Rotary drilling

Rotary drilling

The idea of rotary drilling was to create a hole in the earth by circulating fluid out the end of a

hollow string of pipe washing the cuttings back to the surface. This principle worked very well

until the formations encountered began to take or absorb more water than was being pumped

down the drill stem. This presented a problem of loss of circulation. Many methods of overcoming this problem were tried and as a general solution a mixture of clay and water was used to seal the formations and prevent water loss. In early construction, drillers would locate a clay bank from which they would secure enough clay to make a slurry which would prevent the loss of the drilling fluid (Fig. 3). This principle was employed until the present method of rotary drilling was established. It consists of a drilling machine which has a hoisting rig which usually has two drums. The principle of this type of drilling involves the use of hollow drill fools

through which a drilling fluid, consisting of refined clays in a mixture of water, is circulated.

As previously stated the main purpose is to remove or float the cuttings to the surface. Once

the fluid is at the surface its velocity is reduced to allow the settling of the cuttings in the mud

ditch thereby allowing the reverse of the fluid for the further removal of cuttings. We have,

therefore, arrived at the general principle of rotary drilling. We must now begin to explore the

principles which enable us to construct a successful well by this means.

The first order of business is to have a ma chine capable of drilling to the depth and diameter contemplated. Then it is important we level the machine to allow the drilling of a verti cal hole in which we may set our casing and work. The machine will allow the operator to perform this function. We must realize that if the machine is not level then we tend to have a drill hole which will not follow a plumb and ver tical line. Immediately upon the beginning of the set up we must begin to prepare the drilling fluid. This is very important as with the modern fluids they require a time for swelling. If they are used before the time necessary we tend to pump or place undeveloped fluids opposite formations — thereby creating a situation which will allow them to swell in place. The modern clays for drilling require curing time and therefore if ignored they tend to cure in place. If this happens we can see what happens when we attempt to develop the formation. I only mention this and attempt to emphasize at the beginning how important it is to prepare this drilling fluid before using same.

 The next principle is the drilling of the actual hole. We begin by allowing the drill string to penetrate the formation which consists of a rotating or revolving of the drill tools and concentrically pumping the drilling fluid through the hollow string. This allows the fluid to discharge out of the bit and circulate to the surface. The viscosity or thickness of the drilling fluid will carry the cuttings to the surface due to the velocity. The construction of the mud ditches on the surface reduces the velocity and thereby allows the cuttings to settle. This enables us to reuse the fluid over and over again. It is very important that the sand or cuttings settle in the slush pit to prevent the pump from circulating high volumes of cuttings which will cause excessive wear in the pumps.

Rotary drilling as previously stated consists of the removal of formations by pumping a drill ing fluid through a string of hollow tools remov ing the formations encountered by the bit. The

principle of water well drilling is to construct a plumb or straight hole. In rotary drilling this is

accomplished by not running a slack line. In other words we must not allow the full weight of the string to apply upon the bit. If this should happen we will cause a bow in the line of tools and thereby cause the bit to wander or move in a direction other than plumb. The proper way to

maintain a straight or plumb hole is by the use of drill collars in deep holes. A drill collar is a

section of very heavy wall pipe that consists of considerable steel which allows weight to be

exerted directly upon the bit and also causes the string to be pulled taut or plumb. In order to get

the designed penetration from the bits used we must apply a certain force or pounds per square

foot of area drilled. In other words a certain force is necessary to penetrate the formations.

From the driller's manuals we see that pressures vary depending upon formations encountered.

It is very important to mentally visualize the formations encountered. This must be done by the drilling or reaction of the machine itself. This is a matter of experience from the watching of the machine operate in various formations. Sand, for instance, will tend to cause a chatter and drill rather fast; while clay will cause the machine to run rather smooth and drill slow. Other than this we find the drilling procedure to be a combination of varying drilling conditions. This varying speed of the drilling vs operation of the machine through experience will enable the driller to determine the formations pcnetrated.

In recent years the use of the electric log enables us to confirm various formations penetrated to a much greater degree than before. The types of pumps used in the drilling depends upon the formations and depths to which the drilling will be done. A centrifugal type of pump will allow us to have rather high volumes of fluid to relative shallow depths but is limited when the friction loss in the drill pipe becomes high due to long lengths or small size. When drilling to deep depths it is necessary that high pressure piston pumps be used. They do require careful design of the mud pits together with good mud control to prevent high sand contents in the fluid. High volumes of sand in this type of pump will cause rapid wear of the pistons and liners.