Section 5Mixing and Using Drill Mud
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 (Driscoll, 1986). 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 (Moffat, 198?), it can be adventageous to use synthetic muds (polymers) such as Revert when drilling into marginal aquifers (see Footnote #1). Because it is very concentrated, powdered polymer can be shipped at relatively low cost into countries where bentonite is not available.
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 (see Appendix H).
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 (Driscoll, 1986). 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 (Driscoll, 1986).
During the drilling process, solids accumulate in the drilling fluid - especially when drilling silt, clay or weakly consolidated shale (Driscoll, 1986). 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 (Driscoll, 1986).
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 (see Appendix G-1).
If drilling stops for more than a few minutes and the water recedes down the hole, the well may cave-in! (see Appendix G-2). 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).
Viscosity is a measurement of a fluid's resistance to flow: the greater the resistance, the higher the viscosity. The viscosity of drilling mud is influenced by the gelatin-related density and the solids content. The viscosity can be controlled by adding drilling mud and adjusting the pH. The viscosity should be adjusted depending upon the type of material being drilled, the drilling rate, and the hole size.
Different types of clay have a wide range of hydration potential. The more the clay hydrates, the more it expands and has more lifting ability. Selling clays like bentonite and montmorillonite are preferred because the clay particles are much thinner and come apart more easily than those of other clays. When properly hydrated in water, these clays can swell to approximately 10 times their original volume. Bentonite and montmorillonite hydrate only in fresh water.
Viscosity can be measured with a Marsh Funnel. The procedure is as follows:
The following guidelines can be used to assess whether drill mud is thick enough:
References and Footnotes 1 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.