For many operators, coiled tubing (CT) has become the standard conveyance method for almost all fishing and equipment retrieval operations for situations where wireline is not a viable option. When compared to a workover rig, the advantage of using CT is that it can be run in and out of the wellbore much faster than jointed pipe. This makes it a great option for simple pick-up jobs.
Needless to say, CT also may be used for more complex jobs with proper planning. This can save the operator time and money, especially when multiple runs are required to retrieve a fish or downhole equipment. Downfalls to using CT for fishing operations include lower tensile pipe yields compared to jointed pipe, limited rotational capabilities and the limited number of pipe “cycles” available during jarring operations.
As the CT is run into the wellbore, it is continuously fatigued as it unwinds off the reel and as it bends over the gooseneck into the injector. The CT is again exposed to the same fatigue spots as it is pulled out of the wellbore. As pumping operations begin and internal CT pressures begin to increase, the CT will fatigue exponentially as pressures climb toward the maximum operating limit.
Additionally, larger diameter CT will fatigue faster than smaller diameter CT. Lastly, the steel grade of the CT will affect how quickly the pipe will fatigue. For example, 100,000-psi grade CT will fatigue faster than 80,000-psi grade CT. Although lower tensile-yield CT will fatigue slower and is necessary for sour gas wells, higher tensile-yield CT is needed for deep offshore wells and long horizontal land wells where high pulling weights are common.
Most existing commercial hydraulic jars require the CT to be cycled to fire and reset (also referred to as recocking) the jar in the fishing bottomhole assembly (BHA). Considering that each successive firing and resetting of the jar requires pipe cycling, a limit is set on how many times this process can occur. This helps to prevent a large fatigue “spike” in the CT. If not monitored closely, spikes will diminish the life of the CT string quickly.
All of this cannot go without saying that fatigue on the CT costs money, a cost that is paid for by the operator or absorbed by the CT company. A hydraulic jar for CT that can be fired and reset without cycling the CT can greatly suppress financial impact by allowing extended jarring operations beyond what the industry has seen.
After engaging the fish, the jar can be fired by applying overpull to the string. After the jar fires, overpull on the string is maintained as it is not necessary to release the string tension for the resetting process. The jar encompasses internal overbalanced pistons that create an area for fluidic pressure to act upon.
To reset the jar, CT pressure is increased to a predetermined amount that is dependent on the amount of overpull on the string. When the correct amount of differential pressure is reached, the jar will overcome the tension force being exerted and reset hydraulically (Figure 1). After the jar has been reset, the CT pressure is relieved, which will allow the jar to stroke upward again and fire. Each successive resetting and firing of the jar can be performed without ever cycling CT, therefore extending CT string life and reducing cycle costs.
Land case study
An operator in southern Louisiana had a wireline tool string that was stuck in 4.5-in., 15.1-lb casing in the build section of the wellbore. It was unknown at the time how much force would be required to free the wireline tool string, so the decision was made to use CT instead of a workover rig for the operation. A 2-in. diameter, 100,000-psi grade CT was used for the fishing operation.
After initially washing over the fish, a fishing BHA was deployed that consisted of the hydraulic reset jar, standard 150-series overshot and other necessary BHA components. The fish was successfully engaged with the overshot around 2,758 m (9,050 ft) rotary kelly bushing. An overpull of about 12,000 lb over string weight was maintained on the CT until the jars fired.
Since the fish did not come free after the first jarring attempt, 12,000-lb overpull was maintained on the CT and a simple calculation was done to determine that approximately 1,400-psi differential pressure at the BHA was required to overcome the tensile force (overpull) to reset the jar.
The pump was brought online at 2 bbl/min and maintained at that rate for two minutes to ensure that the jars had been reset. To fire the jars again, the pump was brought offline and all CT pressure was relieved while still maintaining 12,000-lb overpull. After a few minutes, the jars fired for the second time. The wireline tool string was still stuck, so the jar resetting process was repeated.
The jar was successfully fired and reset hydraulically 39 times before the CT was ever cycled again. After the 39th time, the wireline tool string parted in half and the CT was pulled out of the hole. Once at surface, the fish was removed, and the overshot and jar were replaced for a second run in hole.
The remaining fish was successfully latched, and jarring operations continued while initially maintaining 16,000 lb of overpull. For the resetting process, approximately 1,850-psi differential pressure at the BHA was necessary to overcome the tensile force to reset the jar. In summary, the jar was fired and reset a total of 62 times with a maximum attempted overpull of 31,000 lb to fire the jar. At this force, a differential pressure of about 3,550 psi was required to reset the jar hydraulically. On multiple occasions after firing the jar, a maximum overpull of 41,000 lb was attempted. All attempts were unsuccessful at retrieving the remaining fish. Although more force was required to retrieve the fish than what was achievable with CT, it was well known that all attempts with CT were exhausted and that the limiting factor of the fishing operation was not due to the limited number of cycles. The hydraulic reset jar was fired a total of 101 times and was partially successful at retrieving the fish. This would have been an impossible task for a standard hydraulic jar due to the cyclic fatigue generated during the standard jarring operation. The financial impact of using CT was minimized due to the minimal amount of string life used by the hydraulic reset jar.