Coiled tubing
water or diesel. The viscosity of gelled fluids slows the fall of particles, making these fluids highly effective in vertical and slightly deviated wells. In some cases, adding nitrogen increases a fluid’s capacity to lift solids.
Operators often select coiled tubing as a conduit for pumping cement downhole. It can be used in cement squeezes to seal perforations or casing leaks, for primary or secondary zonal isolation and to set cement plugs for kickoff or abandonment operations.3
The squeeze tech - Screen Gravel
nique plugs casing leaks or existing perforations by pumping specially designed cement slurry under pressure into these openings. A properly designed squeeze job causes the cement to fill the openings between the formation and the casing, forming a seal. Setting a cement plug involves circulating the cement slurry into position using CT and then withdrawing the CT string to a point above the top of cement. A slight squeeze pressure is applied if necessary, the cement is displaced by a tail slurry, and then the CT is pulled out of the hole.
> Gravel-pack washdown. As the gravel-pack screen is lowered toward the top of the gravel, surface pumps are activated. The pump rate is sufficient to fluidize the gravel without causing it to circulate back into the tubing. While the pumps are active, the CT is slowly lowered into the gravel until the screen reaches its setting depth. A ball is pumped through the CT string to release the screen, and then the CT string is pulled back to surface.
The most common application for CT is the cleanout and removal of sand or debris that fills a wellbore. Fill material can seriously impede production by reducing the flow of oil or gas. It can also block the passage of slickline or wireline tools during well operations or prevent downhole sleeves and valves from opening or closing. Common sources of fill are sand or fine material produced from the reservoir, proppant materials used during hydraulic fracturing operations, organic scale and debris from workovers. Fill removal typically involves circulating a cleanout fluid, such as water, brine or diesel, through a jet nozzle run on the end of the CT. As the fluids are circulated, they carry the debris to surface through the annulus between the CT string and the completion tubing. Sometimes, an acid or solvent is pumped to chemically break down the fill before it is circulated out of the wellbore. Gelled fluids may also be employed to provide greater solids-carrying capacity than
Compared to a standard workover rig, the CT approach to cementing offers a range of advantages: • There is no need to pull completion equipment. • The operation can be conducted without kill - ing the well.
• The cement can be placed accurately, thus reducing contamination of the slurry. Treatment programs often use CT to convey stimulation fluids that boost production by restoring or improving the permeability of a reservoir. In a matrix treatment, fluids are pumped into a reservoir at a pressure below the formation fracture threshold (see “Options for High-Temperature Well Stimulation,” page 52). This technique pushes the fluids through the open pore spaces without initiating a fracture. Completion operations—CT can facilitate the installation of production tubing and associated completion equipment. In certain wells, a string or section of CT remains in the borehole as a permanent part of the completion. CT completions can provide a low-cost approach for prolonging the life of old wells. Typical installations include velocity strings, tubing patches and through-tubing gravel packs (GPs). For example, in some wells operators choose to permanently install CT as a velocity string inside existing production tubing. This approach is used when a reduction in the gas-to-liquid ratio of produced fluids or a decrease in bottom- hole pressure causes a decline in production. A decrease in fluid flow velocity results as gas content declines, and the ensuing increase in
fluid slippage causes the well to load up. The velocity string reduces the cross-sectional flow area of the tubular, thus yielding higher flow velocity for a given production rate and allowing fluids to be carried out of well.
Coiled tubing may serve both as a conveyance and a medium for patching production tubulars. A CT tubing patch can be positioned in a completion to cover mechanical damage or erosion in tubing, to permanently shut off a sliding sleeve, or to isolate perforations. Packers set at the top and bottom of the patch hold it in position and provide the seal between the existing completion and CT string.
Coiled tubing is often used in completion programs to convey tools, fluids and materials. Frequently, wells drilled in unconsolidated sands require the wire-mesh screen of a GP to prevent sand production. Common GP installations involve a washdown procedure.
First, the CT string is run to the GP depth, then gravel is pumped through the coiled tubing. The CT string is then retrieved to surface and a GP screen assembly is attached (left). As the cylindrical screen is run to the top of the gravel, fluid is pumped through the CT to agitate the gravel and allow the screen to settle into place across from the perforations. The CT string is then retrieved to surface. The GP keeps the sand in place while allowing formation fluids to flow. Should sanding begin later in the life of a non-GP well, coiled tubing offers a means of installing a through-tubing GP completion, in which GP screens are installed through the existing production tubing to maintain the original completion hardware.
ACTive Sensors and Telemetry
The success of fill cleanouts, matrix stimulations, cementing jobs and other CT applications is closely linked with the ability to model and evaluate the behavior of downhole parameters such as temperature and pressure. In other applications such as perforating operations or installation of packers, tubing patches or whipstocks, the ability to accurately control depth is absolutely necessary. However, CT crews and operators often depend on indirect surface indicators to infer these critical parameters. For conventional CT jobs, downhole pressure is often estimated from surface readings of pressure at the pump or wellhead. However, subtle pressure changes can be attenuated through thousands of feet of wellbore fluid before reaching surface. Thus, surface indicators may sometimes be misleading.
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Oilfield Review
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