Guidelines for Minimum Standards in Water Well Construction, Province of British Columbia — June 1982
Part 2 — Technical Information Appendices
APPENDIX 9: WELL TYPES AND WELL CONSTRUCTION METHODS*
1. Types of Wells
Water wells can be broadly classified into the following groups:
Dug Wells: Large diameter dug wells are built to obtain water from unconsolidated materials that normally occur in the first 50 feet beneath the surface. In this type of well that may range from 3 to 20 feet in diameter, the walls of the hole are lined in such a way as to prevent cave in of the walls yet still to allow seepage of ground water into the well base to produce a reservoir of water.
Radial Collector Wells: This type of well usually is constructed of a concrete shaft or caisson from which horizontal intake pipes project radially.
Naturally Developed Screened Wells: This type of well is built to obtain water at various depths from unconsolidated formations which can be stabilized naturally by development following the installation of the casing and screen.
Artificial Filter Pack Wells: This type of well is built to obtain water at various depths from unconsolidated formations that cannot be stabilized by the use of a casing/screen combination and natural development only, but in addition require the use of material (sand, gravel, rock) that is coarser than the formation material in the screened interval. This added material acts as an additional screen to prevent the finer aquifer materials from entering the well.
Open Hole — Partially Cased Wells in Consolidated Formations: This type of well is built to obtain water at various depths from consolidated formations (bedrock) either fractured or unfractured. It is not cased or screened in the producing zone, for no formation stabilization or filtering is needed, but it may be cased through the formations above the producing zone if it is unconsolidated or unstable to prevent borehole collapse or to exclude water of undesirable quality.
Open Hole — Partially Cased Wells in Unstable-Consolidated Formations: An additional well type is built to obtain water from unstable potentially caving formations. This type is usually screened through the producing zone and may, under some conditions, require the addition of coarse filter material. The well may not be cased through all the formations above the producing zone, except to exclude water of undesirable quality.
2. Well Construction Methods
Well construction methods are many and varied and range from simple digging with hand tools to high speed sophisticated drilling equipment. The most common construction methods are listed below:
Digging Method:
In this method the hole is constructed by digging to the desired diameter and depth with hand or power tool. The dug out materials are removed by lifting them from the hole in some type of container. The hole is shored, staved, or cased as the depth is increased. When casing is used, a common practice is to add casing at the surface, allowing it to sink by its own weight as the hole is excavated below the bottom of the casing.
Boring Method:
In this method the hole is constructed by the use of a selected diameter hand or power auger which is turned to bore the hole to the desired depth. Cuttings are removed by pulling and emptying the auger or bucket or by the screw action of the auger flite itself.
Driving Method:
In this method the hole is constructed by forcing a casing equipped with a drive point into the ground by a series of blows, either manually or machine-delivered, on the top of the casing. Driven wells (or "well points") should be installed only in soft formations that are relatively free of cobbles or boulders. They are feasible only where lifts are shallow and the quantity of water desired is small. Well points can be installed by hand or machine.
Jetting and Hydrauticing Method:
The jet drill is basically a combination percussion unit and pressure pump. The pressure pump is mounted on the machine together with pipe and hose connections leading from the pump to the drill pipe, with a return hose from the well casing to a pit or other suitable container. A suction hose returns the water from the pit or container to the pump. The drill pipe consists of a small diameter standard pipe with a bit or chisel attached to the bottom section. Water is forced down through the drill pipe by means of the pressure pump and out through the holes in the bit. This water, being under pressure, carries the cuttings to the surface through the space between the casing and the drill pipe. The. lifting and dropping action of the drill pipe chops up the material in the hole and loosens it so that it may be washed to the surface. This method uses a short, fast stroke and is very effective in soft ground, sand and gravel, or other loose unconsolidated formations. This method is best suited for smaller holes of from 2 to 4 inch diameter. A percussion machine is also used for drilling holes by the hydraulicing method. The difference between this and the jetting method is that with the hydraulicing method no pressure pump is needed. The hydraulicing unit utilizes a bit with an opening at the top and a valve seat and ball check valve above it. Water is directed into the hole by gravity in the space between the drill pipe and the casing. The up and down motion of the drill pipe acts as a displacement pump. The ball-check valve opens on the down stroke and draws material and fluid into the drill pipe. The valve closes on the up stroke, holding the slurry within the pipe. Eventually the pipe fills up and the mixture of fluid and cuttings are discharged at the surface. Like jetting, the hydraulicing method is fast and efficient in relatively soft formations such as clay and sand.
Cable Tool Method:
The cable tool method is used to construct wells by alternately lifting and dropping a set of drilling tools suspended on a wire cable so that with each stroke the drill bit strikes the bottom of the hole. The repeated action of the percussion drill permits bit penetration of the underground formations. The loosened material and drill cuttings are mixed with drilling water by action of the bit and the resulting slurry must be removed from the drill hole by a bailer or sand pump. In drilling a dry hole, water must be added periodically to replace that removed with the drill cuttings. Tools for drilling and bailing are carried on separate lines or cables. Each cable is spooled on a separate drum. In cable tool or percussion drilling there are basically three major operations: first, the drilling of the hole by chiseling or crushing the rock, clay, or other material by the impact of the drill bit, second, removing the cuttings with a bailer as cuttings accumulate in the hole; and third, driving or forcing the well casing down into the hole as the drilling proceeds. Well casing used in most percussion type drilling operations usually ranges from 4 to 24 inches in diameter. This casing is used to keep the well bore from collapsing and to prevent surface or sub-surface leakage of water or contaminants into the well bore.
