Deepwater system saves time, liner size in a tight-tolerance, extended-reach well
Ken Whanger, Evan Lowe - Weatherford International Ltd.
Tight-tolerance, extended-reach deepwater drilling poses multiple hazards, including high surge pressures, excessive mud losses, and potential formation damage. In a tight-hole in the Green Canyon block of the Gulf of Mexico (water depth about 5,600 ft or 1,707 m), the operator was unable to drill a directional exploration well through an extreme lost-circulation zone at about 25,000 ft. (7,620 m).
One possible solution was to drill through the problem zone with liner (DwL), but there were significant challenges. The casing size above the zone was 135⁄8 in., and the operator required the next casing to be 11 7⁄8 in., leaving little annular space. Radial annular clearance between casing strings is approximately 0.125 in. (1⁄8 in. or 3.175 mm). Cuttings and debris in the drilling fluid would be an issue, and hanger components would have to be compact yet with enough wall thickness to provide the requisite torque, tensile, burst, and collapse strength. Overall, the tight-tolerance system would impose significant restrictions on torque and circulation capacity.
Drilling with casing or liner is common in some areas – it is routinely used in South Texas – but it is much less common in the Gulf. Where it is used, the general practice is 95⁄8-in. liner in 135⁄8-in. casing, which leaves adequate annular space but limits hole depth. In this case, the operator could not sacrifice the liner size.
DwL advantages
Liner is more cumbersome than drillpipe, but DwL can offer advantages beyond that of navigating a lost-circulation zone. It reduces the total number of trips to help limit borehole instability issues and to reduce potential formation damage from trip swabs and surges. It also eliminates a wiper trip between drilling and setting the casing. This alone can save a day of rig time, or more, in the deepwater GoM.
DwL reduces exposure to risk by requiring fewer personnel on the rig floor, and also drills with lower mud weights. Because the liner diameter is larger than that of drillpipe, holes are straighter, which reduces the torque, drag, and cleaning problems of corkscrewing. Like conventional drilling, DwL can be used in underbalanced conditions.
Finally, DwL generally leads to better cement jobs. Circulation is uninterrupted until cementing begins, and the high-quality, near-gauge borehole optimizes hole cleaning, cement placement, and bonding. Cementing can begin as soon as the casing reaches total depth and the more uniform borehole helps prevent channeling.
DwL generally includes drilling virgin hole with a drillshoe at the end of the liner and with the liner-hanger assembly at the top of the liner. When drilling is complete, the process begins setting the hanger and releasing the running tool, cementing the liner, pressure testing the liner, setting the packer at the top of the liner and pressure-testing it, reverse circulating excess cement out of the hole, and pulling the tool out of the hole.
A closer view of the components of the premium liner system used in this job shows that the string to the left comprises rental running tools, while the red string to the right represents the purchased liner system that remains in the hole.
Although DwL systems appear to have interchangeable parts, the tool string must be designed as a whole. It is virtually impossible to tweak one component without affecting almost every other aspect of the system. The overall goal is to reduce risk and provide value, so issues like early tool release or improper liner or packer setting must be prevented. This is not the place to save pennies and risk dollars.
At the top of the liner hanger system is the polished bore receptacle (PBR), which contains the packer actuator and is topped by a patented floating junk bonnet (FJB). While the junk bonnet might seem inconsequential, it can be critical. When drilling fluid and cement are circulated, the annular velocity in the limited space between liner and casing is high, and debris is carried up as long as circulation is maintained. When fluid reaches the drillstring just above the top of the liner hanger system, the annular space increases, velocity drops, and suspended solids fall out and settle. The junk bonnet is to prevent these solids from entering the PBR, where they might settle around the running tools to prevent them from being retrieved. The FJB is designed with reamer blades on top to back-ream itself out of the PBR when being pulled after the cement job.
The FJB in this GoM case is a proprietary tool that sits on a column of clean water preinstalled in the PBR, so the FJB always remains at the top of the toolstring. A conventional junk bonnet typically moves during liner hanger setting, potentially allowing debris to settle inside the PBR around the running tools.
The packer actuator inside the PBR sets the liner-top packer after cementing. As the toolstring is raised, the packer actuator snaps out. The drill string can be set down and the dogs will engage the top of the PBR. Drillstring weight can then be transferred through the PBR to the packer below, setting the packing element and slips.
