Drilling fluid (often called “mud”) is used to:-
- lift soil/rock cuttings from the bottom of the borehole and carry them to a settling pit.
- allow cuttings to drop out in the mud pit so that they are not re-circulated (influenced by mud thickness, flow rate in the settling pits and shape/size of the pits).
- prevent cuttings from rapidly settling while another length of drill pipe is being added (if cuttings drop too fast, they can build-up on top of the bit and seize it in the hole).
- create a film of small particles on the borehole wall to prevent caving and to ensure that the upward-flowing stream of drilling fluid does not erode the adjacent formation.
- seal the borehole wall to reduce fluid loss (minimizing volumes of drilling fluid is especially important in dry areas where water must be carried from far away).
- cool and clean the drill bit; and .
- lubricate the bit, bearings, mud pump and drill pipe (Driscoll, 1986) .
Always start drilling with clean water as the drilling fluid; keep it as clean as possible during drilling to minimize subsequent well development problems In clay-rich formations, the water will quickly mix with natural clays in the borehole to form a thin clay slurry While this “natural mud” can be used for drilling the 10 cm (4 in) pilot hole, it should be replaced with clean water or a drilling mud prior to the water bearing zones being reamed-out to 15 cm (6 in) If this is not done, the natural clays will be pushed into the aquifer and will not break-down with development, thus seriously restricting well yield.
In sandy soils, bentonite clay (sodium montmorillonite) must be mixed with the drilling water to increase its viscosity and keep the borehole from collapsing (just a small (amount of bentonite is required.
While better than natural clays, bentonite does not readily break down its cohesive structure and it can be difficult to remove from the borehole and aquifer Since this can keep boreholes from reaching their potential yield , it can be adventageous to use synthetic muds (polymers) such as Revert when drilling into marginal aquifers Because it is very concentrated, powdered polymer can be shipped at relatively low cost into countries where bentonite is not available.
Whenever using synthetic drilling polymers, however, it is extremely important to flush all the polymer out of the borehole as soon as possible Some polymers have an organic base which can act as a bacterial food source If left in the borehole, nuisance and health-related bacterial populations can grow rapidly and permamently affect the taste, odour and safety of the well water To avoid these problems, flush as much polymer out of the borehole as possible before floating in the gravel pack Break-down can be enhanced by adding 500 to 1,000 ppm chlorine to the drilling fluid during the flushing process .
If bentonite or polymer is not available, it is best to determine (from the government or other knowledgeable organization)where there is a good supply of clay suitable for drilling (one that is relatively pure and has little or no sand) Make sure that you evaluate the suitability of local clays prior to drilling .
Drilling mud is created by thoroughly mixing water with clay to a desired consistency Pumping water through the by-pass hose on the 3-way valve and recirculating water back through the pits will help ensure that the clay and water are thoroughly mixed.
After the fluid is mixed, sufficient time must be allowed to elapse to insure complete hydration of the clay prior to it being circulated into the hole If this is not done, the clays may swell in the hole or in the aquifer itself If this happens, it may be impossible to remove them after the casing is installed and the well may never reach its potential yield.
Drilling fluids must be mixed thick (viscous) enough to bring soil cuttings up from the bottom of the hole to the surface, yet not so viscous as to prevent their settling out in the mud pits It is, therefore, very important to understand the properties of drilling muds and their proper use.
The ability of a fluid to lift cuttings increases rapidly as viscosity (the degree to which a fluid resists flow under an applied force) and up-hole velocity are increased After cuttings are brought to the surface, however, it is essential that they drop out as the fluid flows through the settling pit The desired results are obtained by properly designing the mud pits, controlling the viscosity and weight of the drilling fluid and adjusting the pump speed.
During the drilling process, solids accumulate in the drilling fluid – especially when drilling silt, clay or weakly consolidated shale The thickness of the drilling fluid often needs to be adjusted during drilling by adding more water and/or removing some of the accumulated cuttings from the settling pit.
Fluid which is too thick will be difficult to pump and will cause unnecessary wear of the mud pump since cuttings will not have settled out of the mud before the mud is pumped back down the borehole It will also make it difficult to remove the mud from the borehole walls and adjacent aquifer during well development The rate of penetration is also potentially reduced.
If the mud is too thin, cuttings will not be brought to the surface and the drill bit and drill pipe may get stuck in the borehole by settling cuttings In addition, thin mud can result in excessive migration of mud into the formation, thus decreasing the potential yield of the well.
Once the well is started and the fluid is being pumped, it is important to keep the well and mud pits full of water and complete the drilling and installation of the casing before the well is allowed to run out of water from the drilling process If return circulation of drilling fluid out of the borehole is suddenly lost, ensure that you take immediate action.
If drilling stops for more than a few minutes and the water recedes down the hole, the well may cave-in! To minimize caving risk, keep the drill pipe in the well (several metres off the bottom) and re-fill the well through the drill pipe Do NOT pour water down the open hole since this may actually cause a cave-in! If the drilling stoppage lasts long, pull the drill pipe out of the borehole to ensure it is not jammed and lost (the ( drill string can even be pulled-out by hand using pipe wrenches.
References and Footnotes:-
Drilling polymer’s (such as “Revert” made by Johnson & Johnson 612-636-3900), are organic drilling fluid additives which take the place of native clay or bentonite When Revert is mixed with water in a ratio of about 7 1 Kg per 1000 litres (6 lb per 100 gal), a bright blue viscous fluid forms Revert prevents caving, drops cuttings in the mud pit better than bentonite mixtures Biological breakdown causes it to change (“revert”) to a fluid as thin as water after several days (the fluid becomes light grey when reversion to a water-like fluid is imminent) After the fluid has reverted, it can be thoroughly flushed from the well and the well can be developed as easily as if only clear water had been used in drilling Dry Revert powder can be put in a coffee can and slowly sprinkled into a barrel of water which is agitated using a paddle mixer (such as a paint mixer attached to an electric drill) If fed too fast it will tend to form lumps If more than 3 ppm iron is present, pre-treat the water with about 75 lb calcium hypochlorite per 1,000 gals of water (50 ppm chlorine) to oxidize any dissolved iron.