Drilling and Excavation Technologies for the Future (1994)

Drilling involves a set of processes for breaking and removing rock to produce boreholes, tunnels, and excavations. In general, the object of drilling is to reach a target in the subsurface. The target may be a small feature at considerable depth or increasingly—particularly in petroleum industry applications—at substantial horizontal distances from the drilling site. The paramount objectives of drilling are to reach the target safely in the shortest possible time and at the lowest possible cost, with additional sampling and evaluation constraints dictated by the particular application.

Technology to drill holes and to excavate tunnels and openings in rock is vital for the economic and environmental well-being of the United States. During this century, U.S. technology has dominated the worldwide drilling industry and much of the excavation and comminution industries. In the committee's view, this U.S. dominance is likely to erode without continued technological advances.

With this concern in mind, the Geothermal Division of the U.S. Department of Energy asked the National Research Council to establish a committee to examine opportunities for advances in drilling technologies that would have broad industrial, environmental, and scientific applications such as energy exploration and production, mining, tunneling, water well drilling, underground storage, and environmental remediation. This report is the result of the committee's deliberations.

Drilling is a key technology in several applications of strategic or societal importance, including

• energy and mineral production,

• environmental protection, and

• infrastructure development. 

Each of these applications would benefit from improvements in the basic system that breaks rock and removes debris from drill holes and excavations.

The drilling system comprises two principal parts: first, the mechanical, electrical, and hydraulic components, and second, the interactions between these components and the rock. Significant technological advances in drilling technology are likely to be obtained only through improvements in both aspects of the system. The goal of these technological advances, simply stated, is to drill and excavate safely at the lowest possible cost and in the shortest possible time.

There are both short- and long-term benefits from research and development (R&D) programs aimed at developing advanced drilling systems. Improvements in individual system components could be incorporated into conventional drilling systems almost immediately, providing short-term (less than five-year) payoffs. Long-term payoffs will come from advances in basic R&D to assemble these individual components into a smart drilling system—a system capable of sensing and adapting to conditions around and ahead of the drill bit.

1. R&D in advanced drilling technology is needed to improve the drilling system. R&D should result in reduced costs and drilling times to more effectively achieve various drilling goals.  

   Drilling involves a complex set of mutually interacting, consecutive component operations (mechanical, hydraulic, and electrical) that must function in unison. An integrated systems approach is needed to ensure that these component operations function near peak performance with a minimum of discontinuities (failures in components that lead to system breakdowns). A long-term R&D effort could provide significant improvements in drilling technology through advances in understanding basic physical and chemical processes related to rock breaking and rock removal, and particularly through the development of flexible smart drilling systems incorporating improvements in sensing and guidance of autonomously advancing drilling units. Significant improvements in drilling technology could also be realized over the short term (less than five years) through incremental R&D on many of the rate-limiting processes and critical components of current drilling systems. A well-coordinated, 

    incremental R&D effort could accelerate attainment of the primary goal of development of the smart drilling system.

2. The principal thrust of an R&D program should be on the development of the smart drilling system. A smart drilling system is one that is capable of sensing and adapting to conditions around and ahead of the drill bit in real time (i.e., while drilling), with minimal operator intervention. Such a system must be capable of assessing the mechanical properties of rock through measurement of physical and chemical (e.g., mineralogical) properties concurrent with drilling. The development of a smart system will require concerted technological advances in several areas, including the following:

• Development of precise connections between measurable properties and local drilling resistance: Such connections must be established based on the existing understanding of the connection between rock constitution and comminution mechanisms. Many physical and microstructural properties such as porosity, elastic properties, and wave attenuation can readily be measured locally. These must be associated more precisely with factors that govern the drilling resistance of rock through directed mechanistic studies and modeling to develop automated response characteristics of a smart system.  

• Development of sensors for the smart drilling system: The smart drilling system requires sensors that are capable of detecting and measuring the following:

  1. Conditions at the drill bit: Sensors are needed for in situ measurement of pressure (including pore pressure), temperature, permeability, mineralogic and chemical composition of the rock and heterogeneities, borehole fluid composition (at the part-per-million level for environmental applications), stress state, and rock strength.  

  2. Conditions ahead of the drill bit: Sensors are needed that measure rock properties (such as porosity, elastic properties, and wave attenuation) ahead of the drill bit to adjust drilling parameters, such as the weight on the drill bit and the rotary speed, and to avoid potential problems (e.g., blowouts or loss of circulation) while drilling. 

3. Spatial position of the drill bit: Sensors are needed that are capable of detecting the position of the drill bit in space in order to steer the bit around undesirable zones and reach desired targets.

