Well Drilling including geothermal and water is often the least understood. This article provides the basics of equipment and methods used for drilling and completion of geothermal and water wells. Due to cable tool is very old and less efficient way of drilling, it would be talked about here. Rotary and DTH drilling will be focused.

Water Well and Geothermal Drilling Meothod Category

Rotary Drilling

  • Directional Drilling
  • Reverse Circulation Drilling
  • Core Drilling

DTH(Down The Hole) Drilling

Rotary Drilling

The drill bit, usually a tricone roller, is rotated by the hollow drill collar and drill pipe. Torque is applied through the rotary table and kelly. Drilling fluid is circulated down the drill pipe and out openings in the bit where it cleans cuttings from beneath the bit, cools the bit and carries cuttings to the surface where they are separated from the fluid. Weight on the bit is applied by the heavy drill collar assembly. The drill pipe is held in tension by the traveling block. Too much weight on the bit tends to drill crooked holes and, in some formations, slows down drilling because of insufficient cleaning action at the drilling face. See figure 1.1

The other type of rotary rigs with top head drive do not use a rotary table and kelly. Instead, a hydraulic motor that travels up and down the mast supplies torque directly to the drill pipe. Often a much shorter and lighter collar is used, and the rigs have pull-down chains to utilize part of the rig’s weight at shallow depths.

Reverse Circulation

In reverse circulation, drilling fluid (usually water or very thin mud) flows down the annulus, up the drill pipe to the suction side of a pump, and into the tank or pit. Cuttings are lifted inside the drill pipe that has a smaller cross section than the annulus. Suction lift of the pumps limits this method to approximately 450 ft depth at sea level. The method that is preferred for geothermal wells utilizes an air pipe inside the drill pipe to provide the lift, and a cyclone or similar separator to separate air from the water and cuttings mixture. The air lift greatly increases depth capacity. Fluid level in the annulus is maintained at or very near the surface. The drill pipe is similar to conventional air drilling pipe.  Shown as figure 1.2

Dual Tube Reverse Circulation

The drill pipe is double wall, usually flush jointed. Drilling fluid can be air, foam or light bentonite, or polymer muds. Fluid is circulated down between the pipe walls, through a bit sub, inward across the bit, picking up cuttings, and up through the inner pipe. The bit is normally one nominal size large than the outer pipe; therefore, a good seal between the pipe and well wall is obtained (Figure 1.3).



Core Drilling

Core drilling is basically an exploration method. This technique is widely used in mineral exploration, civil works foundation investigation, and wells for scientific investigation. It is also used for geothermal test and/or temperature gradient well drilling where accurate and complete lithology are required.

The core bits (Figure 1.4) are usually faced with a powder metallurgy diamond grit material. Water or thin bentonite drilling

fluid is circulated for bit cooling and drill rod lubrication. Surface returns of drilling fluid are desirable, but drilling without returns is practical, because the cuttings are very fine and not as likely to stick the downhole string as in conventional rotary drilling. Fluid circulation rates are low because the annulus is small and drilling fluid is not a major expense.

Directional Drilling

Ordinarily, a well is drilled as straight and plumb as reasonably possible, particularly for direct use projects. This makes well completion and pump installation much easier and more economical. Directional drilling is often used in geothermal electrical generation reservoirs where there are economics realized by drilling several wells from one drill pad and steam gathering systems are simplified. To date, the only directional drilling for direct use projects has been to sidetrack junk in a hole, i.e., twisted off drill pipe that cannot be fished out. Directional drilling could be used to intersect a fault for increased production, or to parallel in close proximity to a fault to reduce the possibility of fault movement shearing off a casing. However, the economics of direct use projects usually will not permit the additional expense. Figure 1.5 shows the directional drilling tools with stabilizer.

Down The Hole Drilling

One of the more popular methods for drilling water and geothermal wells is the air hammer method. It is especially suited to drilling hard igneous and metamorphic formations. It is a percussion method.

A pneumatic hammer, similar in action to a large jack- hammer, operates at the down hole end of the drill pipe from 100- 450 psi even higher compressed air. The hammer face has tungsten carbide inserts to provide chipping capabilities. Air hammers are available in 3 in. to at least 20 in. diameter and will provide between approximately 800 to 2,000 strokes/min. The drill pipe and hammer are rotated slowly so the inserts continually strike a new surface to provide even penetration and drill a straight hole. Hammer exhaust or excess air or both is directed to clean the chips away as they are formed, providing a clean surface and increasing drilling rates from 50 to 100% faster than tricone rollers. The exhaust air carries cuttings up the annular space and out the hole(Figure 1.6)

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