Coil Tube Drilling: The Gas Drainage Saviour?29 September 2008
Coil tube drilling could be the answer to draining gas from mines in a more efficient, accurate and safe way. Paul French explains why.
Deep in the heart of Queensland's Pinjarra Hills, researchers from CRCMining are developing a technology that they hope will dramatically impact on one of coal mining's biggest headaches: gas drainage.
The levels of gas which exude as coal is extracted often limits the rate of mining and sudden releases from pressurised pockets can cause mines to be shut down until gas levels return to acceptable limits.
As production rates grow and higher gas levels are witnessed more frequently, traditional drilling techniques are struggling to cope. This is now causing costs to spiral out of control and exceed the current inseam estimates of between $50 and $75 a metre.
However, CRCMining's new coil tube drilling (CTD) system, based on systems used in the petroleum industry for well completion and intervention work on deep gas wells, could be the answer. It has the promise to increase safety, significantly reduce the problems associated with difficult drilling conditions and slash costs by at least 15%.
"The current push to increase the production rate and reliability of longwall miners can only be supported if the rate of development can be increased to provide an adequate float and an adequate rate of gas drainage," says Dr Joe Cronin, programme leader for coal production at CRCMining. "If CTD is successful, it could provide a key enabling technology to support this effort."
He adds: "CTD offers the potential to develop a continuous advance, semi automated drilling system for degassing underground coal panels. Such a system will provide step change benefits in both the safety of, and the productivity for, drillers and the downstream mining process."
Coils in practice
The first application of a coiled continuous drill string was by the California Oil Company in 1962. However, this unit demonstrated limited success as the strength of the tube and the high number of butt welds led to an extremely limited fatigue life. Modern steelmaking processes and the ability to fabricate continuous lengths of tube up to 500m have enabled the technology to flourish in more recent times.
Coiled tube technology was then put into practice during project PLUTO (pipelines under the ocean). This involved the laying of fuel pipelines across the English Channel to support the D-Day invasion. Long sections of pipe were welded together, coiled around 13m drums and then laid in a continuous string from barges towed by tugs.
Then, in the 1980s, coil tube was used as a means of decreasing the time and cost of work-overs on conventional oil and gas wells. Drilling with coiled tube was first undertaken in the early 1990s, and today CTD is well established as a surface-based technology. The idea to adapt the technology for mining came in a lightbulb-over-the-head moment to CRCMining researchers.
"The idea for the use of coiled tube for inseam gas drilling came up a few years ago when the team were testing a high-speed, cross-panel water jet drill," says Cronin. "This rig used flexible continuous hosing to provide the high pressure water for the cutter. When one of the engineers started to wish for a less flexible hose, a drilling contractor suggested coiled tube instead of steel hose. The resulting discussion led to the idea of using CTD underground."
Change in direction
CRCMining is a government-backed research centre supported by four universities and 14 industry partners including BHP Billiton, Rio Tinto and Xstrata. The centre is known for researching and developing innovative technologies for the mining industry and has already pioneered the introduction of the revolutionary tight radius drilling (TRD) technology.
CTD builds on the work of the TRD and was initially developed in conjunction with BHP Billiton Illawarra Coal in the grassy mines of the Illawarra region of New South Wales. BHP and CRCMining spent $100,000 developing a performance specification and CTD concept and have conducted basic investigations into the coal life, machine cost, likely performance and potential risk areas of the technology.
"The work completed with BHP Billiton Illawarra Coal involved extensive modelling of the proposed system which included the business case, engineering modelling and safety analysis," says Cronin. "The results were sufficiently encouraging to take the proposal to the Australian coal industry's research programme to fund the current stage of development."
Gas drainage is currently achieved by drilling boreholes from the surface or inseam drilling underground. The problem is that, as pressure on gas drainage is increasing due to rises in productivity and the amount of gas found in coalmines, current drilling techniques have not changed since the 1980s.
Because the drilling process is labour intensive it leads to a large number of handling injuries. It also limits drilling productivity and contributes to the relatively high cost per metre drilled. Costs are driven up further because a monopoly situation exists for the supply of down-hole survey and electronic components, so replacement costs are usually in excess of $300,000, a high-risk capital investment considering the frequency of equipment being lost down-hole.
Current equipment is not suitable for real-time monitoring of drill rig operating parameters, or the integration of down-hole geological sensing probes.
Therefore, important geological information suitable for input into the mine geological database is not collected as part of the drilling process.
Finally, difficult drilling conditions are expected to become more prevalent with the move towards deeper reserves. But conventional underground inseam drilling equipment becomes unstable if there is a disparity between borehole and formation pressures, leading to an increase in the time and cost of drilling, potential delays in development through poor drainage and a higher incidence of stuck drill strings.
The solution to these problems is, according to CRCMining, CTD. Essentially, CTD turns a batch process into a continuous process with no manual handling requirements. In a conventional directional drill rig, straight lengths of drill rod are added to the string as the hole is drilled. Each time a new section is added, the drill string is stopped, clamped, the drill head retracted, a new rod inserted and screwed into the string and then the head is started again to drive the string further into the hole.
In the CRC coil tube rig, the drill string is a continuous 700m metal tube that is wrapped around a drum. As the hole is drilled, the coiled tube is unwound from the drum, straightened through a set of rollers and fed into the hole by the injector.
The CTD could cut drilling costs by at least 15% by saving time on assembling the drill string from 3m rods and facilitating the automation of the drilling process. It is safer to use than conventional drilling methods because it eliminates the need to handle rods manually and the continuous drill string allows the pressurisation of the hole during the drilling process. This has the potential to significantly reduce the collapse of holes in difficult drilling conditions.
"CTD does not solve the gas drainage problem, it is hoped that the technology will make existing practices more efficient in terms of cost and safety," says Cronin. "The main promise of CTD is in the continuous nature of the process which offers faster and continuous drilling rates, vastly decreased manual handling, continuous mud flow and potential for pressurised holes. The continuous process allows for automation and reduces the number of average errors, allowing for more accurate steering."
Cracks in the pipes
The two key challenges facing the CTD technology are both to do with tube size. The design challenge is to develop a rig that is compact enough to be readily transported around the underground mine workings and able to be operated in the stubs. Because of this need, the solution has to have the smallest possible tube diameter. The drilling challenge is to configure a down-hole assembly and tube combination that can quickly and reliably drill holes to the required specification.
Other identified challenges include making the CTD comply with electrical and control equipment regulations for use in gassy environments as well as developing a survey system capable of continuous operation.
"Extensive discussions with CTD companies in the US and Canada suggest that while significant research is required, there are no issues that are seen to be insurmountable in the development of a prototype," says Cronin.
He adds: "The project is more about integration and scale reduction than the development of bleeding edge technology. Fundamentally, it is a machine design problem, taking into account the safety and handling requirements of an underground coal operation. Again, given the underground and mining engineering experience held by CRCMining, we are confident we have identified the key development challenges and the potential solutions to overcome them."
CRCMining is currently working on a test plan for the risk mitigation phase of the technology. By the end of the year, they expect to have completed this process and hope to document the prototype phase of the project and seek funds for construction, estimated to be in the region of $1.5m, in the second quarter of 2009. Construction of the prototype would then commence later that year with full prototype testing commencing in early 2010.
"The ultimate aim of CTD is to provide an enabling technology for rapid development in longwall mines," says Cronin. "Will every underground coal mine in the world convert to CTD? Definitely not! However, if the technology turns out to be reliable and ultimately incorporates a high degree of automation, it will be extremely attractive to gassy mines."