Profiling the in situ compressibility of cretaceous shale using grouted-in piezometers and laboratory testing

Grouted-in vibrating wire pressure transducers (VWPs) can be used to measure the in situ constrained (1-D) compressibility (mv) of deep claystone aquitards through measurement of barometric loading efficiency. Measurements of in situ mv for a Cretaceous shale in southern Saskatchewan, Canada, were undertaken using data collected from 27 VWPs installed in multiple boreholes at two sites over depths of 10 to 325 m below ground. The measured mv profiles at both sites produced similar trends of decreasing mv with increasing depth. 1-D consolidation testing was used to measure pre-consolidation pressure (Pc’), compression index (Cc), and the swelling index (Cr) on nine core samples collected from Site 1. These tests yielded Cc values ranging from 0.1–0.5 (\( \bar{x} \) = 0.29 ± 0.12), and Cr from 0.03–0.07 (\( \bar{x} \) = 0.05 ± 0.02). Laboratory measurements of Cc and Cr were used to estimate variations in in situ mv with depth. A theoretical relationship between in situ void ratio (e) and effective stress (σ′) was determined using the laboratory determined Pc’ values, compression indices (Cc, Cr), and measurements of in situ e. Varying the values of Pc’, Cc, or e exerted minor influences on these profiles relative to Cr. The resulting theoretical patterns of in situ mv with depth (or σ′), exhibited a similar pattern to the laboratory and field observations, however to replicate the in situ profiles the Cr values had to be an order of magnitude lower than the laboratory values. The good agreement between the theoretical and measured mv profiles with depth highlight the potential to combine in situ measurements of mv with laboratory consolidation test results to characterize the mechanical properties of deep claystone aquitards and potentially improve upon our understanding of how the stress history of these formation has resulted in their present day geomechanical properties.