DRILLING AND BORING OF ROCK
In the smart drilling approach, whose development is recommended in this report, the process of local rock fracture, comminution, or other novel forms of rock removal is the first, and perhaps most crucial, step in a long succession of processes of reaching a target stratum or resource reservoir. As described in Chapter 5, mechanical fracturing of rock is still the most effective means of advancing the drill head. Thus, in the proposed ideal smart drilling approach, the drill must sense the type of rock or stratum ahead of the drill bit; recognize its resistance; and automatically adjust the drilling process in terms of rate, contact pressure, and so forth. If necessary, a smart drill will divert around a particularly difficult heterogeneity or seek alternative directions to avoid premature termination of the drilling operation.
Although identifying and evaluating rock ahead of the drill bit will require sophisticated advances in sensing and guidance, the actual fracturing, or comminution, of the rock requires a thorough understanding of the mechanical properties of rock and its response under the drill bits or cutters. Currently, there is insufficient understanding of both the interaction of cutting tools with the rock and the possible variations of such interactions based on rock type.
This chapter presents a summary of the current understanding of the structure and mechanical properties of rock, with particular emphasis on its fracture in compression, both under quasi-homogeneous stress fields and under conditions resembling the interaction of rock with the drill bit. The chapter is partly abstracted from a 1983 report by the National Materials Advisory Board (NMAB, 1983), but new developments have been added.
Types of Rock
The term ''rock" includes a great variety of material types with distinctive characteristics. Rock is one of the materials for which fracture behavior under compressive stresses has been studied most thoroughly. For example, granitic rocks can behave in a brittle manner up to a confining pressure of 1 gigapascal (GPa), whereas carbonate rocks become plastic at moderate pressures of about 100 megapascals (MPa). Extensive crystal plasticity is observed in rock salt at moderate stress at room temperature, whereas most quartz-bearing rocks do not show significant dislocation activities up to about 400°C.
Rocks tend to be permeated with pores and microcavities, which were either formed during the inception of the rock or produced by its subsequent stress history. The porosity and microcavity morphology of rocks are as important as the mineralogic composition itself. Collectively, the microcavities cause nonlinear behaviors in many mechanical properties. These manifest themselves in the stress or pressure dependence of strain, velocity of sound, stress wave attenuation, and fracture behavior. Microcavities also introduce a scale effect into the prediction of mechanical behavior, and heterogeneities in the form of distribution of microcavities are the principal source of scatter in test results. Thus, this relatively readily probed characteristic of rocks may become an important indicator of mechanical properties to be sensed by the smart drill.
Rock has a finite hydraulic conductivity, which implies that a portion of the void space forms an interconnected network. Petrological and geophysical evidence indicates that rocks are saturated with water to a depth of tens of kilometers. Pore fluids play a significant role in engineering applications for energy resource recovery or dam construction. The effect of pore fluids on fracture behavior can be either mechanical through pore pressure diffusion or chemical through stress corrosion (NMAB, 1983). The effect of pore fluid will be an important measurable indicator of the mechanical properties of rock relevant to drilling.
Theoretical Models for the Behavior of Rocks in Compression
Like all solids, rocks can undergo true intrinsic inelastic behavior by dislocation motion, diffusional flow, or analogous processes occurring in glassy media. In most crustal rocks of interest, however, such intrinsic inelastic behavior is exhibited only at elevated temperatures or pressures that are not encountered during mechanical drilling. In most drilling applications, rocks act as purely elastic solids, but the heterogeneities discussed above can affect their elastic behavior. For equiaxed heterogeneities, rock behavior can be readily accounted for by a variety of bounding approaches, or more elegantly by ellipsoidal inclusion models (Chow, 1978) or self-consistent models (Budiansky, 1965). Corresponding approaches that account for the effects of microcracks are also well developed (Salganik, 1973; Budiansky and O'Connell, 1976). Overviews of such self-consistent models have been given by a number of authors (Cleary and others, 1980; Haskin, 1988; Kachanov, 1992). Thus, considerable theoretical and mechanistic methodologies exist that are capable of relating elastic properties to heterogeneities in the rock that govern its fracture behavior.
