永和山構造現地應力與注氣引發斷層再活動評估;In-situ Stress and Fault Reactivation Potential in Response to Fluid Injection in Yunghoshan Structure

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Title:	永和山構造現地應力與注氣引發斷層再活動評估;In-situ Stress and Fault Reactivation Potential in Response to Fluid Injection in Yunghoshan Structure
Authors:	林器暉;Lin,Qi-hui
Contributors:	應用地質研究所
Keywords:	永和山;現地應力;地質力學模擬;斷層再活動;Yunghoshan;in-situ stress field;geomechanical modelling;fault reactivation
Date:	2014-08-08
Issue Date:	2014-10-15 17:19:34 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究利用解析方法模擬二氧化碳注入地層造成孔隙液壓上升，作用在斷層面上的有效正應力減少導致斷層再活動之可能性評估。首先蒐集永和山地區鑽井之井下地質報告及電測資料，求得現地的孔隙液壓、鉛直應力、最小水平應力及最大水平應力隨深度的變化，並利用井徑電測(Caliper log)判釋最大水平應力的方位；研究並蒐集井下地質、震測剖面及構造剖面，利用GOCAD軟體建構永和山氣田地下地層及斷層的三維構造，再使用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)。分析結果顯示永和山地區的應力梯度分別為鉛直應力23.3 MPa/km，最小水平應力18.35 MPa/km，最大水平應力根據臨界應力斷層摩擦理論(μ=0.6)求得34.03 MPa/km。依據井徑電測判釋的最大水平應力方位在桂竹林層平均為147°，打鹿砂層為157°。假設μ=0.6情況下，藤坪斷層相對於鹿廚坑斷層與龍港斷層，滑移潛勢最高，但低於0.6，所以在目前現地應力作用下屬於穩定狀態。藤坪斷層在滑動前所能承受的臨界孔隙液壓在魚藤坪砂岩(約海面下1240公尺)為7.6 MPa，在打鹿砂層(約海平面下3180公尺)為22.2 MPa。除了應力場及斷層幾何的因素外，由於打鹿砂層內含天然氣的柱壓較靜水壓為小，所以可承受相對較大的臨界灌注液壓。針對影響藤坪斷層再活動的各項參數進行參數敏感度分析，結果顯示藤坪斷層在魚藤坪砂岩及打鹿砂岩的深度，敏感度最高皆為最小水平應力。以蒙地卡羅情境分析結果，在魚藤坪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3.35 MPa，相當於二氧化碳柱高481公尺(假設超臨界狀態二氧化碳密度為0.7 g/cm3)。以魚藤坪砂岩在永和山背斜的構造起伏厚度約為250公尺，表示在不超過滿溢點(spill point)情形下進行二氧化碳封存所增加的地層壓力，對藤坪斷層而言，仍然處於穩定狀態；相同地，在打鹿砂岩灌注二氧化碳，最保守灌注壓力為12.06 MPa，相當於二氧化碳柱高1749.30公尺，以打鹿砂岩的構造起伏厚度約為800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，不致造成藤坪斷層重新活動。 ;An analytical method is applied to simulate CO2 injection into reservoir, which could generate high fluid pressure and trigger slip on reservoir-bounding faults. The magnitudes of vertical stress (SV), formation pore pressure (Pf), minimum horizontal stress (Shmin) and maximum horizontal stress (SHmax) are obtained from drilling and down-hole logging data, and 3-D subsurface structure of the Younghoshan filed is modeled using GOCAD software. The risk of fault reactivation is assessed using 3DStress and Traptester. Under current in-situ stress, Tengping fault has higher slip tendency compared to Lungkang and Luchukeng faults, but still less than frictional coefficient of 0.6 a common threshold value for motion on incohesive faults. At the depth of the Yutengping sandstone (1240 mbsl), approximately 7.6 MPa of excess pore pressure would be required to cause the Tengping fault to slip, and at deeper Talu sandstone, approximately 22.2 MPa of excess pressure would be required to cause the Tengping fault to slip. In addition to the stress field and fault plane geometry, the natural gas-bearing Talu sandstone has formation pressure less than the hydrostatic, would take in more fluid injection and pore pressure. Sensitivity analysis of the parameters affecting the slip potential of optimally oriented Tengping fault indicates that Shmin has highest effect on the critical pressure perturbation at the depths of Yutengping and Talu sandstones. Scenarios tests via over 10,000 Monte Carlo Simulations show that in the pessimistic case, the critical perturbation pressure at the depth of the Yutengping sandstone to initiate slip on the Tengping fault is nearly 3.35 MPa (corresponds to a CO2 column height of ~ 481 m). It can be expected that the injection of CO2 will not reach the 3.35 MPa because the structural closure of Yutengping sandstone controls the accumulation of CO2 of 250 m or a pressure of 1.39 MPa. Similar results also indicate that CO2 injection in the deeper Talu sand will not compromise stability of the Tengping fault.
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