩擦係數為0.6，可估算出最大水平應力梯度在走向滑移應力系統(SHmax > Sv >Shmin)的上限約為 29.91 MPa/km(1.32 psi/ft)。利用井徑電測判釋在打鹿頁岩層平均裸孔伸張(最小水平應力方向)的方位角約為35o；換言之，最大水平應力方位角為125o，此一角度與台灣中西部麓山帶地區地震震源機制解以及位於台中大坑車籠埔斷層科學鑽探井所求得的最大壓縮應力方向一致。最後，透過地質力學模型模擬臨界孔隙壓差，可得知於鐵砧山地區打鹿頁岩內之打鹿砂層(T-1A層)深度附近的斷層中，以東西走向斷層最容易重新活動，西北-東南走向斷層次之，東北-西南走向的斷層較為穩定。針對鐵砧山最適重新活動位態的斷層(f1斷層)進行參數敏感度分析，結果顯示敏感度最高者為最小水平應力，敏感最低者為垂直應力。以蒙地卡羅法進行情境分析之模擬結果顯示，在不引發f1斷層再活動之條件下，最低之灌注壓力至少為5.92MPa，相當約石油柱高764.9公尺；若灌注二氧化碳，則相當約862.9公尺。相較於打鹿砂層構造閉合約400公尺(-2500至-2900公尺)之儲集層垂深，顯示本區斷層在天然條件下灌注氣體仍相當穩定，應無斷層再活動之餘慮。The in-situ stresses in the Tiechanshan field were determined using petroleum exploration database. The magnitudes of vertical stress (Sv), formation pore pressure (Pf) and minimum horizontal stress (Shmin) were obtained from formation density logs, repeat formation tests, and hydraulic fracturing including leak-off tests and fluid injection inside casing, respectively. The magnitude of maximum horizontal stress (SHmax) was constrained by both subsurface structural contour map at reservoir depths and frictional limit of critically stressed faults. Results show that Sv gradient is about 23.6 MPa/km (1.04 psi/ft), and Shmin gradient is 18.70 MPa/km (0.83 psi/ft). At the average depth of T-1A Sandstone (~2750 mbsl), the formation pore pressure was about 10% lower than the hydrostatic pressure and equivalent to 0.4 of the overburden pressure. Below the Talu Formation (~3 km below sea level), the formation is transitional from normal to overpressure with amount deviated from normal trend up to 35%. The upper bound of SHmax constrained by strike-slip fault stress regime (SHmax>Sv>Shmin) and coefficient of friction (μ=0.6) is about 29.91 Mpa/km (1.32 psi/ft). The average azimuth of borehole breakouts (parallel to the azimuth of Shmin), judging from four-arm caliper log in Talu Formation, is oriented about 35o; in other words, the maximum horizontal stress is oriented about 125o. Faults with azimuths of E-W and NW-SE are far easier to reactivate than those trending NE-SW in T-1A Sandstone under in-situ stress state. Sensitivity analyses of parameters on the slip potential of optimally-oriented f1 fault indicate that Shmin has higher effect on the critical pressure perturbation and Sv is the lowest one. Scenarios tests via over 10,000 Monte Carlo Simulations show that the lowest critical perturbation pressure is 5.92MPa, which correspond to oil column height of ~765 m or carbon dioxide column height of ~ 863 m. Since the structural closure of T-1A sandstone is less than 400 m (2500-2900 mbsl), it is apparent that the injection of LNG (liquid natural gas) will not compromise the f1 fault stability.
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