The apparent inelastic behavior of rocks known as clastic flow results from brittle fracture processes due to the formation and stable growth of brittle microcracks. This behavior has been dealt with in two ways. The first, a purely phenomenological approach, is used widely by civil engineers for characterizing the related clastic flow of concrete. Specific developments of this behavior by so-called deformation theories (Kupfer and Gerstle, 1973; Bazant and Tsubaki, 1980) or by incremental flow theories (Bazant and Kim, 1979; Nemat-Nasser and Shokovoh, 1980) have been discussed in the above-mentioned NMAB (1983) report. As discussed below, these formalisms are quite useful for predicting the development of shear faulting zones in rock, which is a central mechanism of rock fracture in drilling. They also find ready application in the understanding of chip formation under the drill bit.
Of fundamental interest to drilling is the behavior of fractured rocks in compression. Griffith (1924), who pioneered the understanding of brittle fracture in tension, also was the first to elucidate brittle fracture in compression. He noted that in a solid containing microcracks of many orientations under triaxial compression, local tensile fracture is initiated when shear stresses produced by unequal compression displace surfaces of
preexisting microcracks. These displacements produce tensile stresses near the tips of these microcracks, which result in their growth across the local maximum tensile stress when this stress reaches the cohesive strength (Figure 4.1a). Griffith used this model of crack initiation to show that the uniaxial compressive strength of brittle solids must be many times (about eightfold) higher than their tensile strength, and that this compressive strength could be increased monotonically by the application of a confining pressure. It later became clear that the Griffith model needed modification to account for the frictional resistance on the touching faces of the microcracks; such a modified model was provided by McClintock and Walsh (1962). Figure 4.2 shows the predictions of the models of Griffith (1924) and McClintock and Walsh (1962) compared to experimental
It has been recognized by many investigators (e.g., Brace and Bombolakis, 1963; Erdogan and Sih, 1963; McClintock, 1965; Babel and Sines, 1968; Nemat-Nasser and Horii, 1982; Ashby and Hallam, 1986) that when the local recracking condition is satisfied, wing cracks develop from the extremities of the initial microcracks and extend stably in the direction of principal compression as shown in Figure 4.1b. Since the continued extension of the wing cracks necessitates continued relative translation of the faces of the initial microcracks, and since the total amount of such displacement is limited by their original lengths, overall compressive failure cannot result from the limited growth of wing cracks in "parts" (individual rock pieces or fragments) with dimensions much larger than those of the initial microcracks (NMAB, 1983). Thus, it is remarkable that the agreement between experiments and the simple recracking models is as good as that presented in Figure 4.2.
It is well known that brittle fracture in compression in massive parts occurs through the development of shear faults. This occurs when many developing wing cracks, aligned in a plane, begin to interact strongly en echelon and form a nucleus of a more macroscopic shear fault (Figures 4.3 a-b), which then spreads longitudinally to result in overall fracture. Although the conditions for development of the nucleus of the shear fault have not been well studied, the overall asymptotic conditions for their localization have been developed as a bifurcation model by Rudnicki and Rice (1975). They found that the critical strain at which the shear fault can develop freely depends strongly on the incremental moduli of the material containing the accumulating microcracks. The best agreement between theory and experiment, at least qualitatively, is obtained with a deformation theory approach. However, the actual conditions for fault development depend sensitively on the presence of initial imperfections, as is the case for all bifurcation phenomena (NMAB, 1983). The actual mechanism of brittle fracture in compression is a simple criterion of an equality between a crack driving force KI and a material fracture toughness KIc at the tips of the microcracks, interacting en echelon. However, the development of a shear fault that produces eventual failure or chip formation in drilling obeys a phenomenological pressure-dependent deviatoric stress criterion (NMAB, 1983). Such mechanistic understanding of the development of shear faults should be most useful in the effective control of the smart drilling process.
Experimental Verification of Models
Two Views of Rock Behavior
Two approaches have been taken in the study of the mechanical behavior of rocks that together have contributed significantly to current understanding. The first is a global "applied mechanics" approach commonly used for engineering design and for the analysis of geologic faulting. The fracture process is taken as a discrete event without significant prior deformation and without warning. The only physical quantity of interest is the peak stress, which is of interest as an upper bound on solutions of the relevant boundary value problems. The fundamentals of this approach have been covered by many investigations including Jaeger and Cook (1979), Goodman (1980), Hoek and Brown
(1980), and Germanovich and Cherepanov (1987). A previous report by the U.S. National Committee for Rock Mechanics of the National Research Council (NRC, 1981) also touches on this subject. The second is a "mechanistic" approach that supplements standard deformation tests with nondestructive evaluation and microscopy, aiming at a fundamental understanding of the microscopic mechanism. The evolution of microstructure is treated as a continuous process culminating in the coalescence of microfissures to form a throughgoing fault. This approach was adopted by Paterson (1978) in his extensive review of rock fracture. It is this latter approach that has led to more definitive mechanistic understanding of the clastic behavior of rocks and their eventual fracture, which is discussed in a following section.
