Changes of porosity, permeability and, especially, of the contraction coefficient are also affected by the lithological- petrographic properties of the rocks.
Three-dimensional compression of rocks results in an increase of their volume mass. For instance, the volume mass of sandstones in the Canada submontane region deposits, occurring at a depth of around 3000 m, is roughly 30-35 per cent and that of argillaceous rocks 45-50 per cent greater than the volume mass of analogous rocks lying at the surface.
With growing triaxial compression the Poisson ratio and modulus of elasticity for rocks go up somewhat. But it is the strength and plastic properties that are affected most tangibly by the three- dimensional compression.
Intensification of three-dimensional compression tells above all, on the plasticity of the rock, for many types of rocks, which under atmospheric pressure crumble like brittle bodies, acquire plastic properties when subjected to three-dimensional compression. Under ordinary pressure marble behaves like a brittle body. However, already under a three-dimensional pressure of 23 MPa in evidence is plastic deformation that proceeds with continually diminishing stresses. Under a greater three-dimensional compression (>80MPa) plastic deformation of this marble is attended by its strengthening.
The magnitudes of triaxial compression at which plastic deformation sets out are dissimilar for different rocks. Thus, plastic deformation of uniform medium- and coarse-grained limestones and also of little metamorphosed argillites sets in under a three- dimensional compression of 50-100 MPa; of fine-grained and pelitomorphic limestones, compact afgillites, anhydrites and aleurolites - at 80 -100 MPa and more; of fine-grained dolomites and heavily metamorphosed argillaceous rocks - only at 200-350 MPa. Under a pressure of up to 1000 MPa quartzites behave like brittle bodies, and sandstones with siliceous cement behave almost in the same manner.
For most rocks the yield point, goes up as the three-dimensional compression gains in intensity and the more so the less plastic the rock. In salt rock the yield point is almost unaffected by the three- dimensional compression.
The degree of plastic deformation up to disintegration augments as the three-dimensional compression becomes more intensive, this being most marked in limestones and insignificant in quartzites, with dolomites and sandstones occupying an intermediate position.
The strength of all rocks increases as the three-dimensional compression gains in intensity. The strength of rocks of the same name rise^more sharply with a decrease in the size of grains therein.
^fhree-dimensional Compression causes shifting of rock grains with respect to one another and their convergence. The greater compression, the tighter the grains press against one another, and the higher the interaction forces between them the fewer the possibilities for their relative movement. In rocks plastic deformations occur basically due to a relative movement of grains (in-tercrystal gliding). In a more fine-grained rock there are more intergranular boundaries and, consequently, the possibilities for intergranular gliding are broader and, therefore, the rock has greater plasticity. With an extremely great three-dimensional compression the displacement of grains becomes virtually impossible and plastic deformation sets in owing to gliding inside the grains proper, which is usually accompanied by strengthening of the rock.
Reduced porosity of the rock following an intensified three- dimensional compression, convergence of the grains and subsequent enhancement of the interaction forces between them are factors contributing to an added strength of the rock. Deformation strengthening or strain hardening due to intercrystal gliding causes an increase in the strength of rocks as the three-dimensional compression rises. Intensification of three-dimensional compression leads to "healing" of internal defects in the crystal lattice of the rock and, as a result of this, to a higher strength and a greater degree of plastic deformation prior to breaking.
Let us segregate mentally inside the earth a certain volume of rock. This volume is subjected to the action of a vertically directed force caused by the weight of superjacent rocks. Under the effect of this force the segregated volume of rock undergoes compression (contraction) along the vertical axis, but tends to expand radially (in the horizontal plane). Its radial expansion is resisted by the surrounding rocks. Consequently, the segregated volume becomes subject to the action of a lateral compressive force exerted by the ambient massif.
In mining the pressure originating from the weight of overlying rock strata is commonly referred to as geostatic or rock pressure. The magnitude of rock pressure depends upon the volume mass of this rock and on the depth at which the volume under consideration occurs.
