Stochastic quantification of CO2 fault sealing capacity in sand-shale sequences

Abstract

Structural fault trapping of buoyant fluids, such as hydrocarbons and CO2, relies on fault sealing capacity. The traditional approach to quantifying the sealing capacity of fault zone materials is Shale Gouge Ratio (SGR), which implicitly homogenizes fault properties. However, a large range of fault throw and the presence of fault heterogeneity can lead to different trapping capacities for the same structure. This paper presents a stochastic study to statistically determine the possible range of CO2 column height at a normal fault in sand-shale sequences. We present the procedures of two stochastic models including the continuous shale gouge model (CSGM) and the discrete smear model (DSM). The models are applied in an example case for the High Island field in the Gulf of Mexico (GOM) basin combining borehole geophysical data and measurements from laboratory experiments. Both one-dimensional and two-dimensional cases are discussed. The results show that more than 50% of the realizations predict negligible fault sealing capacity. The CSGM method shows that CO2 column height is overall proportional to SGR and breakthrough pressure. The DSM model suggests that clay smear continuity may significantly affect fault sealing capacity. Large fault throws (>100 m) result in smear breaches and therefore in highly variable maximum CO2 column heights, varying from 0 m to a maximum of ∼95 m. Both the DSM method and the CSGM method proposed in this paper permit explaining geologic uncertainties and the resulting variability of column heights observed in the field. Our results together with the field data from the literature demonstrate the utility of combining both methods to help improve column height prediction. Uncertainty quantification of clay ductility, smear location, and fault heterogeneity is important for determining fault sealing capacity and improving reservoir risk management associated with carbon dioxide geological storage.