The Tuna M–1 reservoir was developed in 1997 from both the new West Tuna platform and the existing Tuna A platform in the Gippsland Basin. The M–1 reservoir is contained within an anticlinal closure with an approximate gross hydrocarbon column of 85 metres. The oil column was originally 12 m thick and is supported by a large gas cap and a strong flank aquifer.Performance from the M–1 reservoir has been good, due to excellent reservoir properties. The combination of conventional and geo-steered horizontal wells has performed well with recovery efficiencies of 70% observed in many parts of the field. Lower than expected performance from the northwestern edge of the oil rim was, however, a significant anomaly, with recovery efficiencies 10% lower than from comparable rock in the southern and eastern parts of the field. The underlying cause of this lower performance was believed to be the result of an anisotropic aquifer response allowing greater pressure support along the northwestern flank of the fieldA re-entry well was drilled from a watered out horizontal well on the Tuna A platform in December 2000. This well was drilled as an oil production opportunity and as a key surveillance data point for the northwestern flank of the field. Results led to further surveillance including contact monitoring and production logging in horizontal wells. In addition to this, simulations were updated to reflect actual performance and surveillance data. Subsequent analysis supported development of a work program for new M–1 drainage points, including additional drill wells and the conversion of existing, watered out horizontal wells to conventional wells. The M–1 redevelopment work has been highly successful with production rates increasing by about 20,000 barrels per day in the first nine months of the program.
