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Information Only - Waste Isolation Pilot Plant - U.S. Department of ...

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X-5<br />

Other applications are exploration, steam injection, environmental observation,<br />

delineation <strong>of</strong> basic production, and injection wells.<br />

Well re-entry application<br />

I) Deepening existing wells (vertical well)<br />

Vertical deepening <strong>of</strong> a well is technically simple to achieve by using a pendular<br />

bottomhole assembly (BHA) to control the hole direction. The weight on bit (WOB) is<br />

provided by a long BHA that maintains the neutral point (NP) in the BHA.This<br />

technique ensures a straight, vertical wellbore because the string is always put into<br />

tension in conjunction with the stiff BHA.<br />

2) Drilling lateral drainholes (deviated .well)<br />

In lateral diainhole drilling, a whipstock is set in the bottom <strong>of</strong> the wellbore and a<br />

window is milled or cut through the casing. The drilling BHA is equipped with a<br />

directional measurement and control system that allows the drilling <strong>of</strong> the lateral<br />

drainhole into the producing formation. The force necessary to push the BHA and the<br />

coiled tubing around the buildup radius and into the deviated section is limited by the<br />

force that can be exerted on the coiled tubing in the vertical section.<br />

New well applications<br />

Even though coiled-tubing drilling (Crn) is much simpler than conventional<br />

drilling, a new well using ern still requires a small rig to spud the well and set the<br />

surface casing. The rig provides equipment-support and pressure-control capabilities<br />

required for drilling. The prospect <strong>of</strong> low cost, low-impact exploration, observation and<br />

delineation wells has led oil companies to increased spending on crn development<br />

during the past decade. In addition, the drilling <strong>of</strong> slimhole-injectionlproductiol1-weUs<br />

using crn techniques is expected to be a viable option in marginal fields.<br />

The limitations <strong>of</strong> the coiled-tubing technology are in directional applications<br />

because it does not have the capability to apply enough weight on the bit. Depth<br />

limitations due to tensile strength <strong>of</strong> the tubing are crucial. Coiled-tubing operations have<br />

been performed at depths up to 22,000 ft. The advantages are: nO pipe connections,<br />

better well control, safer operations, less tripping time, and a smaller rig (Leising, 1992).<br />

Future applications and development will make crn a viable option in marginal<br />

fields at depths up to about 20,000 ft. The usage <strong>of</strong> crn is another option when normal<br />

drilling is not economical. This <strong>of</strong> course depefl.ds on the availability <strong>of</strong> theCTDunits. It<br />

is also estimated that re-entry drilling will be dominated by crn in the future, if the<br />

economics are correct and CTD units are available.<br />

Underbalanced drilling<br />

In underbalanced drilling the wellbore hydrostatic pressure is less than the<br />

formation pressure. It is usually performed using either air, air/mud, foam. or mist<br />

mixtures as the drilling fluid. Underbalanced drilling is not a new concept but has been<br />

<strong>Information</strong> <strong>Only</strong>

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