CHARACTERIZING THE HYDROGEOLOGIC PROPERTIES OF THICK CLAYSTONE AQUITARDS IN THE WILLISTON BASIN USING GROUTED-IN PRESSURE TRANSDUCERS
Clay-rich (argillaceous) aquitards with low hydraulic conductivity (K ≤ 10-8 ms-1) are widespread throughout the world. Managing and protecting groundwater resources is often dependent on accurate determinations of the hydrogeologic properties of a formation (bulk compressibility, α and K). However, characterizing such deposits has been difficult due to challenges associated with the slow response times of field-based methods, and the difficulty of collecting representative and competent core samples for laboratory analysis. Laboratory tests also tend to overestimate α and underestimate K. Due to these challenges, aquitards remain one of the least understood areas of hydrogeology. This work presents results of in situ measurements of α and K of Cretaceous-aged claystones obtained through the measurement and interpretation of pore pressure responses recorded by grouted-in vibrating wire pressure transducers (VWPs). Ten VWPs were installed in one borehole to a total depth of 325 m BG near the northeastern portion of the Williston Basin (Site 1). The loading efficiency (γ) was determined based on pore pressure responses to barometric pressure fluctuation and was found to decrease with increasing depth (0.93 to 0.6). The γ was then used to calculate the constrained one dimensional compressibility, mv (2.5×10-7 to 2.2×10-6 kPa-1), which also decreased with depth. Measurements of mv from laboratory testing on core samples from the same borehole were as much as an order of magnitude greater than the in situ estimates. The vertical and horizontal hydraulic conductivity (Kv and Kh, respectively) of the claystone at Site 1 was evaluated at different spatial scales using numerical modeling and laboratory analysis. The results highlight the importance of accounting for scale (laboratory vs field) and consideration of the presence of possible secondary features (e.g. fractures) in the aquitard. The findings from Site 1 suggest that fully-grouted VWPs may provide an alternative method to determine the mv and K of overconsolidated aquitards and become a useful tool to increase our understanding of the hydrogeology of aquitards. This conclusion was supported by the results obtained from an additional 27 VWPs installed in 5 boreholes at Site 2, also located in the Williston Basin.
Aquitards, in situ, pressure transducers, compressibility, hydraulic conductivity
Doctor of Philosophy (Ph.D.)