Direct Rotary Drilling
The direct rotary drilling method was developed to increase drilling speeds and to reach greater depths in most formations. The bore hole is drilled by rotating a bit, and cuttings are removed by continuous circulation of the drilling fluid as a bit penetrates the formation. The bit is attached to the lower end of a string of drill pipe, which transmits the rotating action from the rig to the bit. In the direct rotary system, drilling fluid is pumped down through the drill pipe and out through the ports or jets in a bit, the fluid then flows upward in the annular space between the hole and the drill pipe, carrying the cuttings and suspension to the surface. At the surface, the fluid is channeled into a settling pit or pits where most of the cuttings drop out. Cleaning fluid is then picked up by the pump at the far end of the pit or from the second pit and is recirculated down the hole. For a relatively shallow wells, 150 to 500 gallons portable pits may be used, much larger portable pits ten thousand and twelve thousand gallons, are used for deeper wells. Mud pits may also be excavated for temporary use during drilling and backfilled after completion of the well.
Before 1920, the type of rotary drill used in water well drilling was commonly called a whirler. This equipment used it well casing itself as the drill pipe. The lower end of the pipe was fitted with a serrated cutting shoe with an outside diameter a little larger than the drill pipe couplings. The sawteeth of the shoe cut and loosened the materials as the pipe was rotated. Water was pumped under pressure through the pipe to lift the cuttings to the surface. Native clays and silt were depended upon to seal the borehole wall to maintain circulation; prepared drilling fluids were not used. The method with suitable for drilling only relatively small diameter shallow wells and unconsolidated formations that did not contain cobbles or boulders.
In the 1930’s, shot-hole rotary drills, used for seismograph work and oil exploration were successfully adapted for drilling small down there water wells. Shot-hole machines, however, could not drill the large diameter holes necessary for water well work because the mud pump and drill pipe were generally too small to circulate enough drilling fluid to efficiently drill even an 8 inch well. In time, truck-mounted portable rigs for drilling large diameter water wells were developed from oilfield exploration technology.
The components of rotary drilling machine are designed to serve two functions simultaneously, operation of the bit and continuous circulation of the drilling fluid. Both are indispensable in cutting and maintaining the borehole. For economic and efficient operation, rotary drillers must acquire considerable knowledge concerning these factors and how they relate to various formation conditions.
Indirect circulation rotary drilling for water wells, two general types of bits are used, the drag bit, and the roller cone bit, usually called a rock bit. Drag bits have short blades, each forced to a cutting-edge and faced with durable metal. Short nozzles direct jets of drilling fluid down the faces of the blades to clean and cool them. Drag bits have a shearing action and cut rapidly in sands, clays and some soft rock formations, but they do not work well in coarse gravel or hard rock formations.
Roller cone bits exert a crushing and chipping action, making it possible to cut hard formations. The rollers, or Cutters, are made with either hardened steel teeth or tungsten carbide inserts of varied shape, length, and spacing, designed so that each tooth applies pressure at a different point on the bottom of the borehole has the cone rotates. The teeth of adjacent cones intermesh so that self-cleaning occurs. Long, widely spaced teeth are used in bits designed to cut soft clay formations, whereas shorter, closer space teeth are used for denser formations. Some roller bits are made with carbide buttons for particular dense formations such as dolomite, granite, chert, basil, and quartzite.
The tricone bit, used as an all-purpose bit in every type of formation, has conically shaped rollers or spindles and bearings set an angle to the axis of the bit. Another design has four rollers; two are set at an angle and to our normal to the vertical axis of the bit. The cutting surfaces of a roller bits are flushed by jets of drilling fluid directed from the inside center of the bit. The Jets can be sized to maximize the cutting action of the bit. The Jets are also effective in breaking up or washing away soft formation materials.
