Calculating Lag Time for an Air-Drilled Gas Well

I've been doing this for so many years that I have forgotten the formulas, but it works like this: cfm/annular space= annular velocity. Take away the the 'sink' rate of your cuttings. That is your bottoms up time. If you really want to know the bottoms up time, inject some dye into the air-stream and watch for it, and look for the cuttings behind it.

Hope this helps...

Rockbit,

I don't have clue about geology. 30 years in electrical engneering and power plant ...but I know an expert when I read one.

I am sure you response deserved a" Very Good answer Rating" from your peers!!!!

annuluar volume / flow rate will give up lag time. When drilling with air there should not be much sag. Basic mudlogging. Fluid phase in mud will affect results but if you are using just air there should not be too much of an issue

(but if you are using just air there should not be too much of an issue)

exactly what do you mean by this

Philo,

Sink rate in air drilled holes is not affected much by the viscocity of the circulating medium, in this case air. It is, however affected by cuttings size. In highly fractured formations, large cuttings will exhibit sag. In dense, slow drilling hard rock, making mostly dust, practically no sag at all.

Injected dye or easily detecable tracer gas, such as oderant will tell the tale.

Correct. But as you know most "air drilled" wells have some fluid component in the circulation system that may affect the carrying capacity of the medium.

As you so rightly state when the rock is reduced to effectively powder there would be practically no sag whatsoever. With larger fragments then there might be a drop out rate.

This can be calculated I'd guess but the simplest means it to do a lag test with a coloured medium as you suggest.

Lag time =

(Hole ID) sq - ( pipe OD) sq divided by 1029.46 then x the hole length

that is annulus vol

find out pump output and multiply output by annulus vol

that gives you number of strokes

then find out the pump stroke per min

then times stroke per min by the strokes

that gives you your lag time

Sorry guest, not so. Pump stroke calculations are meaningless on air drilled holes, ther's no pump (excluding the mist pump) They use compressed air, not mud.

this is a formula for gas wells right

There is a way to calculate lag time for Air drilled wells.

Firstly we need to calculate the scfm to Gallon per minute.

1 scfm = 7.48gpm

Also rig pumps would be pumpin water along with the air. That gpm needs to be added to the air gpm and then we can calculate the lag time by dividing the total GPM by the total hole depth.