Drilling advances

	Vol. 227 No. 2
LES SKINNER, PE CONTRIBUTING EDITOR

Extended reach drilling. Extended reach drilling (ERD) has been identified as a key technology by most major operators as well as many independents. In a nutshell, it is defined as drilling a directional well from one point to hit a very remote geological target at some specified depth. The “reach” is that lateral distance between the surface and the final bottomhole location.

Generally, the reach to depth ratio is a measure of success. If a well has an R/D ratio of 1.0, for example, its departure from the surface hole is equal to its depth. Most directional wells are drilled with the upper portion of the wellbore vertical. Then, at some specified depth, the hole is “kicked off” and angle is built to some maximum, and that angle is held to total depth. The higher the R/D ratio is, the greater the required deviation angle. For the longer reach wells where the R/D ratio is 3, 4 or 5 (depending on true vertical depth), the end of the well approaches 90°.

These are not just complicated directional wells. They are composites of multiple technologies all of which must be applied properly, with excellent quality control, at the same time. A failure in any of them either dooms the well mechanically or limits the possible reach.

So, what’s involved in ERD? Answer: the entire summation of current drilling technology. Here are a few requirements for a long-reach ERD well:

An exceptionally smooth borehole. In the past, directional wells were drilled by kicking off a cement plug or a whipstock with no control either in deviation angle or azimuth. The results were wells with severe “doglegs,” abrupt wellbore direction changes.

Advances in directional drilling tools provided the industry with bent subs and adjustable bent motors such that the angle could be modified at the surface before the tools were run. Then, through a system of “sliding” and “rotating,” the desired wellbore path could be achieved. This drilling yields a series of curved and straight wellbore segments with a dogleg at each transition.

Recently, the directional drilling industry has developed rotary steerable tools that allow the deviation angle and azimuth to be continuously altered throughout the angle-build section. Once an angle-hold portion of the wellbore is reached, the same tools can prevent further deviation or azimuthal changes. These systems report to surface detectors for a continuous reading of progress and, importantly, can be changed by sending a signal to the tool. The result is an incredibly smooth wellbore with little tortuosity.

Clean holes. In conventional drilling, thin cuttings beds and a few sloughed cavings can be tolerated. Not so in ERD. Drilling fluids must ensure that cuttings are continuously cleaned from the newly drilled hole and that borehole stability is maintained. Without good cleaning, drilling problems are bound to occur such as packoffs, lost circulation, twist-offs, stuck pipe, high torque and drill string lockup (not being able to run the pipe to bottom).

Multiple casing string or liners. Assume a well is planned to reach a reservoir at a true vertical depth of 6,000 ft. It may require only three or four casing strings to adequately protect all the zones in the wellbore. Not too bad, right?

Now assume the same well is to be drilled with an R/D ratio of 3.0. Its bottomhole location is 18,000 ft laterally away from the surface location, and it is 6,000 ft deep at its endpoint. Depending on the specific wellpath, the well can be 21,000 to 24,000 ft long. Three or four casing strings may not be sufficient to seal off all the intervals. If just that many could do the job, they would be incredibly long. “Incredibly long” normally implies thick-walled, high-tensile strength, special connections, hard-to-locate and very, very expensive.

The problem is: How big should the well start at the surface? What are the trade-offs in cost between hole size, casing/ liner segment lengths, costs for mud and cement, wellheads and rig time? Needless to say, designing one of these long-reach wells is not a simple task.

Friction-reducing appliances. The drilling industry has developed an impressive array of tools to reduce torque and drag. Some designs have supports that hold the pipe off the wall providing a limited number of contact points (like the blades of a reamer). Others have sleeves that intentionally contact the wall with internal bearings that allow the pipe to turn inside the sleeve. Others have centralizers equipped with low-friction outer surfaces such as ball bearings, polymers and special metal alloys.

The friction factors reported in the literature achieved by these devices are almost unbelievable (as low as 0.04 in casing and 0.10 in the open hole).

Lubricants. Various chemical solutions, oils and greases have been used to free stuck pipe. Some now that give the term “slick” a new meaning and many can damage the objective formation. So, the selection of the proper lubricant is crucial both to the drilling process and to later production.

One “lubricant” involves mechanical friction reduction. These are beads or compounds that allow the pipe to slide over their surface when they are placed in the wellbore. For years, stuck coiled tubing was freed by circulating glass beads. These provided tiny bearings that allowed the tubing to slide more easily. Other beads have been used including metals, ceramics and polymers.

Quality control and supervision. In the past, a minor error in a directional drilling program could be tolerated. In ERD work, any error can have a significant negative impact on the reach attainable by a long-reach well. Just getting close won’t do the trick with these wells. Precision is imperative if the well is to make it to bottom. Very tight performance limits, precise instrumentation and vigorous oversight are required during every phase. One “minor” error in the upper part of the hole can result in a well that will be too short by several thousand feet.

Why bother with ERD in the first place? Simple. Many reserve accumulations cannot support a separate drillsite, and they will not be developed if the economic justification does not exist. If, however, they are accessible through a long-reach ERD well from a drillsite even several miles away, not only are the reserves economically viable, but they go a long way toward supplying a crucial commodity to an energy-hungry world.