When hole in enlargement becomes necessary, to other types of bits are used, reamers and under reamers. A reamer is used to straighten, clean, or enlarge a borehole. This tool sometimes consists of a 10 – 20 foot section of drill pipe with specially hardened surfaces or vertical ribs. Other types of reamers are constructed of flanges welded on short sections of drill pipe and mounted between the bit and stabilizer. In the under-reaming process, the borehole diameter is enlarged beneath the permanent casing. Under reamers are particularly useful when a filter pack must be placed around the screen, but the cost of drilling the entire borehole at the larger diameter required for the filter pack would be prohibitive.
The bit is attached the lower end of the drill pipe, which resembles a long tubular shaft. The drill string using consist of four parts, the bit, one or more drill collars or stabilizers, one or more length of drill pipe, and in table drive machines, the Kelly. Selection of the bottom hole assembly will depend on the physical conditions of the geological materials, these include dip of the formation, presence of faults or fractures, and drillability of the formation.
Each drill collar is a heavily walled length of drill pipe, one or more drill collars are used to add weight to the lower part of the drill stem assembly. The concentration of weight just above the bit helps to keep the hole straight, and provide sufficient weight for the bit to maintain the proper penetration rate. Drill collars fitted with stabilizer bars or rollers are even more effective in drilling straight boreholes.
Stabilizers are an important component of the bottom-hole tools. To be effective in maintaining straight holes in soft formations, the stabilizer must have large wall contact. Increased contact can be achieved by using stabilizers with longer and wider blades or by using longer stabilizers. The flow of drilling fluid upward around the stabilizer was not be restricted too much, however, because the cuttings may pack around the stabilizer. This leads to sticking and a possible loss of circulation if back pressure builds up. Weakening of the formation structure can also result from the pressure increase. Accumulation of cuttings around the stabilizer may also cause local zones of erosion in the borehole wall. In relatively hard formations, the stabilizer can perform satisfactorily with less wall contact.
Drill pipe is seamless tubing manufactured in joints that are usually 20 feet long, although other lengths are available. Each joint is equipped with a tool-joint pin on one end and a tool- joint box on the other. Outside diameters of drill pipe used for direct rotary drilling generally range from 2 3/8 to 6 iches. High circulation rates for drilling fluids in water well drilling require that the drill pipe diameter be adequate to hold friction loss in the pipe to an acceptable level so as to reduce the power required for the pump. For efficient operation, the outside diameter of the tool joint should be about two-thirds the borehole diameter, this ratio may be impractical, however, for holes larger than 10 inches.
In table-drive machines, the kelly constitutes the uppermost section of the drill string column. It passes through and engages in the opening in the rotary table, which is driven by hydraulic or mechanical means. The outer shape of the kelly may be square or hexagonal, or round with lengthwise grooves or flutes cut into the outside wall.Made about 3 feet longer than one joint of drill pipe, the kelly has an inside bore that is usually smaller than that of the drill pipe because of the heavy wall thickness required. The square, hexagonal, or grooved circular section of the kelly works up and down through drive bushings in the rotary table. With the bushings properly in place around the kelly, the entire drill stem and bit are forced to turn with the rotary table. While rotating, the kelly slips down through the drive bushings to feed the bit downward as the hole is drilled. The lower end of the kelly is provided with a replaceable substitute joint (sub), called a “kelly saver,” that connects to the drill pipe. The sub saves the tool joint on the kelly from excessive wear resulting from the screwing and unscrewing of innumerable sections of drill pipe. The upper end of the kelly connects to a swivel (by a left-hand threaded joint) that is suspended from a traveling block in the derrick. A heavy thrust bearing between the two parts of the swivel carries the entire weight of the drill string while allowing the drill pipe to rotate freely.
Some rotary drilling machines use a top-head drive to rotate the drill string. In this system, the rotational unit moves up and down the mast; energy is obtained from a hydraulic transmission unit powered by a motor-driven pump.
