Ula field is operated with Water Alternating Gas (WAG) injection. Since 2014, all Jurassic field production is attributed to WAG tertiary recovery mechanism. A 2006 study which focused on core from the 7/12-A-9 A surveillance well demonstrated that theoretical WAG performance was a reality at reservoir conditions. Residual oil saturation of 35-50% after water flood (Sorw) could be reduced to a residual oil saturation after miscible flood (Sorm) of 5-10% in the most efficiently flooded layers.

The evidence and pattern of flooding from A-9 A was overwhelming and enlightening. Clear demonstration of laboratory-based miscible flood theory was proven at reservoir conditions. A stratigraphic WAG distribution in the reservoir was identified which prompted modification of the reservoir model and led directly to the establishment of a new field injection strategy that has been followed ever since.

The Ula infill drilling (UID) programme of 2019-20 was based upon the replacement and repositioning of established WAG wells. This would re-focus the WAG sweep to new, un-swept areas of the field, and protect the crestal offset well locations by installation of new well stock. 7/12-A-7 D was designed as a new Southern Crest Producer to replace the aging A-12 A. The optimal location was decided c.400m north of A-12 A and 40m up-dip, within the established A-7 C - A-12 A WAG segment, as proven by 4-D seismic and production history response.

The location provided a new opportunity to evaluate the WAG efficiency at reservoir conditions, this time on the crest and close to a major producer. Sufficient uncertainty in log-based data and saturation measurement accuracy supported a repeat of the A-9 A core-based data acquisition programme.

Results from the core analysis demonstrate a high WAG efficiency in specific layers but confirm the challenge of being able to effectively flood a more substantial profile of the reservoir column. The outcomes are broadly in line with expectations. WAG is proven and the depth of WAG sweep is encouraging, covering 1/3 of the reservoir column. Residual oil saturation after water sweep (no WAG) is also demonstrated to reach low levels, supporting continued water flooding on the flank.

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