Cable Tool Drilling
Developed by the Chinese, the cable tool percussion method was the earliest drilling method and has been in continuous use for about 4000 years. Using Tools constructed of bamboo, the early Chinese could drill wells to a depth of 3000 ft although construction took two to three generations. Cable tool drilling machines, also called percussion or spudder rigs, operate by repeatedly lifting and dropping a heavy string of drilling tools into the borehole. The drill bit breaks or crushes consolidated rock into small fragments, whereas the bit primarily loosens the material when drilling unconsolidated formations. In both instances, the reciprocating action of the tools mixes the crushed or loosened particles with water to form a slurry or sludge at the bottom of the borehole. If little or no water is present in the formation, water is added to form a slurry. Slurry accumulation increases as drilling proceeds and eventually it reduces the impact of the tools. When the penetration rate becomes unacceptable, slurry is removed at intervals from the borehole by a sand pump or bailer.
A full string of cable tool drilling equipment consists of five components: drill bit, drill stem, drilling jaws, swivel socket, and cable. Each component has an important function drilling process. The cable tool bit is usually massive and heavy so as to crush and mix all types of materials. The drill stem gives additional weight to the bit, and its length helps to maintain a straight hole when drilling in hard rock.
Drilling jars consist of a pair of linked, heat-treated steel bars. When the bit is stuck, it can be freed most of the time by upwards blows of the free-sliding jars. This is the primary function of the drilling jars; except in unusual circumstances, they serve no purpose in the drilling operation itself. The stroke of the drilling jars is 9 to 18 in and distinguishes them from fishing jars which have a stroke of 18 to 36 in or longer.
The swivel socket connects the string of tools to the cable; in addition, the weight of the socket supplies part of the upward energy to the jars when their use becomes necessary. The socket transmits the rotation of the cable to the tool string and bit so that the rock is cut on each downstroke, thereby ensuring that around, straight hole will be cut. The elements of the tool string are screwed together with right hand threaded tool joints of standard API design and dimension.
The wire cable that carries and rotates the drilling tool is called the drill line. It is a 5/8 to 1 inch left hand lay cable that twist the tool joint on each upstroke to prevent it from unscrewing. The drill line is grieved over a crown shield at the top of the mast, down to the spudding sheave on the walking beam, to the heel sheave, and then to the working line side of the bull reel. Bull reels are generally set up with a separator on the drum to provide a working line side and a storage line side.
Bailers used to remove the mud or rock slurry consists of a pipe with a check valve at the bottom. The valve may be either a flat pattern or a ball and tongue pattern called a dart valve. A bail handle at the top of this tool attaches to a cable called the sand line. The sand line is threaded over a separate sheave at the top of the mast and down to the sand line reel. The diameter of the sand line can vary according to the anticipated loads.
Another type of bailer is called the sand pump or suction bailer. This bailer is filled with a plunger so that an upward pull on the plunger tends to produce a vacuum that opens the valve and sucks sand or slurry cuttings into the tubing. The sand pump can have a bit bottom, but more often in water well drilling it has a flat bottom with a flat type valve. Some sand pump bailers have a flat bottom for slurry release.
The characteristic up and down drilling action of a cable tool machine is imparted to the drill line and drilling tools by the walking beam. The walking beam pivots at one end while its outer end, which carries a sheave for the drill line, is moved up and down by a single pittman connected to a crankshaft. The vertical stroke of the walking beam, and thus the drill tools, can be varied by adjusting the position of the Pitman pin or the bull gear and equipment connection to the walking beam. The number of strokes per minute can be varied by changing the speed of the drive shaft. The bull gear is driven by a pinion mounted on a clutch. This clutch, the friction drive for the sand line, (on smaller cable tool rigs only) and the drive pinion for the drill line reel are all mounted on the same drive shaft assembly.
Another drum, called a casing real, is frequently added to the basic machine assembly. The casing real is capable of exerting a powerful pull on a third cable, the casing line. This cable is used for handling pipe, tools, and pumps, or other heavy hoisting. It may be used to pull a string of casing when the cable is reeved with blocks to make two, three, or four parts lines. Reinforcement of the derrick by means of a stiff leg may be required to utilize the maximum pull that may be applied.