Conventional Fluid Rotary Drilling Method:
In the conventional mud-rotary method of drilling, drilling is accomplished by rotating a drill pipe and bit by means of a power drive. The drill bit cuts and breaks up the rock material as it penetrates the formation. Drilling fluid is pumped through the rotating drill pipe and holes in the bit. This fluid swirls in the bottom of the hole, picking up material broken by the bit, then flows upward in the well bore, carrying the cuttings to the surface.
The drill pipe and bit move progressively downward, deepening the hole as the operation proceeds. At the land surface, the drilling fluid flows into a settling pit where the cuttings settle to the bottom. From the settling (mud) pit the fluid overflows into a second pit from which it is picked up through the suction hose of the mud pump and recirculated through the drill pipe. In the rotary drilling method the well casing is not introduced into the hole until drilling operations are completed, the walls of the hole being supported by the pressure (weight) of the drilling fluid.
Reverse Circulation Drilling Method:
In reverse circulation drilling, instead of circulating the drilling fluid through the drill pipe and up the outside of the pipe, the process is reversed. Fluid is fed down through the space between the wall of the hole and the drill pipe and it is then pumped up, together with the cuttings, through the hollow part of the drill pipe and out a discharge pipe. With the addition of air (applied via a compressor through piping along the drill stem) drilling depths have been increased in one instance, from 450 to 1,000 feet. Of particular importance is the use of a light (nearly clear) drilling fluid which eliminates the need for a viscous and heavy drilling mud used in conventional rotary drilling. The mud sometimes tends to seal-off water-bearing formations. The use of a relatively clear drilling fluid is possible because drilling is rapid; however, a substantial quantity of fluid must be on hand to maintain an open hole.
This method is used for rapid drilling of large diameter holes in soft formations where small boulders are encountered. Boulders up to six inches in diameter can be brought up to the surface through the hollow drill pipe. Such performance is possible because of the extremely high velocity of the fluid as it is drawn up through the drill pipe by the suction pump. In the reverse circulation method, holes 16 inches to 72 inches in diameter have been drilled. The walls of the hole are held in place by the pressure of the fluid against the sides of the hole.
Air Rotary Drilling Method:
In the air rotary method of drilling, air serves as the fluid and excavation is accomplished exactly as is done in the conventional rotary method. The bit cuts and breaks up the formation.
Air is forced down through the drilling pipe and out through holes at the bottom of the rotary bit. A stream of water is often introduced into the air system to help cool the drill bit and control dust. The air serves both to cool the drill bit and force cuttings up and out of the hole. The cuttings move up in the annular space between the drill pipe and the wall of the hole, and are collected at the top. Air is used principally in hard clay or rock formations, because once the air pressure is turned off, loose formations tend to cave-in against the drill pipe. This method is not generally recommended for drilling in unconsolidated materials because the quality of the samples are usually poor. Foaming additives are occasionally used to increase the uphole carrying capacity of the return air.
Down-the-Hole (Down-Hole Hammer, Hammer Drilling) Method:
The down-the-hole method involves a pneumatically operated bottom-hole drill that efficiently combines the percussion action of cable tool drilling with the turning action of rotary drilling. The pneumatic drill can be used on a standard rotary rig with an air compressor of sufficient capacity. It is used for fast and economical drilling of medium to extremely hard formations. Fast penetration results from the blows transmitted directly to the bit by the air piston. Continuous hole cleaning exposes new formation to the bit and practically no energy is wasted in re-drilling old cuttings. Down-the-hole drilling is generally the fastest method of penetration in hard rock. The bit is turned slowly (5 to 15 rpm) by the same method by which the drill bit in the fluid or air drilling operation is rotated. Foaming additives are occasionally used to increase the uphole carrying capacity of the return air.
Top Drive Air Rotary with Casing Hammer Method:
This is a relatively new and popular method and it is designed for fast penetration rates. Sand and gravel, boulders and glacial till are drilled and cased simultaneously. Screened wells can be drilled, cased and developed at high speed. The drill pipe is inserted inside the casing and picked up simultaneously with a special adapter when adding drill pipe and casing. After making up the tool joints with the hydraulically powered breakout tongs, the casing is welded and drilling continues. The air operated hammer of one manufacturer develops up to 7,200 foot pounds of driving force at 70 blows per minute with a 24 inch stroke. The hammer can be cleared from over the hole for pulling pipe and for conventional rotary drilling.
* From EPA-570/9-001.
It is impossible to describe in this section all types of wells or even a majority of those in common use.
The AWWA Standard for Deep Wells list 11 types of drilled wells and this publication should be consulted for details.
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