RIASSUNTO
Chambers, M.J. BP Exploration (Alaska) Inc. SPE Member
Abstract
A successful technique for placing accurate amounts of sand inside casing was developed for the Western Operating Area (WOA) of the Prudhoe Bay Unit (PBU) on the North Slope of Alaska. Using coiled tubing and a multistage pumping schedule, a sand plug can be laid in one, time-saving operation.
The experience gathered from more than 100 field operations performed in the WOA during 1991 and 1992, in varying downhole conditions, provided for the refinement of the operational steps; allowing the first and each succeeding stage to be adjusted, in situ, for the manner in which the sand plug 'behaved' as it was placed.
Introduction
In a multiple zone completion, when it becomes necessary to isolate a deeper producing zone from a shallower one, a barrier must be placed between them to prevent fluid communication. See Fig. 1. In Prudhoe Bay, the primary use of this technique is to provide temporary isolation between the zone to be stimulated or cement-squeezed and the bottom perforated or leaking interval. Because the lower zone is not going to be permanently abandoned in most cases, the isolating mechanism must be relatively easy to remove and be non-damaging to the perforations.
In the past, the main technique for accomplishing zone isolation, through tubing, was to set an inflatable bridge plug (IBP) on coiled tubing or electric line. Often, the IBP was then covered with a sealing material, such as sand or cement, with a wireline dump bailer.
In the WOA, laying a sand plug with coiled tubing, has proven to be less expensive than setting an IBP. Cost savings come in the form of eliminating the IBP setting, rental, and removal charges; and all too often, the costs associated with repeated setting attempts, fishing, and possibly a rig workover.
The sandback technique can also be applied to placing a sand cap on top of an IBP to improve the differential pressure rating of the plug and to prevent small leaks or movement which can lead to gross element failure.
Rig workovers (RWO) frequently involve the cutting and milling of tubulars resulting in large amounts of dirty fluids being lost to the formation. Sand plugs combined with lost circulation material can be placed by coiled tubing across the producing zones prior to a rig workover to limit fluid loss which can lead to formation damage. Prior to using sand plugs, the normal RWO procedure was to bullhead kill the well and then spot a sized-salt pill across the perforations.
ADVANTAGES
There are several advantages to laying a sand plug with coiled tubing. Long intervals can be covered with sand in a short period of time and the sand top can be accurately placed within a selected depth window. The strength and permanence of sand plugs can be adjusted through the use of different types and sizes of sand, and by placing cement or scale caps on the sand plug.
P. 809^