In both the rotary table and top-head drive mechanisms, the driller can determine the rotation speed depending on the resistance of the formation and the rate of penetration. For shallow boreholes of 200 to 400 feet, pull-down pressure may be applied to the bit. Down-hole pressures on the bit can be increased beyond the weight of the drill string by exerting a pull-down force derived from the weight of the drilling rig. The chain assemblies (or cables) on the mast are used to transfer part of the weight of the drilling rig to the drill string. Caution should be used to avoid excessive pull-down pressure (weight) because hole deflection (crooked holes) may result. To avoid crooked holes, many drillers will use drill collars that concentrate additional weight on the bit rather than exert pull-down pressure. Rotation speed is adjusted to the pull-down or existing pressures on the bit. In general, the higher the pressure on the bit, the slower the rotation should be. In most deep direct rotary boreholes, the driller must hold back (suspend) part of the drill string weight from the swivel so that the weight on the bit does not become excessive. In general, the driller may start holding back when the weight of the drill string exceeds 10,000 pounds, although the exact figure depends on the bit being used. Bit manufacturers usually indicate the optimum pressure that an individual bit should exert against the formation for maximum cutting rates.
Adding drill rods (pipe) to the drill string or removing rods to change bits or take split-spoon or core samples is a major part of every rotary drilling operation. “Tripping in” and “tripping out” are the terms used to describe the process of running the bit into or pulling the bit from the hole. Most newer drilling rigs have been designed to make this process as fast and automated as possible. With some new machines, it is possible to pull back a 20 foot rod and remove it from the drill string in approximately 30 seconds. In general, top-head drive machines, especially those equipped with carousels (drill rod storage racks mounted on the mast), offer an advantage in rod handling speed, although recent modifications in table-drive machines have enabled this type of rig to match the speed of the top-head drive rotaries.
When a rod is to be added, the swivel is just above the rotary table (in a table-drive machine). Usually the driller will circulate the drilling fluid for a few minutes to make sure that most of the cuttings are out of the hole to prevent the bit and drill string from sand-locking when the circulation is stopped to add a drill rod. The kelly is raised until the joint between the kelly sub and the uppermost drill rod is just above the drive table.
Slips are placed in the table to hold the drill string. The kelly is then disconnected and placed out of the way momentarily. A sand line (cable) is joined to another rod section using a quick- release elevator (clamp). The rod is hoisted into place above the rod held in the table and the two are threaded together, usually with the aid of automatic pipe clamps. The slips are removed and the string is lowered by the sand line until the top (tool-joint box) of the just-added drill rod is just above the table. The slips are reinserted, the elevator is removed, and the kelly is rethreaded to the drill string. After lowering the kelly into the drive table, drilling can continue.
In top-head drive machines, no kelly is required and therefore the bottom sub of the hydraulic drive motor is connected directly to the drill rod. Additional rods can be taken directly from a carousel by the top-head drive unit. If the machine is equipped with side storage racks, a sand line must be used to raise the drill rod into position.
Internal pressure created by the drilling fluid can cause a momentary but forceful surge of drilling fluid out of the drill string at the point where the kelly is disconnected from the upper drill rod. Drillers usually break this joint slowly to allow the pressure to dissipate so that drilling fluid is not expelled violently. Occasionally during the addition of a drill rod, drilling fluid may continue to overflow from the top of the rods. Confining pressures within permeable material in the borehole may be causing this flow, but it is more likely that clay “collars” packed around the drill rods are falling deeper into the borehole, thereby pushing drilling fluid back up the center of the rods.
Direct rotary drilling, the most common method, offers the following advantages:
- Penetration rates are relatively high in all types of materials.
- Minimal casing is required during the drilling operation.
- Rig Mobilization and demobilization are rapid.
- Well screens can be set easily as part of the casing installation.
Major disadvantages include the following:
- Drilling rigs are costly.
- Drilling rigs require a high level of maintenance.
- Mobility of the rigs may be limited depending on the slope and condition (wetness) of the land surface.
- Most rigs must be handled by a crew of at least two persons.
- Collection of accurate samples requires special procedures.
- Use of drilling fluids may cause plugging of certain formations.
- Rigs cannot be operated economically in extremely cold temperatures.
- Drilling fluid management requires additional knowledge and experience.