Update: Relative Permeabilities for Post-Waterflood Depressurisation

This article is an update of a previous IOR eNewsletter article on gas recovery from deep post-waterflood depressurisation . Matthew Goodfield ( matthew.goodfield@ecltechnology.com ) of ECL Technology, presents the results from a study conducted under the DTI's SHARP programme on a field-scale simulation to examine the effect of a revised interpretation of the laboratory data [1]

Earlier Work

A series of four depressurisation experiments were performed as part of a joint industry project using core from two UKCS oil fields [2,3].

In a previously reported interpretation of Experiment 4 [4], the high gas saturations could only be explained by an unconventional gas relative permeability function, in which very low gas relative permeabilities persisted to high gas saturations.

An alternative match was found in a re-interpretation of the experiments without the need to use very low gas relative permeabilities. This revised match gives a qualitatively better representation of the evolution of the gas saturation profile along the core.

Figure 1 illustrates the potential increase in gas recovery from post-waterflood depressurisation when using the revised gas relative permeabilities rather than those obtained in the original study

Figure 1: Potential Increase in Gas Recovery from Post-Waterflood Depressurisation

Figure 2: Very Low Gas Relative Permeabilities Obtained in the Original Interpretation Compared with the Revised Interpretation

Field-Scale Simulation

A field-scale simulation model has now been used to examine the potential for post-waterflood depressurisation. The simulation model represents a typical UKCS reservoir geology with limited aquifer support. The initial saturation pressure varies with depth. Simulations have been performed using both the original and the revised interpretations of the gas relative permeabilities.

The average reservoir pressure is maintained at the initial value of approximately 5000 psia during the waterflooding phase then drops at a rate of approximately 1.5 psi/day during the depressurisation (Figure 3). Simulations with both the sets of relative permeabilities reached pressures substantially below the bubble point. The simulations show that if the revised gas relative permeability curves are correct then using the original curve could give a large under estimate of the ultimate gas recovery from a post-waterflood depressurisation (Figure 1). The incremental gas recovery 10 years after the start of the depressurisation is 30% of gas initially in place (GIIP) for the revised gas relative permeabilities compared with only 5% of GIIP for the original relative permeabilities.

Figure 3: Drop in Average Reservoir Pressure During the Post-Waterflood Depressurisation

References

1. Goodfield, M. and Goodyear, S.G.., "Relative Permeabilities for Post-Waterflood Depressurisation", SPE 83958, to be presented at Offshore Europe 2003 in Aberdeen, UK, 2-5 September 2003.
2. Naylor, P., Fishlock, T.P., Mogford, D. and Smith, R.A., "Relative Permeability Measurements for Post-Waterflood Depressurisation of the Miller Field, North Sea", SPE 63148, SPE Annual Technical Conference and Exhibition held in Dallas, Texas, 1-4 October 2000.
3. Naylor, P., Mogford, D. and Smith, R.A., "Development of In-situ Measurements to Determine Reservoir Condition Critical Gas Saturations During Depressurisation", SCA2000-37, 2000 International Symposium Abu Dhabi, 18-22 October 2000.
4. Drummond, A., Fishlock, T.P., Naylor, P., and Rothkopf, B., "An Evaluation of Post-Waterflood Depressurisation of the South Brae Field, North Sea", SPE 71487, SPE Annual Technical Conference and Exhibition held in New Orleans, Louisiana, 30 September-3 October 2001.

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