Graduate Student Seminar

Posted on 2019-04-01 in Events
Apr 5, 2019

Please join us for graduate student seminars this Friday April 5 starting at 3:30 pm in rm 155 Geology:

3:30 pm

Yichuan Wang, PhD Candidate

Using time-lapse seismic impedance for monitoring geological CO2 sequestration in Weyburn oilfield, Saskatchewan

Since early 1990s, time-lapse seismic imaging became an effective approach for monitoring hydrocarbon reservoirs during enhanced oil recovery. At present, seismic monitoring is also the standard tool for monitoring processes of geological CO2 storage. In this study, I investigate the use of time-lapse acoustic impedance (AI) for such monitoring, using the data from the Phase I of IEA GHG Weyburn-Midale Monitoring and Storage project in southern Saskatchewan. In Weyburn Field, CO2 was injected into the Midale reservoir zone (Vuggy limestone and Marly dolostone), and the goal of seismic monitoring was in detecting and measuring its propagation through the reservoir.

The AI gives direct connections to the elastic properties of rock and its density; however, evaluation of its subtle time-lapse variations is complicated by the relatively elaborate character of AI inversion from seismic data. Three enhancements of this procedure are proposed in order to accurately measure the time-lapse AI variations within the reservoir. First, following careful equalization of the monitor seismic datasets with the baseline one, time-lapse reflectivity differences are evaluated from the results of this equalization, and without subtraction of the reflectivity from different years of acquisition. Second, a robust approach to AI inversion is used, utilizing the seismic, well-log data, and also the spatially-variant stacking and interval velocities derived during seismic data processing. For time-lapse analysis, it is particularly important that this inversion method requires no subjective selections of parameters and inversion algorithms. Third, the time-lapse AI difference is obtained directly from the baseline AI and reflectivity difference without the uncertainty-prone subtraction of AI volumes from different seismic vintages.

High-quality baseline and time-lapse AI volumes are obtained for the Weyburn study area. Within the reservoir zone, time-lapse variations are observed in the time-shift, reflectivity-difference, and AI-difference images. Reductions of the AI by about 2 to 4.5% and negative time shifts from about 0.5 to 1.5 ms over three years of injection are correlated with injection patterns and interpreted as caused by increases in CO2 content and/or reservoir pore pressure.

4:00 pm

Matthew Nadeau, MSc candidate

Mg isotope tracing of paleofluid migration during dolomite formation in the Williston Basin

Dolomite in the Williston Basin is more common below the Devonian Prairie Evaporite than above. It is therefore not surprising that brine reflux is the most often cited model for dolomitization in the Williston Basin. However, 87Sr/86Sr ratios of two prominent dolomite bodies, the Ordovician Red River Formation and the Devonian Winnipegosis Formation, are higher than contemporaneous seawater. Instead suggesting that the dolomitizing fluids moved upwards from deeper levels in the basin where formation waters in the present day are known to be very radiogenic. Here we explore the possibility that Mg isotopes may trace the direction of fluid-flow during dolomitization. Dolomite preferentially takes up light Mg isotopes during its formation, thus enriching the dolomitizing fluid in heavy isotopes. Accordingly, dolomite should increase its δ26Mg value in the direction of paleo-fluid flow. We looked for gradients in δ26Mg values that may be preserved in the Red River and Winnipegosis dolomites, and found δ26Mg values increasing from the deep center (–1.9‰) to the edge of the basin (–1.4‰) in all directions. We interpret this finding as evidence that the dolomitizing fluids that dolomitized both units ascended, rather than descended, through the Paleozoic carbonate succession, which is consistent with the radiogenic Sr isotope signatures in the dolomites. The burial history of the basin is punctuated by two heat flow anomalies, one in the early Carboniferous and the other in the early Cretaceous. Fluid movement at these times are indicated by the resetting of thermal remnant magnetization in dolomite and evaporate minerals in the basin. We speculate that one or both of these heating events triggered Mg-bearing formation waters to ascend from the fractured crystalline basement through a series of down-to-the basement faults located in the center of the basin. The ascending fluids then flowed laterally along the most permeable sedimentary units to the basin margins, partially dolomitizing the units composed of carbonate sediment. If this interpretation is correct, the Prairie Evaporite basin was not the source of the dolomitizing fluids, but the salt deposit blocked the upwardly flowing fluids from dolomitizing the overlying carbonates.