Assessing the predictive capability of the empirical Shale Gouge Ratio–buoyancy pressure calibration: implications for estimating CO 2 column heights

Abstract

Abstract Current workflows for estimating the maximum height of sequestrated CO 2 trapped by fault-rock seal are based on an empirically derived predictive relationship between fault-zone shale content estimated using the Shale Gouge Ratio (SGR) algorithm and buoyancy pressure. The predictive capability of this relationship has been assessed using observed in-place column height data from 25 fault-bounded traps containing gas or oil only. Two-thirds of gas-only traps have an observed trapped column that is more than 67% of the maximum predicted column height. By contrast, only 14% of oil-only traps have an observed column height of more than 67% of the maximum predicted column height. The existing SGR–buoyancy pressure calibration relationship is more optimal for low density (<450 kg m −3 ) buoyant fluids but is less so for higher density fluids (>650 kg m −3 ) such as oil. A new SGR–buoyancy pressure relationship has been derived for oil-only data. Fault seal envelopes for oil-only data exhibit a ‘reclined’ trend which implies a limited increase in seal strength and hence column height, with increasing SGR. As sequestrated CO 2 is likely to have fluid properties (interfacial tension, wettability and density) similar to that of oil in the subsurface, an empirical calibration relationship based on oil-only data should be used as a first-order predictor for CO 2 column heights.