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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/68375

    Title: 錦水構造現地應力與注氣誘發斷層再活動分析;In-situ Stress and Fault Reactivation Potential in Response to Fluid Injection in Chinshui Structure
    Authors: 宋家宇;Sung,Chia-yu
    Contributors: 地球科學學系
    Keywords: 錦水氣田;二氧化碳;現地應力
    Date: 2015-08-18
    Issue Date: 2015-09-23 11:30:39 (UTC+8)
    Publisher: 國立中央大學
    Abstract: 錦水氣田位於台灣地質分區中的西部麓山帶,其含氣地層之打鹿砂岩目前天然氣已趨於耗盡,因此未來將評估實施CO2激勵採收,以提升舊有氣田之採收率。將氣體注入於地下地層之中將導致地層內孔隙液壓升高,伴隨有效應力的下降,可能使得岩石產生水力破裂並造成斷層剪力破壞而重新活動,進而導致氣體沿斷層洩漏之風險。本研究以定量方式估算研究區域之現地應力狀態並利用解析方法模擬二氧化碳注入地層造成孔隙液壓上升導致斷層再活動之風險。首先由密度電測、聲波走時和現地的孔隙液壓、地層滲漏試驗和鑽井時漏泥的泥漿比重,分別求得地下現地應力狀態包括鉛直應力 (Sv)、地層孔隙液壓(PP)及最小水平應力(Shmin)隨深度的分佈。並利用前述資料以及安德森臨界摩擦理論推求最大水平應力(SHmax)。分析結果顯示錦水地區的應力梯度分別為鉛直應力23.02 MPa/km (1.02 psi/ft),最小水平應力18.05 MPa/km (0.80 psi/ft),地層孔隙液壓於海平面下深度2公里以上為靜水壓狀態,2公里以下為超孔隙液壓狀態,於中地塊地層孔隙液壓梯度2公里下增為19.47 MPa/km (0.86 psi/ft) ,而南地塊地層孔隙液壓梯度2公里下增為16.62 MPa/km (0.73 psi/ft),此外於打鹿砂層深度由於此地層含有天然氣,而天然氣之密度較低於地層流體,因此獲得較低之孔隙液壓梯度為12.95 MPa/km。最大水平應力根據臨界應力斷層摩擦理論(μ=0.6)於走向滑移斷層應力機制下求得於中地塊為15.07 MPa/km (0.67 psi/ft),南地塊則為18.55 MPa/km (0.82 psi/ft)。應力方位依據井徑電測判釋出井孔崩落方位,進一步獲得最大水平應力方位平均為170°,而其結果與遠場大地構造擠壓方向一致。
    地質力學模擬利用3DStress和Traptester軟體計算斷層面上各區塊因注氣引發重新滑動的潛勢(slip tendency, Ts)及臨界灌注液壓值(Pcp)。分析結果顯示於現今應力場作用下,中地塊之F2斷層所具有之滑移潛勢最高,其最高潛勢可達0.39,但其滑移潛勢值仍低於理論斷層發生再活動之值0.6,表示在現今大地應力作用下,斷層屬於穩定狀態。臨界孔隙液壓模擬結果顯示,當斷層摩擦係數為0.6時,於打鹿砂岩以F2斷層最先破壞,且斷層能承受之最低灌注量為14.5 MPa。針對影響F2斷層再活動的各項參數進行參數敏感度分析,結果顯示F2斷層在打鹿砂岩的深度,敏感度最高之參數為最小水平應力。以蒙地卡羅情境分析結果,在打鹿砂岩注氣不引發此斷層區塊滑動,可承受最保守的灌注壓力為8.85 MPa,相當於二氧化碳柱高1290公尺(假設超臨界狀態二氧化碳密度為0.7 g/cm3)。以打鹿砂岩的構造起伏厚度約為750公尺,表示在此條件下進行二氧化碳激勵天然氣採收所增加的地層壓力,將不致造成F2斷層重新活動。
    ;The Chinshui gas field located in the fold-thrust belt of western Taiwan was a depleted reservoir. Recently, CO2 sequestration has been planned at shallower depths of this structure. CO2 injection into reservoir will generate high fluid pressure and trigger slip on reservoir-bounding faults. We present detailed in-situ stresses from deep wells in the Chinshui gas field and evaluated the risk of fault reactivation for underground CO2 injection. The magnitudes of vertical stress (Sv), formation pore pressure (Pf) and minimum horizontal stress (Shmin) were obtained from formation density logs, repeat formation tests, sonic logs, mud weight, and hydraulic fracturing including leak-off tests and hydraulic fracturing. The magnitude of maximum horizontal stress (SHmax) was constrained by frictional limit of critically stressed faults. Results show that vertical stress gradient is about 23.02 MPa/km (1.02 psi/ft), and minimum horizontal stress gradient is 18.05 MPa/km (0.80 psi/ft) or equivalent to 0.8 of Sv. Formation pore pressures were hydrostatic at depths 2 km, and belowe depths 2 km, formation pore pressure increase with a gradient of 19.47 MPa/km (0.86 psi/ft) in the middle block and 16.62 MPa/km (0.73 psi/ft) in the south block. The ratio of fluid pressure and overburden pressure (λp) below depth 2 km is 0.59. Lower than normal pressures (average 12.95 MPa/km) are observed in the gas-bearing reservoir of the Talu-sand. The upper bound of maximum horizontal stress constrained by strike-slip fault stress regime and coefficient of friction (μ=0.6) is about 15.07 MPa/km (0.67 psi/ft) in the middle block and 18.55 MPa/km (0.82 psi/ft) in the south block. The orientation of maximum horizontal stresses was calculated from four-arm caliper tools through the methodology suggested by World Stress Map (WMS). The mean azimuth of preferred orientation of borehole breakouts are in about N80。E. Consequently, the maximum horizontal stress axis trends in N170。E and sub-parallel to the far-field plate-convergence direction.
    Geomechanical analyses of the reactivation of pre-existing faults was assessed using 3DStress and Traptester software. Under current in-situ stress, the F2 fault in middle block has higher slip tendency, but still less than frictional coefficient of 0.6 a common threshold value for motion on incohesive faults. At the depth of the TL-sand, approximately 14.5 MPa of excess pressure would be required to cause the F2 fault to slip. Sensitivity analysis of the parameters affecting the slip potential indicates that Shmin has highest effect on the faults stability. Scenarios tests indicate that 8.85 MPa excess pore pressure would be required to cause the optimal oriented F2 fault to reactivate. This corresponds to CO2 column heights of 1290 m, whereas the height of structural closure of the TL-sand does not exceed 750 m. The results also indicate that CO2 injection in the Chinshui gas field will not compromise the stability of faults.
    Appears in Collections:[Graduate Institute of Geophysics] Electronic Thesis & Dissertation

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