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(1) Rotary Table. The rotary table is a rotating platform that transmits torque to the drill rod through the kelly. The kelly, which is attached to the mud swivel, is the uppermost section of the drill string that passes through the rotary table. The drill string may be square, hexagonal, or round with grooves or flukes on the outside wall. The drive kelly bar slides through the rotary table while rotating. By removing the kelly bar, you can add drill pipe and work the pipe through the open hole in the rotary table. The rotary table normally is a mechanical, positive drive mechanism.

(2) Top Head. The top-head drive uses a power swivel. Torque is applied at the top of the drill string. The top-head mechanism moves down along the rig mast as the boring is advanced and is raised to the top of the mast to add a length of drill pipe. Top-head-drive drill rigs do not use a kelly bar. Most top-head drives are powered by hydraulic motors capable of variable speeds rather than positive constant rotation.

(3) Feed Drive. Rotary rigs are equipped with a mechanism to apply a downward thrust to the drill string. This mechanism is called a pulldown or feed drive.

Generally, two roller chains apply the thrust for rotary tables. The chains are attached to the kelly swivel and extended over sprockets at the top of the mast and under the rotary table. On older rigs, the sprockets under the rotary table are powered mechanically through a PTO and clutch. The pulldown chains on modem drill rigs are powered by a hydraulic motor, which provides better thrust control.

The thrust mechanism on most top-head rigs is a pair of roller chains consisting of two chain sections. One end of each section is attached to the swivel at the top of the kelly bar. The other ends are dead-headed to the top and bottom of the mast. Sprockets at the top and bottom of the mast act as idlers. Chains are powered in either direction by hydraulic rams. These rams apply thrust and are used in a hold-back mode to reduce the bit load of the weight due to the drill string. This chain mechanism is also the main hoist for lifting the drill string.

(4) Mud Pump. A mud pump (Figure 5-1) on a rotary drill is usually a positive-displacement, double-acting piston pump with capacities ranging from one to several hundred GPM at pressures up to several hundred psi. Power may be provided through a mechanical PTO and clutch, with or without a separate transmission. Power may also be provided by a separate engine or a hydraulic or air motor. Other types of pumps are often used successfully, but their limited pressure capacity may jeopardize the success of the drilling operation. Most well-drilling machines have dual piston, double-acting, positive-displacement mud pumps. Pump capacity (volume and pressure) can limit the effective depth of a drilling operation. The horsepower required to drive a mud pump often exceeds the power required to hoist and rotate the drill string.

(5) Hoists. Man drill-head hoists (draw works) are mechanically or hydraulically driven wire-line winches. On top-head rigs, the pulldown chains are used as the main hoist. Many drill rigs have auxiliary hoists for handling pipe and other equipment and for bailing. The bailing drum usually has less lifting capacity and a faster spooling rate than hoisting drums. Bailing drums can spool several hundred feet of wire line, which is sufficient to reach the bottom of most wells. Auxiliary hoists may be powered mechanically or hydraulically.

(1) Tricone Roller Bits. These bits bit consists of three cone-shaped rollers are best suited for brittle or friable materials. The tricone with steel teeth milled into the surfaces. Tooth locations are designed so that as the cone rotates, each tooth strikes the bottom of the hole at a different location. Drilling fluid is jetted on each roller to clean and cool it. The cutting action is a progressive crushing under the point load of each tooth. Roller bits designed for rock, rocky soil (gravel), and soft formations (shale) have long teeth. The bits for harder formations have smaller, stronger teeth. The gauge teeth on bits designed for very hard rock are reinforced with webs. For extremely hard formations, milled teeth are replaced with connected carbide buttons.

(2) Drag Bits. These bits are used in soil and other unconsolidated materials. The blades are designed so that they cut into the formation with a carving or scraping action. Drag bits may have multiblade, hardened-steel, finger-shaped teeth or may have connected carbide-reinforced cutting edges.

(1) Bit Design. Rotary drill bits are designed to cut specific material types. Choose the drill bit based on the anticipated formation. Either drag or rotary bits are available at a drill site. Normally, you will use drag bits for beginning a borehole in unconsolidated overburden materials. These bits are used for hard, medium, and soft rock and are part of the drilling-rig equipment. When drilling in soft rock, use a medium soft-rock drill bit. In very hard rock, stop mud rotary drilling; install casing to the rock layers, use a down-hole air hammer. The objective is to produce a hole quickly and efficiently. Be careful that you do not penetrate too quickly. Producing cuttings faster than you can remove them can cause serious problems, such as the drill string sticking in the hole, excessive completion delays, and loss of equipment and the hole.

