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 vol ume 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 t ie hydraulic and pore pressures. When this pressure difference 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, rooks 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 solid 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 microfractures is quickly covered by adsorption 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, норовий 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-combinations 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 coursc 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 ceitain 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 ab rasive prope rties 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 frictior 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 deternined 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 metal?, 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 find 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 £it 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 nonuniform 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.