Scale Tubing wall
> Scale buildup in wellbore tubulars. Variations in produced water quality, especially when injecting seawater for pressure maintenance, contribute to the formation of hard inorganic scales in perforations and wellbore tubulars. Insoluble scale buildup reduces the ow area, restricting or preventing production, and contributes to gas-lift problems, and failure of subsurface safety valves and other equipment.
0.75 mm
Drift ring Jet nozzle Rotating head
Tubing wall Scale
> Mechanical scale-removal. The Jet Blaster tool consists of a rotating head with opposing tangentially offset nozzles and a drift ring. The jet nozzles remove scale from tubular walls while the drift ring allows the tool to advance only after the internal tubular diameter is clean. Blaster services include three mechanical scale-removal techniques: the Jet Blaster method uses nonabrasive uids for removal of soft scales; the Scale Blaster method adds the abrasive Sterling Beads system to remove hard scales; and the Bridge Blaster method uses abrasive jetting and a powered milling head when tubulars are completely plugged.
Based on real-time monitoring, cleanout operations removed 16,500 lbm [7,484 kg] of bauxite in 12 hours. A subsequent CT run tagged the gravel-pack assembly rope socket, confi rming that the well was clean. After gravel-pack screens were replaced, well production increased from 0.5 to 2.5 MMcf/D [14,320 to 70,600 m3/d]. Downhole deposits of inorganic scales in wellbore tubulars are a serious well-intervention problem (above left).8
Scale buildup changes the Sand particles Limestone particles Glass beads Sterling Beads particles
> Microscopic views of Sterling Beads shape and various particle abrasion effects. The spherical Sterling Beads particles have a high fracture toughness, low friability, and are acid soluble (top). This nontoxic material matches the erosive performance of sand on hard, brittle scales, but does not cause excess damage to steel during prolonged jetting in one spot. Angular sand and calcite particles gouge steel surfaces, which can cause ductile tubular failures (middle left and right). Glass beads and round particles tend to bounce off steel surfaces, creating large, deep craters that eventually may result in erosion through tubular walls (bottom left). Sterling Beads particles shatter on impact, creating only small pits (bottom right).
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surface roughness of tubulars, increasing frictional pressure and restricting production. Additional scale growth decreases tubular flow area, prevents access to deeper sections of a well and ultimately may block the tubing completely. Extremely hard, insoluble scales, such as stron- tium or barium sulfate, may form when injected seawater breaks into a well. In Brazil, Petrobras used abrasive-jet CT technology to clean heavy barium sulfate scale from production tubing in an offshore well.9
well was located on a fi xed offshore platform and no workover rigs were available, so tubing replacement was not an option. CT provided a means of conveying mechanical scale-removal tools and circulating cleanout fluids without a conventional rig.
Methods such as chemical dissolvers,
slickline brushes and downhole motors had suc- cessfully removed scale in other area fields. In some of these cases, however, residual debris fell
to the bottom of wells and blocked the perfora- tions, requiring additional cleanout operations. Schlumberger Blaster services use high-pres- sure jetting technology to remove downhole deposits (above). This specialized system uses solvents or special abrasive material to remove scale without damaging tubulars or com- pletion equipment, such as profile nipples, subsurface safety valves or sliding sleeves. This technology comprises three techniques— Jet Blaster, Scale Blaster and Bridge Blaster scale- removal services.
The
Jet Blaster techniques use conventional fluids or scale-dissolving solvents with a radial jetting tool. The Scale Blaster approach uses the Sterling Beads safe hard scale-removal system developed at Schlumberger Cambridge Research in England to remove hard, inert scales (left). By properly selecting particle hardness, shape, size, density and fracture toughness, researchers achieved unique properties that remove scale without damaging steel surfaces.
The Bridge Blaster technique combines a positive displacement motor (PDM) and a 15
⁄ 8 -in.
tapered mill with the radial jetting tool and the Sterling Beads system modifi ed to prevent PDM clogging. This system drills scale deposits or cement plugs through tubing without damaging wellbore equipment. The small tapered mill partially removes the scale deposit while jetting removes the rest. Removal rates are higher than with conventional milling.
Oilfi eld Review
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