This paper reports a laboratory study of the gas breakthrough pressure for different gas/liquid systems in the Mississippian-age Midale Evaporite. This low-permeability rock formation is the seal rock for the Weyburn Field in southeastern Saskatchewan, Canada, where CO 2 is being injected into an oil reservoir for enhanced recovery and CO 2 storage. A technique for experimentally determining CO 2 breakthrough pressure at reservoir conditions is presented. Breakthrough pressures for N 2 , CO 2 and CH 4 were measured with the selected seal-rock samples. The maximum breakthrough pressure is over 30 MPa for N 2 and approximately 21 MPa for CO 2 . The experimental results demonstrate that the Weyburn Midale Evaporite seal rock is of high sealing quality. Therefore, the Weyburn reservoir and Midale Beds can be used as a CO 2 storage site after abandonment. The measured results also show that the breakthrough pressure of a seal rock for a gas is nearly proportional to the interfacial tension of the gas/brine system. The breakthrough pressure of a CO 2 /brine system is significantly reduced compared with that of a CH 4 /brine system because of the much lower interfacial tension of the former. This implies that a seal rock that seals the original gas in a gas reservoir or an oil reservoir with a gas cap may not be tight enough to seal the injected CO 2 if the pressure during or after CO 2 injection is the same or higher than the original reservoir pressure. Therefore, reevaluation of the breakthrough pressure of seal rocks for a given reservoir is necessary and of highest priority once it is chosen as a CO 2 storage site.

ABSTRACT:In this study, different interpolation techniques in a geographical information system (GIS) environment are analysed and compared for estimating the spatial distribution of precipitation in the province of Ontario, Canada. A high-resolution regional climate modelling system [Providing Regional Climates for Impacts Studies (PRECIS)] is used to simulate the present and future (2071-2100) precipitation events for 12 meteorological stations over Ontario. The results verify that for the present case PRECIS simulates well the precipitation events when compared with observed data. The future precipitation events can be projected after the validation of PRECIS. Six interpolation methods are then used to generate spatial distribution of precipitation based on the projections of future precipitation of 12 meteorological stations; they include inverse distance weighting (IDW), global polynomial interpolation (GPI), local polynomial interpolation (LPI), radial basis functions (RBF), ordinary kriging (OK), and universal kriging (UK). Cross-validation is applied to evaluate the accuracy of interpolation methods in terms of the root mean square error (RMSE). The results indicate that LPI is the optimal method with the least RMSE for interpolating the PRECIS precipitation. LPI is then used to analyse spatial variations of the average annual precipitation for the period of 2071-2100 over Ontario.

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In this study, a bivariate hydrologic risk framework is proposed through coupling Gaussian mixtures into copulas, leading to a coupled GMM-copula method. In the coupled GMM-Copula method, the marginal distributions of flood peak, volume and duration are quantified through Gaussian mixture models and the joint probability distributions of flood peak-volume, peak-duration and volume duration are established through copulas. The bivariate hydrologic risk is then derived based on the joint return period of flood variable pairs. The proposed method is applied to the risk analysis for the Yichang station on the main stream of the Yangtze River, China. The results indicate that (i) the bivariate risk for flood peak-volume would keep constant for the flood volume less than 1.0 × 10 5 m 3 /s day, but present a significant decreasing trend for the flood volume larger than 1.7 × 10 5 m 3 /s day; (ii) the bivariate risk for flood peak-duration would not change significantly for the flood duration less than 8 days, and then decrease significantly as duration value become larger. The probability density functions (pdfs) of the flood volume and duration conditional on flood peak can also be generated through the fitted copulas. The results indicate that the conditional pdfs of flood volume and duration follow bimodal distributions, with the occurrence frequency of the first vertex decreasing and the latter one increasing as the increase of flood peak. The obtained conclusions from the bivariate hydrologic analysis can provide decision support for flood control and mitigation.

The formation of the world‐class, high‐grade unconformity‐related uranium deposits in the Athabasca Basin (Canada) requires circulation of large amounts of fluids, the mechanisms for which are still not well understood. Recent studies advocate thermal convection as a possible driving force for the fluid flow related to uranium mineralization; however, little is known regarding how basement faults, which are spatially associated with most unconformity‐related uranium deposits, influence fluid convection and how this may affect the localization of mineralization. This study addresses these questions through simulations of thermal convection with various configurations of basement faults using the FLAC3D software. Modelling results indicate that the location, spacing, orientation and thermal conductivities of basement faults influence the size and location of thermally driven fluid convection. In a model with a single isolated fault, the fault coincides with an upwelling plume and the dip angle of the fault does not affect the fluid flow pattern; when the fault is moved laterally, the upwelling plume shifts accordingly. In the case of two vertical faults, the faults may either coincide with upwelling flow between two convection cells or be located below individual convection cells, depending on fault spacing. In the latter case, fluid may flow into and out of individual fault zones. Similar results were also obtained for models with two nonvertical (i.e. dipping) faults. Convective flow can penetrate the uppermost basement when the permeability is less than two orders of magnitude lower than that of the overlying sandstone. In this case, the basement faults not only can control the location of ascending flow, but also can passively act as fluid conduits of either flow from the basin into the basement (ingress), or flow from the basement into the basin (egress), depending on their thermal conductivities and relative locations in the models.