Percussion drilling in old mine workings 1

[groblert]

Good day

We are a geotechnical engineering company and we are busy with a project where we will need to do percussion drilling into old bord and pillar coal workings. We are concerned about the possibility of drilling into methane and are concerned that this might cause combustion. Can anyone advise me on the risk of such an occurrence and possibly supply me with some mitigating measures?
I will really appreciate your help.

Regards

 

[miningman]

Unless you can find historical references that prove that the area is known to be non-gaseos ( highly unlikely) or now known to be flooded you WILL hit methane. p In this case combustion is the least of your worries. I forget the exact numbers but at something like 4%-12% concentration in air methane will detonate ie explode. The force of these explosions are way more vilent than conentional explosives.... I can see your drill rig becoming air borne

 

[groblert]

Thank you for your response miningman. Much appreciated.

I am aware of the severity of methane explosions in general.
When doing rotary core drilling, one has drilling fluid/water constantly flowing at the drill-bit, which should prevent ignition. I am concerned about doing percussion drilling into these areas though.
Any mitigating measures to prevent ignition that you can recommend?

Thank you in advance!

 

[miningman]

Oh I can think of several possible mitigation measures but would need more data. How long are the proposed holes??? Are you targeting mined out areas or the pillars?? Geographically where are you?? Is the old mine in a remote area where unintended flying objects could be tolerated or are you say 20metres from a schoolyard? Do you have a drill specced out yet?? Is remote operation of this drill feasible??

 

[groblert]

The holes will be between 28m and 60m.
The project is in South Africa in a remote area, so some flyrock won't be a problem. Hangingwall conditions are probably sandstone.
The last mining that took place in the area was in 1980, so we need to determine amongst others the hangingwall conditions (and obtain samples for laboratory testing), general lithology of the area and we want to use some down the hole photography (if possible) to view the current conditions in terms of pillar scaling.

Yes, the specs have been submitted to the drilling contractor, but if we need to change any of this to allow for remote drilling to decrease the consequence in the event of an accident I am sure we will be able to somehow include this.

I have thought of first drilling a rotary core hole and then inserting a methane sensor to at least give us an indication of possible risk.

I would really like to hear your thoughts.

 

[miningman]

Your idea of using a core drill initially should probably be expanded. One issue is that the methane concentration is not likely to be homogeneous. Get hold of the old mine plans and figure out where the back is at the highest elevation. This is where I would expect the highest concentration . Use an HQ bit at least to obtain a 3 inch diameter hole. Dont use this for your methane probe but leave it open to allow some of the gas to disapate....... you might even want to burn it off but that requires a totally separate discussion. Then try to determine one or two locations where the length of the holes would be at low end of your expected range. This should allow for the generation of a natural ventilation pressure which would start to flush the methane from the workings. Introduction of compressed air or a fan to to increase the pressure would improve flow. By all means use your percussion drill but at your proposed hole length it is unlikely you will be using an old airtrack. Definiotely utilize remote control drilling for say the last three metres before break thru, and be prepared to write off the drill if you happen to generate an incandescent spark at breakthru. By all means flush the drill string with either water or water mist during drilling. If using a drilling mud , incorporate calcium carbonate into the mud mix... this material can quench a dust explosion before it really gets going.

 

[groblert]

Thank you again for your advice miningman.
One of the possibilities we are also considering is to proceed with the first couple of meters with percussion drilling, to about 5m above the workings and then use diamond drilling for the final 5m (with calcium carbonate in our drilling fluid).
This obviously has some logistical implications, but do you think it can work?

 

[miningman]

It can be done , there are some drill rod stability issues to overcome, but you really should focus on methane drainage before you get too excited about other details