Xstrata Coal’s gas drainage policy is to achieve the lowest gas threshold limit possible before mining.
Oaky Creek’s gas reservoir varies from 1 cubic metre per tonne to 15m³/t and permeability is extremely variable, adding to the problem. Conducting gas drainage well in advance of mining couldn’t be achieved through underground gas drainage.
The solution was to drain the gas from the surface and AJ Lucas Coal Technologies was awarded the contract.
As surface to in-seam (SIS) drilling for gas drainage is still a developing field, an important element of the contract was its provisions for continuous improvement and allowing different approaches to draining the gas.
So while Lucas’ tender was based on well-proven medium radius drilling (MRD) techniques, there was an opportunity to develop better techniques.
Working in conjunction with the mine’s technical services group, a new method for removing the gas was designed and engineered. It proved to be safer, more efficient and cost-effective than any previous technique.
In 1991, AJ Lucas was the first Australian company involved in horizontal directional drilling (HDD).
“Above all, our success in HDD relies on engineering,” said Andy Lukas, the company’s technical director and father of Australian HDD.
“Every record we’ve set, every challenge we’ve met, has been a direct result of our precise understanding and analysis of the forces, the materials and the geology involved. We’ve worked out what questions to ask, then searched out – or invented – the answers.”
Over 10 years Lucas has used HDD to install water, sewer, gas, oil and telecommunications conduits under every kind of obstacle. Five years ago it set about applying this expertise to coal mines.
Coal mine methane is becoming an important resource and draining it poses a significant cost and time factor for mine operators.
"Our goal from the outset was to find an economic way to drain and collect the gas from the surface," said Lukas.
"This served the mines’ need to minimise underground drilling and associated production delays and made the gas available as an energy resource rather than a waste product.”
Lucas Coal Technologies was established in 2000 to pursue this goal.
The company began with conventional underground in-seam (UIS) drainage to build its understanding of the mine environment and coal seam characteristics, recently tackling the challenge of surface to in-seam (SIS) drainage, according to Lucas general manager Mark Tonkin.
“After several years of work and practical experience in drilling in coal horizontally from the surface, we had developed SIS techniques that were ready for commercial deployment," Tonkin said.
"Oaky Creek was the perfect opportunity to put our money on that confidence.”
Most attempts to drain gas from the surface have been made using mineral exploration rigs. Many medium radius drilled (MRD) wells have been designed to suit the capabilities of these rigs.
But mineral rigs are extremely limited, according to Tonkin. In shallow coal seams they can only drill up to 1200m in-seam.
As longwall blocks can be 3km in length, costs could be reduced significantly by eliminating wellhead infrastructure and increasing the percentage of drilled metres in coal, versus wasted overburden drilling.
“Designing MRD programmes around mineral rigs is back-to-front. Our approach was to engineer an MRD program from first principles, then determine the best rig to do the job,” Tonkin said.
This attitude led to the development of an alternative drilling rig that provides 65 tonnes of force, 27kNm of torque, the ability to handle larger diameter pipes, and precise control.
“Everything we needed to carry out our engineering design - and an industry first,” he said.
Risk management was also a factor in the new rig’s development.
One major risk was losing tools in the holes. Using proven oilfield technology and a rig with a very large margin of power and torque, minimised the risk of getting downhole equipment bogged in the longwall.
The brief from the Oaky North mine was to reduce gas concentration to as low as what was reasonably achievable. A 12-month timetable was drawn up, drilling 17 holes totalling 19,338m in-seam using proven MRD technology with vertical wells to extract the gas.
In line with the contract’s allowance for continuous improvement, the mine’s management was keen to explore ways to further improve and accelerate gas drainage.
“Obviously the longer the holes we can drill, the fewer times we need to move and re-set the rig, so the faster we can drain the gas,” Tonkin said.
"Then we had another thought: if we could use the same vertical well to drill two horizontal drainage bores, we could double the production of the well and save yet more rig moving time. We nicknamed it the ‘pitchfork’ design."
Further engineering work determined that 2000m bores could be achieved with an adequate safety margin, and techniques were devised to drill three lateral bores from a single cased well - the “devil’s fork”.
Lucas presented this plan to Oaky Creek management, who immediately saw its potential and put their support behind it.
The project went ahead using the new plan. In eight months, 14 straight, two pitchfork and one devil’s fork wells were installed, fed by 34,291m of in-seam bores - a 75% increase in two thirds the time. Gas production has varied depending on hole length and the gas content of the particular part of the seam. The best rates have been around 200 litres per second against 450kPa wellhead pressure from 10m3/t areas.
Gas content in the production areas has been reduced to below 6 m3/t.
Overall, the change in methodology and design allowed savings of about $5 million over all the coal metres drilled.
Key technical achievements include:
“The success of this project affirms two of our basic principles: that engineering is the way to solve drilling challenges and that working as partners with clients delivers better results for both parties. And in this case, for the whole industry,” Lukas says.
The new technology is now integrated throughout AJ Lucas Coal Technologies’ operations.
