ABSTRACT:

The Annabella gas field was developed in 1991, with seven wells completed in dualselective completions, mainly in gravel pack, using Inside Casing Gravel Pack (ICGP) and Open Hole Gravel Pack (OHGP) techniques. The initial production at start-up was about 550,000 Sm3/d. Production declined to 180,000 Sm3/d in 1994 and 90,000 Sm3/d in 2004, when a workover project was planned. The reasons for production decline were mainly related to water break-through with consequent formation fines migration, completion screen plugging, erosion and hydrates tubular deposition. The intervention's initial target was the execution of three side-tracks from existing wells to drain reservoirs with low recovery factors. The expected production target was around 450,000 Sm3/d, with a planned water production of around 70 m3/d. To improve completion longevity and efficiency, the ICGP-Frac&Pack (F&P) technique was selected. The expected advantage in choosing the ICGP-F&P technique was in placing a fracture that could change the flow regime from radial to bi-linear to reduce matrix velocities by several orders of magnitude and then reducing the risk of fines production. The limit could have been the presence of layers with high water saturation close to the boundaries. A massive F&P operation could have caused those zones to increase the water production, leading to hydrates deposition. Consequently, in the planning phase, particular care was taken to control growth and define proper fracture geometry. The fluid chosen was a polymer-free ViscoElastic surfactant which was selected to limit fracture extension. A new type of fluid-loss control valve was installed to stop fluid losses on the last layer of the dual-selective gravel pack completion. The valve controlled fluid loss and eliminated the need for pumping fluid loss pills. It enabled installing the final upper completion and tubing hanger as per standard procedure. This led to wells with higher flow efficiencies and reduced clean-up times in a safe operating environment. This paper explains the Annabella workover experience with particular reference to the fracture design, which led to a remarkable production increase with no evidence of fines migration and less-than-expected water production. The successful installation of a newgeneration fluid loss control device will contribute in designing future dual-selective gravel pack completions in the offshore Adriatic Sea area

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