8 Read Text 3:
Text 3
In the course of drilling there arise diverse complications: absorption (circulation loss) of drilling fluid or of cement slurry, oil-gas and water showings, caving-in and crumbling of rocks, stickings and tight pulls of the drilling string and others. In many cases it is easier to avert the occurrence of a trouble than to eliminate it. Quite frequently one complication developing in the well and not eliminated quickly enough becomes aggravated by other types and, at times, occasions the appearance of new troubles. Therefore, great attention should be given to prevention and quick elimination of developing complications.
Circulation losses occur:
- when the rock has open fractures, caverns and other major channels, and the pressure exerted by the mud on the borehole walls exceeds the formation pressure in a given horizon;
- when pressure exerted by the mud on the borehole walls is high enough to open up closed microfractures existing in a given rock, or
to rend it and thus cause the development of new fractures.
Mud absorption increases the total consumption of the drilling fluid required for sinking a well; the rate of its deepening is slowed down, for the drilling crew is forced to spend a part of their working hours in preparing and treating an additional volume of the mud fluid, which increases the cost of drilling.
In the event of circulation loss the level of the mud fluid settles several scores and even hundreds of metres below the mouth of the well. Because of this the backpressure on the borehole walls declines and this can give rise to an inflow of brine water and gas coming from a horizon with a higher coefficient of abnormality. It is important to forestall the development of circulation loss and, if it occurs, to control it as quickly as possible.
Natural jointing, cavemosity, firmness of rocks and formation pressures are factors common to nature, they are outside the power of man and cannot possibly be controlled in the course of drilling. To preclude and eliminate absorption in the first of the above named cases it is necessary that the resistance of natural fractures and channels to spreading of mud fluid over and along them be in excess of the difference between the pressure exerted by this mud on the walls of the fractures and the formation pressure in a given horizon.
An analysis shows that circulation loss may be attributed to the following causes:
• an increased density of the mud fluid surpassing the limit;
• excessively high hydrodynamic pressure arising during backwash of the well along the section from the absorbing horizon to the mouth of the well and occasioned by a high flow velocity, too small a clearance between the pipe string and the borehole wall, or an improper choice of rheological properties for the drilling fluid;
• high hydrodynamic pressure at the time of re-established circulation, caused by an elevated gel strength of the mud;
• high hydrodynamic pressure arising during an attempt at a
very quick restoration of circulation and conditioned by the
inertia of the drilling fluid;
• high hydrodynamic pressure developing when running in a pipe string at a great speed, or with excessive acceleration;
• high hydrodynamic pressure arising during flushing of the well, or in the course of round trips, if the pipe string or the bit becomes subject to balling.
With known pressure gradients of rock fracturing and abnormality coefficients (factors), it is possible in many cases to forestall the above troubles through proper control and adjustment of the drilling fluid properties (by reducing density, gel strength and the yield point, plastic viscidity), by restricting the speed and acceleration in round trips and also by an adequate choice of the casing program.
With absorption ensuing when drilling in a fissured or cavernous rock one must, if necessary, minimize the circulation loss through adjusting mud properties (by reducing density, additing plugging materials, raising viscosity), quickly traverse the whole of the difficult zone and then, if needed, seal it off with some or other plugging material, or as the last resort, by means of the casing string.
If several absorption horizons may be opened up in the course of drilling a well, the question of sealing off each individual horizon before drilling in the next one, or of sealing operations to be effected following the opening of all or several horizons is decided upon by taking account of the circulation loss intensity and its influence on the technical arid economic efficiency of boring operations (drilling rate, the cost of well).
When a horizon with a high absorption intensity is drilled in, the outflow of mud to the ground surface stops altogether, its level in the well goes down and settles at a depth of tens and even hundreds of metres from the well mouth. In this case the intensity of the circulation loss can be determined by means of special investigations only. Circulation losses of such a great intensity are commonly referred i;o as complete, or catastrophic.
The following are the measures used for eliminating circulation losses:
• the density of the drilling fluid is reduced, through aeration, for instance;
• the flow velocity of the mud fluid in the annulus is brought down, especially if the clearance between the borehole walls and the pipe string is insignificant;
• a certain volume of mud fluid with high gel strength and rapid structurization is squeezed into the absorption horizon and the well is left alone for a few hours;
• a small amount of fibrous or granulated materials is added to the mud fluid to plug up joints and fissures less than 1-2 mm wide and channels with an equivalent diameter of 4-5 mm;
• boring is done without allowing the mud fluid to escape to the day surface, and it is presumed that the drilling cuttings by entering the channels and fissures of the absorption horizon gradually plug them up;
• a certain volume of quick-setting cement slurry or of some other low-density plugging mixture, often with an addition of fibrous or granulated materials, is forced into the absorption formation;
• coarse-grained inert materials (sand, gravel, etc.) are washed into large channels, cracks and caverns to plug them up and to achieve a significant diminution of the absorption intensity, these being then cemented with a quick-setting plugging mixture;
• high-molecular compounds capable of becoming polymerized upon contact with formation water, hardening and tightly plugging up the absorption channels, are squeezed into the bed;
• absorption zones are overlapped with case pipes (as a last resort).
