Instead of drilling from an expensive offshore platform, a major Gulf of Mexico (GoM) operator wanted to reach its offshore reserves from an onshore surface facility. The well would have to reach a measured depth (MD) of 30,880 ft (9,412 m) with a true vertical depth (TVD) of only 11,591 ft (3,533 m); and engineers were concerned about running a 95?8-in. casing string nearly 27,000-ft (8,230-m) long. The challenge — could the team get the casing all the way to total depth (TD), could they rotate the casing during cementing, and most importantly, could they pull the casing out if problems developed?

Reducing drag

One way to get around the bend, or build a section of a horizontal well, is to “float” the pipe into the hole to reduce drag. Two float valves — one at the bottom, the other higher up — trap air so the pipe floats through the heavy drilling fluid in the hole, rising to the top of the horizontal section instead of sinking to the bottom. This process works for short pipe lengths; however casing flotation systems rely on two valves, and if one does not work, the entire job could be jeopardized.

Flotation jobs are often run slick, without centralizers, because this frees the pipe to move around. If the casing unexpectedly floods in the lateral section with no centralization, it sinks in the cuttings bed and can become stuck and unable to trip in or out. Even if the string makes it to TD, as soon as the valves are opened for cementing to proceed, the casing sinks to the bottom where it can not be rotated — and that makes for a poor cement job.

It is impossible to circulate drilling mud because the float valves seal off the pipe. Due to the irregularity of most well bores, a pipe can often hit a ledge and get stuck. In this case, the normal practice would be to pick up the casing string, start circulating, and then allow the drilling fluid to clear out the hole and lubricate the passage of the pipe.

This option is eliminated when a casing string is being floated in.

There is also the issue of pipe buckling. A floating pipe is essentially weightless, which means it is pushed into the hole instead of dropping in under its own weight. As the pipe goes deeper, the increasing hydrostatic pressure outside the pipe adds to the stress, and the pipe can buckle. To compensate, operators use heavier-walled pipe — but this pipe is even more rigid and likely to get stuck in curved sections of the well. The longer the well is, the greater the risk.

Finally, the air trapped in a floating casing must be removed from the well by flowing up through the casing, expanding as hydrostatic pressure decreases. Depending on the length of the string, removing the expanding air column could result in significant rig downtime and an increased risk of the pipe becoming stuck as a result of mud gelling.

Reducing torque

Aside from drag, there is also the question of torque, which can play out in drilling and in running and cementing liners and casing. When the liner or casing is stationary during cementing, cement always channels to the high side, with poor results. Rotation solves the problem by distributing cement evenly around the pipe.

A substantial amount of torque is required to rotate 95?8-in. casing or larger. The feat could become impossible as cement and extended pipe length are added. Even if the drilling rig has the torque capacity in the top drive, the strain on the pipe can exceed the strength of the drillpipe and casing connections.

There are some relatively simple, non-engineered solutions to reducing torque, but the effectiveness of these solutions is limited. For example, Teflon buttons designed to reduce frictional drag are infamous for shearing off in the hole.

Reaching farther

Weatherford’s engineered solutions for torque-and-drag reduction include LoTAD, LoTORQ, and LoDRAG roller centralizers. These tools have been commercially available for more than a decade but are gaining recognition with the growing importance and challenges of extended-reach-drilling (ERD) wells.

Essentially, the tools provide further reach for horizontal wells. ERD wells drilled from land to offshore reservoirs have specifically benefited. A major operator used the LoTAD and LoTORQ systems on various completion and liner runs in the Sakhalin I project.

Weatherford engineering is involved in evaluating and simulating the well, based on planned and actual North Slope for the hole trajectory. The graph shows the torque-and-drag analysis of an 85?8-in. liner run in a 24,000-ft (7,315 m) MD, ERD well.

The red lines show the maximum torque that the drillpipe and casing connections can withstand. (Drillpipe connections are typically stronger than casing connections.) The sloping lines show torque as the liner is rotated during cementing. Cement is inside the casing near the bottom of the string, just before entering the annulus. This situation causes the greatest amount of friction and requires the greatest torque to rotate.

The blue line (without roller tools) shows that rotation during cementing would break both drillpipe and casing connections. Using Weatherford’s mechanical friction reduction tools helps alleviate torque. The ideal engineered solution for cementing and rotation involves the LoTAD systems in the drillstring and the LoTORQ systems on the casing, as shown by the green line. Here torque is well within the limits, even for the near worst-case friction scenario represented by this graph.

GoM operator gets its answer

Using Weatherford’s LoTORQ and LoTAD systems enabled the operator to rotate and cement 26,591 ft (8,105 m) of 95?8-in. casing at a record-setting depth. The ability to place extended-reach wells in the reservoir from an existing onshore facility reduced drilling and production costs dramatically. In addition, the casing string was retrievable at all times, greatly reducing risk.

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