Coiled Tubing "Vacuum" Restores North Slope Wells

Abstract

Technology Update The North Slope of Alaska is home to thousands of oil wells, most drilled horizontally and then stimulated with hydraulic fracturing or allowed to flow through natural formation fractures. Wellbore plugging caused by proppant flowback and formation fines can be a problem in these wells, with severe impact on production. For most of these wells, conventional coiled-tubing (CT) cleanouts are possible, using nitrogen to lift the cleaning fluid and suspended proppant to surface. Some of these wells, however, are so underpressured that normal CT cleanouts are technically and/or economically unfeasible because of the massive amounts of nitrogen required to achieve returns to the surface. A Vacuum Cleanout Method One alternative is to clean out the fill by means of concentric coiled tubing (CCT) with a downhole-vacuum tool. The BJ Services Sand-Vac cleanout tool is based on a jet pump comprising a high-pressure nozzle, suction port, and diffuser (Fig. 1). High-pressure fluid is accelerated through the nozzle, creating a pressure drop that essentially draws in wellbore fluids, jetted fluids, and suspended solids. A diffuser allows the velocity of the combined flow to decrease and the pressure to increase. This pressure gain is sufficient to drive the column of return fluids back to the surface. To run the jet pump, power fluid is pumped down the center of a CCT string, with a portion of the fluid allowed to exit an external nozzle to fluidize wellbore solids. Returns come up the larger annulus between the inner and outer strings. Because the CCT string provides the additional flow path, the wellbore is not exposed to the return pressure and, therefore, nitrogen is not required. All of the supplied fluid is returned to the surface, leaving none in the well. This technology was born in the Canadian heavy-oil fields, and it has been proved in a decade of cleanouts around the world—especially in low-pressure, horizontal wells and in areas where nitrogen logistics are difficult or simply too expensive to justify. Alaska Gas Wells In 2006, for example, this technology was employed on the Tyonek platform in the Cook Inlet basin in southern Alaska (Rafferty et al. 2007). This platform ties together 14 wells that produce from the Cook Inlet and Beluga reservoirs, which have bottomhole-pressure (BHP) gradients of less than 0.1 psi/ft. Both reservoirs are composed of highly permeable, friable sandstones that tend to slump into unconsolidated sand piles upon exposure to water. Water breakthrough, therefore, creates major sand-production problems for wells in this area.