Fault seal first-order analysis - SW Hub: structural analysis for CO2 containment risk for the SW Hub, Western Australia

[摘要] The Mandurah Terrace in the onshore central Perth Basin has been proposed as an environmentally suitable site for CO2 injection i.e. the SW Hub) with the reservoir and seal represented by the Triassic lower Lesueur Sandstone (Wonnerup Member) and the and the Late Triassic upper Lesueur Sandstone (Yolgerup Member) and the Early Jurassic basal Eneabba Formation, respectively.Prior investigations in the SW Hub indicate that multiscale faults affect the target CO2 storage reservoir and the potential top seals and it is known that changes in the pore pressure and stress field cause by fluid injection could potentially alter the initial seal of a reservoir by either overcoming the faults membrane seal capacity leading to across- or along-fault circulation of CO2 or by triggering slip on pre-existing faults leading to the potential along-fault circulation of CO2. This project relies on the available subsurface data and carries out a first-order assessment of the CO2 containment potential for the SW Hub. A new geological model consistent with the integration of the latest 2D seismic survey with the vintage seismic surveys and available geophysical data has been built and integrates five stratigraphic horizons tied to formation tops in Harvey-1 (Neocomian UC, top basal Eneabba Shale, top Yalgorup, top Wonnerup and top Sabina sandstone) and 13 main faults that can be correlated between at least two seismic lines and that show constancy in dip, strike orientation and offset. This represents a first-order geological model and the acquisition of additional seismic and well data is critical to constrain the structural framework.The membrane fault seal capacity has been assessed using the Shale Gouge Ratio (SGR) predictive algorithm that can be calibrated to derive a fault-seal failure envelop and to calculate a maximum fluid column height trap by a fault without leaking. Three across-fault CO2 migration scenarios are investigated: (1) between two self juxtaposed Wonnerup Members, (2) between the Wonnerup Member and a juxtaposed thief zone in the Yalgorup Member and (3) between the Wonnerup Member and the Eneabba Formation lying above the Yalgorup Member. The likelihood of lateral migration of CO2 across faults between the Wonnerup Member and any interbedded sandstone (i.e., thief zones) in the Yalgorup Member can be locally high to the south of the SW Hub (faults F1, F11 and 14). The likelihood of lateral migration of CO2 across faults within the Wonnerup Member can be also locally high to the south of the SW Hub with potential of westward migration beyond F1 if the CO2 column exceeds the local offset. One fault (F10) juxtaposes the Wonnerup with the overburden Eneabba, however SGR values on the fault plane suggest an average to low likelihood of across-fault migration and the supported CO2 column before breaching the membrane seal is between 110 and 1100m.The relationship between the modelled faults and the present-day stress field has been investigated to define critically stressed fault segments most likely to be forced into failure with pore-pressure bulid-up and be associated with an increase in structural permeability (i.e., along-fault flow). The slip tendency values are minimum for faults striking parallel to SHmax and Shmin (105 and 195 respectively) and increase for strike orientation 50-80 (SW-NE) and 130-160 (SSE-NNW). However the slip tendency magnitude for the SSE-NNW-oriented faults in the SW Hub are low (typically between 0.15 and 0.3) suggesting a low risk of fault failure under the present-day stress. The critical pore pressure perturbation required to induce failure on a particular fault orientation is primarily depth-dependent with an increase in depth resulting in an increase in stresses that will prevent failure to occur. Therefore the smallest critical pore pressure perturbations required to reach failure stress are located to the north of the SW Hub along faults F1, F2 and F10 and once converted to an equiv

[发布日期] 2013-05-08 [发布机构] CSIRO

[学科分类] 地球科学(综合)