Materials handling involves the relocation of materials when a hole is drilled or excavated. The current practice of materials handling depends on the type of hole being constructed (e.g., wells, mine shafts, or tunnels); the site conditions; and the size, orientation, and length of the hole. In general, materials must be transported from the bit face to the surface and from the surface to disposal. Materials handling limits the rate of hole advance when materials cannot be transported to the surface as rapidly as they are mined or when they cannot be moved from the surface to a disposal area as rapidly as they are brought to the surface. Surface disposal problems often relate to environmental problems associated with toxic materials (liquids or solids). The problems that restrict the movement of materials from the bit face to the surface are often the result of material movement interfering with other functions that occur in the same space. For example, in a tunneling operation, the handling of large amounts of materials might interfere with other uses of the tunnel such as transporting personnel and supplies. In drilling, the materials may not fit in the annular space between the hole and the drill pipe.
Status of the Field for Drilling Wells
Rotary drilling, the most common process for drilling wells, uses circulating fluids (both liquids and air) to remove the drilled solid cuttings. Other removal methods include screw augers, which continuously remove solids; buckets; and bailers, which allow water and solids to fill a tube that is periodically retrieved and emptied. These methods are seldom used and are not be described here. In the circulating fluid method (see Figure 2.1),
the fluid is picked up by the pump, circulated through the surface piping, and sent down the inside the drill pipe and drill collars, where it exits the bit and entrains the drilled cuttings. The fluid carries the cuttings to the surface for separation and disposal. The behavior of the cuttings in the circulating fluid depends on fluid rheology, as shown in Figure 9.1. The different behaviors of the fluid help determine the degree of difficulty in removing the cuttings from the hole (Walker and Mayes, 1975).
The ability to use fluids to transmit solids depends on fluid properties such as density, viscosity, and velocity (Bourgoyne and others, 1986). Because cuttings are usually more dense than fluids, they will fall under the influence of gravity. As the density difference between fluids and solids decreases, the rate of fall slows. Consequently, the higher the fluid density, the easier it is to remove the solids from the hole. Similarly, as the viscosity of the fluid increases, the difference in velocity between the fluid and solids decreases. Consequently, the higher the fluid viscosity, the easier it is to remove solids from the hole. The higher the fluid velocity, the more rapidly solids are removed from the hole. The velocity must, at a minimum, exceed the rate of fall of the solids for the solids to have a positive velocity out of the hole.
Unfortunately, fluid properties such as density, viscosity, and velocity serve other purposes that limit the range of properties that can be used. For instance, if the density is too high, circulation will be lost; if it is too low, the well will flow. Higher viscosities reduce the drilling rate and can also cause greater friction-induced pressure buildups, which in turn lead to loss of circulation. Excess velocity can erode the hole wall and produce pressure drops that may lead to loss of circulation.
Circulating fluids serve purposes other than removal of drilled cuttings, namely, density for well control, chemical stabilization of the rock, prevention of filtrate invasion of the rock, cooling and lubricating the bit, suspension of cuttings and weight materials, and corrosion prevention (Rodgers, 1963). These functions must be maintained as the removal of drilled solids is accomplished. Interference among these functions is one of the major problems faced when using a circulating fluid to remove drilled cuttings.
Once the cuttings have been circulated to the surface, they must be separated from the fluid. Figure 9.2 shows an example of a system used to remove drilled cuttings from the fluid. The drilling fluid containing
solids is routed from the well to a shale shaker, where the large solids are removed. As the fluid is routed to each successive piece of equipment, progressively finer solids are removed (desander, desilter, mud cleaner, and centrifuge). Desanders and desilters work on a hydrocyclone principle shown in Figure 9.3, whereas centrifuges work on mechanically induced centrifugal forces as shown in Figure 9.4. Proper design and installation of this equipment are critical to the operation of the system (Ormsby, 1973). Solids that remain in the fluid beyond this point cannot be removed mechanically. If they become a problem, the fluid must be discarded and replaced or treated with chemical flocculants.
Other problems are present within the current system that impede performance (e.g., by limiting the speed at which a hole is drilled) or increase the cost of materials handling, including the following:
Lost circulation: When circulation of the fluid is lost, the drilled cuttings are not removed. They can accumulate and halt the drilling operation.
Incompatibility: The drilled cuttings may not be compatible with the other functions of circulating fluid, such as density for well control or hole stabilization.
Environmental disposal and contamination: The fluid can be environmentally hazardous (oil), or the drilled cuttings themselves may be hazardous (e.g., they may contain heavy metals). Disposal of the fluid or cuttings then becomes difficult and expensive, and mixing of the fluids and cuttings contaminates both systems.
Information pathway blocked by choice of fluid: The fluid system must aid in obtaining information from the well while drilling (such as directional and logging information) and during non-drilling time (such as characterization of the formations by electrical logging, stress state
measurements, or fluid analyses). Using air and aerated fluids interferes with this information transfer (particularly measurement while drilling), and the use of oil or brine interferes with other measurements (such as electrical logging).
Hostile environment: Hostile environments (high temperatures or pressures) interfere with the effective removal of material.
