2
ADVANCED DRILLING SYSTEMS

Introduction

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.

The Drilling System

The principal elements of current drilling systems are shown in Figure 2.1. The drill head comminutes rock at the end of the borehole. In most applications, the drill head is a drill bit that breaks rock by mechanical or mechanical-hydraulic action. The rate of breakage is governed by bit design, including the bit's effectiveness in breaking rock and resisting wear, and by rock type, temperature, pressure, and operating procedure (the experience and aptitude of the driller can have a considerable impact on how fast and effectively drilling proceeds). The drill is powered either from the surface through a drive string or by a downhole motor.

Rock breakage by the bit is followed by transfer of rock fragments to the surface. In tunneling applications, transport of waste material can involve elaborate mechanical systems; in petroleum and geothermal drilling, fragmented rock is typically transported to the surface by drilling fluid (mud) or air. The drilling fluid may provide power to drive the drill;



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2 ADVANCED DRILLING SYSTEMS Introduction 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. The Drilling System The principal elements of current drilling systems are shown in Figure 2.1. The drill head comminutes rock at the end of the borehole. In most applications, the drill head is a drill bit that breaks rock by mechanical or mechanical-hydraulic action. The rate of breakage is governed by bit design, including the bit's effectiveness in breaking rock and resisting wear, and by rock type, temperature, pressure, and operating procedure (the experience and aptitude of the driller can have a considerable impact on how fast and effectively drilling proceeds). The drill is powered either from the surface through a drive string or by a downhole motor. Rock breakage by the bit is followed by transfer of rock fragments to the surface. In tunneling applications, transport of waste material can involve elaborate mechanical systems; in petroleum and geothermal drilling, fragmented rock is typically transported to the surface by drilling fluid (mud) or air. The drilling fluid may provide power to drive the drill;

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FIGURE 2.1 Principal components of current drilling systems (Bobo and Hoch, 1957). it also acts as a coolant for the cutters, a conditioner for stabilizing the borehole (i.e., preventing borehole collapse and blowouts), and in some advanced applications, a medium for transmitting information to the surface by pressure pulses. In most drilling systems, there is little or no downhole sensing of rock or bit conditions, and guidance systems, if present, are primitive. Although measurement-while-drilling and logging-while-drilling technologies exist, in most cases target acquisition requires interruption of the drilling process to insert special tools to obtain rock samples (core) or borehole measurements (e.g., pressure tests or well logs). Sensing the condition of the drill

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bit or wellbore commonly amounts to nothing more than an attempt (in many cases, unsuccessful) to recognize the warning signs of incipient loss of function or catastrophic failure. Key processes of the typical drilling system can be subdivided into two groups (Table 2.1). The first group includes processes such as rock breakage, debris removal, and maintenance of borehole stability. These are the rate-controlling processes or system bottlenecks, and they limit the rates at which the other processes can operate. In present drilling practice, the rate and cost of a single process in this group may dominate the system. In the committee's view, improvements in these processes can be achieved through increases in their rates and reductions in their cost. Important, but evolutionary, improvements are possible through advances in these individual processes and in their integration. The second group includes processes such as drill bit sensing (e.g., monitoring bit wear), rock properties sensing and evaluation, drill bit or drillstring steering, and wellbore damage sensing. Three factors make revolutionary advances in these processes possible and perhaps even likely: (1) these areas of drilling technology are relatively undeveloped, and therefore significant advances are possible; (2) these advances will be driven by the changing nature of drilling targets (e.g., smaller, deeper, harder to detect); and (3) this development will be facilitated by technological advances in related fields (e.g., computer science, microelectronics) that can be readily adapted to drilling. As described in the following chapters, important recent advances in sensing while drilling and directional drilling show that rapid and striking advances are already under way. This report focuses on the development of a smart drilling system—a self-guided drilling system that has the potential to operate in a way that permits rapid, efficient, and damage-free detection and acquisition of targets without costly breakdowns and discontinuities. Smart Drilling Systems and the Systems Approach The smart drilling system is a system capable of sensing and adapting to conditions around and ahead of the drill bit to reach desired targets. This system may be guided from the surface, or it may be self-guided, utilizing a remote guidance system that modifies the trajectory of the drill when the parameters measured by the sensing system deviate from expectations.

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TABLE 2.1 Key Elements of Drilling Systems and Areas of Possible Evolutionary and Revolutionary Improvement DRILLING SYSTEM PROCESS CURRENT STATUS ANTICIPATED LEVEL OF IMPROVEMENT Rock breaking Key element in drilling process: bottleneck to increased drilling rate Evolutionary Debris removal Potential bottleneck, especially in tunneling Evolutionary Borehole stabilization Discontinuous process Evolutionary Drill bit sensing and evaluation Technology not available Revolutionary Rock properties sensing and evaluation Some measurement-while-drilling capability now exists Revolutionary Drill bit positioning and steering Notable recent advances in steering Revolutionary Borehole sensing Technology not available Revolutionary The smart drilling system does not currently exist, but it is presaged by recent dramatic advancements in directional drilling and in technologies of measurement while drilling. Rapid innovation in microelectronics and other fields of computer science and miniaturization technology holds the prospect for greater improvements—even revolutionary breakthroughs—in these systems. The development of smart drilling systems has the potential to revolutionize drilling. Research in this area will have a significant impact on drilling success and overall cost reduction. Such ''smart" systems are in

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creasingly needed to overcome the drilling challenges posed by small, elusive, easily damaged subsurface targets. This is particularly true in applications where the identification of small or difficult-to-predict drilling targets and formation damage are key issues in drilling success. In the development of smart drilling systems, the required improvements in the sensing elements of the system will have an impact on other processes such as rock breaking, debris removal, and borehole stabilization. Revolutionary advances in these fundamental processes might be possible as information about the subsurface environment becomes available in real time. The most effective means of operational implementation of the smart drilling systems and all other secondary goals recommended by this committee is the adoption of an integrated systems approach to all major drilling practices. In the integrated systems approach, all component parts and processes are designed to function in unison at an optimum level of performance without excessive redundancies and without overloading any of the interdependent component parts and processes. At present, this is rarely the case in drilling. The driller must adjust drilling parameters to changing conditions based on very limited information—with some modifications guided primarily by experience and intuition. Further, a host of discontinuities can and do interrupt these processes. These discontinuities include (1) tool-bit wear; (2) degradation and loss of effectiveness of the drilling fluid; (3) damage to the wellbore (e.g., borehole breakouts); and (4) time out for well testing and wireline geophysical logging. Discontinuities are costly and time consuming and, in the worst case, catastrophic (e.g., lost circulation, borehole collapse). Adherence to a systems approach in all instances should reduce costly discontinuities to acceptable levels. References Bobo, R. A., and Hoch, R. S., 1957, Keys to successful competitive drilling - part 5-A: World Oil, v. 145 (4), p. 91-98.