(2) Weight on Bit. Adding weight on the bit increases the torque required for rotation. Too much weight can cause excessive penetration and produce cuttings that are too large and heavy. Large cuttings are difficult to wash out and may cause gumming and premature failure of the bit. Insufficient weight reduces or stops penetration and can produce fine cuttings. In cohesive soils, fine cuttings may thicken the drilling fluid and fail to settle in the mud pit. How weight is applied can also cause serious alignment problems and difficulty in well construction. Rotary-drilled boreholes spiral slightly and are seldom straight. Once spindling occurs, weight added by pulling down with the drill rig bends the string and magnifies the deviation. You should never use the chain pulldown to advance the hole beyond the first run (20 feet). Ideally, keep the drill string in tension. Add drill collars (heavy wall drill steel) at the bottom just above the bit. See Table 5-3 for drill collar weights.

Bit weight required to cut rock depends on the design of the bit and the strength of the rock. Roller bits need a minimum of 2,000 psi of bit diameter for soft rock and shale and a maximum of 6,000 psi of bit diameter for hard rock. Before drilling, add drill collars instead of drill pipe until the load is sufficient for reasonable cutting. As you dig deeper and add drill pipe, you may have to hold back on the drill string. See Table 5-4 for weight on bit and rotary speed.

(3) Drill Steel. Drill rods, collars, stabilize, subs, and bits are available in different sizes and materials. In most drilling systems, drill rods are either steel or aluminum and come in lengths of either 5 or 20 feet.

(4) Rotation Speed. Rotation speed is determined by the weight on the bit and the material being drilled. Try to regulate the speed to produce the correct size cuttings. Experience will help you determine and regulate rotation speed.

(5) Fluid Requirements. Fluid requirements depend on size, weight, nature of cuttings, and circulation velocity. Velocity depends on capacity and condition of the mud pump, annular area in the borehole, and the stability and permeability of the formation. See paragraph 5-1e for more information on drilling fluids.

Use the following calculation and Figure 5-4 to estimate the mud-pump output and velocity and the hole size requirements:

V = (D² - d²)2

where--

(7) Stabilizers. Unless the drill string includes stabilizers (Figure 5-5) for large drill bits, drill a pilot hole, using drill collars as stabilizers. The initial pilot hole (6- to 7 1/8-inch diameter) will be straighter and easier to sample. The location of aquifers will also be easier to determine. Use a larger bit to ream the hole to the desired size. Use overreaming bits, if available, because they follow the pilot hole best.

(1) Polymers. Polymer fluids are water-based and very low in solids. The polymer admixture can be organic, inorganic, natural, synthetic or synthetically formulated natural polymers. Polymer additives are formulated for various drilling-fluid purposes and can be used alone or to enhance clay muds. Polymers, containing salt and other contaminants, are available and are compatible with water. Polymers are more sensitive to pH than are bentonite muds. Change the pH to effect desirable changes in the polymer fluid. Drilling-fluid weight impacts drilling rate and high-density drilling fluid reduces drilling rote. There is strong indication that the solids of a fluid have a similar effect as density. Polymer fluids are very different from clay muds because a large part of the polymer is soluble in water and becomes a solution when mixed with water. Long, complicated molecular chains tie up the water and can build viscosity without solids. In water-well drilling, many polymers are manufactured for producing drilling fluids, such as E-Z Mud, Revert, and Poly-Sal. E-Z Mud is a synthetic, inorganic polymer. Revert is a natural, organic polymer fluid derived from the guar plant.

Polymers are generally best mixed through a mud gun. Polymers used for special purposes are available from the manufacture complete with specifics on how to use the product. Most polymers can hydrate more water than a high-grade bentonite. Up to ten times more bentonite is needed to build the same viscosity in a given amount of fluid, depending on the quality of the polymer. A polymer does not fully hydrate as quickly as bentonite. Mix the polymer very slowly through the mud gun a minimum of four hours before using for more complete hydration. The fluid will thicken as hydration continues, so do not mix to the desired viscosity. Some polymers possess physical qualities that can result in unusual hydration, gelling and viscosity. Follow the manufacturer's recommendations for hydration. Factors that affect the viscosity are quality of polymer, concentration and size of colloid, metallic ions in mixing water, temperature, rate of shear, and pH.

Water is the primary building block for drilling fluids. Water quality affects the overall performance of drilling fluids. The action of bentonite in water is seriously impaired by dissolved acids or salty substances. Acidic water usually contains dissolved metals that cannot be used unless treated. Hard water affects the suspending and sealing qualities of bentonite. You can test the pH level by using paper pH strips. The pH level should be 8 to 9. If the water is too acidic, treat it with soda ash at a ratio of 1 to 5 pounds of soda ash per 100 gallons of water. Following treatment, retest the pH level as before.

Do not use water from wetlands, swamps, or small ponds for mixing drilling fluids because the water may be contaminated. If you use water from these sources, chlorinate the water before making the drilling fluid. Be careful because chlorine removes metallic ions that are necessary for viscosity in polymers. Adjust the pH of drilling water to 7.5.

