We present the appropriate drilling methods for the construction of groundwater collection wells. There are different drilling methods for different geological conditions from hard rock (granite, dolomite) to loose sediment (alluvial sand, gravel).

Each specific drilling method prevails in certain areas. There is no single method that is best suited for all geological conditions, and each method requires different equipment to meet the job requirements.

This section also outlines the basic drilling principles, some applications, and practical limitations of the main drilling methods to geological conditions. Finally, a summary table summarizes the advantages and disadvantages of each drilling method outlined at the end of the article.

1. CONTROL CIRCULAR DRILL METHOD

1.1 Rotary drilling using gas and drilling fluid

The forward circulation drilling method was developed to increase drilling speed and depth in different geological conditions. A drill hole is drilled by rotating and slowly advancing with the drill bit attached to the end of the drill rod. Drilling mud is brought up by continuous streams of gas or drilling fluid in the space between the drill rod and the borehole wall.

In gas rotary drilling, the gas flow moves the drilling mud in the hole. An air compressor provides a stream of air that is connected to the bore or tip of the drill rod. When drilling down, the gas flow out at the tip of the drill cools the drill bit and carries the drilling mud out of the hole. Drilling mud after blowing up is collected. Add a small amount of water or a mixture of water and surfactant to the drill air system to control dust, lower the temperature of the drill bit, and cool the drill bit.

Gas drilling is practical for use in semi-consolidated or consolidated geological conditions. Usually use a moderate strong current when drilling to give enough time to clean the air back into the borehole and not to let the humus layer stick to the surface of the borehole wall.

The basic functions of a drilling fluid are:

• Transport drilling mulch from the bottom of the pit to the surface.
• Supports and stabilizes the borehole wall, preventing wall collapse or cleft formation.
• Seal the borehole wall to reduce fluid loss;
• Cool and clean the drill bit;
• Allow the humus to separate from the drilling fluid at the surface; and 
• Lubricate drill bits, bearings, fluid pumps, and drill rods. 

Usually the hydraulic pressure exerted by the drilling mud will be greater than the static pressure of the system, so that the fluid can penetrate excessively into the borehole wall. The equilibrium liquid pressure depends mainly on the drilling mud density. Good drilling fluid properties control will reduce both the frequency of failures and the drilling time. Experience and knowledge of drilling fluid additives is essential to minimize drilling failures and maintain borehole control.

Drilling mud must be cut small enough that the solution can be carried away. Air bubbles (fluid blowing) can be used to increase the carrying capacity of drilling mud, thereby increasing drilling speed and reducing fluid loss to the environment. Suggestions for appropriate drilling velocities and the use of different drilling admixtures are discussed in the drilling fluids section  .

1.2  Underbalanced Rotary Drilling

1.3  Downhole Hammer Drilling

Another air-circulating rotary drilling method is the chisel system. At the end of the rig there is a hammer system that shoots (hits) the rock while the lever is rotated slowly. The hammer head is made of alloy steel and tungsten, the super heavy hammers easily destroy the contact position. Rotating action increases borehole drilling and straightening – even in extremely hard rock environments. Drilling depth and speed into hard rock is greater than other drilling methods.

The chisel method is optimal while drilling in very hard media such as basalt, quartz, and granite..

1.4 Drilling methods using struts

The strut drilling method used for drilling in unconsolidated or loose environments is considered ideal when using struts. The four common methods used for this type of drilling are:

Use hammer to close

A ledge on the hammer strikes a control mount at the end of the strut, to push the strut into the ground when drilling is carried out. After the drilling process, the strut is still in the borehole and is difficult to recover.

2. REVERSE CIRCULAR DRILL METHOD

In reverse rotation drilling, the flow of drilling fluid is reversed and the drilling mud moves upward inside the drill rod and is discharged into the settling pit. Centrifugal pumps are frequently used because they treat drilling mud to minimize wear.

The fluid returns to the borehole – thanks to gravity – down a vortex to the bottom of the borehole. It returns to the drill rod, with more of the drilling mud washed away, through the drill holes. Drilling mud spontaneously develops as the clay and silt rotates as drilling is carried out, but where necessary, polymer additives are used to reduce friction and control water loss in the soil to dehydrate. In deeper wells with deep still water, a special drilling fluid is often used.

