The Stag field operated by Apache Energy Limited is located on the North-West Shelf Australia. Waterflooding is used to maximise production rates and reserves recovery. An unusual failure of the reservoir occurred in late October 2000, between a subsea horizontal injector and a platform based horizontal producer, 2130 ft apart. In effect, a reservoir short-circuit was created between the wells. This reservoir failure circumvented the waterflood and had the potential to isolate reserves between the two wells. Analysis of the failure data indicates the short-circuit was caused by high stresses in the weak reservoir formation. The failure appears to be a wormhole like failure.

A tracer study was implemented to provide information on the short circuit dimensions and to design a remedial treatment. The tracer took 2 hours 50 minutes to travel between wells indicating direct communication and a volume of the short circuit flow path in the order of 150-190 bbls.

Lack of isolation options through the uncemented pre-perforated liners would make any re-mediation difficult and expensive. This provided motivation to first attempt remediation by a comparatively inexpensive polymer treatment injected through the subsea injector and into the short-circuit. Following the polymer treatment, a cement squeeze through the producer was performed to block the short circuit.

This paper reviews the unusual reservoir failure, the tracer study, polymer treatment and subsequent cement shut-off. Included is detail on treatment designs, implementation utilising existing platform equipment and interpretations of the results.

This paper is a helpful case study to those designing and implementing water shut offs, identifying waterflood problems and testing low cost re-mediation alternatives. The paper raises an alternative failure mode that could be experienced in waterflood operations in weakly consolidated reservoir rock.

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