Below the PBR is the model “R” setting tool with a “float nut” that threads into the packer body. This connection carries all liner-hanger system components and liner downhole. It has two obvious and critical jobs — not to release the liner-hanger prematurely, and to release without problems when required.
Conventional liner running tools release from the liner hanger by dropping a setting ball and activating a hydraulic mechanism. If the hydraulic mechanism fails, a backup mechanical release allows the tool to be unscrewed by rotating to the left. Any left-hand rotation risks backing off the drillpipe. In addition, there are cases where this mechanical backup released accidently when the string was picked up to make a connection, because residual torque in the drillstring popped the running tool back to the left, causing it to release and drop the liner.
The “R” setting tool used for this 11 7⁄8-in. liner avoids the potential of accidental release. It is designed with a “drill down” feature that releases only when two specific events occur – hydraulic pressure against the setting ball deactivates the drilldown feature, and right hand rotation while in compression releases the setting tool.
While drilling the liner into the well, the torque path through the running tools is conveyed by the thrusting cap, which sits at the bottom of the tool and fits into castellations (think of a chess Rook) at the top of the liner-top packer. This “notched” connection transfers torque from the drillstring and running tools to the liner system and allows the tool and liner to rotate in either direction without danger of accidental release.
Pressuring the toolstring to a pre-determined level hydraulically unlocks the drill down feature. However, this alone is not sufficient for release. When the drill down feature is unlocked, the torque path changes to allow the float nut on the running tool to release from the liner. To do so, the tool must be placed in compression and rotated to the right to release the liner.
The hydraulic pressure that unlocks the drill down feature also sets the slips on the liner hanger, so when the right-hand rotation takes place the liner is hung firmly in position.
This tool will not release without hydraulic pressure, compression, and rotation to the right. It cannot be released accidentally by residual left-hand torque, compression, or tension, which can be critical in extended-reach and tight-tolerance situations. This technology has been used to run 13 of the 20 longest extended-reach liners.
After setting the liner and releasing the running tool, the string is picked up to confirm release of the running tools, after which weight is set back down and the system is pressured to blow out the setting ball. Next is to circulate the well and perform the cement job. When the cement plug has landed and is holding pressure, the tool string is picked up so that the packer actuator reaches the top of the PBR, where it snaps into place. When weight is set back down, it energizes the packing element, which seals the liner annulus to the casing.
The packer also isolates the cement and prevents gas migration or flow while the cement sets. After a quick test to verify that the packer is set, the tool string can be pulled out, and the liner is ready for a clean-out trip and subsequent drilling.
One often-overlooked component is the wiper plug at the bottom of the running tool string. Conventional wiper plugs are shear-pinned to the end of the tool string “slick stinger” that runs through the liner hanger system. When the tool string moves up or down, the wiper plug moves with it. This means the plug can be damaged or prematurely sheared by junk at the bottom of the hole, either of which can make it difficult or impossible to pressure up the tool to release it. The wiper used for this case attaches to a proprietary stationary jointed system, so that it does not move until being launched.
Job details
For this DwL case, Weatherford provided the entire liner running and cementing system, except for the sacrificial casing drillshoe. Operations took place from a semisubmersible rig. The job started at 24,730 ft (7,538 m) MD, just above the problem formation, and reamed the 11 7⁄8 in. liner-hanger assembly inside 135⁄8 in. pipe to 24,868 ft (7,580 m) MD (138 ft or 42 m), while rotating it at 60 rpm with a flow rate of 1.5 b/min synthetic oil drilling fluid. Annular clearance was approximately 0.125 in radially.
The liner then was drilled into virgin formation, going 176 ft from 24,868 ft MD to 25,044 ft (7,633 m) MD (the target depth) with a rotation speed of 40 to 86 rpm and flow rate of 1.5 to 3.5 b/min. The maximum deviation in the well was 30°, and total reaming and drilling time was 18 hours. There were no returns during the process—everything went into the lost-circulation zone. The hanger was set, the running tool released, and the liner shoe cemented into place. Plugs were bumped, and the liner top packer was set and tested to 2,100 psi (145 bar) for 30 min. The liner-hanger running tools were pulled out of the hole, and the operator continued drilling to the target depth.
This floating junk bonnet has been pulled from a 30,000-ft (9,144-m) hole and still retains some of the junk it prevented from falling into the hanger assembly.
