In-situ stress and fault reactivation associated with LNG injection in the Tiechanshan gas field, fold-thrust belt of Western Taiwan

Abstract

The Tiechanshan gas field located in the fold-thrust belt of western Taiwan was depleted and converted for underground storage of Liquefied Natural Gas (LNG) decades ago. Recently, CO2 sequestration has been planned at shallower depths of this structure. We characterize the in-situ stresses from over 40 wells and assess the leakage potential through fault reactivation in response to LNG-injection increased pore-pressure. The formation pore pressure (Pf), vertical stress (Sv), and minimum horizontal stress (Shmin) are measured from repeated formation tests, density logs, and hydrofrac data including leak-off tests and fluid injection. Formation pore pressures are hydrostatic above depths of 2km, and increase with local gradients of 14.02 and 21.26MPa/km above and below 3.2km, respectively. Extremely high pore pressures (λp=0.8) are observed at depths below 3.8km. Lower than normal pressures (average 9.47MPa/km) are observed in the gas-bearing reservoir of the Talu A-sand. The gradient of Shmin is ∼17.46MPa/km or equivalent to 0.74 of Sv (∼23.60MPa/km). A detailed structure contour map of the top of the A-sand, combined with the measured Shmin and Sv, show that the stress state in the Tiechanshan field is predominantly strike-slip stress regime (SHmax>SV>Shmin). An upper-bound value of the maximum horizontal stress (SHmax) constrained by frictional limits and the coefficient of friction (μ=0.6) is about 27.36MPa/km. Caliper logs from two wells show that the mean azimuth of preferred orientation of borehole breakouts are in ∼028°N. Consequently, the maximum horizontal stress axis tends 118°N, which is sub-parallel to the far-field plate-convergence direction. Geomechanical analyses of the reactivation of pre-existing faults at the depths of the LNG reservoir sand indicate that all faults are stable at the present stress state. Sensitivity analyses indicate that ∼5.9MPa excess pore pressure would be required to cause the optimal oriented f1 fault to reactivate. This corresponds to LNG column heights of ∼760m (density=∼790kg/m3), whereas the height of structural closure of the A-sand does not exceed 400m. Therefore, it is unlikely that LNG injection will reactivate f1 fault.