Drilling and Casing Techniques

A. Introduction

Small diameter well sinking techniques include:

Large scale techniques are covered here in enough detail to understand their basic operating principles. These methods use large, expensive, complicated machinery whose detailed operation can be adequately understood only with the benefits of hands-on training. However, the basic drilling techniques used with these larger rigs are, in many cases, the same as those which can be used with smaller equipment and may serve to better illustrate the possible variations of each technique.

Small scale techniques are covered later in enough detail to enable interested persons to perform them.

B. Drilling Techniques with Large Scale Equipment

1. Types of Rigs

These techniques all use a specially made unit known as a "rig" which includes all of the different power systems needed to operate one or a variety of types of drilling tools. The cost of these rigs ranges from about $20,000 to $500,000 and up. Depending on the type of rig and the drilling method, holes can be sunk with diameters ranging from 4 cm to 1.4 m. (See Fig. 11-1.)

There are two basic types of large drilling rigs, percussion rigs which give an up-and-down motion to the tools and rotary rigs which turn the drilling tools.

By far the most common type of percussion rig is the cable tool. Drilling tools are suspended on a cable which is alternately pulled and released to create the up-anddown motion of the tools. When drill cuttings have accumulated to the point that they impede the action of the bit, it is removed from the hole and a bailer is lowered, usually on a second cable, to pick up the cuttings. This is one of the two most common methods of well drilling and is known as cable tool percussion. Cable tool rigs are also adaptable to other forms of percussion drilling such as hydraulic percussion and jet percussion. These other percussion techniques are not, however, commonly practiced.


FIG. 11-1. CABLE TOOL PERCUSSION DRILLING RIG

Unlike percussion,there are several different relatively common types of rotary rigs. The biggest of these, and perhaps the most complicated of all well drilling machinery, is the rig capable of hydraulic rotary drilling and its variants. This and the cable tool are the two most commonly used water well drilling rigs.

Other types of rotary rigs are those that use augers and core drills. Auger rigs are designed to drill relatively large diameter holes to a relatively shallow depth in non-caving formations. Core drills are designed to drill very small diameter holes to great depths and recover samples of all the materials drilled through. Both of these kinds of rotary drills have been used to drill water wells, although both have limitations for such use.

NOTE: When considering using this equipment, carefully check the manufacturer's size specifications. There has been much confusion and delay resulting from the fact that tools and equipment are commonly manufactured in both English and Metric sizes, which are not generally compatible. To compound the problem there are many different kinds of thread patterns in use which can be cut on either English or Metric pipe sizes.

2. Drilling Variations

There are a number of possible drilling variations given the two basic methods, percussion and rotary, depending on whether or not a fluid is used and which way it is circulated.

The following two techniques are not really primary drilling techniques since they are not often used with large drilling equipment to sink wells from the ground surface. They are, however, frequently used in well drilling operations to sink the well screen and casing into place in the aquifer after a hole has been drilled down to the water table by any of the above techniques. For more information, see Chapter 14.


FIG. 11-2. DRILLING METHODS. THE CHOISE OF DRILLING METHOD DEPENDS MAINLY ON WHAT MATERIALS ARE AVAILABLE.

3. Drilling Depth

Assuming that the rig and tools are appropriate for the particular ground formation being penetrated, the depth that can be drilled is limited only by the ability of the rig to lift the tools from the hole.

For example, one particular manufacturer provides the following suggested depth capabilities for their topofthe-line hydraulic rotary drilling rig fitted out with various drilling tools.

drilling method

hole diameter

depth

auger

900 mm

12 m

continuous flight auger

300 mm

15 m

reverse circulation

600 mm

150 m

air rotary

170 mm

450 m

The same company's core drilling rig is capable of drilling a 48 mm diameter hole to 1400 meters.

C. Overview of Small-Scale Techniques

1. Introduction

The small-scale techniques given here have been adapted for use with a minimum amount of equipment and expense. These techniques may require only hand tools, pipe, and a great deal of labor, or they may require a tripod, a motor pump, drilling pipe, and drill bit plus whatever material will be permanently installed in the well. Wells workers may select from a variety of equipment varying in cost and technical difficulty, all of which are less expensive and less complicated than a drilling rig.

