Underbalanced Drilling: General Issues

Coiled Tubing for Underbalanced Drilling

Coiled tubing is ideally suited for drilling underbalanced. The continuous nature of the coiled tubing string and the exclusive use of a mud motor for drilling provides several advantages over conventional jointed pipe drilling for underbalanced applications:

• Drilling with a mud motor prevents the formation of key seats, thus eliminating one cause of stuck pipe.
• Because the coiled tubing string is not rotated, mechanical damage caused by pipe rotation (e.g., pasting of cuttings against the borehole wall) is not an issue.
• Making connections during conventional jointed pipe operations requires stopping and then re-starting the pumps. The gas and liquid phases separate when pumping stops, and then two-phase circulation resumes once the connection is made and the pump is re-started. This continual cycling results in pressure surging of the wellbore and can lead to overbalanced surging of the reservoir. Additionally, halting the pumps can result in the formation of a cuttings bed. When drilling resumes, the cuttings bed may be gradually ground up into the borehole wall. With coiled tubing UBD, these problems are avoided the drill string is continuous, allowing the bottom hole pressure to be kept consistent, with no forced surging of the reservoir. And with properly designed wellbore hydraulics, the formation of a cuttings bed can be avoided.
• The continuous pipe provides constant well control and improved worker safety with minimal pipe handling and faster trip times.
• Coiled tubing has a high resistance to sulfide stress cracking and can be left in the hole as a production string.
• The electric cable inside the coil provides real time monitoring of down hole information (e.g., annulus pressure, directional data, logging while drilling).

Of course, theres are also limitations iinvolved in coiled tubing drilling. If borehole stability problems develop, coiled tubing cannot be rotated. Neither can it withstand the physical abuse that conventional drill collars and drill pipe can. Therefore, in formations prone to sloughing or washing out, coiled tubing should not be the automatic first choice. Due in part to logistical concerns and also due to the current size and weight of coiled tubing, generally, horizontal lengths and hole sizes are smaller than those achieved with larger conventional equipment.