Even though these problems are present, current solids control systems are capable of separating the cuttings that come to the surface through mechanical or chemical (flocculation) separation. Current fluid circulation methods are capable of moving the cuttings to the surface. The problems are almost always related to other competing functions that the fluid system must fulfill, which may limit the allowable velocity, viscosity, density, or particle size.
Status of the Field for Mining and Tunneling
In mining and tunneling, the volumes of materials to be moved are greater than those in drilled wells by three or four orders of magnitude. In addition, individual pieces of material—often called muck—typically have dimensions up to several inches on a side. Removal becomes a problem of mass movement of very coarse materials at rates up to a few hundred cubic yards per hour. These materials are handled by three systems:
a system that removes materials from the advancing face, which may include buckets, augers, conveyors, or pipes;
a system that moves cuttings laterally from the face to a shaft or other access point, which may include trains, trucks, conveyors, earth movers, or pipes; and
a system that moves cuttings vertically at the shaft, which may include lifting muck cars, buckets, skips, conveyors, or pipes.
Although the practice is not common, muck can be brought to the surface in a pipeline entrained in fluid. In these cases, the problems encountered are similar to those found in drilled wells. The much greater volume of coarse materials from mining or tunneling is usually carried to the surface as loose or broken material. At the surface, the materials are
stored in stockpiles or bins until they can be removed and used or disposed of in an environmentally acceptable manner. Problems with the present removal systems include the following:
High-volume, reliable removal: With the volumes of muck removed in mining and tunneling being hundreds of cubic yards per hour, current systems are marginal in their ability to keep up with a rapidly advancing tunnel boring machine or mining operation, especially in small tunnels. Experience at the Superconducting Super Collider, for example, indicates that state-of-the-art tunnel boring machines may produce muck faster than the best available mucking system can remove it. Any breakdown in the material removal system causes immediate cessation of underground operations.
Compatibility with other functions: Workers and material must use the same work site entrances (shafts) that are used to bring excavated materials to the surface. Safe and efficient shared facilities must be provided.
Environmental disposal: Mucking or surface workings are often noisy and dirty operations handling large volumes of materials. At a remote mine, this may be of less concern, but at projects in populated areas (such as urban infrastructure projects), these operations can be disruptive to the environment and to other surface activities. Any new system must be compatible with environmental requirements while handling large volumes of materials.
Priorities for R&D
For well drilling, mining, and tunneling operations, materials handling cannot be considered in isolation from the drilling system. For well drilling, cuttings are usually small and mixed with a transporting fluid, whereas for tunneling and mining, the muck usually contains large pieces and requires mass movement techniques. Advanced materials handling systems must be designed to:
remove muck or cuttings at the face;
transport the muck or cuttings from the face to the surface; and
provide for recovery, decontamination, and disposal.
All of these functions must be compatible with other drilling or tunneling activities.
The development of improved materials handling systems requires advancements on several fronts. These are, broadly, the following:
Improved drilling fluids: In the short term, research is needed to develop environmentally ''friendly" drilling fluids, such as foam and airmud fluids, with material removal properties equivalent to those of oil-base muds, which are the current state of the art for rapid removal of cuttings but present difficult disposal problems. In the long term, these efforts should lead to the development of drilling fluids that can be varied through the density range between gas and rock and can be adjusted easily as needed to remove cuttings while balancing the pore pressure to prevent loss of circulation. Such fluids must be capable of transmitting acoustical energy for data transmission. They should also have sufficient viscosity at low shear rates to carry large-size cuttings, while having a low viscosity at high shear rates to prevent adverse effects on penetration rate.
Improved muck removal systems: Current muck removal systems cannot efficiently handle more than about 400 ft of tunnel advance per day. In the short term, these systems should be improved to handle tunnel advance rates of at least 500 ft/day. Long-term (decadal) efforts should anticipate significant improvements in tunneling capabilities that will result from the development of advanced tunneling technologies; such systems should be capable of handling tunnel advance rates up to 2,000 ft/day in ideal materials.
Contaminated materials handling: Research on contaminated materials handling should focus on the development of automated systems for reducing the volume of contaminated waste through either separation or decontamination techniques. Long-term research efforts should be undertaken to minimize waste handling through in situ disposal of muck or cuttings.
Bourgoyne, A. T., Jr., Millheim, K. K., Chenevert, M. E., and Young, P. S., Jr., 1986, Applied Drilling Engineering: SPE Textbook Series, v. 2, Richardson, Tex., SPE, 502 pp.
Moore, P. L., 1974, Drilling Practices Manual: Houston, Tex.,PennWell Publishing.
Ormsby, G., 1973, Proper rigging boosts efficiency of solids-removing equipment: Drilling, SPE Reprint Series, no. 22, Richardson, Tex., SPE, p. 147-153.
Rodgers, W. F., 1963, Composition and Properties of Oil Well Drilling Fluids: Houston, Tex., PennWell Publishing.
Walker, R. E., and Mayes, T. M., 1975, Design of muds for carrying capacity: Drilling, SPE Reprint Series, no. 22, Richardson, Tex., SPE, p. 139-147.