The 5,000-year-old copper mining industry is taking a page from its young upstarts in the oil business on how to deal with stressed out underground mines.

Hydrofracking solved the problem of extracting unconventional sources of oil such as shale, and it’s now helping solve problems associated with super hard rock in PT Freeport Indonesia’s Deep MLZ mine.

In underground mining, being in hard, massive rock normally is a blessing, but that’s not the case for block caving.

“In block caving, we rely on the rock breaking along natural fractures once a certain span is achieved,” said Mark Johnson, President-Freeport-McMoRan Indonesia. “When this natural breaking didn’t happen as expected in the DMLZ, we began seeing higher stresses in the supporting rock until it couldn’t take it anymore, resulting in a sudden release of stress or mine-induced seismicity.”

With three previous block cave mines under its belt, PTFI had plenty of underground experience and expertise, but had never dealt with a problem quite like this.

“We realized our existing tools no longer were capable of ensuring we met our production targets while maintaining effective safety controls,” said Matt Sullivan, Manager-Geotechnical Monitoring. “We looked to other mines who have successfully dealt with seismicity and rock bursting, and saw how hydrofracking had provided the solution to re-engineering their problem.”

Hydrofracking uses pressurized water to make fractures in the rock. Those fractures help reduce the size of a seismic event by  allowing movement in the rock to occur, reducing the strain the rock accumulates when it’s highly stressed.

Strong is not always a good thing

Though it seems counterintuitive, you want the rock above you in a block cave to be highly fractured so the strain of its weight can cause caving of the material to occur and the resulting loads to be spread in a more uniform fashion.

The strength and hardness of rock in the DMLZ led to series of seismic events caused by the undercutting process.

Those events increased the weight of rock that has to be carried by the pillars around the cave. When seismic activity damaged ground support, PTFI leadership suspended production in the DMLZ until the problem could be rectified.

“Because the rock is very strong, it doesn’t show signs of this increasing load until it suddenly fails,” said Ian Edgar, PTFI’s Vice President-Underground Engineering. “These failures required us to upgrade and repair the ground support in the DMLZ that had been damaged.”

In addition to the beefed-up ground support, changes were made to the way undercutting was sequenced to help manage the risks of the mine-induced seismic activity.

“The repair work was challenging, but the DMLZ teams all worked together very well to ensure the work was completed safely, on schedule and with good quality,” said Ferry Widiyanto, Manager-DMLZ Development.

Those steps significantly improved conditions in the mine, but it became clear getting the DMLZ up to full production would require PTFI to embark on the hydrofracking program – something never done at any of its underground mines.


Getting the hang of it

With support from engineers at Freeport-McMoRan’s Corporate office in Phoenix, PFTI’s underground group began a hydrofracking program, teaming up with drilling contractor PT Pontil Indonesia and Australian equipment manufacturer ACIM – both experts in seismic activity.

“ACIM was identified to have the right expertise and experience to build a set of fracking equipment capable of a fracking the tough ground of the DMLZ,” Edgar said.

The site engineering team developed the specs for the equipment, and the first of three hydrofracking pumps was commissioned earlier this year.

To date, PTFI’s team has drilled and hydrofracked four holes in the DMLZ and are now fracking a fifth hole.

“We are ahead of schedule for the number of fracks, but slightly behind on the number of holes due to very poor ground at the collar of the holes from the DOZ level, which are approximately 500 meters above the DMLZ,” Johnson said.

Edgar said the team will continue to refine the hydrofracking operating parameters at the DMLZ including finding answers to the following:

  • What size fractures are ideal
  • How long will they take to form
  • How much pressure is needed to form a fracture
  • How much water is used to make a fracture

“The fracking equipment is very sophisticated, and the Pontil team operating it has become proficient with it very quickly,” Edgar said. “It achieved a larger fracture in less time than we had originally expected.”

That larger fracture has allowed the team to increase the original planned drill hole spacing by 30 percent, which means the team will create the needed fractures faster than they had estimated, Edgar said.
The process is still in its early stages, as the Pontil team eventually will ramp up to six drill rigs and three hydrofrack pumps, but the early take on PTFI’s first crack at hydrofracking is two thumbs up.
“The initial results look promising with some seismicity moving above the cave,” Edgar said. “What’s also promising is the scientists studying the details of the seismicity indicate the rock is changing in the way we want.” (Stauffer)

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