Laboratory Experiments on Clastic Flow and Fracture of Rocks
As noted previously, the true plastic response of rocks by dislocation motion occurs at such high temperature and pressure as to be of no relevance to the problem of mechanical rock drilling. Many constituent minerals in rocks can, of course, undergo twinning that, for all practical purposes, is not a thermally assisted rate process but usually requires very high stresses. Moreover, twinning is a very inhomogeneous form of deformation, and in the absence of local plasticity, it can at best influence only the fracture behavior of rocks. Thus, experimental studies carried out in the laboratory have been devoted largely to understanding the complex processes of microcracking under compression that result in clastic flow, which is a forerunner of the eventual shear faulting process discussed previously.
The clastic flow response of some common rocks such as Indiana limestone (Myer and others, 1992), Berea sandstone (Myer and others, 1992; Wong and others, 1992), and Carrara marble (Fredrich and others, 1989) has been investigated in the laboratory in a series of elegant experiments. These experiments have identified the key microstructural damage processes, which include fracturing of weak interface boundaries and grain boundaries; Hertzian diametral fracturing of grains; relative sliding across separated interfaces or boundaries, which act as inclined planes that "jack open" tensile wing cracks running parallel to the principal compression direction; and finally, en echelon action of interacting wing cracks that results in zones of shear faulting. In porous rock, pores may be
crushed closed, which results in substantial permanent compression strains that locally stiffen and strengthen the rock.
In laboratory simulations of borehole breakout experiments on externally pressurized thick-wall cylinders of both Indiana limestone and Berea sandstone (Ewy and Cook, 1990a,b), the localization of en echelon damage has been followed in detail, leading to local breakout (see also Dyskin and Germanovich, 1993; Dyskin and others, 1993). These breakout experiments have demonstrated the anisotropic nature of such rocks and have also demonstrated a substantial size effect on rock strength. In particular, rocks in such breakout experiments exhibit a two- to threefold increase in local tangential splitting strength over laboratory-size cylindrical rock samples undergoing fracture by vertical spalling. This size effect, which clearly can be important in the actual fracture response of rock under drill bits, is not well understood and needs further study.
Compressive strength measurements of about 140 MPa, which have been obtained from the laboratory borehole breakout simulations, are quite high. Even when divided by 2.5, for example, to account for the size effect, these effective strengths are between 5 and 6 times higher than the often-quoted estimates of upper crustal effective strengths of about 10 MPa, which are commonly assumed to be proper for rock strength governed by frictional resistance (Zoback and others, 1993). Nevertheless, the strength levels measured in the laboratory may be more appropriate for understanding the local fracture strength of rock in the small volumes subjected to contact pressure by a drill bit. Clearly, these factors must be well understood to at least develop proper strength scaling relations that will be relevant in any definitive model of drilling.
Effect of Confining Pressure and Temperature on Strength
The brittle strength of rocks shows a strong pressure dependence because of its fractured state. It is not uncommon to achieve a tenfold increase in strength by a moderate increment in mean confining stress, as shown in Figure 4.2. Depending on the signs of the principal stresses, a sample can fail in tension or in shear. However, only shear fracture that occurs when the principal stresses are all compressive (Figure 4.4) is considered here. The fracture angle is defined as the angle between the shear fault surface and the maximum principal compressive stress 1.
Most of the empirical fracture criteria discussed earlier are formulated from conventional triaxial test data with the implicit assumption of the independence of the fracture phenomenon on the intermediate principal stress 2. However, Mogi (1972) concluded from his "true" triaxial tests on cube-shaped samples that this is only an approximation. He found that when the minimum compressive stress3 is kept constant, an increase in 2 results in an increase of 1 at failure (Figure 4.5a) and, furthermore, that the fracture angle decreases with increasing 2 when 3 is fixed (Figure 4.5b). The fracture surface, however, always contains the 2 direction. Ductility also is found to decrease with an increase in 2. Similar conclusions were drawn by Handin and others (1960, 1963) from torsion tests on hollow cylindrical specimens.
