How Mud drilling is done.

Besides the cooling and lubrication of drilling bits, which has already been mentioned, the addition of special muds or other additives to circulating water provides the following significant advantages when drilling in unstable formations:

• By using fluids of a density higher than that of water itself, significant hydrostatic pressure is applied to the walls of the borehole, preventing the formation from caving in
• The liquid forms a supportive ‘mud cake’ on the wall of the borehole, discouraging the collapse of the formation
• The liquid holds cuttings in suspension when drilling is halted for the addition of drill pipes
• The liquid removes cuttings from the drill bit, carries them to the surface, and deposits them in mud pits

Drilling mud – a partially colloidal suspension of ultrafine particles in water – fulfills these functions by virtue of its properties of velocity, density, viscosity, and thixotropy (ability to gel or freeze when not circulated). Water by itself exerts hydrostatic pressure at depth in a borehole, but at shallow
depths this may not be sufficient. Among additives for increasing the density of water, salt is one of the most convenient; but one of the most widely used is a natural clay mineral known as bentonite (calcium montmorillonite), which swells enormously in water. A slurry consisting of water and bentonite combined in the proper proportions has a higher viscosity than water and forms a mud cake lining in the borehole. However, a major disadvantage is that the mud needs to be mixed and left for some 12 hours before use to allow the viscosity to build up.

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The normal bentonite mud mix is 50 kilograms per cubic meter of water (a 5% mix), or 70 kilograms per cubic meter, if caving formations are expected.

Natural polymers (also known as Mud chemicals) provide a more practical solution for water boreholes, but they are relatively expensive, so should be used with care. One example of such a polymer, used in oilfield and water drilling, is guar gum, an off-white colored powder extracted from guar beans. It is an effective emulsifier used in the food industry, so is biodegradable, and will lose its viscosity naturally after a few days. Polymers are best mixed by sprinkling the powder into a jet of water, to prevent the formation of lumps. The polymer mud should be mixed during the setting-up stage – a minimum of 30 minutes is usually required – so that it has time to ‘yield’ (build up viscosity).

The normal mix for a polymer is one kilogram per cubic meter of water; for drilling in clay formations, use up to 0.5 kilogram per cubic meter, and for caving formations, use one to two kilograms per cubic meter.

Besides the usual mud properties, polymer drill fluids also coat clay cuttings, preventing the formation of sticky aggregates above a drill bit (known as ‘collars’), which can hold up drilling while they are removed (a simple remedy for clay aggregation is to add salt to the drilling fluid). Another advantage of polymers is that they make it possible when it is clay that is being drilled through, to distinguish genuine formation samples from the mud. Degradation of polymer muds is accelerated by high ambient temperatures, acidity, and the presence of bacteria (using the polymer as a food source): polymer-based mud might last only two or three days in tropical conditions, and can cause bacterial infection of the borehole. It could be that natural polymer powders have a limited shelf life, and this should be checked before purchasing stock from a supplier. Food-grade bacterial inhibitors have been used as additives to prevent the breakdown of polymer-based muds. When using polymers, observe the manufacturer’s guidelines. Foaming agents are also widely used as drilling fluid additives, normally in air drilling.

Learn more about Mud Chemicals here >>>

– Checking the viscosity of drilling mud
Every mud additive (bentonite, mud, salt, etc.) must be mixed into the circulating water to provide the correct viscosity. This can be done initially in a specially prepared pit, but as drilling proceeds, and especially if groundwater is struck, the mud will become diluted, and more mud or additive powder will have to be added. Too low a viscosity may result in fluid seeping into the formation, and it may later be difficult to remove the fine mud particles from the wall of an intersected aquifer, reducing the efficiency of the borehole. ‘Thin’ mud may also cause cuttings to fall back onto the drill bit, causing it to stick in the hole. The viscosity of drilling mud can be easily and frequently checked by means of a simple viscometer known as a Marsh funnel.

Extremely porous or fissured formations can cause a loss of drilling fluid (mud); it is possible that the entire mud circulation might disappear into a cavity. This could put a stop to drilling altogether, if increasing fluid viscosity by adding more additive has no effect. If the area from which fluid is being lost is not likely to be part of an aquifer, fibrous materials such as sawdust, dried grass, or cow-dung could be introduced into the mud, while ensuring that a pumpable circulation is maintained. Such additives can block large pores and cavities permanently, which is why they should not be used to cure losses in a water-bearing zone.

Photo Credit: Regan

– Mud pits
To mix the mud, as described previously, mud pits are required. This can be combined with a ‘suction pit’ or sump from which a mud pump will take the circulation supply. Second, a larger, ‘settling’ pit is essential, in which mud returning to the surface from the borehole’s annular space will be allowed to drop its load of drill cuttings. The two pits and the borehole are usually connected by shallow channels or ditches and a weir. Mud pits are most commonly dug in the ground alongside the rig, but some contractors can supply steel tanks, which are their equivalent. If dug in soft soil, pits may be lined with plastic sheeting, clay or cement. Mud circulation through pits must be slow and steady, to settle the cuttings and to make collecting formation samples (normally taken from a channel close by the borehole) easier. The mud pump inlet and strainer are held by rope above the bottom of the suction pit, so that mud that is as clean as possible can be recirculate into the borehole via the drill pipes. Optional extra ‘swirl pits’ may be included between the borehole and the settling pit to further aid settlement of debris.

The capacity of the suction pit should be roughly equal to the volume of the hole being drilled; the capacity of the settlement pit should be at least three times that.

To roughly calculate pit volumes, given hole diameter D in inches (drill bit size):
Borehole volume and suction pit volume = D2H/2000 in cubic metres (or D2H/2in litres), where H is depth of hole in metres.
Settlement pit volume should be ~0.002D2H cubic metres (or 2D2H litres).

Before selecting the mud rotary drilling method, there are economic factors you may need to consider:

1. The cost of the bentonite product, water supply, and mud mixing equipment
2. The need and the price for geophysical logging the borehole
3. Whether or not in-the-ground mud pits can be dug to hold the cuttings
4. Whether or not the well development water can be discharged to the ground
5. Disposal of the drilling mud and cuttings when the well is completed
6. The time it takes to develop the well as compared to other methods of drilling.

Mud rotary well drillers for decades have found ways to make this particular system work to drill and construct domestic water wells. In some areas, it’s the ideal method to use because of the geologic formations there, while other areas of the country favor air rotary methods.

Some drilling rigs are equipped to drill using either method, so the contractor must make the decision as to which method works best in your area, for your well, and at your point in time.