The pressure produced as a result of resistance of the
surrounding rocks to radial deformation of the segregated volume is called lateral pressure. Its magnitude is a function of rock pressure. Since both rock and lateral pressure increase with depth, the strength of a rock of the given mineralogical composition accrues as well. As the temperature rises so does also the ability of rocks to undergo plastic deformation, but at the same time the yield point and the strength of the rock decline. At times the influence exerted by the temperature is so considerable that the effect of the strength increment due to an elevated three-dimensional compression is cancelled out altogether. Thus, the strength of some argillaceous rocks, limestones and even dolomites at a temperature of 300°C and under three-dimensional compression of 200 MPa proves lower than at a temperature of 24°C and under a pressure of 100 MPa. A temperature rise adversely affects most particularly the strength of a number of chemogenie rocks (bishofite camallite, halite).
With rising temperature the elastic properties of rocks experience but an insignificant change. The modulus of elasticity goes up slightly, while Poisson's ratio remains practically almost unchanged.
3 Read Text 1 again. Find, read and translate the sentences in Text 1 in which we learn about shifting of rock grains, changes in the crystal lattice of the rock, rock pressure, plastic deformation.
4 Look through Text 1 again. Give the main points of each passage of the text. Use: “deals with”.
5 Give English equivalents of the following:
залишкова деформація, стискуваність, коефіцієнт ГІуассона, кремнисгий, межа текучості, переміщення, посилення, решітка, пласт, верхній пласт, піднімати, сатурований.
6 Make a written translation of the following:
Reduced porosity of the rock following an intensified three- dimensional compression, convergence of the grains and subsequent enhancement of the interaction forces between them are factors contributing to an added strength of the rock. Deformation strengthening or strain hardening due to intercrystal gliding causes an increase in the strength of rocks as the three-dimensional compression rises. Intensification of three-dimensional compression leads to "healing" of internal defects in the crystal lattice of the rock and, as a result of this, to a higher strength and a greater degree of plastic deformation prior to breaking.
Unit 3
The Effect of Saturating Fluid on the M echanical Properties of Rocks
1 Learn the meaning of the following words, word-combinations and word groups:
traverse, seal off, film, framework, stem, invade, viscosity, infiltration, wettability, fracture, preclude, screening, wedging.
2 Read Text I.
Text 1
The majority of sedimentär/ rocks that have to be traversed in sinking wells are porous bodies. The voids in rocks are filled either with a dropping fluid (crude oil, water), or with gases under a certain pressure, which subsequently we shall call pore (or formation) pressure. The pore pressure always stands below the rock pressure.
The influence of fluid saturating a rock on the mechanical properties of the latter consists in causing a direct change in the magnitude of the triaxial compression and makes itself felt in two ways.
Let us imagine a sample of rock sealed off on all its sides by an impervious film. If the sample is not saturated with fluid then, on building up a confined hydraulic pressure P, it will become subject to elastic deformation and the volume of voids will diminish somewhat. In this case the rock's framework (skeleton) will be acted upon by an external force whose magnitude is fully determined by the hydraulic pressure.
On the other hand, should the sample be saturated with fluid under an initial pore pressure Pp. then, on undergoing deformation under the effect of external hydraulic pressure P that contracts the volume of voids, the pore pressure of the saturating fluid will be going
up. Therefore, the rock's framework (skeleton) will be acted upon by an external compressive force stemming not from the hydraulic pressure, but only from the difference between the hydraulic and pore pressures. When this pressure d ifference is equal to zero the rock’s framework will not be exposed to triaxial (uniform) compression and mechanical properties of the rock will remain unchanged even under fairly high hydraulic pressure.