Another commonly used axillary hosting device on a cable tool machine is called a cat head. Use of this drum requires that a heavy line of manila rope be carried on a separate sheave at the top of the derrick. This line may be used for handling light loads and alternately lifting and dropping tools such as a drive block or bumper which are used to drive or lift casing. Synthetic ropes made with nylon or Dacron are considerably stronger than manila rope, but they are not resistant to abrasion or heat and therefore cannot be used with cat head. Two or three loose turns of the free end of the rope are wrapped on the cat head. When a cat head is rotating, the drill holes on the free end of the rope, causing the coil to tighten and grip the cat has. This raises the load at the other end of the rope. When the driller reduces the pulling pressure on the rope friction between the rope and a revolving cat head is reduced and a load descends at a controlled rate. The cat has is a live drum, that is, there are no such as to engage or disengage during use.
Every cable tool machine has certain independent limits on bore hole depth and diameter. For example, if a hole is relatively small in diameter it may be drilled to relatively great depth. In large diameter holes, the weight of the drill string and cable may become so excessive as machine cannot function, thereby eliminating well depth at the initial diameter. Collapsing formations may further limit the effective depth for large-diameter casing, because considerable friction develops between the casing and borehole wall while the casing is being driven. In many cases the casing size is progressively decreased as a hole is deepened, thereby reducing friction and also the weight of the drilling tools. Friction between the borehole wall and casing can be reduced by the addition of drilling fluid slurry around the outside of the casing during driving. This small amount of slurry will also decreased the energy required for pulling back casing to expose screens set within a casing. In water well drilling, the depth capability for cable tool rigs ranges from 300 to 5000 feet.
The drilling motion of the cable to the machine must be synchronized with the gravity fall of the tools for effective penetration. Several factors (thickness of the slurry in the borehole, whip in the cable, hole alignment, and rocks protruding in the borehole) may interfere with the free gravity fall, and a driller must adjust the motion and speed of the machine to the vertical movement of the tools. Effective drilling action is obtained when the engine speed is synchronized with the fall of the tools and the stress of the cable, well paying out of the correct amount of cable to maintain proper feed of the bit. The bit should strike the bottom of the hole at the extreme elastic limit of the cable and immediately snap upward so that a sharp blow is given to the earth material by the bit. This requires some resilience and elasticity and the cable and certain parts of the rigs mechanism. An elastic snubber or shock absorber is usually installed in the mounting of the drill line crown ship to provide part of those resilience in the system. The shock absorber compresses as the walking beam completes its up stroke and starts his pull on the cable. Cable tension then reaches its maximum because it tools are still moving downward. The shock absorbers rebound helps to lift the tool sharply after they strike bottom. The objective is to give the tools that peculiar whip at the end of the stroke which is essential to rapid drilling. At the surface, the cable will appear to be constantly in tension. When properly done, this technique conserves power and increases drilling speed. The shock absorber also dampens the vibration that occurs when the drill bit strikes the bottom of the hole, it protects the Derek and the rest of the machine from severe shock stresses.
Drilling consolidated formations
Most boreholes completed in consolidated formations by the cable tool method are drilled open hole, that is, no casing is used during part or all of the drilling operation. When drilling in consolidated rock, the cable tool bit is essentially a crusher. Its performance depends on the energy it can deliver to the bottom of the hole when the proper drilling motion is maintained. Factors that affect drilling rate or efficiency are, resistance of the rock, dip of the rock structure, weight of the drill tools, length of stroke, strokes per minute, diameter, sharpness, and shape of it, clearance between the tool string and the hole, and density and depth of the accumulator slurry. Each driller relies on the drilling machine manufacturer for guidance on these factors, and adds to this basic knowledge from personal experience.
Drilling unconsolidated formations
Drilling in unconsolidated formation differs from hard rock drilling in two ways. First, pipe or casing must follow the drill bit closely as the well is deepened to prevent caving and keep the borehole open. Usually the casing has to be driven by an operation similar to pile driving. Second, the drilling action of the bit is largely a loosening and mixing process. Actual crushing is of little importance except when a large stone or Boulder is encountered.
A drive shoe made of hardened and tempered steel is attached to the lower end of the casing string this is to prevent damage to the bottom of the casing when it is being driven. For the pipe driven operation, a drive head is fitted to the top of the casing to serve as an anvil and protect the top of the casing. Drive clamps constructed of heavy steel forging made in halves are attached to the square near the top of the drill stem. Drive clamps act as the hammer face, and the tools provide the weight for driving the pipe. The tools are lifted and dropped by the spotting action of the drilling machine.