To avoid subsidence (due to hydrostatic pressure imbalance) it is necessary to maintain a positive fluid pressure in the borehole at all times - even when drilling is suspended. Reverse circulation is not usually performed when the still water level is less than 3m from the ground. A substantial amount of backup water should always be available when drilling into permeable sand and gravel (at least 3 times the amount of material removed during drilling).

3.  DUAL-WALL AIR ROTARY DRILLING

The double bore well drilling was developed in the mining industry to obtain accurate geological samples from known depths. Unlike the traditional reverse circulation method, the drilling fluid does not flow down the outside of the drill bit, but is contained between the two walls of the pipe and comes into contact with the borehole wall only in the area near the cutting edge. The double-bore drilling method is applied to the construction of water wells in all geologic formations, and larger boreholes are being built to drill large boreholes in difficult conditions.

Using a reverse circulation system can increase the capacity of a rig allowing large diameter wells to be drilled. Depending on the type of rig used, holes from 508 mm to 762 mm can be drilled. If the capacity of the rig is large enough, the bore can be from 762 mm to 1520 mm in unconsolidated geological environments.

4. NON-CIRCULATED DRILL METHOD

4.1 Cable punching drill

Developed by the Chinese, the cable breaking method has been in continuous use for about 4,000 years.

Cable hammer drills, also known as percussion or vibrating rods, work by repeatedly lifting and dropping a heavy drill bit into the drill hole. The act of reciprocating the tool mixes humus with water to form a solution: water is added if little or no water is formed. Drilling mud accumulates gradually during drilling, and affects tool operation to some extent is periodically removed by a pump or bucket to speed up drilling again.

Small diameter holes can be drilled to great depths, while drilling large diameter holes is limited by the weight of the rig and cable. Depths for cable dam rigs range from about 300 ft (90 m) to 1,500 ft (500 m).

Most boreholes are completed in the consolidated medium through the cabling method being drilled without the use of spools during the drilling process. When drilling in an unconsolidated environment, the pipe or sleeve must follow the drill bit to prevent collapsing and to keep the borehole open. To prevent damage, the steering base (including hardened steel and heat-resistant steel) is attached to the strut. When wall friction prevents penetration of the strut, a smaller strut can be inserted to continue drilling, in some cases two or three downsizing is required to drill to the desired depth. .

The cable hammer drilling method still exists because it is easy to use when drilling in a variety of geological conditions. It is a convenient method when drilling in rough rock; Muddy sand; basalt; or strongly disturbed, broken, cracked, or caved rock formations. In thin aquifers and small reserves, cable dam drilling allows to identify areas that could be missed using other drilling methods.

4.2 Bucket Augers drilling

The BA drilling method uses a large diameter cavity box with a bottom twist blade to excavate materials. The material is collected in a cylindrical bucket after being twisted with a blade at the bottom. The bucket is attached to the lower part of a kelly rod and is rotated by a large gear of the rig.

This method is mainly used in clay strata because it does not use support pipes. Drilling in sandy strata below the groundwater table is difficult, but not impossible using drilling fluids.

Many reverse circulation drills are equipped with air compressors, and when drilling to the required depth, turn on the blower (typically 45 ft (13.7m) to 60 ft (18.3 m)) through the mud pump. , drilling mud is blown up and pumped out of the borehole. Compressed air is delivered through a line made of plastic or metal. The gas lines are suspended inside the drill rod or the outer line of the drill rod. Most of the drill rods used are threaded and connected pipes.

Reverse circulation is an economical method when drilling large wells in unconsolidated strata. Most wells using this method are 20 inches (508 mm) or larger in diameter. The filter layer is often used because of the relatively large bore diameter.

A major disadvantage of the BA drilling method is that it is limited to use at shallow depths - typically less than 100 ft (30 m), and in unconsolidated deposits or loose sedimentary rocks.

5. ASSESSMENT THE RELATIVE EFFECTIVENESS OF METHODS

Selecting the best, most efficient drilling method requires an understanding of the geological conditions of the formation and the limitations of the rig and drilling method. A lot of problems can happen when drilling, so drillers need to be prepared to deal with when drilling in a variety of geological conditions or trying to drill when it is beyond safe conditions.

The table below provides relative efficiency ratings for different drilling methods when used in different geologic formations. However, the relative performance differences between drilling methods depend on the driller's level of experience, the lenses in the drilling area, and the groundwater pressure conditions.