• Development of control systems for accurate positioning and steering of the drill bit and for automatically adjusting drill parameters (e.g., load and torque on the drill string, flow rates of drilling muds and fluids), according to local conditions, is necessary to optimize rock breakage and rock removal. This will require precise information at the bit-rock interface of the rock breaking mechanism, rock strength, pressure, temperature, and stress state.  

• Development of improved methods for steering the drill bit: A number of mechanical methods for steering are currently available, but large turning radii often preclude their application in a smart drilling system. To enhance steering capabilities, R&D is needed to develop downhole motors, flexible drill strings, and guidance techniques for smart drilling systems.  

• Continuous monitoring of the state of the entire drilling unit, including wear of tools, state of other mechanical components, flow of coolant, and the like, is required to anticipate the occurrence of possible discontinuities.  

• Development of improved telemetry methods for transmitting real-time borehole data to the surface: The use of advanced sensors for real-time, downhole measurements will require significant improvements in data telemetry. Such telemetry is essential for monitoring the smart system from the surface. At present, the most advanced telemetry systems utilize mud-pulse technology and are capable of transmitting data at only a few bits per second. Rates on the order of kilobits per second or higher will be required for advanced smart drilling systems. Telemetry is a rate-limiting step in present drilling systems, and it will become more so as smart drilling systems are developed.  

• Development of means for continuous and instantaneous support of the rock around the borehole: The support provided by the rock itself should be used, where possible, in lieu of casing the hole as a separate operation.

1. Although the principal thrust of the proposed R&D program should be on the smart system, the program should also facilitate incremental improvements in all consequential aspects of present 

    drilling technology. This should result in more immediate attainment of greater efficiencies and cost savings. Such additional R&D should focus on the following problems:

• Novel drilling technology, with a focus on the physics of rock removal to reduce energy requirements for drilling: Hybrid systems that combine novel technologies to lower the drilling resistance of the rock and mechanical methods (e.g., fluid jets coupled with conventional rotary technologies) to break and remove the rock are especially promising. Initial attempts to develop combined novel-mechanical cutters should be conducted with mining or oil field bits because of their smaller size and lower cost. Once this technology is developed in drilling applications, the transfer to tunneling and excavating applications should be attempted.

• Improved cutter materials and bearings: Conventional drill bits have steel, diamond, or carbide cutters that remove rock by impact or shearing processes. New wear-resistant, diamond-coated cutters are finding increased use in hard, abrasive rocks. Advances in new wear-resistant materials are rapidly applied to cutters and high-speed bearings; as such, additional R&D on hard materials and their applications is encouraged.  

• Improved bits for drilling in heterogeneous materials: Bits with polycrystalline or natural diamond cutters have the potential to drill much faster than conventional steel or carbide bits because they can operate at much higher rotary speeds and weight-on-bit loads. R&D is needed to utilize these wear-resistant cutter materials in multipurpose bits that can effectively drill through alternating layers of soft and hard rock.  

• Development of environmentally benign drilling fluids: R&D is needed on the design of nontoxic drilling fluids and foams as alternatives to oil-base fluids, which may be both toxic and difficult to remove from the drillhole. These new drilling fluids must have (1) filtration control to minimize fluid invasion and damage to permeable zones and (2) lubrication to prevent differential-pressure sticking of the drill pipe against the borehole wall. They also must provide adequate hole-cleaning capabilities in horizontal and high-angle wells. Improvements in understanding the fundamentals of cutting transport, flow visualization, air/foam behavior, and fluid viscoelastic behavior will aid the development of such fluids.  

• Development of durable, compact, high-power downhole motors for directional and extended reach drilling: Technology now exists to build downhole motors to increase drilling rates by factors of two to four by increasing the power delivered to the drill bit; the development of 

such motors would be a short-term improvement. Additional areas for R&D include improved air-drilling motors and higher-power motors for hard-rock drilling.

1. This R&D program should be a national effort. Both the public and private sectors should benefit; resources and guidance for the program should be shared, where appropriate. This program should have the following characteristics:

   • Integration of industry, university, and government perspectives should be achieved.  

   • Federal support should serve primarily as a catalyst, with industry providing both technological and financial support. The percentage of R&D support from the federal government and industry could be project specific. The actual R&D should be done by the best-qualified institutions whether in the private sector, universities, or government laboratories.  

   • Finally, a long-term commitment is needed to accomplish the objectives of the program.

The program should be structured with shared research objectives among the federal and industrial partners. Support of projects should be based on a peer-review process and assessment of how the results would contribute to overall program goals. Competition for research funds should be open to industry, national laboratories, and universities.

Attainment of the proposed enhanced drilling capabilities through both short-term and long-term R&D requires a long-range administrative structure that combines the discipline, mission orientation, and flexibility needed to nurture the required scientific and technological innovations.¹