Drilling Fluids & Chemicals
It often becomes necessary to introduce fluid to the hole, even whilst drilling with air as the main flushing medium. Water, quite obviously, the easiest and most inexpensive fluid to use, but it often becomes impractical due to it's physical limitations. In these situations the physical characteristics of the water are modified to suit ground conditions with the additives. The three main additives are: Foam, Hydratable clays and Polymers.
Foam is created by mixing a chemically stabilised detergent with water, with the aim of reducing the surface tension. Foam is most commonly used with air-flush drilling, especially where there are large influxes of ground water, or in sticky formations which might cause the hole to produce collars behind the bit or hammer. The foam mixture is injected into the air stream, either as fine mist or in larger quantities. It helps facilitate the drilling of deeper holes using air and in penetrating less cohesive/stable formations. Modern foaming agents are bio-degradable, of obvious advantage when water well drilling.
Hydratable Clays - Mud:
Drilling mud is usually produced by mixing Hydratable clay with water to produce a fairly high viscosity colloidal suspension. The most widely used clay is bentonite. Mud has several advantages over the usage of water alone:
- Mud has the ability to gel and suspend cuttings in the hole - even when pumping has stopped.
- In less stable, permeable formations it produces a wall-cake by filtering water out of the colloidal solution and coating the borehole with a film of clay.
- It has a higher density than water and therefore a higher hydrostatic pressure gradient, which can help minimise the influx of water from the formation whilst drilling and acting as a "support" to the borehole walls.
Mud does also have some disadvantages;
- Mud requires a long settlement time at the surface in order to release the cuttings from suspension, or the use of specialised equipment such as shale shakers, de-sanders etc.
- A Wall-Cake can completely mask an aquifer, preventing water from entering the borehole, making the development of a water well extremely difficult.
To overcome some of the problems associated with mud drilling, drilling-specific polymers have been developed. They are intended to have the same effect on the physical properties of water as mud, but are easier to handle on the surface, stabilise the borehole wall with out producing a thick filter cake, and are usually biodegradable.
Modern drilling polymers, such as DANDOPOL are supplied in liquid form to allow mixing without the need for expensive specialised equipment.