The effect of temperature increases is to stabilize postfailure behavior (Wong, 1982). In general, in comparison with pressure, temperature has a relatively small effect on the brittle fracture of dry rocks.
(a) Dependence of compressive strength 1 on transverse compressive stress 2 for different levels of the third principal compressive stress 3 from tests in triaxial compression. (b) Dependence of the fracture angle, between the shear fault plane and the direction of principal compression, on the principal stress difference from tests in triaxial compression (Mogi, 1972).
Effect of Pore Fluid
Experiments on sedimentary rocks by Handin and others (1963) and on crystalline rocks by Brace and Martin (1968) show that if the sample is "drained," Terzaghi's principle of effective stress should apply to fracture (Jaeger and Cook 1979). If pore fluid diffusion is relatively slow (so that pore pressure is no longer uniform), it is necessary to take into account microcavity deformation and permeability. Theories for such so-called poroelastic behavior have been developed primarily in petroleum engineering (Biot, 1941) and have been reviewed by Rice and Cleary (1976; see
also Atkinson and Craster, 1991; Detournay and Cheng, 1991). Clearly, these developments will be of importance in tailoring the drilling process to local requirements.
If the interstitial fluid is not inert relative to the mineral constituents of the rock, the pore fluid can exert a chemical effect in addition to the purely mechanical one discussed above. An evident weakening effect in water-saturated samples has been observed in calcite by Rutter (1972) and in quartz by Scholz (1968c). The observed behavior is usually attributed to stress corrosion cracking (Atkinson, 1982). Mizutani and others (1977) investigated the strengthening effect of a sample when placed in a high vacuum similar to the lunar environment. A limited amount of available data indicates that the stress corrosion cracking effect is reduced by an increase in pressure (Kranz, 1980) or a decrease in temperature (Kranz and others, 1982).
Effect of Size
Because of their brittle nature, the compressive strength of rocks depends on size, as pointed out above (see also NMAB, 1983). Size effects are particularly important in relating laboratory fracture experiments both to failures in massive crustal formations and to the high-gradient local fracture processes under the drill bit.
Evolution of Sources of Shear Faulting
The sequence of processes of microcrack interactions leading to the evolution of shear faults has been studied in the laboratory both by techniques of systematic sectioning experiments and by a variety of acoustic techniques. For example, monitoring the acoustic emission (AE) from the compressed samples by multiple probes has permitted determination of the spatial correlation of signals emanating from microcracking events leading to formation of shear faults. Such sophisticated AE studies (Mogi, 1968, 1972; Scholz, 1968a, b; Byerlee and Lockner, 1977; Lockner and others, 1977; Rothman, 1977; Bailey and others, 1979; Lockner and Byerlee, 1980; Sondergeld and Estey, 1981) support the microstructural studies discussed previously and show that existing imperfections are likely sites of AE activity. Microcracking is found to
intensify in the neighborhood of previous microcrack sites and is often persistent in a locality until final shear faulting sets in (Sondergeld and Estey, 1982).
Recent experimental observations on the evolution of microcracking damage in rocks and scanning electron microscopy (SEM) experiments provide graphic means for understanding shear localization measurements by AE. Central features in the development of a faulting nucleus are the en echelon interaction of microcracks and the planar form of dilatancy they introduce into the fracture problem. Tapponier and Brace (1976) concluded that dilatancy is primarily a consequence of two types of cracking: (1) widening and extension of preexisting discontinuities such as grain boundaries, cracks, and pores; and (2) initiation and propagation of cracks at heterogeneity sites. These observations clearly demonstrate that a continuum description of the fracture localization behavior of rocks is adequate only over a so-called representative continuum volume element large enough to smooth out grain-scale inhomogeneities. This presents a problem of how to translate laboratory findings in quasi-homogeneous deformation fields to the high-gradient fields under a drill bit or cutter, for eventual application for control of cutting operations by a smart drill.