Temperature affects the viscosity and stability of some polymers. Consider the following examples:

• Seven pounds of Revert per 100 gallons of water at 45°F yields Marsh funnel viscosity of 125 seconds per quart. The same mix at 85°F yields 70 seconds per quart.
• A 0.87 weight mix of Revert at 68°F F yields a viscosity above 90 seconds per quart for three and a half days. The same mix at 80°F maintains viscosity of 80 seconds per quart for only two days.

Polymer drilling fluids can break down viscosity. Without treatment, the viscosity of some polymers (Revert) completely breaks down in one to six days depending mainly on temperature. You can correct this by adding chlorine. Revert requires Fast Break; E-Z Mud needs sodium hypochlorite at a ratio of 2 quarts for every 100 gallons of water. Other polymers, such as E-Z Mud and Poly-Sal, maintain their viscosity for long periods of time since natural breakdown is not significant. Table 5-6 lists additives for drilling fluids.

Polymer drilling fluids have virtually no gel strength because the colloids are nonionic and exhibit no attraction for each other. Most of the drill cuttings should be washed out of the borehole before circulation is stopped. A polymer fluid does not have thixotropic properties to hold cuttings in suspension. Shakers, desanders, or desilters are not needed when using polymer fluids. Since most of the cuttings are dropped out, friction and wear in the pump are minimized and the drilling rate is not impeded by cuttings or high density. A major advantage of polymers is the lack of mechanical wear to the drill-rig mud system.

Some drilled fines (clay, silt) circulate back down the hole. Recirculated solids are much less a problem with polymer fluids than with clay fluids. Revert will not hydrate in water containing any appreciable amount of borate. However, Borax can be used to produce a gel plug in hydrated guar-gum polymer. With a pH of 7.5, the borate cross-links the polymeric chains and forms a strong three-dimensional molecular gel. If a strong gel plug is necessary to get through a lost circulation zone, mix 1 cup of borax in 5 gallons of water and pour slowly into the pump section while pumping at idle speed. When the berated fluid (stringy gelled mass) recirculates, stop pumping for one-half hour. Resuming normal drilling should be possible after wasting the borated fluid. Repeat the procedure, if necessary. Although the polymer fluid is not thixotropic and has no gel strength, it thins somewhat while being pumped.

Polymer fluids build a type of membrane on the wall different from clay mud. Unnatural clay particles (bentonite) are not introduced into the hole. Since the polymer fluid is partly a thick solution, infiltration into the permeable wall is reduced. However, insoluble portions of some polymer colloids do exist. The insoluble and cuttings are surrounded with thick coatings that are more impermeable per unit thickness than a bentonite filter cake. The insoluble and cuttings seal the wall of the hole with a thinner, less active layer. The impermeable layer performs the same function as the filter cake in clay muds but does not restrict the annulus.

The colloids in the polymer fluid are nonionic, have no chemical interaction, and are easier to remove in water-well development. When the viscosity of the fluid is broken, much of the cohesive function of the thin film becomes a water-like liquid and is washed out of the water well. Field testing of polymer fluids, using the falter press and the Marsh funnel, yields different results. The mud balance for measuring fluid density does not change. Weighing of the polymer fluid is limited. You can add sodium chloride to the fluid to bring the weight up to about 10 pounds per gallon. The addition of heavy solids (ground barite) is ineffective because of the polymer fluid's lack of thixotropic qualities.

(2) Mud Products. Commercially processed clays for drilling are bentonite and attapulgite. Bentonite is superior except in brackish or salty water. (Use attapulgite in these waters.) Bentonite forms naturally from decomposition of volcanic ash when ground. Bentonite consists of aggregates of flat platelets in face-to-face contact. Bentonite is mined in many states, but the best grade (Wyoming bentonite) is mined only in Wyoming and South Dakota. Wyoming bentonite contains sodium montmorillonite (the active part of the clay mineral) and is small in size, which is important in building viscosity.

Mud drilling fluids should be mixed with a mud gun. When agitated and sheared with water, the bentonite platelets absorb more than 25 times their own weight in water, separate, and swell. The amount of surface area wetted determines the ability of the particle to build viscosity. One ounce of Wyoming bentonite dispersed in water has more surface area than five football fields. Interparticle activity between platelets gives the mud its gel properties. The chemical composition of the mixing water affects the ability of bentonite to develop desirable qualities. These qualities can be manipulated by adding small amounts of various chemicals.

(3) Mud Viscosity. True viscosity is a term relating only to true (Newtonian) fluids, such as water, and is a proportional constant between shear stress and shear rate in laminar flow. Drilling muds act differently in that the proportion between shear stress and shear rate is reduced when shear rate is increased. Drilling muds are thixotropic. The viscosity of a drilling mud refers to the thickness of the mud while flowing. Gel strength is the term used to describe the thickness of drilling mud at rest. Gel strength develops over a short period of time.

Yield point is the mud quality broadly included in viscosity. You need more stress (pump pressure) to cause the gelled drilling mud to start flowing than to sustain flow once the gel is broken. The stress required to initiate shear or flow is the gel strength of the mud. The stress required to maintain shear is the viscosity. You want a higher yield strength with respect to the gel strength so the mud becomes very thin in flow shear.

