DISCUSIÓN

Although the reasons for drilling short-radius wells usually are based on a physical constraint of some sort, there is frequently some choice about just how short the radius is. This decision is critical, not just because it affects how the build section is drilled, but because the radius affects how the rest

of the well can be designed and drilled. Drilling efficiency in the lateral and lateral-reach capability are the factors that are most affected by the radius of curvature in the build section.

The lateral section of a short-radius well is drilled either in the sliding mode or with very slow drillstring rotation. If build rate and the resulting pipe stresses preclude rotation, the lateral must be drilled entirely in the oriented mode. Though the motor used to drill the lateral has a fixed build rate, the well can be kept within a small TVD plane by alternating orientations to the right and left, resulting in a sinusoidal profile when viewed in the horizontal plane (Figure 2-25).

Figure 2-25 Drillstring rotation enables smoother well profiles, further extension This procedure is acceptable in many cases, but the increased drillstring drag resulting from this technique will eventually limit the length of lateral that can be drilled. Oriented drilling will also progress at a much slower ROP than when the drillstring is rotated.

If the build rate and the resulting drillpipe stresses are low enough to allow limited rotation, drilling the lateral with rotation is certainly preferable to sliding. In addition to improved ROP and reach capability and the ability to drill a straight hole, rotating results in better hole cleaning and a reduced chance of stuck pipe. However, depending on drillpipe size and the radius of the curvature, the pipe is stressed somewhere between its endurance limit and its yield point. Since damage from fatigue is a function of the stress level and the accumulated number of cycles, at some number of cycles, the drillstring will fail. Figure 2-26 provides guidelines for limited rotation for various radii of curvature and drillpipe sizes and grades, based on both calculations and historical data. However, cycles-to-failure are difficult to

quantify because of variations in material strength, previous stress history, corrosion, surface finish, and wear.

Figure 2-26 General guidelines for limited drillpipe rotation in high-build-rate wellbores

While cycles-to-failure cannot be predicted precisely, one thing is certain:

the longer the radius of the well, the lower the chance of failure, and the more contingency options that are available. The radius of the build section directly affects the motor's ability to rotate, and thereby affects drilling efficiency in the lateral. Since the lateral comprises 80% to 90% of footage drilled in short-radius wells, it is the main factor in drilling costs. Drillpipe damage, ROP, and trouble time all factor heavily into short-radius economics. Some operators have settled on an established radius of about 75 ft or greater as optimum from the viewpoint of overall economics. At or above a 75-ft radius, drillstring damage is normally minimal, and limited rotation can be used to maximize ROP and minimize hole problems. Also, above a 75-ft radius, some completion equipment, such as screens or packers can also be used.

Even if the build rate allows limited rotation, the length of the lateral that can be drilled will eventually be limited by drillstring drag, which is normally exacerbated by buckling. In the oriented mode, laterals of up to 1000 ft have been achieved, although 500 ft may be a more realistic expectation.

During rotation, laterals of over 2000 ft have been achieved, with 1000 ft as a realistic expectation.

Milling

When the short-radius well is a re-entry of an existing well, a milling operation of some type is required. The choice of section milling or window milling will depend on several considerations.

For section milling, a packer is typically set below the kickoff point, and an under-reaming casing mill is used to remove existing casing and cement. It is usually desirable to mill enough casing (normally about 60 ft) to eliminate magnetic interference and allow the directional assembly to be oriented with MWD or a steering tool instead of a gyro. If the wellbore is angled enough to allow gravity toolface orientation, the milled section can be shorter. The optimum circulating fluid rheology and very high annular velocities are required for removing milled cuttings. Screens and magnets should be used in the surface equipment for removing all cuttings from the drilling fluid.

Metal shavings can damage drilling motors if they are recirculated into the system or are allowed to enter from reverse flow into the string during tripping. After milling, a hard cement plug is set from the packer to a distance of a hundred feet or so above the kick-off point. After the cement is allowed to harden, it is then dressed with a mill or rock bit down to the kickoff point. The short-radius BHA is then tripped in, and the kickoff is initiated. To prevent the bit from preferentially drilling into the cement instead of the formation, the cement plug must be at least as hard as the formation.

When a casing is window-milled, the whipstock is oriented and set through the use of a gyro. The whipstock is carried in on the starter mill, but one to two extra mill runs may be necessary for enlarging and dressing the window.

After milling, the short-radius BHA is run into the hole and oriented with a gyro, and drilling begins. A whipstock set in a milled window has the advantage of providing a means of positive curve initiation and easily allows tools to re-enter the new wellbore as long as the whipstock is in place. A flow-through whipstock permits continued communication with the casing below the kickoff point to be maintained in the event that the lower wellbore is still producing. However, whipstocks usually require more milling trips and the expense of gyro orientation. Whipstocks occasionally slip after being set, causing errors in the heading of the new well or re-entry problems in the milled window.