I recently read about safe mud window - it states that the mud pressure in a wellbore should be high enough to prevent well collapse, and should not exceed the formation's fracture pressure. But consider deep mine tunnels - there is no "mud pressure" at all, just air. A mine's depth could exceed 3km, and its tunnels withstand this overburden rock mass. So why do wellbores collapse with a lack of pressure, but deep mines do not?
Drilling mud is only used when drilling for fluids that are pressurized such as oil, gas and sometimes water. It is not used when drilling for minerals such as gold, iron ore, copper.
Exploratory drilling minerals will use a combination of:
- Rotary air blast drilling (RAB) - where compressed air is forced through the drill steel to flush drill cutting out of the hole between the drill steel and the wall of the hole.
- Reverse circulation drilling (RC) - where the air flow is opposite to RAB holes. Compressed air is forced between the gap between the drill steel and the wall of the hole. The drill cutting exit the hole through the hollow center of the drill steel.
- Diamond drilling - uses cylindrical drill bit with diamonds in the cutting edge to collect a solid sample of rock from the drill hole.
Because rock containing minerals rarely contains pressurized fluids, if the rock is competent the drill holes remain open.
Drilling for oil and gas and sometimes water requires the drill steel to pierce rock that is pressurized by fluids. To counter the pressure of the fluids so the hole remains open, such holes require drilling mud.
The other reasons for using mud when drilling for oil and gas are:
•To lubricate the drill bit.
•To remove drilled material away from the drill bit and transport them to the surface.
•To counteract the fluid pressure in the rock.
•Stabilize the wellbore
If a well could be drilled without a drilling fluid, formation fluids, which are under their fluid pressure,would spurt out of the borehole (blow-out). The density of the drilling fluid used in a particular borehole is designed to generate a drilling mud pressure (due to the weight of the drilling mud in the borehole above a given depth) that counteracts the fluid pressure in the formation and prevent blow-outs. This is usually successful, but because sudden increases in over pressure can be encountered the drilling mud pressure is kept higher than necessary as a safety feature. When the mud pressure is greater than the formation fluid pressure, the well is said to be over-balanced.
Mine and civil engineering tunnels are not simply dug and left to stay as they have been excavated.
Civil engineering tunnels can be lined with concrete, as can mine shafts. Mining tunnels tend not to be concrete lined due to the cost of the concrete. Instead they are reinforced by using rock bolts, cable bolts and shotcrete, as required.