Clarifying Coiled Tubing

Coiled Tubing-based fracks can offer quicker well completion times, precise stimulation and the potential for increased production. Many Bakken operators have used the method to put more fracks in the well, but there are challenges.
By Luke Geiver | November 11, 2014

Coiled tubing is taking the guess work out of the high-stakes hydraulic-fracturing game happening every day in the Bakken shale play. When used to deploy fracture isolating tools known as bottom hole assemblies, coiled tubing allows completion crews certainty, allowing them to better understand when and where discrete fractures are placed along the lateral of the well bore.  Many exploration and production firms have implemented the use of coiled tubing-based fracks in 2014 operations, including Whiting Petroleum. When the Bakken’s largest operator unveiled its second-quarter results earlier this year, the company announced that due to better-than-expected well results, its 2014 production guidance had increased. Instead of the 18 percent production increase over 2013 it had expected in 2014, newly implemented well completion technologies used in early 2014, including coiled tubing, had caused Whiting’s production guidance to increase for 2014, versus the previous year, to 20 percent. The exploration and production firm specifically called to attention its use of a coiled tubing frack system for producing a record number of frack stages. NCS Energy Services was responsible for the record. (In its Q3 announcement, Whiting again pointed out its success with coiled tubing fracks).

The record breaking frack job reached 93 discrete fracture stages, a number nearly double, (sometimes triple), of that capable by other methods. But, although coiled tubing deployed frack systems can offer operators pinpoint stimulation, improved completion cycle time efficiencies, faster screen out recoveries and record breaking wells, there are drawbacks.

Anatomy Of A Coiled Tubing Frack Job
Coiled tubing spools are transported via truck to the well site. The spools house thousands of feet of metal tubing with diameters ranging from one to four inches. Unlike wireline, coiled tubing can be pumped down a well bore. Coiled tubing doesn't depend on the weight of gravity for downward movement. For a coiled tubing-based frack job, a bottom hole assembly (BHA) or tool is commonly attached to the end of a coiled tubing string. The energy service firm running the coiled tubing string can inject the string deeper into the well or retrieve it upward to place the BHA in a desired position. Proppant and water is pumped down the annulus between the coiled tubing and the casing.

Although cemented liners are not necessary for a coiled tubing frack job, many Bakken operators have chosen to add cement to the well bore. Coiled tubing frack sleeves with frack ports can also be placed in the well bore casing string to give the BHA a predetermined segment of the well to situate in. Along with the BHA, a coiled tubing procedure requires a lubricator, injector head, tubing reel, support pumps and a specialized well head tree. The overall well footprint of a coiled tubing operation can be less than that of other fracture methods.

To create Whiting’s record-breaking well, NCS Energy deployed its Multistage Unlimited frack-isolation system that was recently awarded 2014’s best for completion technology by World Oil. The system included cemented casing sleeves. According to the company, the full-drift casing sleeves are equipped with the same specifications as the host casing and are all identical for a given well. They are handled like casing joints and are made up into the casing string at planned frack-initiation points. Using coiled tubing equipped with an NCS BHA, the process includes many steps.

After the coiled tubing moves the BHA into the well, a resettable bridge plug in the BHA is pulled into the frack sleeves already placed in the well casing. The weight of the coiled tubing and pressure present in the annulus push the frack ports in the casing sleeves open. A fracture fluid is then pumped down the annulus, exiting out of ports on the BHA and through the opened frack ports into the rock formation. Following the pressure pumping for the individual stage of the fracture job, a pull on the coiled tubing then opens an equalizing valve in the BHA that retracts the bridge plug. With the bridge plug retracted, the tool string can be relocated to the next preplaced frack sleeve for the next frack.

The BHA includes a frack sleeve locater that matches the profile of a receptacle in the casing sleeves helping the coiled tubing operators to locate with certainty each fracture initiation point along the casing. And, for operators who want to add unplanned fractures into the wellbore, the BHA includes a standby jet perforating system through which frack fluid can be pumped into the casing. NCS is also able to measure pressure readings to determine the placement of each frack.

The Coiled Tubing Advantage
Monte Madsen, operations manager for Whiting’s Northern Rockies unit, says the NCS coiled tubing frack approach is a new tool in Whiting’s arsenal and that although technology like NCS’s has been available since 2011, the ability and adoption of it is still growing. Madsen talked about Whiting's use of coiled tubing during the North Dakota Petroleum Council's annual meeting in September.

The advantages of the completion method are already undeniable. Because the system doesn’t require the use of dissolvable balls or plugs, when completions in a well are finished, there is no need to mill-out existing plugs. “There is nothing to run in the well between fracks, nothing left in the well and nothing to drill out, just a full-open, production-ready wellbore,” he says of its system.

When used with frack sleeves, the BHA assembly can be placed into predetermined sections of the well with certainty, an element of the system that isn’t as present with other techniques. The single point injection system built into the BHA also allows operators to control frack volume which helps to prevent fracture growth into neighboring wells, water zones or other formations where no natural frack barriers exist.

To relocate the BHA and shift the frack sleeves open takes roughly 9 minutes, according to the company. Madsen says that for plug and perf operations, the rule of thumb for the number of completed fracture stages per day is 8 to 9 stages. “We have easily done 16 to 18 sleeves opened, pumped and completed in a 24-hour period,” he says of the company’s use of the NCS system.

Because the system pumps liquid down the annulus of the coiled tubing, it also reduces the amount of water that needs to be introduced into the well, an element of the approach that cuts water volume needed and water costs associated with each well.

Although production increases will vary from well to well, one feature of the coiled tubing frack that most operators favor, including Whiting, is the time savings.

“A key benefit of coiled tubing fracks are cycle-time efficiencies in high well density areas,” the company says. “Without the need to drill out the plugs, we have been able to accelerate production by five to seven days per pad.”

The use of coiled tubing is not without flaws. To date, one of the main limiting factors for operators looking to deploy it in the Williston Basin is the length offerings available for coiled tubing spools. Because laterals are surpassing the 2-mile mark in some cases, coiled tubing cannot be used due to the length limits of some systems. Well path variability is also an issue. A well’s path, Madsen says, is not a level line. Instead, he says, think of using coiled tubing like pushing rope through a tube. Because the path of the well varies, it can be difficult to navigate the tubing through the well. Erosion from sand slurry-induced friction can also occur on the well bore and tubing. Coiled tubing frack jobs aren’t less expensive than most jobs either. Several coiled tubing providers exist in the Williston Basin, with more on the way, however. From Minot, North Dakota Halliburton offers coiled tubing services along with Integrated Production Services and Coiled Tubing Solutions. C&J Energy Services operating out of Williston, North Dakota and Dickinson, North Dakota also offer the service along with several others.

For operators looking to increase well-cycle efficiency or in some cases production, technologies like NCS’s have proven themselves. Although every operator may not be looking to place 93-plus discrete fracture stages in an individual horizontal, there are three main elements of a record-breaking well for NCS. First, an operator needs to commit to undertaking completion activities that have the highest probability of a known result. Second, the process of completing a well must involve diagnostics to measure and quantify any downhole activities that may produce variable results. And, third, an operator must separate what it knows from what it thinks it knows.

Author: Luke Geiver
Managing Editor, The Bakken magazine