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Shale gas wells drilled underbalanced with nitrogen during coiled tubing drilling operations

Posted on 20 December 2010

Two successful drilling operations using the COLT, a coiled tubing drilling (CTD) bottomhole assembly designed for re-entry and through-tubing markets, could open up CTD opportunities that not currently commercially viable. Developed by AnTech Ltd, the assembly was used to complete two underbalanced drilling operations in the shale gas reservoirs in the northeastern US. The primary aim was to assess how simple the tool was to use, control and steer. The secondary aim was to gauge its reliability and ability to drill with gaseous drilling fluids in high vibration environments.

The first field test took place on a well that had originally been drilled conventionally with jointed pipe but well problems meant that casing could not be run to total depth. AnTech was offered the opportunity to re-drill the reservoir section, which would require drilling out a cement plug at the bottom of the casing, creating a new well trajectory and drilling a new horizontal section. AnTech designed custom alignment tools and tool handling equipment with hydraulically-powered pistons to control deployment.

Drilling operations were performed in three stages:  drilling the plug at the casing shoe, then the curve or “build” section, and the horizontal.  First, the COLT was run into the hole and drilling commenced upon reaching the plug. The COLT drilled through the plug and continued drilling for 165 ft with a straight motor and a tri-cone bit.

After drilling the plug, the operator set a well path for the team to follow. The challenge was to drill a build section that would effectively bring the well into a horizontal configuration. Setting a bend angle of 1.5° on the motor and aiming the tool face toward the desired direction, a build rate of 20° per 100 ft was achieved, which is well within the operational limits of the tool. The operation was carried out using formation water as the drilling fluid. A 3⅛-in. motor and a PDC bit were used to drill 46 0ft to bring the wellbore horizontal at the desired depth.  This demonstrated that the tool was easy to control and changed direction as intended.

The final step was to drill horizontally until the wellbore intersected the gently dipping formation above. While the previous two sections were drilled with formation fluid, the challenge was to see how the tool performed when drilling underbalanced with nitrogen. Aerated fluids, while creating the desirable underbalance condition, increase vibration and make it more difficult to see motor stalls by monitoring surface pressures. The immediate effect of the underbalance condition doubled the rate of penetration as the overbalanced pressure was reduced. The wireline telemetry used in the COLT meant that the signal transmission was not affected by the change in fluid, and the fast signal transmission and downhole weight and torque sensors meant that stalls could be avoided by monitoring the downhole signals. The tool’s performance was not adversely affected by the increase in vibration, which was monitored real-time on the tool. The borehole target was reached as planned.

Following the first operation, another operator retained AnTech to use the COLT to extend and drill a sump in a nearby commercial well. The well was efficiently drilled in one run, with an extension and an openhole sidetrack for a sump. The COLT was used in combination with a 3⅛-in. motor which had a 3⅜-in. bearing section. It was drilled with an air-foam mix, at an initial flow rate of 700 scf/min and 10 gpm which was then increased to 900 scf/min and 10 gpm. At a true vertical depth of 1,340 ft, the lateral was extended by 215.5 ft and a 255-ft sump was successfully drilled.

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