The method of the circulation loss elimination or a combination of such methods is chosen depending on the specific absorption intensity and with due regard for a possible technical and economic effect they may yield. If the well opens up several highly intensive absorption zones, the best result is, as a rule, achieved when each zone is sealed off individually, being preliminarily isolated from other pervious horizons by means of a packer.
9 Fiind in Text 3 English equivalents for the following:
запобігати, усувати, ліквідовувати, посилювати, перевищувати, протитиск, передбачати, переважати, зворотний потік води, відновлювати, операція опускання-піднімання, утворення сальників, градієнти тиску розриву порід, герметично заробляти, тампонуючий матеріал, тампонуюча суміш, у крайньому випадку, за допомогою, швидкість буріння, вартість свердловини, пластова вода, щільно закупорювати, канал поглинання.
10 Find in Text 3 Infinitive constructions and translate them into Ukrainian.
11 Find in Text 3 conditional sentences and translate them into Ukrainian.
12 Make a written t ranslation of the following:
The method of the circulation loss elimination or a combination of such methods is chosen depending on the specific absorption intensity and with due regard for a possible technical and economic effect: they may yield. If the well opens up several highly intensive absorption zones, the best result is, as a rule, achieved when each zone is sealed off individually, being preliminarily isolated from other pervious horizons by means of a packer.
13 Translate the following sentences into English using the words and expressions from Text 3.
1) При поглинанні у багато разів збільшуються загальні витрати промивальної рідини.
2) Іноді при зниженні рівня рідини оголюються стінки свердловини, складені нестійкими породами.
3) Природна тріщинуватість, кавернозність, міцність гірських порід і пластовий тиск: - це природні чинники, незалежні від волі людини, тому управляти ними в процесі буруння неможливо.
4) Щоб запобігти поглинанню промивальної рідини, дуже важливо знати величини градієнтів тиску розриву і коефіцієнти аномальності продуктивних та всіх інших горизонтів, розкритих свердловиною.
5) Поглинання великої інтенсивності звичайно називають повним, або катастрофічним.
14 Supply a heading for Text 3.
15 Imagine that you are asked to make a report on measures used for eliminating circulation losses. Write the main points of your report and illustrate each point with the material from Text 3.
Unit 20
Gas-, Oil-, and Waiter-Showings
1 Learn the meaning of the following words, word-combinations and word groups:
hoisting, single-cone roller bit, diamond-set bit, balled up bit, tool joint, pressure differential, permeability, bulk, menace, influx, overflow, breakdown, blowout, reliability, sediment, foliation, rupture, sinking of the well, check val ve, stand idle, fluid jet, union, hook up, batch, stand pipe.
2 Read and translate Text 1: Text 1
The formation fluids and gases can enter the well, first and foremost, in the event when formation pressure even in one of the horizons is higher than the pressure exerted on it by the mud fluid. The inflow of fluids and gases can take place when a bed displaying an elevated abnormality factor is being drilled in, with poor control over the density and degassing of the mud fluid, with the mud level in the well going down as a result of the circulation loss, or when hoisting a pipe string without addition of the mud, with a rapid hoisting of a pipe si ring (especially with a diamond-set or a single-cone roller bit, with a balled up bit or tool, joints,, with a packer, etc.).
T he inflow intensity depends upon the pressure differential, the permeability of the bed area adjoining the well bore, the properties of formation fluids and gases, as well as on other factors, and quite frequently the inflow is very significant.
A certain amount of formation fluids and gases gain access into the drilling fluid together with drilling breaks. Their bulk is roughly proportional to the drill speed in rock. The formation gas can also force its way into the well owing to diffusion via permeable walls of the borehole. As a rule, the inflow intensity of the formation fluids and gases is not high and does not present any menace, provided there are no lengthy intervals in the mud circulation and the mud undergoes proper degassing on the surface. In case of protracted intermissions in circulation a certain amount of gas can enter the mud fluid from the top portion of the gas-bearing sand.
The influx of non-gassy or little gassy formation fluids (commonly brine water) becomes ostensible by the overflow through the mouth on discontinued flushing, and, at times, even in the course of it, as well as by an increased volume of mud in the receiving tanks of the circulating pumps.