The primary function of viscosity is to help lift drill cuttings from the borehole. Other mud characteristics affecting lifting capacity are density, velocity, and flow patterns. Gel strength holds the cuttings in suspension at the bottom of the hole when circulation is stopped. The stress (hydraulic pressure) required to break the gel strength to initiate cumulation can be detrimental. Required bottom-hole pressures can cause fracturing or opening of fractures in the formation, resulting in loss of drilling fluid, formation damage, and borehole wall damage. Down-hole pressure required to continue circulation depends on friction, density (or weight of fluid column), and viscosity of the mobilized fluid. These pressures can also cause serious problems. Therefore, it is desirable to use a drilling mud of relatively low density and viscosity, moderate gel strength, and high yield point relative to the gel strength (a very thin fluid in circulation).

(4) Mud Testing. The Marsh funnel (Figure 5-6) is routinely used to give an indication of thickness or apparent viscosity of drilling fluid. The Marsh funnel is 12 inches long and 6 inches in diameter and has a No. 12 mesh strainer and a 1,000-milliliter (ml) cone. The funnel has a 2-inch-long, calibrated, hard-rubber orifice with an inside diameter of 3/16 inch. The funnel's cup is marked with a capacity of 1,000 ml. Use the following procedure for the Baroid Marsh funnel:

• Hold or mount the funnel in an upright position, and place a finger over the hole.
• Pour the test sample, freshly taken from the mud system, through the string in the top of the funnel until the level touches the top of the screen.
• Immediately remove the finger from the outlet tube, and measure the number of seconds for a quart of mud to flow into the measuring cup.
• Record time in seconds as funnel viscosity.

NOTE: Calibration time for fresh water at 70°F is 26 seconds.

• The funnel viscosity measurement obtained is influenced considerably by the gelation rate of the mud sample and its density. Because of these variations, the viscosity values obtained with the Marsh funnel cannot be correlated directly with other types of viscometers and/or rheometers. Graduated in cubic centimeters (cc) and fluid ounces, the 1,000-cc measuring cup is designed specifically for use with the Baroid Marsh funnel viscometer. A quart volume is clearly marked on the measuring cup.

The desired mud consistency depends on many factors. The nature of the formations will dictate mud qualities. However, you will not know all the conditions before you start to drill. The inexperienced driller must be careful because mud with the correct thickness often is too thin. You can adjust the viscosity by adding water or clay. A good range for drilling muds is 32 to 50 seconds per quart. Viscosity is influenced by mud density, hole size, pumping rate, drilling rate, pressure requirements, and geology. Considering the thixotropic qualities of drilling mud, a funnel viscosity of 100 seconds per quart may be no more viscous than a funnel viscosity of 50 seconds per quart if both fluids are in motion.

You can use test readings as an indicator of changes in mud that might lead to problems. Therefore, conduct Marsh-funnel tests before beginning operations and record the findings. Take mud samples for each test from the same location in the circulating system just before returning to the hole. The apparent viscosity of the drilling mud in motion affects carrying capacity, the pump pressure (hydrostatic down-hole pressure) required for circulation, and the ability to drop cuttings in the settling pit. These characteristics are also intrinsically involved with well hydraulics, density of mud, density and size of cuttings, and particle slip.

(5) Density. The carrying capacity of a mud is affected by its density and the density of the drill cuttings. If the cuttings are denser than the fluid, they will descend. The magnitude of the difference in density, particle size, and fluid viscosity affect the rate at which a particle descends. Particle slip denotes this downward movement through the fluid. Ignoring thixotropy, the actual downward particle slip is constant regardless of velocity of flow. However, when the upward velocity of fluid exceeds the downward particle slip, the new movement of the particle is upward. Up-hole velocity plays a major role in determining the carrying capacity of the cumulating fluid. The practical limits of up-hole velocity depend on pump size and capacity, inside diameter (ID) of the drill string, jet size in the bit, viscosity of the fluid, cross-sectional area of the annulus, and stability of the borehole wall. Up-hole velocity is not as simple as the comparison of pump capacity, drill string ID, and annulus. Up-hole velocity is not a constant.

The density of the drill fluid serves other purposes in rotary drilling. Heavy fluids can control (hold down) formation pressures encountered in drilling. You can build heavy mud by adding a weighing material such as ground barite (specific gravity 4.25). Prepare drilling mud in excess of 20 pounds per gallon by using barite. First, mix bentonite and water to build viscosity. Then, add finely ground barite so the mud will hold the barite in suspension. Use heavy drilling mud only when absolutely necessary to control pressures since the muds have disadvantages. High-density mud increases pressure on the formation by the weight of the fluid column. Figure 5-7 shows the nomograph for determining the hydrostatic head produced by drilling fluids. The increased pressure is further increased by the pump pressure required to mobilize the fluid in circulation. The increased pressure can cause formation damage and loss of circulation. In formations that are strong enough to withstand the pressures without being damaged, the drilling operation can still suffer.

