Coiled tubing (CT) has long been used to meet various needs in the oil and gas industry.
In some instances, it is used to actually drill a well.
A far more common application is for well workovers, including drill hole cleanouts and fishing operations to retrieve tools dropped down the wellbore.
The continuous length of pipe is coiled around a take-up reel and is unwound during drilling, then later rewound as the drill string exits the drill hole.
This is in stark contrast to the usual connecting and disconnecting of rigid pipe as drilling occurs.
Currently, there are about 2,000 CT drilling rigs worldwide in the oil and gas industry, according to AAPG member Richard Hillis, CEO of the Deep Exploration Technologies Cooperative Research Centre (DET CRE) in Australia.
The organization was established in 2010 under the Australian government’s CRE program, which provides funding to build critical mass in research ventures between end users and researchers to deliver significant economic, environmental and social benefits across Australia.
Hillis noted that CT drilling is suited to key niche areas, such as shallow gas wells in Alberta.
Thousands of wells are drilled there annually during continuous drilling programs requiring no mobilization and demobilization.
Given its unique aspects and applications, only 8 percent or so of the existing coiled tubing fleet is involved in drilling.
Hillis and numerous fellow researchers at the DET CRE are working diligently to prove the effectiveness of CT to increase Tier I mineral resource discoveries. These types of discoveries are critical to maintaining the world’s inventory of these resources, and the ongoing decline in the grade of those being mined must be reversed.
Because remaining prospective and underexplored areas increasingly tend to be obscured by deep, barren cover, Hillis is promoting a step change in mineral exploration techniques.
“This may be provided by ‘prospecting drilling,’” he said, “that is, extensive drilling programs that map mineral systems beneath cover, enabling geophysical and geochemical vectoring toward deposits.
“The technological platform for prospecting drilling must include low-cost drilling due to the dense subsurface sampling required,” he noted.
Enter CT drilling technology, with its continuous drill pipe spinning off of the reel in a timely manner.
“A key positive aspect of coiled tubing drilling is potentially faster and cheaper drilling because connection of drill pipe is not required,” Hillis said. “This means the drill bit spends more time drilling at the bottom of the hole.”
Indeed, even changing the drill bit entails only a speedy round-trip to the surface and back down the borehole.
Even so, there are issues to address.
The DET CRE researchers are evaluating the prime challenges to the use of CT drilling in mineral exploration:
- Its rate of penetration in hard rocks.
- The durability of CT.
- The recovery of cuttings.
The lack of core from CT drilling means that rock characterization must depend on another approach.
Hillis emphasized that the ultimate platform for prospecting drilling would be CT drilling augmented by downhole and top-of-hole sensing.
“The first manifestation of real time downhole sensing is our newly-developed autonomous sonde that is deployed by the driller and logs natural gamma radiation as the drill rods are pulled,” he noted.
“Field trials of real-time downhole LWD (logging while drilling) and top-of-hole sensing have demonstrated cost effective, rapid, repeatable and accurate determination of petrophysics, geochemistry and mineralogy, with the necessary depth fidelity, during conventional diamond drilling,” he said.
“These techniques can be modified to complement coiled tubing drilling.”