Barque and Clipper are two low permeability sandstone reservoirs in the southern North Sea basin. Gas initially in place amounts to some 1800 Bcf (5.1 × 1010 m3), of which 900 Bcf (2.5 × 1010 m3) is presently estimated to be recoverable. The formations are Permian age Rotliegend dunes, which have suffered diagenetic alteration due to palaeoburial, with a consequent sharp reduction in permeability.

From the beginning of appraisal, evaluation of the hydrocarbon resources has been complicated by the heterogeneity of the fields. Observed production rates ranged from less than 1 MMscf/d (2.8 × 104 m3/d) to over 50 MMscf/d (1.4 × 106 m3/d). Conceptually this variation was explained as production from a tight matrix with in situ gas permeability typically from 20 – 40 μd (20 - 40 × 10−6μm2), mediated by a network of natural fractures. Indeed coring has shown natural fractures, of varying cementation, to be present throughout the reservoirs.

During this study, drawdown and build-up data gathered over a 20 year period were critically re-examined applying derivative and classical techniques. Previously unexplained behaviour was shown to result from multiple porosity systems, the incomplete transitions between such systems or hydraulic fracture wall skin. Consequently, it was possible to correlate welltest derived permeabilities to critical point dried and stress corrected core measurements. Finally the geometric average of matrix permeability, calculated from petrophysical measurements, was established as the best estimator of in situ permeability.

The greater understanding of the production processes, resulting from this study, impacted directly on the decision to proceed with the £450 million (US$845 million) development of the Barque and Clipper fields. First gas is due October 1990.

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