The pressures from the mud and the formation should be balanced so that the borehole bottom exposed to the drill bit is near surface eruption from pore pressure. That balance makes the formation easy to fracture and enhances the cutting rate. If the mud pressure is much lower than the formation pressure, the borehole can be unstable. If the mud pressure far exceeds the formation pressure, the cuttings may be suppressed and reground by the bit. The result of recutting reduces bit life and lowers drilling rate. Mud density is increased by drill cuttings; the mud rising in the hole is heavier than the mud returning to the hole.

All cuttings should be removed from the drilling mud in the settling pits and not recirculated. Although 100 percent removal is unrealistic, well-designed mud pits and mechanical screens, desanders, and desilters materially aid in removing cuttings from the mud. Water weighs 8.34 pounds per gallon. Clean, low-solid bentonite mud can weigh 8.5 to 9.0 pounds per gallon; try to maintain that weight. Increasing density of the mud during drilling indicates that the mud contains native solids. The drill's penetration rate could be exceeding the combined effort of the mud pit, desanders, and fluids to effectively separate solids from the drilling mud. To correct this problem, slow down the penetration rate and run the desanders to remove the solids.

You can determine the density or weight of the drilling mud using a mud balance. Fill the mudbalance cup with mud, and place the inset lid in the cup. Excess mud will be displaced through a hole in the lid. Clean the outside cup area, place the assembly on the center pivot, and balance it using sliding weight. You read mud density as pounds per gallon and pounds per cubic feet. If you know the mud weight that enters and exits the drill hole, you can evaluate the efficiency of the mud pit and mechanical separators, determine when to clean the mud pit, and tell how well the mud is cleaning the hole. If you take samples from only one location, take them from the return end of the pit.

(6) Filter Cake. Filter cake consists of solids from the drilling mud deposited on the borehole wall as the water phase is lost into the formation. Desirable properties are thinness and impermeability. Drilling mud is a colloidal suspension that can be separated by simple filtering. With the hole kept full of mud, the hydrostatic pressure inside usually exceeds the formation pressure. Occasionally, an artisan aquifer is penetrated, with formation pressure higher than the hydrostatic in-hole pressure. When the borehole pressure is higher than the formation pressure, the drilling mud tends to penetrate more permeable formations. Solids from the mud filter out and deposit on the wall, and the liquid phase of the mud (filtrate) enters the formation.

Filter cake can be compacted against the wall by the excess hydrostatic pressure in the borehole. If the drilling mud is a well-conditioned bentonite and water mixture, most of the solids plastered against the wall will be flat platelets of highly active clay. The filter cake is self-regulating based on its degree of impermeability. As long as filtrate can pass through, the filter cake continues to thicken. A thick cake detrimentally increases down-hole cumulating pressure by restricting the annulus, making it difficult to pull the drill string because of the physical size of the drill collars and bit. In deep and somewhat deviated holes, the danger of key seating increases. Because a thick filter cake is of a lower quality and depends on its thickness to be effective, it is more easily damaged and eroded. A thick filter cake may indicate that the fluid has a high percentage of native solids. You may have to clean these solids from the mud before drilling progresses.

A thin, highly impermeable filter cake bonds well to the wall and provides a surface for the hydrostatic pressures to act against to support the wall. Filtrate loss into the formation can account for significant fluid loss, if the consistency of the drilling mud is not good. Good consistency does not necessarily mean thick; it has to do with the bentonite content and the quality of falter cake. If a permeable formation is encountered with pore spaces too large to be plugged by the fine bentonite particles, the drilling mud will enter the formation. That mud loss can take the entire output of the mud pump. The drill cuttings being carried up the annulus can sometimes be beneficial. The cuttings are coarser than the bentonite particles and may help bridge across formation pores. If you use this technique, maintain the normal drilling rate to supply the cuttings. Slow down the pumping rate to reduce pressure on the formation while bridging the open spaces. With sufficient bridging, a suitable filter cake follows, circulation is regained, and normal drilling operations are resumed.

In the field, you can test the filtration properties and filter-cake thickness using the filter-press kit. This kit consists of a press with a mounted pressure gauge and a CO₂ charging system that is used to simulate the hydrostatic pressure inside a 200-foot hole. By placing a sample of drilling mud in the press and charging the system, you can forma filter cake. The filter cake should be less than 2/32-inch thick.

(7) Salty Environment. A high chloride content in the mixing water causes bentonite to react anomalously or not react at all. The ground bentonite remains agitated; it does not disperse, hydrate, or swell. In salt water, bentonite is an inefficient clay additive for drilling mud. The dissolved salt is an electrolyte that changes the interparticle activity of bentonite. If you add sufficient amounts of salt water to a fresh water and bentonite mud mixture, the dispersed platelets will form lumps. Viscosity and filtrate loss increase and the mud's ability to build a thin, impermeable filter cake decreases.

