Antech completes underbalanced drilling operations in US shale gas reservoirs


Antech completes underbalanced drilling operations in US shale gas reservoirs

AnTech Ltd announced that it has completed two successful drilling operations with the COLT, a coiled tubing drilling (CTD) bottom hole assembly (BHA) designed for the re-entry and thru-tubing markets. The COLT withstood the rigors of the punishing vibration produced when drilling with aerated fluids, while responding promptly to changes in direction and speed. 

AnTech carried out the operations using the COLT on two wells in shale gas reservoirs in the northeast of the United States. The primary aim was 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.

Following design and testing in the lab at AnTech’s headquarters in Exeter, England, the COLT was put through its first field test in a well.  The well had originally been drilled conventionally with jointed pipe, but well problems meant that casing could not be run to total depth (TD).  To remedy this, 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.

In temperatures as low as -20°C (-4°F), the AnTech team gathered at the wellsite. Equipment was assembled, including a rig with 2-3/8” diameter coil with e-line, surface infrastructure and fluid tanks, a workshop trailer and a crane. After carrying out preliminary procedures, calibrating and assembling the equipment, the team lowered the tool into the hole with the crane.  To ensure that the BHA would be accurately aligned, AnTech designed custom alignment tools and tool handling equipment with hydraulically-powered pistons to control deployment. By using the crane, the system was set up and deployed in just over two hours.

Drilling operations were performed in three stages:  1) drilling the plug at the casing shoe, 2) then the curve or “build” section, and 3) 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 165ft 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 deg on the motor and aiming the tool face toward the desired direction, a build rate of 20° per 100 feet 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 3-1/8” motor and a PDC bit were used to drill 460ft 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 (ROP) 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 (WOB/TOB) 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 3-1/8” motor which had a 3-3/8” bearing section. It was drilled with an air-foam mix, at an initial flow rate of 700scf/min and 10gpm which was then increased to 900scf/min and 10gpm. At a true vertical depth (TVD) of 1340ft, the lateral was extended by 215.5’ and a 255ft sump was successfully drilled. 

CTD is the only drilling technology that guarantees that underbalance is maintained 100% of the time. There are no joints to make and break, and downhole pressures are monitored on continuously to ensure that they do not fall above or below planned values. In addition to the fast running speeds, continuous pressure containment and no personnel required on the rig floor, CTD is an attractive proposition. In some cases it is the only viable way to drill a re-entry.

Each well test demonstrated a new directional CTD BHA that offers new alternatives for the CTD market. Drawing upon the experience gained in these operations, AnTech has since implemented design improvements and used it to create POLARIS, a new simpler tool scheduled to debut in 2011. It is suitable for commodity drilling markets, such as Shale Gas, Coal Bed Methane, and Underground Coal Gasification where a highly specified tool such as the COLT is not required.   

With the commercial advantages they provide, the next generation of CTD tools could open up CTD to opportunities that are not currently commercially viable.  AnTech’s aim is to make CTD a reality for more operators. These operations illustrate that this is achievable. The COLT and POLARIS will help grow the CTD industry with economical and technically-advanced solutions. With the new tools entering the market next year, it is an opportunity for Coiled Tubing Drilling to realise its full potential.  


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