A Comparison of Coreflood and Digital Rocks Methods to Study the Remobilisation of Residual Gas Through Depressurisation

A series of coreflood experiments were conducted as part of a broader study of the remobilisation of trapped gas. A well drilled beneath the producing zones logged hydrocarbon gas saturations of up to 50% in sandstones of comparable rock quality to the overlying zones. A subsequent well test found that only the brine phase flowed, suggesting that the hydrocarbon gas is in an immobile, residual state. To assess the feasibility of producing gas from this residual zone, a Digital Rocks (DR) study was devised to replicate the complex reservoir behaviour and evaluate the mechanisms that occur as brine is produced under pressure depletion until the immobile gas phase starts to flow. DR was chosen to study this process because the pore pressure in the exploration well was ~20,000 psi, higher than the maximum operating pressure of the coreflood equipment available. To test the validity of the DR model, a series of corefloods was performed at reduced pressure using model fluids. One coreflood was designed to mimic the reservoir mechanisms as closely as possible, while a second was modified to allow a robust comparison with the DR simulations. The DR model was validated by successfully replicating the laboratory tests so that it could be used to predict gas remobilisation at full reservoir conditions using hydrocarbon gas.