Abstract This paper presents relationships for estimating horizontal stresses based on the assumptions that the in situ stress state in a petroleum basin is controlled by the bounding normal or thrust faults at a limit equilibrium and that the fault block is linear elastic and plane strain condition applies in the direction parallel to the strike of the fault. These relationships are an extension of an earlier study and include the effect of residual friction angles on the estimation of horizontal stresses at depth. The result shows that re-orientation of the minimum principal stress is possible after faulting depending on the Poisson's ratio of the formation. Predictions based on the relationships are compared with the stress data obtained in normal and thrust fault conditions as well as with the change in the minimum horizontal stress induced by the pore pressure depletion. The results show that to match the field stress data, a relatively low residual friction angle (100 – 300) on the fault is required. This is further supported by the numerical modelling of the in situ stresses in the Cusiana field in Colombia, and is consistent with the residual friction angles measured in laboratory or back-calculated based on earthquake mechanism. P. 59
Integrity failure of wells in gas resource developments poses a potential risk to groundwater resource quality and quantity by enhancing the connectivity and fluid migration between a gas reservoir and overlying aquifers, which may support groundwater dependent ecosystems or be used as a water source. This study assessed the potential for impact on groundwater resources from flow pathways created by well integrity failure in decommissioned coal seam gas (CSG) wells, deep water bores or gas wells repurposed as water bores and legacy coal exploration drillholes using an example based on a proposed CSG development near Narrabri, Australia. The study only considered the flow of water. A multi-stage screening method for likelihood and consequence assessment was used, which included (i) a semi-quantitative risk prioritisation of potential pathways for inter-aquifer leakage and development of metrics for impact assessment, (ii) assessment of the consequences of each of these pathways through an analytical model, and (iii) numerical groundwater modelling of single and multiple leaky wells. All three approaches indicated that increases in inter-aquifer leakage, drawdown in upper aquifers are not likely to be significant for high flow leaky gas wells (effective well conductivity, Kw <10−1 m/d) based on conditions reported in previous studies and for the case study. It is theoretically possible that extremely high flow leaky wells (Kw >102 m/d) could have an impact where aquitard conductivity is 10−4 m/d or less, or well failure density is higher than 1 well per km2, however Kw values for compromised gas wells of >1 m/d have not been identified in previous studies. The most extreme case tested, open legacy coal exploration drillholes or petroleum bores repurposed into water bores across an aquitard, should it occur, has the potential to deplete or contaminate groundwater resources in connected aquifers, the magnitude of which will depend on aquifer and production zone transmissivity. However, current Australian regulations for petroleum exploration make this an extremely unlikely case. More effort is required to determine Kw and failure rates of gas wells, water bores and exploration drillholes in Australian conditions to better quantify the potential risks associated with leaking infrastructure.
Coal is usually found in many sandstone-hosted uranium deposits in western China. However, it remains unclear whether coal organic matter is related to the mineralization of uranium in sandstone. In this study, the organic matter of coal containing lenticular carbon plant strips and carbon clastics was analyzed in different alteration zones in the Daying sandstone-hosted uranium deposit located in the northern Ordos Basin. According to geological and geochemical characters, Daying sandstone can be classified into three alteration zones: the oxidation zone, the transition zone, and the reduction zone. The results are as follows. Firstly, the maceral organic matter of the coal at Daying is mostly vitrinite, indicating the characters of humic coal. The maturity of the organic matter is low, which is in the transitional evolution stage of immature lignite or lignite-long-flame coal. Secondly, the kerogen is mainly type III (humic type), which can be easily transformed into humic acid with a strong adsorption capacity for uranium. Thirdly, the vitrinite reflectance (Ro) of the coal organic matter in the transition zone is higher than that of the other alteration zones. Meanwhile, there are bright white bands in the microscopic components of the organic matter of the coal in the transition zone (mineralized zone) and a certain composition of the sapropelite has strong fluorescence. All three aforementioned phenomena are related to the radioactivity of uranium, and each of them possesses the potential for application in mineral exploration. Fourthly, the extraction and separation of humic substances indicates that humic acid plays a key role in organic matter-related uranium mineralization. In the transition zone, uranium can co-precipitate as a humate, and the transition zone’s organic carbon content increases. Therefore, the organic matter in coal contributes to sandstone-hosted uranium mineralization, providing a further guide to prospecting methodologies.
Summary The creation and propagation of hydraulic fractures (HFs) emanating from a well in a naturally fractured rock is important not only to the success of fracturing treatments, but also for interpretation of the data from diagnostic fracture injection tests (DFITs). In this paper, we consider the reservoir rock to consist of an impermeable rock matrix and a system of discrete natural fractures (NFs) that are permeable. The well is assumed to intersect two sets of NFs at their midpoints, and injection into the wellbore might open the NFs and/or create new fractures that extend along the maximum-principal-stress direction. Both new fractures and pre-existing NFs can act as either a main HF or a fluid-loss path. In this near-well transient-fracture analysis, the NFs are short segments characterized by size, orientation, and aperture. A fully coupled HF model is used to investigate the interaction between the fractures to determine how the fluid injected is distributed to the fractures for a range of stress, fluid-injection-rate, and NF-geometry conditions. We find that a more-isotropic stress condition and a lower value of the fluid-viscosity/injection-rate product favor propagation of NFs. These conditions cause the NFs to accept more fluid, and, as a result, the growth of new fractures is suppressed. The post-shut-in pressure responses for the cases with propagating new fractures and nonpropagating NFs are studied.
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