The need for more accurate placement of straddle packers led to the design of a tool for use in coiled-tubing fracturing operations.

Fracturing through coiled tubing is especially well suited to shallow wells with multiple thin zones; these wells can be fractured in a much shorter time than with conventional methods, often within one day.
The techniques used in coiled-tubing fracturing (CTF) are similar in various parts of the world. A large-diameter coiled-tubing string achieves sufficient flow rates to properly fracture the zones. The most common coiled-tubing strings are 23/8 in. or 27/8 in. in diameter. Most of the coiled-tubing units (CTU) have an integral mast or derrick to support the injector head and the lubricator. A lubricator is used so that the tools can be retrieved from the well in an under-balanced condition.

Most tool strings used for CTF operations have some type of straddle packer, which may include cup packers, mechanical packers or a combination of both. The left side of Figure 1 shows a typical straddle packer.

When using cup packers, standard operating procedure calls for reverse circulating while tripping in the well, helping to reduce the friction between the top cup packer and the casing wall. The reverse circulation also helps remove formation fines that have migrated into the well after perforating.
The ability to reverse circulate is also very important when moving the tool from one zone to the next, as any sand that accumulates around the tool can be cleaned off prior to moving the tool.

Straddle packer depth control

Proper placement of the straddle packer across the perforated interval is crucial. If the packer is set too low, it can result in lower production. Fluid or sand communication around the top cup may also occur, causing tool string sticking problems.

The same types of problems can result from setting the packer too high. This can lead to lost production from untreated perforations, communication problems, and damage to the lower portion of the straddle packer if sand communicates around the lower packer element.

Depth inaccuracy is a common problem with coiled tubing. Most CTUs use a simple wheel counter or an optical encoder, or a combination of both, to keep track of the length of coiled tubing inserted into the well.
Wheel counters roll along the surface of the coiled tubing and measure its length. Normal wear, stretch and slippage can result in significant depth discrepancies - as much as 4 to 6 ft (1 to 2 m) in a 5,000-ft (1,525-m) well.

It is important to note that even if CTU depth counters were perfect, the possibility of off-depth open-hole logs can cause depth discrepancies between the wireline logs and the coiled-tubing tool string.

Wireless casing collar locators

To address the inefficiencies of coiled-tubing depth counters, wireless casing collar locators (WCCL) were developed about 3 years ago. These tools are battery-powered devices that indicate the location of casing collars or anomalies by sending a pressure pulse to the surface. No cable is required for power or data transmission. The right side of Figure 1 shows a cutaway of the DepthPro WCCL tool for coiled tubing.

A coil and magnet arrangement, similar to wireline collar locator tools, detects changes in metal mass associated with the collar or anomaly. A small voltage pulse is induced in the coil as the WCCL passes a collar. This voltage pulse is analyzed by a circuit board in the tool and acts as a switch to route battery power to the downhole solenoid valve.

As the log is being run, an incompressible fluid is pumped through the WCCL at 0.5 to 1.0 bbl/min. Once energized, the solenoid valve routes fluid to a sliding piston, blocking the flow path. After 3 seconds, power to the solenoid valve is switched off, and the tool reopens. A pressure spike can be seen clearly on a pressure-vs-depth log. The resulting log is then correlated back to previous wireline logs.

These first WCCL tools were developed primarily for depth control before setting plugs or perforating. They are limited to flow rates well below normal fracturing rates, and the flow path follows a somewhat tortuous route, which is not desirable when pumping sand slurries. A WCCL suited to CTF operations required a new design.

New tool development

The following requirements were established for the CTF WCCL tool design:
• bore size adequate to handle 12 bbl/min of sand-laden fluid;
• capable of working in casing as small as 41/2 in.;
• must allow reverse circulation while running in the well;
• must be rated to 300°F and 15,000-psi bottomhole pressure; and
• does not require a ball drop to switch from logging to fracturing mode.

Most of the CTF well candidates are shallower than 10,000 ft (3,050 m); therefore, the temperature rating of 300°F would be enough to service these wells. Readily available AA alkaline batteries work well for bottomhole temperature up to 250°F. Above that temperature, lithium batteries are used.

