Tool depth is another critical parameter that may be subject to error or misinterpretation. For instance, accurate placement of gun strings is essential to the success of any perforating job. Poor depth control could mean a missed target or poor contact with the hydrocarbon interval or— even worse—perforating into a water zone. In the past, a dedicated tie-in run was required for depth control. The CT was run in the hole with a memory gamma ray and casing collar locator (GR-CCL) tool to identify a known reference point in the well. Typical reference points included the bottom of the well, a known restriction, a distinctive piece of completion equipment or a short length of pipe called a pup joint. Reference points such as TD or a known restriction were found by tagging, or gingerly setting down on them. Others, such as pup joints or completion intervals, were located by the GR-CCL tool. Upon reaching the reference point, a mark was placed on the coiled tubing at surface to flag the amount of tubing that had been spooled off the reel. The CT GR-CCL was then pulled out of the hole, and perforating guns were installed. However, other factors enter into the depth- tie-in process: The length of the interval from the downhole correlation point to the pay zone must be considered, along with a host of new details. Between the memory run and the perforating run, a change in tools will be accompanied by a significant change in bottomhole assembly (BHA) dimensions and weights, fluids, friction, debris and coiled tubing deformation. Any one or combination of factors could alter the placement of the downhole measure point with regard to the surface reference flag. Tubing-depth errors as high as 0.3% were not uncommon.4
The industry is quite aware of the potential for trouble caused by reliance on surface indica - tors. The range of problems is wide and varied: • packers and plugs that are set off depth as a result of poor depth control
• perforating guns that are detonated off depth because of poor depth control
• guns that fail to fire owing to pressure discrep- ancies—discovered only after coiled tubing has been retrieved to surface
• wells that fail to perform as expected as a result of insufficient underbalance prior to perforating. To address such issues, a special BHA was developed along with an advanced fiber-optic telemetry system and a surface control system for assessing downhole job performance. The
ACTive monitoring system incorporates sensors inside the BHA to measure temperature, annulus pressure and CT pressure (right). Depth control is handled by a fully configurable casing collar locator, also carried in the BHA. The CCL is sensitive enough to detect collars at any logging speed and is capable of detecting flush-joint connections at logging speeds of 15 ft/min [4.6 m/min]. This sensitivity has also helped operators find other anomalies, such as perforations and casing defects.
Designated as the PTC (pressure, temperature, CCL), this BHA and associated telemetry system give operators access to fundamental information that might only be seen in surface measurements many seconds later— or not at all. Measurements made downhole, at the point of application, help CT crews more accurately control depth and respond to parameters as they change during the course of a treatment. The PTC assembly is made up beneath the CT head at the terminus of the CT string. It can be run with other CT tools such as a perforating gun, multilateral locator tool, inflatable packer and jetting tool, and it is designed to withstand high tensile loads, torque and pressure.
-in. [5.4-cm] OD BHA comes with a built- in check valve. The minimum flow-through restric tion is 0.688 in. [1.7 cm], which allows ball-operated tools to be run below the fiber- optic sub.
⁄8 A rugged high-bandwidth fiber-optic telemetry
system is placed inside the CT string to convey PTC measurements to surface. Optical fiber offers a number of advantages over other hard- wired transmission media. In contrast to electric wireline, glass fibers are used to carry signals in the form of light pulses, resulting in faster transmissions and immunity to electro magnetic interference. Certain wavelengths of these light pulses are sensitive to changes in temperature, and this characteristic is exploited to create an intrinsic sensor that measures temperatures along the length of the fiber (see “Downhole Temperatures from Optical Fiber,” page 34). The ACTive configuration uses four optical
fibers: Two strands are dedicated to the PTC, one is used for measuring temperature, and one strand is set aside as a spare. These strands are
This tool has two distinct sections: the CT head, which provides a CT connector while housing the termination for the optical fiber, and a power supply for electronic components. The 21
Coiled tubing (CT) CT connector CT head
Electronics and sensors
Casing collar locator
Crossover sub
> ACTive BHA. Downhole temperature and pressure sensors, along with a casing collar locator, are key components of the ACTive BHA. This assembly is only about 7 ft [2 m] long, and it is made up between the CT head and the crossover sub to provide measurements just above the downhole tools deployed by the CT string. Optical fiber inside a protective metal carrier (inset) conveys data to the surface.
enclosed in a protective and flexible INCONEL steel carrier. The carrier and fibers are run through the CT reel to the head of the tool at the end of the CT string. The fiber-optic carrier, with an outer diameter of just 0.071 in. [1.8 mm], has a negligible impact on the CT internal cross section and so does not affect pumping rates. It is very lightweight, weighing nearly one- twentieth of an equivalent length of electric
3. Primary zonal isolation is typically established by an inflatable packer, while secondary zonal isolation is established subsequently, using cement.
4. Rangel PD, Sorman I, Blount CG and Woods N: “Fiber- Optic-Enabled Coiled-Tubing Operations on Alaska’s North Slope,” paper SPE 106567, presented at the SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, The Woodlands, Texas, USA. March 20–21, 2007.
Winter 2008/2009
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