The SEM observations show, for example, that quartz, which comprises about one-third by volume of granite, has limited participation in the localization process in the initial postfailure stage. In other words, localized deformation extending over a continuum element with grains of all major mineral types is not observed until the sample has been deformed well into the postfailure stage. In this limiting sense, SEM observations agree with the theoretical prediction of localization analysis outlined previously for frictional, dilatant materials. Such analyses (e.g., Rudnicki and Rice, 1975; Rudnicki, 1977) predict that the onset of localization under axisymmetric compression should occur when the sample has been deformed well into the strain-softening stage—another factor of importance in the mechanistic rationalization of the drilling process.
Surface-Active Agents in Rock Fracture
In brittle fracture of solids, the work of fracture results largely from the surface free energy of the solid (Griffith, 1920). Even in pseudoductile fracture where the actual fracture process is one of extension of brittle cracks, the surface free energy is the overall factor that scales to specific
fracture work, even though other energy-dissipating phenomena such as frictional rubbing and true plastic deformation are present (Rice, 1965). Under these conditions, the presence of surface-active agents, which significantly reduce the free energy of the surfaces that have been created by fracture, can have dramatic effects on the overall specific work of fracture, known collectively as Rehbinder effects (Rehbinder and Shchukin, 1972).
There are many examples of dramatic reduction of the local work of brittle separation by active liquid environments in laboratory experiments (Westwood and Pickens, 1983). Although the fundamental mechanisms of these effects are not fully understood, they are often well characterized, and some have been used with mixed success in near-surface drilling (Zoback and others, 1993). Some evidence (Kranz, 1980) suggests, however, that such Rehbinder effects are considerably reduced when the part is under pressure or when the surface-active agent cannot penetrate to the crack tips, particularly when the key shear faulting events are of a subsurface nature. Consequently, it is not clear whether these effects are present in deep drilling environments. Surface-active agents can also promote stress corrosion cracking of drilling equipment. However, this problem could probably be avoided by careful process planning.
Another important potential application of surface-active agents is their use to influence wear rates of diamond tool bits in cutters (Cooper and Berlie, 1978; Mills and Westwood, 1978). In a detailed experimental study, Cooper and coworkers (Cooper and Berlie, 1978; Cooper, 1979) have established that few, if any, surface-active agents have produced unambiguous weakening of a variety of rocks, including marble and granite. They have shown that these agents can either increase or decrease wear rates of the cutters, and that oxidizing agents in particular promote increased diamond wear. The best understanding of this effect is that these agents promote effective nucleate boiling heat transfer between the hot diamond and the cooling fluids, and that this appears to have a major beneficial effect in most cases.
Whatever the mechanism of the effect of the environment on the rock destruction process, be it by changing the strength of the rock or affecting the wear process of the tool, the effects are sufficiently dramatic, when present, that they merit further special consideration to exploit their full potential.
Fracturing of Rock in Drilling
Phenomenology of Drill-Rock Interaction
One of the important practical applications of fracture of brittle solids in compression is the penetration of drilling tools. Examples include percussive and rotary drilling, drag bit drilling, and ploughing or planing of rock formations (e.g., coal). Despite the extreme commercial importance of tool penetration into brittle rock, details of the penetration mechanism are poorly understood. Much of the past work on this problem has been directed toward semiempirical drilling models specific to a particular formation. Although such models have some engineering utility, they generally are not consistent with recent developments concerning the constitutive behavior of rock, and they are not based on the mechanistic processes described above. Hence, their range of applicability is quite limited. The following sections review the current practice in phenomenological theories for tool penetration into rock and the attempts at developing mechanistic models for chip formation. Quite clearly, a detailed understanding of these will be of key importance in the establishment of controlled cutting strategies for exploitation of the full potential of the smart drilling process.
Quasi-static tool penetration tests (Evans and Murrell, 1958; Hartman, 1959; Reichmuth, 1963; Sikarskie, 1966; Singh and Johnson, 1967) have established the features of the penetration mechanism. Figure 4.6 shows a typical set of force-penetration traces of wedge-shaped tools into charcoal gray granite. As the traces suggest, the penetration process occurs in a repetition of two distinct phases. The first is a crushing phase, in which the forces on the tool increase monotonically. Hydrostatic stresses in the vicinity of the tool tip are extremely high; the material under the tip is crushed in the region of very high contact pressures and undergoes a volumetric expansion. The crushed and expanded material behaves in an almost plastic fashion. The tool-bit stresses are transmitted through the crushed zone, resulting in a stress field closely resembling that of a plastic indentation problem. In the second phase, a macrofracture zone is initiated, and with a subsequent load increase, it eventually grows to form a chip. The cycles repeat under increasing total force as the contact area between the tool and the rock increases.
