The Redback gas discovery: a triumph for the formation image log*

Abstract

A thin but highly productive reservoir near the top of the Permian Wagina Sandstone has been responsible for more than 100 bcf of gas from six producing wells, which were drilled along the fault bounded Beharra Springs Terrace in the northern Perth Basin. Recent drilling has targeted the Redback Terrace, located immediately east of the Beharra Springs Terrace with the Wagina Sandstone lying at least 300 m deeper.

Mt Adams-1 was drilled in 1966 at the northern end of the Redback Terrace—it encountered a tight Wagina section, suggesting reservoir quality could be an insurmountable risk on the deeper terrace. This paradigm seemed to be confirmed when Origin Energy drilled Redback-1 in 2004; nevertheless, this well was cased and suspended for a possible future sidetrack.

Redback-1 image logs provided evidence for the presence of a small, sub-seismic fault that could have been responsible for the absence of the high quality reservoir unit in the Wagina Sandstone. This interpretation, however, seemed to be too contrived; hence, it was not widely accepted.

The sidetrack well, Redback South-1, was finally drilled in 2009 and intersected 5.7 m of the high quality Wagina reservoir, confirming the image log interpretation. This reservoir flowed gas at a stabilised rate of 11.8 MMCFD. Subsequently, in 2010, Redback-2 was drilled 400 m north of Redback-1 and encountered 14 m of pay—the thickest reservoir section ever recorded in the region.