Ruta Karolyte, Gareth Johnson, Graham Yielding, Stuart M. V. Gilfillan 

Fault seal analysis is a key part of understanding the hydrocarbon trapping mechanisms in the petroleum industry. Fault seal research has also been expanded to CO2-brine systems for the application to Carbon Capture and Storage (CCS). The wetting properties of rock-forming minerals in the presence of hydrocarbons or CO2 are a source of uncertainty in the calculations of capillary threshold pressure, which defines the fault sealing capacity. Here we explore this uncertainty in a comparison study between two fault-sealed fields located in the Otway Basin, south-east Australia. The Katnook field in the Penola Trough is a methane field, while Boggy Creek in Port Campbell contains a high-CO2/methane mixture. Two industry standard fault seal modelling methods (Yielding et al., 2010; Sperrevik et al., 2002) are used to discuss their relative strengths and applicability to the CO2 storage context. We identify a range of interfacial tensions and contact angle values in the hydrocarbon-water system under the conditions assumed by the Yielding et al. (2010) method. Based on this, the uncertainty related to the spread in fluid properties was determined to be 24% of the calculated threshold capillary pressure value. We propose a methodology of threshold capillary pressure conversion from hydrocarbon-brine to the CO2-brine system, using an input of appropriate interfacial tension and contact angle under reservoir conditions. The method can be used for any fluid system where fluid properties are defined by these two parameters.