A typical drill bit action may see two to four cycles per ''blow." The cyclic behavior is also observed in drag bits (Whittaker and Szwilskie, 1973). This cyclic penetration behavior applies to rock at low confining pressure. At high confining pressure (e.g., in deep drilling), rock develops more prominent clastic flow behavior and the force-penetration curves become smoother (Cheatham and Sikarskie, 1973).
Theoretical "Plasticity" Models for the Faulting of Rocks
The process of chip formation as outlined above involves the repeated application of local shear faulting in the rock under the concentrated pressure of the tool. The mechanics and mechanisms of the evolution of microcracking processes leading to the formation of a shear fault nucleus in a homogeneous compressive stress field under confining pressure apply locally in the rock that is to be chipped—albeit in this case, the local stress field is highly inhomogeneous, and shear faults comprise curved surfaces.
In such situations, more formal phenomenological theories clearly are needed to deal with the problem on the basis of a continuum, without repeatedly coming to grips with the details of faulting. The constitutive localization model of Rudnicki and Rice (1975), which develops conditions of shear localization by treating the brittle rock undergoing microcracking as if it were a pressure-sensitive, somewhat dilating plastic continuum, serves as a guide. These problems may be amenable to analysis using the slip line field method of the mathematical theory of plane plasticity, generalized to deal with the pressure dependence of plastic resistance and dilatancy, and including, if necessary, vertex phenomena on yield surfaces that are known to make the material more prone to localization of deformation. This approach is currently under active development (Mokhel and Salganik, 1993). A number of attempts have been made to model the behavior of granular materials such as soils (Drucker and Prager, 1952; Drescher and others, 1967; Szczepinski, 1971; Mroz and Szymanski, 1978; Spencer, 1982; Anand, 1983). These models have been applied with varying degrees of success to inhomogeneous problems, such as indentation of a half space of sand or soil, but they have not found application to corresponding problems of machining or chip formation in rocks where modifications that take into account the small size of the stressed volume, the scale of heterogeneities, and the effects of pore fluids become necessary. This will be a fertile area for immediate development for application to the smart drilling process.
Specific Models for Chip Formation
Theoretical models using slip line plasticity approaches have been developed specifically for the purpose of dealing with chip formation in rocks by the wedge indentation process. Figure 4.7 shows a schematic two-dimensional view of a wedge penetration model developed by Paul and Sikarskie (1965). The plane strain model has assumed symmetrical distortions and an initial local penetration by crushing that is linear with the applied force. The model further assumes that the chips are planar blocks and that they move out of the way when the Coulomb-Mohr criterion of a pressure-dependent plastic resistance can be counteracted. The model demonstrates that for proper chip formation, the wedge angle shown in Figure 4.7 must be less than a given amount, determined by the
friction angle between the wedge and the rock, and the pressure dependence of the plastic resistance that governs the Coulomb-Mohr criterion. The model is capable of presenting an upper-bound, outer envelope to the force-penetration behavior of a wedge as given in Figure 4.6. The model has been extended to the symmetrical penetration of anisotropic rock by Benjumea and Sikarskie (1969) and to the application of a tilted wedge by McLamore (1971).
The simple wedge penetration model described above is not fully consistent with the actual phenomenon. Altiero and Sikarskie (1974) have studied this in some detail on models made of plaster of Paris to better understand the complex processes of vertical splitting, crushing, and chip formation. On the basis of their observations, Sikarskie and Altiero (1973)
analyzed the wedge penetration process as a quarter-space loading problem, where the free surface and the vertical splitting delineate the quarter-space boundaries as shown in Figure 4.7. The normal and frictional loading of the interface between the rock and the wedge is then considered in detail, the stress distribution in the rock is calculated, and the Coulomb-Mohr criterion is applied to determine the point at which fracturing will begin. The growth direction of the chip was assumed to follow the steepest slope of the Coulomb-Mohr function. This is clearly somewhat arbitrary; the generalized slip line field approach discussed above probably should have been used instead.