Now, let us assume that a previous sample of the rock saturated with fluid is not sealed off (segregated). Then, in producing the hydraulic pressure the pores of the rock will be invaded by that very same fluid which serves to build up this pressure. The lower the viscosity of the fluid, the higher the rate of its infiltration into the sample and, consequently, the speedier the growth of the pore pressure. In this case, too, the rock skeleton is acted upon by the difference between the external hydraulic and pore pressures. If this difference is small, as is usually the case, then during building up of a uniform triaxial hydraulic compression the mechanical properties of the rock experience no noticeable changes.
Hence, if with triaxial hydraulic compression the strength of the rock augments, the pore pressure of the saturating fluid then is a factor contributing to the fall of the y ield point and to lowering the strength of the rock.
The influence of the saturating fluid on the strain properties is in a great measure dependent upon the lyophilic capabilities of the rocks. The more lyophilic the rock, i.e. the greater its wettabi-lity, the higher the scope and rate of its saturation wi th the fluid, and the quicker grows the pore pressure. The fuller the rock pores are filled with the dropping fluid, the less is the change in strain (deformation) properties of the rock.
Molecular forces at the surface of any solid body or fluid are noncompensated and the surface layer possesses an excess surface energy. The greater the excess surface energy, the higher the strength of the rock. Accordingly, through lowering the surface energy it is possible to reduce the strength of a solid body, rocks including.
Surface energy can be substantially lessened by adsorption of water and, especially, of surfactants on the surface of a solid body. This method of reducing the strength of sc lid bodies through adsorption is termed Rebinder's effect.
In rocks, particularly porous ones, there is always a considerable number of weakened bonds (sites of grain contacts, micro-fractures and other defects). In drilling the number of sites with weakened bonds augments owing to the development during operation of the bit of microfractures in an area around the bottom hole. Via these microfractures and voids the adsorption layers of water and surfactant molecules gain access into rocks. During deformation of the rock beneath the bit the microfractures in the rock become deeper and there appear new ones. The surface of newly upsurging micro fractures is quickly covered by adsoiption films of water and surfactants, which prevent microfractures to close on relieving the pressure and preclude reestablishment of bonds lost at the time of deformation. As a result of such a screening and wedging action of the adsorption layers the strength of rocks suffers quite appreciably.
When a rock comes into contact with a fluid and substances dissolved in it, this can also give rise to chemical reactions. A chemical reaction of this kind can result in the rock both loosing and gaining some strength.
3 Give English equivalents of the following:
буріння свердловини, поровий тиск, герметично закривати, непроникна плівка, гідравлічний тиск, насичувати рідиною, в’язкість, зазнавати значних змін, зниження міцності породи, значною мірою, поверхнево-активна речовина, основа свердловини, підвищення рівня води.
4 Find in Text 1 five sentences with adjectives and adverbs used in different degrees of comparison and translate them.
5 Write 8-10 questions covering the main idea of Text 1.
6 Supply a heading for Text 1.
Unit 4
Abrasiveness of Rocks
1 Learn the meaning of the following words, word-comhinations and word groups:
abrasiveness, wear out, alloy, friction, rock-breaking tool, drilling equipment, measurement, a reference ring, horizontal axis, a jet of water, friction route, circumferential speed, establish, tungsten, chilled steel.
2 Read Text 1.
Text 1
By the abrasiveness of rocks is meant their ability to wear out metals and hard alloys in the course of friction. The abrasiveness of rocks manifests itself during interaction with the latter of rock- breaking tools and other components of drilling equipment. The greater the abrasiveness of a rock, the higher the rate of the tool wear- out and, consequently, the sooner it will get out of service. Frequent replacements of rock-breaking tools used in a deep well significantly prolong the period of its construction and heighten its cost. The knowledge of abrasive properties of rocks helps to choose the right type of rock-breaking tool and thereby raise the efficiency of drilling operations.
