Laboratory measurement of residual saturation (Sgr) in core samples is a matter of some debate due to the lack of a commonly accepted general method. Different protocols can lead to different values of Sgr being generated, with the associated uncertainty for simulation input. A previously conducted study indicates to the authors that steady-state displacement and co-current imbibition are the preferred methods to generate resevoir representative values of Sgr. The steady-state technique is designed to yield full-curve relative permeability data in addition to Sgr, but it is both costly and time-consuming. The objective of this paper is to introduce a method that yields both Sgr and predicted water displacing gas relative permeability data in a time-effective and cost-effective manner.
To determine their accuracy, the relative permeability curves derived from this method have been compared to those from actual laboratory test results on core samples from various water-driven gas reservoirs in Australia, New Zealand, SouthEast Asia and the U.S.A. The data predicted using our method have also been compared to relative permeability data predictions using correlations previously developed by other researchers.
The data comparisons indicate that relative permeability curves generated using our new (MAK) method not only correlate well with laboratory-measured steady-state curves, but also more closely approximate the laboratory data that predictions using previously published methods.
In general, laboratory measurements can be categorised into two major groups: steady state and unsteady-state methods1. For water-displacing-gas, steady-state techniques must be used to generate full-curve data. Water and gas are injected simultaneously into the core sample at incrementally increasing rate-gas ratio. As the system reaches pressure equillibrium (indicating steady-state flow) at each incremental ratio, relative permeabilities both water and gas are measured and the increase in water saturation (Sw) determined. Several such measurements are made to generate decreasing relative permeability to gas and increasing relative permeability to water curves versus increasing water saturation.
With the unsteady-state technique, only the diplacing phase (water) is injected into the core sample. Since the mobility ratio is so favourable, only "end-point" can be generated – relative permeability to gas at immobile water saturation (Krg@Swi) at the beginning of the test and relative permeability to water at residual gas saturaiton (Krw@Sgr) at the end of the test. Co-current imbibition is an unsteady-state test.
A previous study by the authors2 demonstrated that steady-state and co-current imbibition methods produced similar values of Sgr. Since co-current imbibition is relatively a faster and less expensive test, it would be our method of choice to generate Sgr data. By contrast, the steady-state test is a slower, more expensive test – but entirely necessary to produce full-curve data in the laboratory.
Several correlations have been developed previously to empirically derive water-gas relative permeability data and therefore avoid expensive laboratory tests. Selected correlations are reviewed below.