Coiled tubing, thru-tubing solutions
An independent Canadian operator has demonstrated that conventional coiled tubing can be used to perform downhole rotational and removal operations. Recent operations, as described by Gordon Mackenzie and Graeme Kelbie of Baker Oil Tools, came as part of one of the operator’s coiled-tubing intervention campaigns in the UK sector of the North Sea.
One intervention was designed to remove a third-party mechanical type retrievable bridge plug. The plug had been set at 12,500 ft MD (10,686 ft TVD) in 5.5-in., 26 lb/ft, L-80 13% Cr tubing. The customer suspected that tubing to annulus communication on the well may have been from one of the gas lift mandrels in the completion string. Subsequent pressure testing operations on the bridge plug negated this hypothesis. Conventional wireline retrieval methods to recover the bridge plug proved unsuccessful. It was thought that a sheath of scale left behind on the tubing from a previous scale milling intervention could have been dislodged while pressure testing the bridge plug, thereby restricting access to the fishing profile.
Baker started the coiled tubing operation by conducting a number of runs with either a bridge plug retrieving tool or a debris clean-out assembly. They recovered scale, and although the bridge plug was latched several times, it proved reluctant to release. This particular iteration of 4140 material plug had not been milled in prior field operations.
Baker used a rotary shoe - 4.375-in. smooth outside diameter by 3.625-in. inside diameter - internally dressed with a Metal Muncher cutting matrix in conjunction with a 2.875-in. OD Navi-Drill VIP workover motor. Their philosophy was that by removing the OD components of the bridge plug they would release the setting force from the tool, allowing them to recover the plug on a subsequent trip. Over eight hours, Baker took two milling trips and believed the plug was free in the well. After a further clean-up run in which they recovered pieces of rubber element, Baker used a hydraulic release pulling tool assembly, which latched an internal profile within the bridge plug to allow for retrieval of the remaining plug components.
“Successful coiled tubing milling/removal of the bridge plug returned the well to its original status with tubing to annulus communication,” says Mackenzie. Further leak detection operations using wireline services confirmed the completion packer was providing an annular communication path. The initial mechanical retrievable bridge plug set in the ID of the 5.5-in., 26 lb/ft, L-80 13% Cr tubing string served to confirm the integrity of the tubing and associated jewelry above its 12,500-ft setting depth. A memory production-logging tool run on wireline confirmed that production flow was bypassing the production packer.
Bridge plug washover assembly was run using a 4.375-in. smooth OD by 3.625-in. ID rotary shoe internally dressed with a cutting matrix in conjunction with a 2.875-in. OD workover motor.
The operator wanted to isolate the well in the liner below the completion. The minimum wellbore restriction above the 7-in., 32 lb/ft (6.094-in. ID) liner was a 4.01-in. ID wireline entry guide. Baker engineered and supplied a high expansion inflatable plugging solution for deployment on the 1.75-in. OD tapered coiled tubing string. They ran a 3.375-in. OD retrievable inflatable bridge plug into the well and set it at 12,800 ft with a hydraulic release tool dictating a final setting pressure of 1,550 psi. With this inflation pressure, the temporary inflatable isolation could support a maximum applied pressure differential of 6,200 psi and a maximum drawdown pressure of 4,550 psi. Once they set the inflatable retrievable bridge pug within the liner string, a surface inflow test of 3,200 psi was conducted. With the success of this inflow test, the well was temporarily suspended in this condition pending further workover operations.
Second intervention
With the second of the planned coiled tubing operations, Baker attempted to clean out a liner obstruction in a well that had been sidetracked with a through-tubing rotary drilling methodology in conjunction with a custom designed 3.375-in. OD workover motor. The clean-out was designed to facilitate further wellbore isolation and perforating below the restriction.
“A number of varying operations had been conducted to access the required perforation depth in the 2.875-in., 6.4 lb/ft, L-80 13% Cr liner,” says Kelbie. “These operations had included competitor attempts with a milling assembly and also with a coiled tubing abrasive pumping type methodology. The restriction was identified at a depth of 14,383 ft MD.”
It was suspected the restriction may have resulted in part from asphaltine deposition. The initial coiled tubing run deployed a hydraulically actuated debris collection system with a 2.125-in. OD coiled tubing workover motor. The debris system incorporated an externally dressed rotary shoe designed for a variety of clean-out options in a single trip in hole. The application of this system allowed the tool string to reach a hold-up depth of 14,588 ft, 205 ft deeper than the original tag depth.
For the second run, Baker ran the coiled tubing bottom hole assembly with another 2.125-in. OD workover motor along with a 2.31-in. OD turbo mill. At the previously reached hold up depth of 14,588 ft, Baker once again tagged the restriction, at which point milling operations started. Within 7.5 hours of milling time, Baker achieved 14,793.5 ft TD. The objective was to have the lowest perforation at 14,586 ft. The second milling trip allowed the objective to be met while also gaining wellbore access further into the previously perforated production intervals.
The operator wanted to perform a permanent plug back operation on the open perforations prior to perforating the next zone. To accomplish the plug back, Baker conveyed their cast iron bridge plug on coiled tubing, set it at 14,620 ft depth, and successfully tested it. After perforating the upper zone with electric wireline conveyed guns, Baker brought this previously non-productive well back online at 9,000 b/d, which decreased over the next few weeks to an average production of 6,000 b/d.
Mackenzie and Kelbie commented that from both an HS&E and economic perspective, this client was able to achieve its coiled tubing campaign objectives by using “fit for purpose” coiled tubing tools with engineering principles centered on providing a thru-tubing solutions ideology.