Methods for assessing abrasive properties of rocks are fairly numerous, but, so far, there exists no universal and generally accepted one. At the basis of most methods lies the measurement of the volume or mass of the metal worn out during friction against the rock under certain conditions constant for a given method. Among laboratory procedures, the one proposed by Prof. L. A. Shreyner is most suitable for appraising abrasiveness of rocks. In essence it consists in that a reference ring made of test material (steel, hard alloy), tightly pressed with its lateral cylindrical surface against the horizontal ground surface of a test rock sample under the action of a pre-set force, revolves about the horizontal axis at a constant speed. The rock sample advances relative to the ring at a certain pre-assigned speed. Disintegration products resulting from the friction of the ring against the rock surface are carried away by a jet of water supplied to the site of the contact. The abrasive properties are judged by the volume of the broken down material of the ring and rock along the friction route of 1 m.
For the majority of minerals and types of rock the voluminal wear-out of the reference ring material has been found to be irrelevant to the circumferential speed, being' directly proportional to the force of pressure between t ie ring and the sample. The proportionality constant between the volume of the worn-out material of the ring along the friction route of 1 in and the force with which the ring is pressed against the rock sample is termed the abrasiveness factor.
It features the abrasive properties of the test rock with respect, the material of the reference ring. Experiments have established that if the reference ring is made of a hard tungsten carbide alloy the direct proportionality remains true even in testing of quartz rocks. But if the reference ring is made of chilled steel such a dependence continues to be true with friction against crystalline rocks save quartz ones. In friction against quart/, rocks the magnitude of the coefficient appears as a function of the pressure force and the circumferential speed. With force surpassing a certain value, the voluminal wear-out increases at a faster rate than does the pressure force.
Yet another characteristic featuring abrasive properties of rocks generally determined by this method is the relative wear-out, i.e. the ratio between the volume of the worn-out material of the reference ring to that of the worn-out rock along the friction route of 1 m.
The abrasive properties of rocks may also be judged by the work spent in wearing out a unit volume of the reference ring material or of the rock itself, or else with reference to relative abrasiveness. By the latter is understood the ratio between the relative wear-out of the test rock and the relative wear- out of gypsum, conventionally taken as a unit of reference. Gypsum is the least abrasive rock. Relative abrasiveness is dimensionless and does not depend upon experimental conditions. It is; convenient for a comparative assessment of abrasive properties common to diverse rocks and minerals.
The abrasive wear-out of metals, and hard alloys depends both on the abrasiveness of the rock and also upon a number of other factors, such as the relation between the hardness of rock and metal (alloy), the roughness of friction surfaces, the contact pressure, temperature, sliding speed and the properties of the cooling medium (lubricant). The abrasiveness of a rock is contingent Upon the microhardness of mineral grains of which it is composed, their size, shape and the nature of the surface. The abras veness of crystalline rocks with regard to chilled steel is proportional to the microhardness of minerals making part of the rocks. By the degree of increasing abrasiveness these rocks the may be arrayed as follows: gypsum < barite < dolomites < limestones < silicious rocks (chalcedony, flint) < magnesioferruginous and feldspar rocks < quartz and quartzites.
If the hardness of mineral grains comprising the rock is inferior to the hardness of the metal, this then results in a tenuous surface wear of the metal due to friction forces and in an intensive wear of the grains and the rock as a whole. On the other hand, if the hardness of mineral grains stands close to that of the metal,:hen there takes place a very tenuous, but voluminal disintegration of the metal due to a high topical concentration of stresses at the contact sites of friction bodies. In the lastly named case of great significance is the initial roughness of the rock and metal. As a general rule, igneous polymineral rocks are more abrasive than the mono mineral ones. This is apparently attributable to the fact that the friction surface becomes more rough because of a nommiform wearing of different minerals making part of the rock.
