Relative Permeabilities for Post-Waterflood Depressurisation

Laboratory depressurisation data [1] has been re-examined. The new interpretation indicates that gas recovery from post-waterflood depressurisation of oil reservoirs could be substantially higher than previously predicted. Matthew Goodfield (matthew.goodfield@ecltechnology.com) of ECL Technology Ltd presents the results of work undertaken as part of the DTI’s SHARP programme.

Laboratory studies

A series of four depressurisation experiments were performed as part of a joint industry project using core from two UKCS oil fields [2,3]. Cores were aged with live crude and following a waterflood were slowly depressurised. The produced fluids were measured, together with in-situ saturation profiles for gas, oil and water.

The laboratory results indicated low critical gas saturations, which decrease for slower depressurisation experiments (closer to field rates). However, the ultimate gas saturation left in the core at the end of the depressurisation was found to be as high as 40% and would represent a significant loss of target gas recovery if this value were directly applicable to the field. This would directly affect the outcome of the potential UKCS depressurisation projects.

Match to measurements

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 (see Figure 1). The match to the measured in-situ gas saturations is illustrated in Figure 2.

Figure 2: Original match to the measured in-situ gas saturations (Click image for larger view)

A key element of the simulation of the experiments is the representation of the core outlet boundary condition. New methods have recently been developed for waterflood SCAL studies influenced by capillary end effects, which allow the observed saturation history to be used as the outlet boundary condition, overcoming the limitations of the conventional zero capillary pressure boundary condition. Analysis of the depressurisation experiments shows that movement of gas in the core could be dominated by capillary forces. The new methods for handling the outlet boundary condition were extended to the depressurisation situation and Experiment 4 was reinterpreted. An alternative match was found without invoking unconventional gas relative permeabilities. This revised match gives a qualitatively better representation of the evolution of the gas saturation profile (Figure 3).

A range of simulations was performed to assess the uncertainty in the inferred gas relative permeability and to highlight that, in future experiments, uncertainty could be reduced by performing a post-depressurisation waterflood to help quantify gas mobility. The new relative permeabilities give much greater gas mobilities and are expected to lead to significantly higher predictions of gas recovery at the field scale than was suggested by the previous interpretation.

Conclusions

• A reasonable match to the experimental data has been obtained without the need for very low gas relative permeabilities or invoking a complex physical explanation involving gas trapping, diffusion etc.
• An important factor in the interpretation of the experiments is the representation of the outlet boundary condition.
• This study illustrates the importance of in-situ saturation data in the interpretation of core flooding experiments.
• A post waterflood should be performed at the end of depressurisation experiments to provide additional information on the gas relative permeability.

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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