This paper will examine an unusual intervention case history for a major UK North Sea operator, where a specialised coiled tubing (CT) conveyed mechanical cutter system was used to allow the recovery of stuck drill pipe.
Previous to the CT being mobilised to the offshore installation, a number of conventional drill pipe recovery techniques had been tried. However, all proved unsuccessful in freeing the stuck DP.
The decision was made to mobilise CT, dictated by both the need to free the High Strength S-135 DP and to confirm the degree of ID access for future intervention options. Ultimately a series of three mechanical cuts were made within the drill string. All cuts were at depths greater than 14,000 feet with 11.8 PPG OBM in the well. The particular challenges presented during this operation and the steps taken to provide a successful outcome will be explored.
During liner setting / cementation operations on the well in question, the drillstring had become stuck in the hole. All attempts to free the drillstring had failed by the time that the cement set fully. This left an overall length of 18,475 feet of plugged drillstring stuck in hole, since circulation had been lost entirely while attempts to free the drillstring were being made.
The wellbore configuration consisted of 9 5/8"casing from 17,848 feet tied back to surface. A 7" × 7 5/8"liner was installed from the plugged back TD of 19,190 feet to inside the 9 5/8" casing at 17, 540 feet.
To operate within this well bore geometry the drillstring was comprised of 16,869 feet of 5.00" OD 19.50 lbs/ft. drill pipe attached directly to a liner hanger running tool assembly. Below this point the remaining 1,606 feet of drillstring was comprised of smaller 3.50" OD 15.50 lbs/ft. drill pipe that was included to allow access into the smaller liner.
Once the drillstring became stuck the drilling operation changed focus to a fishing operation. In so doing attempts to free and recover the drillstring revolved around standard industry practices using electric wireline to deploy assemblies within the drillstring. Primary fishing contingencies for stuck drill pipe generally include the following options:
Stretch calculations can be performed to determine at what point within the well the drillstring is stuck. However this method is not regarded as very accurate since the equation used to determine stuck point does not take into consideration many key parameters.
When possible the stuck point within a drillstring can be accurately determined through the deployment of a Free Point Indicator Tool [FPIT] on electric wireline. This assembly is run inside the drillstring and ‘activated’ at various depths. Prior to activation, tension and/or torsion are applied to the drillstring from surface. If the drillstring is free down to the depth where the ‘activated’ FPIT is located then it will detect either the tension and/or torsion within the drillstring, confirming that it is at least partially free.
At a depth where neither tension nor torsion are detected by the FPIT, the drillstring can be assumed to be stuck since an external sticking mechanism is not allowing applied tension and/or torsion to continue to be transmitted through the drillstring.
Once the stuck point has been confirmed through the use of a FPIT, a back-off charge will be deployed, using electric wireline, to a safe depth above the stuck point. The aim of this assembly is to detonate a controlled explosive charge within the drillstring that will allow a rotary connection to be loosened.
The back-off charge assembly is run in conjunction with a casing collar locator (CCL) to provide accurate depth measurements. By comparing the CCL depth log output with the drillstring tally, the back-off charge can be placed exactly across the desired rotary connection.