Attapulgite is often used for salty formations. The small particles produce a high surface-area-to-volume relationship and good viscosity building. Unlike bentonite, the particle shape is needle-like. Viscosity building in attapulgite depends on the entanglement of these needles. The disorderly arrangement of the particles accounts for the poor filtration qualities of attapulgite. The filter cake is more like a layer of strew or sticks. Attapulgite clay does not have the physical qualities to build a thin, impermeable filter cake. If you can mix bentonite in fresh water first and then add salt water as make-up water, the bentonite flocculates; that flocculation can be reversed by chemical treatment.

(8) Well Hydraulics. You must have a basic understanding of well hydraulics. Fluid is pumped down the drill string, out the ports in the bit, and up the annular space between the drill string and the wall. The fluid empties into the mud pit, through any mechanical solids separating equipment, and is picked up from the pit by the mud pump for recirculation. The system is intended as a conservation system. Except for mud lost into the formation or where artisan water exceeding hydrostatic pressure flows into the hole, the return is largely complete and the mud-pit level does not change. Even if the system is in equilibrium, you need to understand the up-hole rearrangement of flow patterns.

Fluids flow in two distinct patterns. Laminar flow is orderly. The streamlines remain distinct and the flow direction at every point remains unchanged with time. Turbulent flow is disorderly. The flow lines and directions are confined and heterogeneously mixed. The type of flow depends on the cross-sectional area of the fluid course and the velocity, density, and viscosity of the fluid. In water-well drilling, the cross-sectional area of the annulus is usually several times that of the inside diameter of the drill string. Because of the increase in volume in the annular space, flow at the point the fluid leaves the bit is turbulent. The fluid becomes laminar flow when it begins flowing up the annular space. The returning fluid velocity is slower and the drill fluid is more dense and probably has more apparent viscosity, which affects the flow pattern. To clean the hole and carry the drill cuttings out, turbulent flow in the annulus would be better.

Picture the flow in the annulus as a series of nested tubes. Velocity varies as if these tubes were sliding past one another while moving in the same direction. Flow near the wall and near the drill string is at a slower rate than near the center. Cuttings near the center can be vigorously lifted while cuttings near the wall and drill string actually slip in a net fall. Rotation of the drill string changes the flow pattern near the drill string and materially enhances particle lift.

For example, if you use a 3 1/2-inch ID and 4-inch outside diameter (OD) pipe to drill a 9-inch hole and pump 200 GPM, the velocity of the ID pipe is 400 feet per minute (fpm) and the velocity of the OD pipe is 75 fpm. To test cumulation at these rates (bit is 300 feet deep), pump down a marker (strew or oats). The majority of the material should take 4 minutes and 45 seconds to return (300 feet at 400 fpm takes 45 seconds and 300 feet at 75 fpm takes 4 minutes). To clean all the cuttings from a 300-foot depth with an average up-hole mud velocity of 75 fpm will require more than four minutes of pumping. Consider this concept regarding sampling cuttings from the return mud.

If cuttings remain in the annular space between the drill rod and borehole wall when circulation is stopped, they will produce a denser fluid than the clean drilling mud inside the drill rods. The denser mud in the annular space will then flow down the hole and force the clean drilling mud up the drill rods. This causes a geyser effect, and the drilling mud may shoot several feet into the air until the mud columns equalize. (Some drillers mistake this for a caving hole.) If this situation happens when adding drill rods, the circulation time should be increased after drilling down the next rod. Use the following formula to calculate the annular space volume:

where--

V = annular space volume, in cubic feet.
D = hole diameter or bit size, in inches (Figure 5-4)
d = drill hole or drill steel collar diameter, in inches (Figure 5-4)
L = hole length, in feet (Figure 5-4)

(9) Mud Pits. Rotary chilling preparation is the design and excavation of an in-ground mud pit or installation of a portable mud pit and the mixing of the drilling fluid. For standard drilling operations that use well-completion kits, well depths could range from 600 to 1,500 feet. For wells up to 600 feet using the 600-foot WDS, use portable mud pits. For wells over 600 feet, use dug mud pits. In either case, you will have to clean cuttings from the pits as drilling progresses. Design considerations include the anticipated depth and diameter of the drill hole, since the material cuttings from the hole will be deposited in the mud pits.

The volume of the pits must equal the volume of the completed hole. Therefore, during drilling, you will have to clean the cuttings from the pits frequently. If you have a backhoe to dig and clean the pits, size and depth of the pits are not critical. If you must dig and clean the pits with shovels, width and depth are important. Drilled cuttings should drop out of suspension in the mud pit. Therefore, long, narrow pits are better. Figure 5-8 shows a mud-pit layout and a chart depicting mud pit capacities and dimensions.

NOTE: Portable mud pits will require constant cleaning.

