Eastern Star Gas (ESG) recently employed a multilateral well drilling technique to intersect the Bibblewindi-12, Bibblewindi-14 and Bibblewindi-13 vertical production wells, which form part of the drilling program for the company’s Narrabri CSG Project, located in the Gunnedah Basin, New South Wales.
The lateral well technique involves drilling down to, and turning into, a coal seam, then drilling horizontally through the coal. The technique allows a further opening up of the coal seam, improving potential for gas flow.
Mr Sleeman says that ESG is using the lateral technique because of the unique structure of the Narrabri coal, which is characterised by vertical fractures running through the entirety of the coal parallel to each other, with limited secondary cleats connecting across the vertical ones. This is unlike normal coal, which Mr Sleeman says is characterised by a cleating system like bricks in a thick wall. “With a normal coal the cleats represent pathways that water, and later gas, can flow through,”? he explains. “In the case of our coal, we only have the one set of parallel fractures.”?
He goes on to say that putting a hole through the middle of the Narrabri project coals, roughly perpendicular to the fractures, is the best way to communicate with the unique fracturing system and will provide the best rates of CSG production from the area.
“We expect to get between five and ten times more production from a multi-lateral well set, relative to what you would get with a standard vertical well with a fractured stimulation, and there’s scope for future improvement.
Although it costs more to drill a multilateral well than a set of vertical wells, the investment is worth it, says Mr Sleeman. For around three times the cost, the multilateral wells produce five to ten times the production
“When you compare the additional production with the additional cost of the well, the decision to pursue multilateral wells was compelling,”? says Mr Sleeman.
Minimisation of drilling costs is also ESG’s reason for drilling multiple laterals from a single location. “The most expensive part of the drilling process, given that we’re at a depth of give-or-take 1 kilometre, is drilling down to the coal. Once you get into the coal the lateral drilling process is reasonably quick and less expensive.
“We’re doing two laterals at the moment. Looking to the future we’ll probably do three and maybe more laterals from each location.”?
Lengthening the lateral
With the company currently drilling laterals approximately 1 km through the coal seam, Mr Sleeman says that the length of each lateral is ultimately determined by the lifting capacity of the drilling rig. Basically, the rig has to be able to lift everything that is down the hole out of the hole.
When drilling a lateral, it is not only the weight of the drill stem hanging down that has to be taken into account, but also the friction associated with pulling the drill string out of the lateral section of the hole. When drilling the laterals a mud motor, fitted to the end of the drill string is used. The mud motor is driven by high pressure drilling fluids that, in turn, carry cuttings out of the hole. The drill head sits at a slight angle, allowing for steering. This differs from the rotating drill string used when drilling vertical wells.
“With our rig we can probably go to at least 2 km perhaps even 3 km. That’s a future optimisation opportunity, to do longer laterals,”? says Mr Sleeman.
In a first for Australia, ESG adopted a rotating magnet system to assist with steering of the lateral well. With the new technology ESG was able to reliably intersect vertical wells on the first attempt, which is impressive considering the intersection occurs 1 km deep and
1 km away from the drill site.
Laterals for the future
In addition to the pilot well set, ESG has recently completed its second adjacent multilateral well set. The company has raised $50 million via placement to include a further four multilateral production pilots, citing potential locations to the west of its existing pilot, as well as in the north of its permit area.
“The ultimate thing that you’ve got to do with any CSG prospect is identify the technology that works and preferably the one that works best,”? says Mr Sleeman.
He is confident that this is what ESG has achieved with the Narrabri drilling program, adding that the first pilot has made great progress and that the company is looking forward to future possibilities.
“There will be things that we can do in the future, like drilling longer laterals, drilling more of them, and having less vertical wells to intersect, that will allow us to optimise the design.”?