Published by the DTI Oil & Gas Directorate for the reservoir engineering and IOR community in the UK.
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Maggie Thompson reviews a study performed by AEA Technology to look at the possibility of developing clusters of small gas condensate fields through shared facilities. Results of the study were provided to the PILOT clusters initiative (co authors were Trevor.Bennison and Gareth Smith of AEA Technology, Odebrecht Consultants provided economic assessments for the development) (maggie.thompson@aeat.co.uk)

Five Central North Sea reservoirs with large variations in productivity and producing pressures were examined in the study. The inflow performance (IPR) and vertical lift performance (VLP) for each appraisal well was first calibrated to data that had been collected from the DTI well records. Predictions of flow rates, tubing head pressures and temperatures were then obtained by combining IPR and VLP predictions with material balance calculations.

To maximise the economic recovery, low productivity reservoirs needed to be brought on stream early so as to maximise the time available to produce from these fields whilst staging production from the higher productivity reservoirs to maintain an economic output stream. It was not possible to produce all wells at a common flowline pressure and it was therefore necessary to reduce the flowline operating pressures to the lowest pressure stream when combining streams. This introduces a number of inefficiencies; the liquid content in the flowline will be increased, additional Joule-Thomson cooling will occur as a consequence of expansion across the chokes and less pressure will be available at surface for processing functions.

The strength of aquifer influx was expected to affect gas and condensate reserves. Increased pressure support raised plateau production rates for the higher productivity fields but did little to improve the rates from the low productivity fields (due to poor inflow performance) and did not therefore significantly change the timing issues. Aquifer influx will be beneficial in delaying liquid drop out in the reservoir and may allow higher common flowline pressures to be maintained, reducing some of the negative aspects of mixing different pressure streams. It is noted that aquifer influx can also be detrimental if large quantities of reservoir gas are trapped behind the advancing water front.

A number of different development scenarios were considered each involving the mixing of fluids from different pressure and temperature streams. Mixing fluids with similar phase envelopes resulted in a fluid that had a phase envelope approximating an average of the two. However as the phase envelopes for the initial fluids become less similar (e.g. increased variation in LGR), the resultant phase envelope was less likely to approximate the average. In particular it was noted that the dew point of the mixture could in some circumstances be higher than that of either of the initial fluids.

Wax deposition was found to be quite likely to occur in the subsea flowline system and there was a significant risk of hydrate formation. Fluids in the flowlines mix to produce a non monotonic temperature profile with cooling and pressure reduction occurring in the flowline. For a typical case studied, the fluids crossed the hydrate formation line on the final section of the subsea flowline prior to the production facilities. Inhibitor injection or widespread insulation of flow lines would be needed to obviate this problem.

With these physical constraints assumed overcome, preliminary engineering design & economics indicated that the cluster development could be economic with a rate of return of over 20% and would be robust to considerable downside.

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