When the ideal coiled tubing string for a sleeve-shifting operation was too heavy for the platform crane, the string was split and then rejoined offshore. BJ Services’ Godwin Effiong and Greg Dean explain how they kept the job safe.
Efficiency and reliability have made coiled tubing (CT) technology a routine part of oil and gas remediation operations off the west coast of India. For well cleaning, sleeve manipulation and stimulation, for example, CT offers many technical and economic advantages compared with traditional snubbing or bullheading operations.
One of the fundamental advantages of CT is its continuous, non-jointed length; in some situations, however, this advantage can become a limitation. For some offshore wells, especially for long horizontal wells, the CT string required to safely and effectively accomplish a particular job may be too heavy to meet platform or vessel crane lift capacity limitations.
For some recent operations offshore India, BJ Services overcame the weight challenge by splitting the CT string and re-joining it offshore. Ensuring a safe, long-lasting connection required the use of an innovative connector technology.
As in many offshore developments, wells off the west coast of India are relatively deep with long, horizontal legs extending away from the main platform. For a variety of well operations, coiled tubing provides a means of rigless intervention, saving operators time and money.
Coiled tubing requirements vary with force and friction requirements. For example, small-diameter coiled tubing strings are often unable to achieve intervention goals, especially the force requirement for manipulating sliding sleeves, in wells with severe doglegs.
Meanwhile, crane capacities, platform deck load restrictions or weather conditions often limit the weight of the CT string that can be lifted. In the past, service companies have minimized weights by using split-reel systems and thin-walled, high-strength parallel CT strings with constant wall thickness. For very deep cleanouts and for selective stimulation operations, service providers were forced to split CT strings into two (or more) lifts and weld the strings together offshore.
Welding raises safety concerns, however. Orbital girth welds in CT butt joints typically reduce the lifetime of the CT string by 25 to 50% and require a large safety factor because of the large scatter in low-cycle fatigue life. In addition, weld fatigue failures are not obvious on the surface of the CT even if one could monitor the weld area after each run to assess its safety.
Furthermore, welding requires specialty DNV-certified welding and inspection equipment and personnel.
Another possible solution to the weight problem is boat spooling, but this option is unsuitable offshore India because of unpredictable weather conditions, specialty equipment requirements and logistics issues.
As a safer alternative, BJ Services developed the DuraLink spoolable connector, which reliably splices two CT strings flush on the outside diameter of the tubing. In addition to being faster to install than a butt weld and requiring fewer personnel offshore, the resulting joint has a longer safe fatigue lifetime. Plus, the CT crew can visually inspect it after each run for quantifiable indications that it should be replaced.
Other benefits include:
This connector has been particularly welcomed to facilitate coiled tubing cleanouts in large-bore wells. Being able to provide the correct size of CT avoids compromising job quality and represents a fundamental change in how CT operations are planned and conducted.
Of the first 17 operations performed using the DuraLink connector, 15 had been previously considered unfeasible due to the inability to lift CT of the correct size to the platform.
Work offshore India
In some recent operations off the west coast of India, a number of factors hindered successful coiled tubing interventions. Most importantly, the lifting capacity of the platform cranes was limited to 13 tons in calm weather, and platform deck load limits constrained the weight of equipment in particular locations. These limitations prevented many CT operations that required a minimum ‘push weight’ at the bottom of the well to reach and stimulate a horizontal target or to manipulate sliding sleeves.
For example, in two wells with long horizontal segments, water production was a concern. The well had been completed with a series of sliding sleeves and swellable packers, each set isolating a particular zone of interest in the horizontal section. The operator wanted to determine which zones were producing the water and whether the swellable packers were functioning as expected. The goal, therefore, was to use coiled tubing to open and close each sleeve, selectively flowing each zone independently to establish its water contribution and isolation. (It’s worth noting that another enabling factor for this application was the ability of CT to perform such operations in live well conditions.)
BJ Services engineers used proprietary CIRCA tubing force software to determine that 1.5in CT would not provide adequate force to shift the deepest sleeves, but 1.75in tubing could. However, a spool of 1.75in CT long enough to reach those sleeves would have exceeded the platform crane capacity.
Instead, the CT was split onto two reels, each of which was well under the crane limit. The two strings were then joined on the platform using a DuraLink connector.
This presented a new weight problem: how to align the CT reels for work on two different wells on the platform. The combined reel weighed 19.22 tons – too much for the platform crane to move into position for the intervention work. Thus, a track-and-trolley system was designed to be used with a turntable that could align the full reel with the coiled tubing gooseneck. The system also spread the weight load across the platform deck.
In the operation, the connector was installed as planned and the reel adjusted for alignment with the first well. A Roto-Jet high-energy, stress-cycling washing tool was attached as a bottomhole assembly for an acid wash to clean out scale, fill and debris in the well before attempting to manipulate the sleeves. The CT was run into the well and the cleaning operation performed as planned.
However, as the CT was being pulled out of the well, the downhole safety valve malfunctioned and closed on the DuraLink connector. After several attempts to pull the coiled tubing free, the malfunction was discovered and the safety valve re-opened, freeing the CT string. At the surface, the connector and CT were found to be undamaged, but the connector was missing some spacer rings. The rings were not essential to its safe functionality, but the operator decided to defer further operations in the well until the spacer rings could be retrieved (with a junk basket or simple fishing operation), out of concern that the rings might interfere with the movement of the sliding sleeves.
Instead, the turntable was used to position the CT for the second well. The BHA included the Roto-Jet tool and a hydraulic sleeve-shifting tool so the acid wash could be followed immediately by the sleeve manipulation. This time, the operation was performed completely as planned, in six runs all using the same connector. To ensure safe operations, personnel were able to inspect the connector and surrounding tubing for ovality and ballooning each time it passed to and from the gooseneck.
Monitoring for safety
Locating a butt weld on a segment of tubing can be a challenge, and if found it may show no signs of degradation even when it is ready to fail. On the other hand, extensive destructive testing with the DuraLink connector has determined a precise set of parameters that indicate whether a connector is fit for service or in need of replacement. One of these parameters is the OD of the CT at the tubing-connector interface. This section of tubing responds very predictably to the plastic cycling of the connector under internal pressure. Another indication is surface rippling – which provides an early indication that the connector should be replaced.
In the India well, no surface changes were seen. Proprietary Cycle fatigue monitoring software also did not indicate any concerns for the connector or coiled tubing lifetime. As a result, the intervention was performed as planned, including manipulating all of the sleeves as required.
After the operations, the connector segment was removed from the CT and shipped for post-job evaluation at BJ Services’ Coiled Tubing Research & Engineering Centre in Calgary, Canada. After testing on a bend fatigue machine, scientists concluded that the connector could have been safely used for at least five more runs while maintaining a safety factor of 2.
After the two initial operations described above, BJ Services has used the DuraLink connector and track-and-trolley system for interventions in three more wells for the same customer offshore India. In addition, researchers are testing new connector designs that incorporate lessons learned from these experiences and more than 120 CT runs performed around the world using the connector. OE
About the Authors
Godwin Effiong is a district engineer for BJ Services in Mumbai. He has 21 years of experience in the oilfield, with a focus on coiled tubing operations since 1991.
Greg Dean is engineering manager for BJ Services in Mumbai. He has 22 years experience working in the areas of stimulation, sand control, coiled tubing, cementing and completion tools.