Mud pits are part of the circulating system for mixing and storing drilling fluid and for settling cuttings. The ground slope will affect site layout. Pit design can enhance pit performance. Most drillers agree that using multiple pits is best when dropping drill cuttings from the fluid. The volume of the pit should be one and one-half to three times the volume of the hole. This will provide fluid to fill the hole and an excess volume to allow stilling and settlement or processing before returning to the drill string. A volume of three times the hole volume will minimize drilling-fluid and mud-pit maintenance. Figure 5-9 shows a mud pit that is prepared on-site. Figure 5-10 shows a portable mud pit.

If you use a clay-based mud with thixotropic qualities and the mud moves slowly or flow stops, the gel strength can hold the cuttings. High velocity through narrow, shallow trenches holds the cuttings in suspension. If mud runs over one or more wide baffles or weirs, flow shear and velocity are low. These factors enhance cuttings to drop out. If mud processing equipment is available, use it. Recirculating clean fluid reduces power requirements, wear, and erosion and enhances drilling rate. See Figure 5-11 and Figure 5-12 to calculate weir dimensions and volume.

(1) Lost Circulation. Lost circulation refers to a loss in volume of drilling fluid returning to the surface. The implication is that some fluid pumped down the drill pipe is entering the formations. The mud pit will lower, since some of the mud is used in forming a mud cake on the borehole wall; however, increased lowering can indicate circulation loss. Losses can occur through open-graded sand or gravel or open joints in rock. A loss can occur when cuttings are not washed out and the borehole annulus becomes restricted, resulting in increased down- hole pressure. Spudding (raising and lowering the drill string) the hole too violently can cause loss. Spudding helps wash cuttings, but down-hole pressures increase momentarily. Experienced drillers can estimate when spudding is safe. When fluid cumulation is lost and a driller continues to drill, he is drilling blind. An experienced driller that knows the rig can often drill blind successfully, but reestablishing circulation is always safer.

Reestablishing circulation can involve several techniques. You can add commercial items such as chopped paper, straw, cottonseed, and nut hulls to the mud pit. Sometimes, while the loss zone is grouted and redrilled, the grout is lost into the formation. In this situation, you may have to set casing through the loss zone. Occasionally, reducing fluid velocity while continuing to drill will plug the loss zone with drill cuttings. Reestablishing circulation is usually a trial-and-error process. The longer you drill without circulation the more difficult it will be to reestablish circulation.

(2) Fall-In. Fall-in is material that accumulates in the bottom of the borehole after you stop cumulation. This material is borehole-wall material that results from sloughing or caving or cuttings previously carried in suspension. Fall-in occurs when you encounter a loose, unstable formation and the drilling-fluid weight is insufficient to stabilize the formation. If you anticipate or suspect fall-in, raise the drill bit off the bottom of the hole (20-foot minimum) each time drilling is interrupted. This will prevent the cuttings and fall-in from settling back around the bit until the problem is solved.

(3) Stuck Drill String. The drill bit and any collars just above the bit are larger in diameter than the drill pipe. The string becomes stuck when cuttings collect on the bit and collar shoulder. This condition is called sanded in. Be careful because you can break the drill pipe while trying to remove the drill string. Regaining circulation and working the sand out are seldom successful. If the formation will not take the fluid when you engage the pump clutch, the relief (pop-off) valve will operate to relieve the pressure. Little can be done to free the drill string except to wash a small pipe down the annulus to the bit and jet the settled sand back into suspension. When the annulus is too small to pass a jet pipe, a part of the drill string may be lost.

When the annulus is small, excessive up-hole velocity can promote erosion of the filter cake in granular zones and allow caving against the drill pipe. If this occurs, try to maintain circulation and rotation, even if circulation is slight. Where the grains are angular, the drill pipe can become locked while being rotated. This situation is similar to a sanded-in bit. With smooth pipe (not upset), hammering up and down will sometimes dislodge the string. You can reestablish circulation and continue drilling. Be careful because hammering up and down can produce unfavorable compacting of the sand. In a hole of fine-grained soil or shale, where the alignment has significantly deviated and the drill pipe has wallowed into the wall, the pipe can become wall stuck. Pipe friction and relatively high borehole pressure can move the pipe tighter into the wallowed groove as you pull the string. An alert driller should recognize early stages of deviation and take measures to realign the hole.

(4) String Failure. When the drill string parts, leaving a portion in the borehole, the drill string is rung off. The portion in the borehole is a fish and attempts to retrieve the portion is fishing. Fishing tools include a tapered tap and an overshot die (Figure 5-13). Ringing off is normally fatigue failure in the drill-rod joints caused by excessive torque or thrust (repeated flexing and vibration that crystallizes heat-treated tool joints) or by borehole deviation (with flexing of the string). Examine drill rods for signs of failure.

