Coiled-tubing fracturing (CTF) operations are proving to be a safe, cost-effective way to stimulate wells. But, the straddle packers attached to the end of the coiled tubing must be precisely placed over the intervals to be fractured or the stimulation can be compromised. A new tool has been developed to help ensure proper placement of the packers in the wellbore. A log of the casing collars is generated and then correlated to existing well logs. Once the depth corrections are made, fracturing operations can commence.
The use of coiled tubing to fracture wells is a relatively new service that many operators are finding ideally suited for some candidate fields. To date, the number of wells that have been fractured by coiled tubing is estimated to exceed 5,000. Most of these wells have been fractured in the last three to four years, and over one-third were fractured in 2001.
Like many other types of services performed with coiled tubing, the limiting factor for well stimulations has been the size and strength of the coiled tubing itself. As tubing has grown larger and more robust, the opportunities for using coiled tubing have expanded. Nowhere is this more evident than in coiled-tubing fracturing operations.
As fatigue life and strength have improved for large-diameter coiled tubing over the last few years, the economics of fracturing wells with coiled tubing have also improved. Previously, the short life of large coiled tubing made such operations impractical. To pay for the coiled tubing, service companies would have had to charge an inordinate amount to recover costs.
Fracturing through coiled tubing is especially well suited to shallow wells with multiple thin zones. 1 With coiled tubing, these wells can be fractured in a much shorter time than with conventional methods. Often, the fracturing job can be completed in one day using coiled tubing. In Canada, two wells have been fractured in one day using the same equipment and crew. This is possible because the coiled tubing can reposition the packers quickly from one zone to the next and can do this in an underbalanced condition. Conventional fracturing operations with jointed pipe on a rig require that the well be in an overbalanced state before moving the tools to the next job.
CTF operations have been carried out in Canada for several years, and in fact, most of the wells stimulated using this process have been in Canada. However, CTF operations have now been performed in several areas of the United States, most notably in Colorado, Texas, Alabama, and Virginia. In the United Kingdom, CTF operations have been performed in England and Ireland. Virtually all of these jobs have been performed on shallow, multizone onshore wells.
The techniques used in CTF are similar regardless of the field. A large diameter coiled-tubing string is needed to achieve sufficient flow rates to properly fracture the zones.2 Most coiled-tubing strings being used are 2 3/8- or 2 7/8-in. diameter. Most of the coiled-tubing units (CTU) have an integral mast or derrick that is used to support the injector head and the lubricator. A lubricator is used so that the tools can be retrieved from the well in an underbalanced condition.
Most of the toolstrings being used for CTF operations consist of some type of straddle packer.Fig. 1shows one of the more successful bottomhole packer assembly (BHPA) designs. The BHPA has been designed specifically for CTF operations and has these features:
Equalizing valve-Allows pressure equalization before the packer is released and moved to the next zone
Lower slip design-Does not require rotation of the outer mandrel to release, making packer unsetting easier
Cup top packer-Allows reverse circulation while running inhole for washing debris
Multiset-Capable of being set, unset, and moved many times reliably
The BHPA shown has been used successfully during CTF operations on thousands of wells. Currently, it is available in 4 1/2- and 5 1/2-in. sizes.