Other Applications of Rock Fracture
Other applications of rock fracturing are of industrial interest. Hydraulic fracturing is of great interest in the improvement of oil recovery from oil-bearing rock beds. Here the problem is one of tensile fracturing of rock under a large wellbore pressure and the opposing confining pressures of the surrounding rock, made complicated by the flow processes of pore fluids. Problems of mine shaft failures and lateral displacement of large earthworks such as dams and embankments are direct applications of compressive fracturing of rock and soil flow, both obeying the general framework of shear faulting theories. The framework for the solution to such problems exists, and solutions will be urgently required in the context of the development of the smart drilling process.
Priorities for R&D
The rate-controlling process in drilling is, and will likely be for the foreseeable future, the rate of rock removal by the drill bit. Increasing the efficiency of the rock removal rate requires a fundamental understanding of how solid rock is fragmented by drill bits and the numerous factors that control this very inhomogeneous and local process. There are several areas in which additional research would be most beneficial in improving rock removal, particularly in the context of the smart drilling system elucidated in Chapter 2. These are broadly the following:
In situ characterization of rock: The chemical constitution (including mineralogy and petrology), physical characteristics, microstructure, and nature and distribution of preexisting flaws (e.g., faults, joints, and bedding planes) of rock have been studied in great detail for many decades. There have been equally detailed laboratory studies of the mechanical properties of rock over a wide range of temperature and pressure to identify mechanisms for drilling and excavation. In any new thrust to capitalize on the potential of the smart drilling approach, it is essential to tailor the response of the drill to the local rock environment (e.g., temperature, pressure, fracture density, and resistance). This could be achieved if the relevant mechanical properties of the rock could be assessed in situ by making meaningful measurements of its chemical, mineralogical, elastic, and acoustic properties, and if these measurements could be used to make adjustments to the drill bit in order to optimize the rate of rock removal.
Fracture processes: For drilling applications, the relevant rock behavior is purely elastic, with brittle fracture occurring under the drill bit. However, conditions under the drill bit differ significantly from the quasi-homogeneous conditions in the Earth's crust or in typical laboratory experiments. The stress field under the drill bit is highly inhomogeneous over volume elements much smaller than those encountered in the usual laboratory experiments, where rock fracture is subject to the strength-size effects discussed previously. Presently, there is inadequate information on the nature of this inhomogeneous fracture process, both qualitatively and quantitatively. The fracture process should be studied in greater detail to identify the specific factors affecting both the local driving forces of fracture and the response of the rock to these forces. These processes should be modeled to obtain useful scaling laws for drilling practice.
Matching the drilling process to the physical environment: The requirements for effective drilling vary for different environments, for example, near-surface, large-scale excavation; porous and fluid-bearing rock; dense rock in the upper crust; and fractured, hot rock in geothermal reservoirs. The physical conditions and mechanical properties of rocks are well known in these environments, but there is inadequate information on how to optimize the drilling process to best suit these different requirements.
Physicochemical effects on rock fracture and tool wear: A potentially important avenue in the improvement of drilling practice involves the physicochemical aspects of the mechanical response of rock in surface-active fluid environments, to take advantage of the so-called Rehbinder effects. Every brittle fracture process is ultimately based on the severance of chemical bonds. There are many examples of dramatic reduction of the local work of brittle separation in active liquid environments. Although the mechanistic details of these effects are not fully understood, they are often well characterized, and some have been tried with some success in drilling. Because of their attractive potential, these effects should be given special consideration.
Unconventional methods for rock removal: Rock is a brittle solid with a characteristic set of strength-impairing imperfections such as pores, weak boundaries, and microcracks. The tensile strength of unconfined rock is generally low, approaching at best 50 to 100 MPa. When confined, or under pressure, as is the case at great depths in the Earth's crust, these imperfections are rendered ineffective by being pressed shut. Under these conditions, the compressive strength of rock is high and increases with increasing pressure. Thus, drilling rock at great depth by subjecting it locally to greater pressure is probably an inefficient means of removing material. Efforts should be made to identify unconventional forms of material removal that capitalize on the low tensile strength of rocks.
A large number of unconventional methods of rock removal have been studied. When compared with mechanical drilling, these techniques have so far been found to be quite ineffective, as discussed in more detail in the next chapter. Nevertheless, the existing information on them should be reevaluated to reach firm quantitative conclusions concerning their potential, particularly for use in conjunction with mechanical drilling.
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