Most CTF operations have been performed with flow rates of 12 bbl/min or less. Using computer models, it was determined that the 11/4-in. inside diameter (ID) could handle this rate.

Operating principles

To enable reverse circulation while running in the well, a flapper was designed for the lower end of the tool.

The flapper swings up as fluid enters the bottom of the WCCL and closes when reverse circulation stops. When logging is taking place, the flapper is closed and all the fluid being pumped down the coiled tubing exits the tool at the side nozzle port.

As with previous WCCL tools, when a collar is detected, the solenoid valve opens and routes fluid in above the sliding piston. The piston travels down sufficiently to cover the exit port and then retracts when the solenoid is de-energized. The exit port can be configured with different-sized nozzles to allow logging flow rates from 1.0 to 1.75 bbl/min.

When the correlation of wireline and coiled tubing depths is completed, fracturing can begin by switching the tool from logging mode to fracturing mode. To save time and fluid, designers developed a method of shifting to frac mode without having to drop a ball. They developed a sleeve that is held in place with shear pins. To shear the pins, the pump rate is increased from the normal logging rate to about 3 to 4 bbl/min so that back pressure inside the tool will exceed the strength of the shear pins. When the pins shear, the sleeve slides down and blocks the side nozzle port. Onceblocked, pressure continues to build inside the tool. At 3,500 psi, a rupture disc situated in the center of the hollow flapper will break, leaving an unrestricted ID of 11/4 in. through the center of the tool.

Case histories: large-bore WCCL

The first large-bore WCCL tools were assembled in the third quarter of 2001. Lab testing verified proper tool function in the logging mode, and sliding sleeve and rupture disc tests ensured that the tool would switch to frac mode as designed. Finally, the tools were tested on a flow loop with sand-laden fluids at 12 bbl/min. Designers refined the tool at this point to increase part life in the lower section of the tool.
In spite of some minor sticking problems, the tool performed two successful jobs in western Canada prior to the first log example described below. These wells required 5-ft and 8.2-ft (1.5-m and 2.5-m) depth corrections and had 80 tons and 55 tons of proppant pumped during their respective frac jobs.

The first log example (Job A) was also a shallow gas well in Canada (Figure 2). The tool ran a good log with no sticking problems. The CTU counter was only 0.5 ft (.1 m) deeper than the wireline log. After shifting to fracturing mode, 65 tons of proppant were pumped during this seven-stage job.

In a coalbed methane well in Colorado, the CTU counter was perfectly on-depth compared to the wireline log. Five stages were pumped through the WCCL. The total amount of proppant pumped was 65 tons.
In a west Texas gas well, the WCCL log was correlated to wireline logs and found to be 20 ft (6 m) shallow. After attempting to switch to the fracturing mode, crews suspected a packer element leak when pumping down the coiled tubing. They marked the coiled tubing with a paint flag and retrieved the tool from the well. The sleeve had not shifted to the fracturing position, and workers suspected that debris lodged under the flapper had prevented it from sealing. They ran the packer back into the well to the flag depth and completed CTF operations.

In an onshore shallow gas well in Northern Ireland, correlation led the tool operator to correct 48 ft (15 m). This large error was attributed to improper mounting of the CT depth counter. After the operator correlated the depth properly, six zones were fractured with nitrified fluid. The total amount of proppant exceeded 180 tons.

The second log example (Job B) was run in the Republic of Ireland (Figure 3). After logging, crews determined the depth counter was 10 ft (3 m) shallow. After shifting to the fracturing mode, 180 tons of proppant were pumped.

Better placement

The need for better straddle packer placement led tool designers to develop a tool for use specifically in CTF operations. The new battery-powered depth-control tool is run into the well using reverse-circulation, and it logs casing collars in real time for correlation to wireline logs.

After correlating and correcting the CTU counters, the tool can be switched from the logging mode to an unrestricted ID for fracturing without the need of dropping a ball. All these functions can be accomplished in a single trip, saving the operator time and money.

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