These sinking methods may be used in largely consolidated ground formations (see Glossary). The suitability of the different methods varies with the degree of caving expected and whether rock will be encountered.

It is common practice to use more than one method to sink the complete hole. With adequate finances and equipment, it is helpful to use the particular technique most suited to the (1) ground conditions, (2) expected depth of water, and (3) the section of well being worked on.

Each individual small equipment sinking technique is generally limited to use in a certain type of soil. Where the soil layers to be penetrated are similar, using one appropriate sinking technique will be most efficient. However, it is much more likely that you will encounter several different soils, requiring the use of variations on a single sinking technique, or even different techniques. When evaluating the various sinking techniques, choose the one most appropriate to a particular situation. You may also want to consider what other equipment might be needed in different ground conditions.

In many cases, ground conditions will vary between the middle and bottom sections of the well. The same type of soil may surround the complete well, but where that soil is saturated with water (bottom section), its drilling characteristics will usually be different. For example, a sand or combination of sand and gravel aquifer will usually be subject to caving, making it impossible to excavate a hole down into the aquifer without sinking a casing with the drilling tools. Sand that is only damp, on the other hand, will tend to stick together and not cave in.

However, if rock is struck, the well must be moved to a different site where rock might not be encountered, or the workers must switch to drilling tools appropriate for rock. Hand-powered tools commonly used in unconsolidated formations generally gouge and slice through soil to loosen it. This is not suitable for rock, which must be smashed and chipped or broken into smaller pieces in order to be removed from the hole.

2. Sinking Methods and Ground Conditions

There are then three basic types of ground conditions that can be encountered while drilling.

Ground Condition

Suitable Sinking Technique

Rock

percussion

Loose, non-caving

rotary, percussion or sludger

Loose, caving

driven, jetted

Where all three ground conditions are likely to be encountered you should be equipped with the types of tools appropriate to each.

3. Level of Complexitv and Common Uses of Small Scale

Techniques

a. techniques that require the least equipment and expertise

b. techniques that require a minimum of manufactured equipment but require that materials and semiskilled labor be available to produce the necessary equipment.

c. techniques that require at least local manufacture of the necessary equipment and pumps.

D. Small Equipment and Drilling Techniques

Here is basic information on small equipment drilling techniques, which provides the following information on each method.

1. Rotary hand auger


FIG. 11-3. HAND AUGERING

2. Hand percussion


FIG. 11-4. CHOPPING BIT

This method is best suited for drilling through soft rock and well packed, non-caving soils. It can be used in hard rock although progress will be slow and drilling bits will need to be sharpened frequently. It can also be used in caving soils if the casing is driven in as the hole is sunk.

3. Hand percussion and fluid (sludger)


FIG.11-5. BAILER


FIG. 11-6. SLUDGER TECHNIQUE

4. Driven


FIG. 11-7. DRIVEN WELL

5. Jetted


FIG. 11-8. JETTED WELL

E. Casing Installation

1. Overview of Casing Techniques

How the casing is installed in the well depends on such factors as casing materials, and ground conditions.

Before the casing is set into place it is very important that you also consider how the well screen will be attached to it. (See p. 192.) If the casing above is set in place the well screen must later be telescoped into place. Special tools and materials are then required to seal the well screen to the casing.

The same three methods of installing the casing are possible whether or not the well screen is attached.

The casing can be driven into place either after the hole has been sunk as far as possible or as the drilling proceeds. The latter is used primarily with percussion techniques where a bailer can be worked inside the casing to remove loose, caving types of soil as the casing is driven in to reinforce and maintain the hole. In this instance special drive clamps must be attached around the casing pipe so that it can be driven while the bailer is inside it.

When the casing will be driven into place before the well screen is attached, a sharpened coupling can be screwed on to the bottom of the casing to facilitate sinking. (See Fig. 11-9.)


FIG. 11-9. SHARPENED COUPLING

2. Sealing the Casing

Once the casing is set in its final position in the well any space around the casing should be filled in. The top three meters of this space should be filled with mortar or concrete to seal the casing to the ground formation and thus prevent possible contaminants from easily flowing down along the outside of the casing pipe .