Among clastic rocks most abrasive are quartzv sandstones and aleurolites. With the same mineral composition, the abrasiveness of clastic rocks is usually greater than that of the crystalline ones, this being associated with the nature of roughness of the friction surface. The more pronounced the porosity, the larger the fragments and the sharper their edges, the greater is the roughness of the clastic rock. The more marked the roughness of the rock, the smaller, as a rule, is the actual area of contact between the metal and the rock, the contact occurring merely along the summits of rough projections. But with diminishing actual contact area under a given load, the contact pressure mounts and it can reach a magnitude equivalent to the hardness of the metal.
The abrasiveness of sandstones augments parallel with their
diminishing hardness. Most abrasive are quartzy and feldspar sandstones. This finds its explanation in the fact that the hardness of clastic (detrital) rocks is largely determined by their strength of the cementing substance. The lower the strength of the cement, the easier is denudation of the mineral grains that are harder than the rock itself, the greater the roughness of the friction surface, for the intensity of the mineral grains and cement; wear-out is dissimilar because of the difference in their strength characteristics. On the other hand, quartz is the most abrasive and hardest among the rock-forming minerals.
The abrasiveness of aleurolites is somewhat inferior to that of sandstones of a similar mineralogical composition, this being due to a smaller size of their grains.
In their pure form, some sedimentary rocks (argillaceous, calcareous, sulphate) are little abrasive. Their abrasiveness, however, increases with a higher proportion of quartz. With a greater content of quartz particularly intensive is the rising abrasiveness of low-strength rocks. With the proportion of quartz of more than 20 per cent the abrasiveness of low-strength rocks becomes higher than that of quartzites.
3 Give English equivalents of the following:
бурове обладнання, вийти з ладу, часті заміни обладнання, підвищити ефективність, за' певних умов, експериментально доведено, гартована сталь, одиниця зношуваності, шершавість, швидкість ковзання, змазувальний матеріал, охолоджуюче середовище, магнезіозалізистий.
4 Make a written translation of the following:
Methods for assessing abrasive properties of rocks are fairly numerous, but, so far, there exists no universal and generally accepted one. At the basis of most methods lies the measurement of the volume or mass of the metal worn out during friction against the rock under certain conditions constant for a given method. Among laboratory procedures, the one proposed by Prof. L. A. Shreyner is most suitable for appraising abrasiveness of rocks. In essence it consists in that a reference ring made of test material (steel, hard alloy), tightly pressed with its lateral cylindrical surface against the horizontal ground surface of a test rock sample under the action of a pre-set force, revolves about the horizontal axis at a constant speed. The rock
sample advances relative to the ring at a certain pre-assigned speed. Disintegration products resulting from the friction of the ring against the rock surface are carried away by a jet of water supplied to the site of the, contact. The abrasive properties are judged by the volume of the broken down material of the ring and rock along the friction route of 1 m.
5 Supply a heading for Text 1.
6 Make a plan to Text 1 and retell it.
CONTROL TASKS
1 Answer the questions:
1. What is the rock made up of?
2. What coefficient does porosity characterized by?
3. How does the volume mass of porous rocks of a similar mineralogical composition change with depth?
4. What does the strength of a rock depend on?
5. How is the strength of rocks influenced by temperature?
6. Name three groups that rocks may be classified into by the nature of the relationship between deformation and stress under static load?