(5) Deviation. A deviated borehole is called going crooked. If you make the initial setup without plumbing the kelly, you can expect the borehole to go crooked. A crooked borehole usually amplifies other problems and can make a borehole unsuitable for a well. You should always anticipate deviation, since the borehole naturally tends to spiral from bit rotation. Variations in the formation badness may start deviation. Excessive bit load magnifies minor initial deviation. Use all available guides and collars and a reduction in bit load to minimize deviation.

(6) Swelling Soil. The in-hole effects of swelling soil (shale or clay) that absorbs water from the drilling fluid is squeezing. The result is a borehole that is undergauged to the extent that you cannot pull the bit by normal hoisting methods. In such cases, you can cut back through the blockage with a roller rock-bit or a drag bit. Swelling can cause caving and failure of the wall. Keep water out of the formation to prevent swelling. Special polymer drilling fluid additives that limit water absorption are available. High quality bentonite forms a thin but highly impermeable filter cake.

(1) Weight on Bit. The weight applied to the drill must be sufficient to hold it in the closed position while drilling. While the drill bit is splined and can move in relation to the drill, the bit should not move in the splines while drilling. The required weight on the bit (bit load) varies somewhat with percussion drills from different manufacture. With a given drill, the bit load varies with the air pressure used for drilling. The air pressure does not tend to enter the drill bit (open position) however, the energy with which the piston strikes the bit shank is directly proportional to the air pressure applied. Consequently, you will need more bit load to keep the bit in the closed position with higher air pressure. See the manufacturer's manual for specific recommendations for minimum bit load. The range for a 6-inch drill generates about 2,500 pounds total bit load at 125 psi to 4,000 pounds total bit load at 200 psi. Increasing bit load above the recommended minimum does little to increase the drilling rate. To achieve an increase in production rate, add weight if it does not adversely affect the rotation speed or constancy. Too much bit load will--

• Affect the constancy of the rotation, which is detrimental to drilling production rate.
• Damage the drill.
• Cause excessive wear and premature failure of the bit.

(2) Plumbness. Water wells should be plumb and straight. Apply bit load by using drill collars to hold the drill string in tension. Most rigs have a mechanism designed for that purpose. Because the drill string is relatively flexible, loads applied from the top will cause the drill string to flex and can misalign the drill bit. This procedure can cause the drill to deviate from vertical. Continued top loading will exaggerate deviation. Apply bit load by adding heavy drill pipe sections (drill collars) at the bottom of the drill string just above the bit. If you add a sufficient load at the bottom, you can hold the drill string in tension while drilling. The borehole tends to be straight because of the pendulum effect. Borehole deviations from vertical are sometimes associated with fast drilling rates. Such deviations actually result from crowding the bit (trying to increase the penetration rate by overloading the bit).

(3) Air Pressure. Exceeding the 350-psi rating of the drill is not possible with the supplied compressors. The energy output of the drill is proportional to the air pressure delivered. Try to operate the drill at the maximum air pressure available. Lubricate the drill, using an in-line oiler designed for operation at the maximum anticipated working pressure. To control dust, inject a small amount of water into the airline. The injection pump and plumbing can be extremely dangerous if failure occurs because they are subjected to maximum pressure.

(4) Rotation. As drilling progresses, adjust the bit rotation to the penetration rate. As the hole deepens, the bit should make one revolution equal to the length of exposure of the carbide buttons in the bit (about 1/3 inch). The rule of thumb is that the rotation rate (in RPM) should about equal the penetration rate in feet per hour. The rotation rate for percussion drilling is slower than for rotary drilling. For percussion drilling to function at maximum proficiency, the rotation must be constant. As the piston strikes the bit shank (about 1,200 blows per minute at maximum air pressure), the constant rotation allows the bit buttons to chip continuously at new rock. If the rotation rate is not constant (stops and jumps), the buttons hammer several times at one place before jumping to new rock. Erratic rotation produces variable sized chips, which are harder to clean from the hole. Erratic rotation also causes the buttons to penetrate too deep, increasing drag on rotation and excessive wear and button breakage.

There are two main causes for erratic rotation. Top-head-drive rigs rotate the drill string with a hydraulic motor. Even though you can apply tremendous torque, a hydraulic system does not produce positive movement but reacts to drag or resistance to drill string rotation. The hydraulic top-head-drive rig is highly susceptible to erratic rotation. The primary force resisting rotation is weight on the bit. If this weight is excessive, the hydraulic drive produces a series of pressure variations and the bit rotates in a sequence of starts and stops. Even with mechanical drive rotation, which is nearly positive movement, dragon the drill bit will cause twist in the drill string, resulting in erratic rotation of the bit. This occurrence is more pronounced in deep holes.

The secondary cause of erratic rotation is borehole wall friction. You cannot eliminate all friction between the borehole wall and the drill string. The more a borehole deviates from being straight, the greater the friction. It is easy to visualize a distorted drill string caused by excessive top loading and how the distortion can cause increased borehole wall friction. Other contributors to borehole wall frictional rock abusiveness, poor lubrication of the drill string (injected or natural-water condition), and poor cutting removal.