The usual procedure is to drive the casing initially for 3 to 10 feet. Material and the casing is then mixed with water by the drill bit to form a slurry. Most of the slurry is bailed out and the pipe is driven again. Each time that the casing is cleaned out, more water must be added if none is encountered in the formation being drilled. In some cases, the hole is drilled three to six feet below the casing, the casing is then driven down to the undisturbed material and drilling is resumed. Driving, drilling, and bailing operations are repeated until the casing is at the desired depth.
When friction on the outside of the casing increases to the point where the casing cannot be driven a deeper or further driving might damage it, a string of smaller casing is inserted inside the first one. Drilling is then continued inside the smaller casing. The diameter of the well is thus reduced. Two or three reductions may be required in certain cases before reaching the desired depth. If friction problems are anticipated, casing and the upper part of the borehole should be one or two sizes larger than the diameter specified for completion of the well.
When penetrating most unconsolidated formations, driving the casing occupies as much time as the actual drilling and bailing. The physical nature of clay, silt, sand, gravel, and marl profoundly affects the rate at which casing can be driven. The best driving weight and setting of the sliding motion is determined from experience in a given locality.
In some cable tool operations the casing is not driven at all but is pushed into the ground by hydraulic jacks as drilling and bailing proceeds. Casing can also be pulled back by using the jacks. Several advantages of this methods are immediately apparent. Drilling proceeds rapidly because it is not necessary to stop the drilling and bailing to drive pipe. Because download pressure is maintained constantly on the casing during drilling and bailing, caving and over excavation are minimized. Perhaps most important, the jarring action of ordinary driving procedures, which compacts sand and gravel formations near the casing and causes excessive friction, is avoided. 16 inch casing has been hydraulically jacked to depths of a thousand feet in unconsolidated formations, and subsequently pulled back 150 feet to expose a screen. In this case, the jacks provided more than 500,000 pounds of lifting force.
When drilling in shallow sands, the casing may follow the bit down without being driven. In these areas, the casing may have to be held to prevent it from sinking too rapidly and to maintain wellness. Also, these formations may be drilled more rapidly by using the sand pump bailer (suction bailer), to remove the sand without a bit. In Argentina, near Buenos Aires, temporary or surface casing has been installed to depths of 200 feet using this method.
Another drilling technique, called the open hole or reverse cable to a method, has been used for many years in Japan and has been used recently in the western United States. With the borehole full of water or drilling fluid, heavy sand pumps or bailers are operated inside the casing to cut the borehole. Holes to 24 inches in diameter with cobbles to 12 inches can be drilled in this manner. Large diameter irrigation wells have been drilled to 100 feet and screamed within one day using this method. In isolated cases, the whole can be drilled open hole even in completely unconsolidated formations, because the hydrostatic water pressure prevents caving of the borehole walls.
The cable tool method has survived for thousands of years because it is reliable for a variety of geological conditions. It may be the best and in some cases the only, method to use in coarse glacial till, boulder deposits, or rock strata that are highly disturbed, broken, fissured, or cavernous. In situations where the aquifers are thin and yields are low, the cable tool operation permits identification of zones that might be overlooked another drilling methods.
The cable tool method offers the following advantages:
- Rigs are relatively inexpensive.
- Rigs are simple in design and require a little sophisticated maintenance
- Machines have low energy requirements
- Borehole is stabilized during the entire drilling operation.
- Recovery of reliable samples is possible from every depth unless heaving conditions occur.
- Wells can be drilled in areas where a little makeup water exists.
- Wells can be constructed with little chances of contamination.
- The driller maintains intimate contact with the drilling process and materials encountered by keeping a hand on the drilling cable.
- Generally, only one person is needed to operate a drilling rig, although a helper is usually available to assist.
- Because of size, machines can be operated in more rugged, inaccessible terrain or in other areas where space is limited.
- Rigs can be operated in all temperature regimes.
- Wells can be drilled in formations where lost circulation is a problem.
- Wells can be built at any time to determine the approximate yield at that depth.
Some disadvantages of the cable tool method include the following:
- Penetration rates are relatively slow.
- Casing costs are usually higher because heavier wall or larger diameter casing may be required.
- It may be difficult to pull back long strings of casing in some geological conditions unless special equipment is available.
Other drilling techniques have been devised because of some inherit disadvantages of the cable tool method because the method is often slow each cable tool trailer can complete only a limited number of holes per year despite high operating efficiency. In times of high customer demand, the drill or may not be able to take on much new business without adding new machines, and expense the long-term economics of the business may not allow. In addition, drillers experienced with cable tool rigs may not be available.