7. What does the plasticity depend on?
8. What does three-dimensional compression of rocks result in?
9. What does three-dimensional compression cause?
10. When does the abrasiveness of rocks manifest itself?
2 Give English equivalents of the 1 | Allowing: |
гірська порода | когезія |
глинистий | стиснення |
модуль пружності | родовище |
залишкова деформація | крихкість |
інструмент для подрібнення породи | крупнозернистий |
1 density 2 volume mass | a) the proportionality constant or factor between normal stress in the rock and relative deformation that corresponds to it |
b) the magnitude of stresses under which a rock disintegrates | |
3 strength | c) the mass contained in the unit volume of its solid phase |
4 coefficient of anisotropy | d) the mass of a unit volume of dry rock in its natural state |
5 modulus of elasticity | e) the ratio between strength in compression parallel to bedding and that normal to it |
6 Poisson’s ratio | f) the proportionality constant between the unit longitudinal and lateral deformations |
7 stress relaxation | g) the pressure produced as a result of resistance of the surrounding rocks to radial deformation of the segregated volume |
8 abrasiveness of rocks | h) a gradual diminution of stresses in a body undergoing continuous deformation |
9 lateral pressure | i) the ability of rocks to wear out metals and hard alloys in | the course of friction |
4 Tell whether each of the following statements is true or false according to the text. Correct the false statements to make them true:
1 The density of magmatic rocks is, as a rule, smaller than that of sedimentary rocks.
2 Plasticity depends on the mineral composition of rocks and declines with a higher level of quartz, feldspar and other hard minerals.
3 For most rocks the yield point goes down as the three-dimensional compression gains in intensity and the more so the less plastic the rock.
4 The voids in rocks are tilled either with a dropping fluid (crude oil, water), or with gases under a certain pressure, which subsequently we shall call pore (or formation) pressure, or with some metals.
5 Gypsum is the most abrasive rock.,
5 Make a written translation of the following:
The abrasive wear-out of metals and hard alloys depends both on the abrasiveness of the rock and also upon a number of other factors, such as the relation between the hardness of rock and metal (alloy), the roughness of friction surfaces, the contact pressure, temperature, sliding speed and the properties of the cooling medium (lubricant). The abrasiveness of a rock is contingent upon the microhardness of mineral grains of which it is composed, their size, shape and the nature of the surface. The abrasiveness of crystalline rocks with regard to chilled steel is proportional to the microhardness of minerals making part of the rocks. By the degree of increasing abrasiveness these rocks the may be arrayed as follows: gypsum < barite < dolomites < limestones < silicious rocks (chalcedony, flint) < magnesiofermginous and feldspar rocks < quartz and quartzites.
FART 2 DRILL BITS
Uiniit 5
Purpose and! Classification
1 Learn the meaning of the following words, word-combinations and word groups:
bit, design, disintegration, boring bit, fragmentation, chip, shear, drill, hardness, cut, alternate, layer, viscous, contain, purpose, plane, core, periphery, bottom, enlargement, straightening, casing string, cement.
2 Read and translate Text 1:
Text 1
The drill bit is intended for breaking up the rock in sinking of Wells. There exist many types of bits each one of which is designed for disintegration of 'ocks possessing definite mechanical and abrasive properties.
As regards the principle underlying the breaking up of the rock all boring bits may be classified as follows:
1 Bits producing fragmentation and chipping (shearing), destined to drill non- abrasive and abrasive rocks of medium hardness, and also hard tough and very tough rocks.
2 Bits of the cutting-abrasive type of action intended for drilling in rocks of medium hardness and also in alternating layers of high-plastic little viscous rocks and those containing rocks of medium hardness and even hard ones.
As concerns their puipose all the boring bits may also be subdivided into three groups:
- for full hole drilling, disintegrating the rock in a single plane or step-wise;
- for core drilling, breaking up the rock on and along the periphery of the bottom hole;
- for special purpose.
At the lower extremity of the drilling shaft is placed the tool which does the actual boring or dril ling of the hole into the formation.
These tools, which are called bits are classified into three general types: drag type, rolling cutter bits, diamond bits and special bits.
Bits for full hole and core drilling are used in sinking wells, while special-purpose bits are designed to operate inside an already drilled well (enlargement and straightening of the well bore) and inside the casing string (drilling out of the cement stone).
Drag bits. Drag bits have rio moving parts and drill by the shoveling action of their blades on Ihe formation. Their water courses are placed so that the drilling fluid is directed on the blades, keeping them clean.
Bits of this type were widely used for drilling soft formations but in recent years they have been replaced by rolling cutter types. Two or three blade bits are used in drilling.
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