The results presented here are the outcome of a research study comparing two different laboratory testing procedures for the determination of CO2 capillary pressures of low permeable rock types with permeabilities ranging between 1·10-21 and 6·10-21m². A well-characterized core section of the Opalinus Clay (Mont Terri) has been sampled for this study. Laboratory test conditions corresponded to a depth of approximately 1500m, representative of a typical CO2 storage scenario. The experiments were performed on fully water-saturated samples plugs.
Two gas breakthrough measurement techniques were tested: 1) the classically used step-by-step gas pressure increase method yielding a value for the capillary breakthrough pressure, and 2) the imbibition or residual pressure method aiming at the detection of the snap-off pressure on the imbibition path. First, imbibition measurements were conducted, followed by the classical step-by-step approach.
Breakthrough pressures of the different sample plugs were strongly variable. The observed gas breakthrough pressures for the core plugs ranged from 3.4 to 12.3MPa for N2 and from 14 to 17.5MPa for CO2. This is attributed to the likely occurrence of artificially induced micro fissures or natural inhomogeneities and the poroelastic response to changes in effective stress (development of dilatant pathways). As expected, breakthrough (drainage) pressures were always higher than the snap-off pressures on the imbibition path (ratio 1.6:4). Intrinsic water permeabilities remained unaltered in the course of the gas breakthrough experiments. A simple relationship to more readily obtained parameters, such as permeability, could not be obtained.
The results have implications for the prediction of the seal capacity of CO2 storage sites. Determination of capillary pressures from a single plug seems insufficient - likely resulting in over/underestimation of the true value. More research is recommended to understand the governing transport and leakage processes and their lateral variability in caprocks.
“Are we using appropriate sample preparation techniques?” “Is it reasonable to use data gained on material after stress-unloading (after coring)?” “Do we use correct wettability data to convert to CO2/brine conditions?” These are the open questions that need more research focus in order to explain the broad range of critical capillary pressures observed in this study, using drilled twin plugs from a core section only few meters long.