The gas-oil-water showings not only stand in the way of proper drilling, but are causative of breakdowns and accidents. In the case of intensive gas-oil showings, instances of demolished wellhead and drilling equipment, of explosions and fires are of not infrequent occurrence.
To prevent the influx of formation fluids and gases into the well, as well as of blowouts and uncontrolled flow it is necessary:
• to tightly seal the wellhead with preventors, keep an eye on their good working order and proper performance., to check on the reliability of their control system and to correct the disclosed defects in good time;
• to exercise control over the quality of the drilling mud leaving the well. Starting from the moment of nearing a horizon displaying a significantly elevated abnormality factor, one must continually control the density of the mud leaving the well and its gas content;
• to use mud fluids with low water loss (not more than 2-3 cm3 in 30 min), as low gel strength as possible, zero daily sediment and an insignificant yield point, when opening up horizons with a materially elevated abnormality factor (especially gas-bearing ones);
• to increase the density of the mud in the well to a level high enough for maintaining a slight excess pressure in the well over the formation pressure, but invariably below that at which is possible foliation (or rupture) of rocks and absorption of the mud, prior to opening up horizons with an elevated abnormality factor;
• to degas the mud fluid leaving the well; in the event of a considerable increase in the gas content it is advisable to suspend the sinking of the well and, without stopping to flush it, replace the gassy mud with a fresh one with a higher density;
• to keep at the drill rig a spare amount of mud fluid of the quality matching that required for drilling in horizons with an increased abnormality factor, this amount comprising not less than two-three-fold volume of the well;
• to pour into the well additional amounts of the drilling fluid, when pulling up a pipe string, at a rate sufficient to always keep its level near the wellhead;
• to set up a check valve in the bottom part of the drilling string;
• not to allow the well to stand idle for long periods of time without flushing it; in the course of round trips intermediate flushings lasting for 1-1.5 cycles every 500-1000 m are necessary.
A best means for removing gas from the drilling fluid are vacuum degassers. It is also expedient to utilize them for removing air from the mud fluid when this is entrained into the latter, in some types of chemical treatment and also in boring attended by flushing with aerated muds.
When adding mud into the well at the time of the pipe string hoisting it is advisable to make use of automatic units.
If it becomes impossible to preclude the inflow of formation fluids and gas and a blowout occurs, with the drilling string inside the well, one must close the preventer without delay, direct the fluid jet escaping from the well via a side branch pipe of the wellhead hookup equipped with a union and pump in, through the drill pipes, a non-gassy drilling fluid of a higher density so as to raise the pressure on the horizon above the formation pressure level and thus stop the inflow. In case of a blowout with no drilling string in the well an attempt is to be made at running in quickly at least a few stands of drill pipes, then close the preventer and start forcing in a weighted mud fluid. If it is impossible to lower the pipes, the preventer is then to be closed and a batch of mud squeezed into the well through the side branch pipe of the hookup.
3 Find in Text 1 conditional sentences and translate them into Ukrainian.
4 Give English equivalents of this; following:
сальник на долоті, перепад тиску, загроза, аварія, розшарування, поглиблення свердловини, проміжне промивання, зв’язка ланцюгів, промивальна рідина, що обважнює, труба-стояк.
5 Make a written translation of the following:
A certain amount of formation fluids and gases gain access into the drilling fluid together with drilling breaks. Their bulk is roughly proportional to the drill speed in rock. The formation gas can also force its way into the well owing to diffusion via permeable walls of the borehole. As a rule, the inflow intensity of the formation fluids and gases is not high and does not present any menace, provided there are no lengthy intervals in the mud circulation and the mud undergoes proper degassing on the surface. In case of protracted intermissions in circulation a certain amount of gas can enter the mud fluid from the top portion of the gas-bearing sand.
6 Supply a heading for Text 1.
7 Find,, read and translate the sentences in Text 1 in which we learn what to do if it is impossible to preclude the inflow of formation fluids and gas and & blowout occurs.
8 Working in pairs test each other’s ability to describe measures for preventing the influx of formation fluids and gases into the well.
Unit 21
Crumbling and Caving-in of Rocks, Narrowing of the
Well Bore
1 Learn the meaning of the following words, word-combinations and word groups:
caving, debris, viscosity, velocity, funnel, reaming, swabbing, caved interval, redrilling, lapse of time, spud, suspend, logy, remedy, scrape off, sheath, keyseated portion, keyseating, keyseated interval, fishing.
2 Read and translate Text 1: Text 1 Caving and Freezing of the Drill Column
Caving zones and sections in which the formations tend to squeeze into the well are often the source of serious trouble. Sometimes caving can be prevented by the use of the correct type of mud fluid, but once a bad cave has occurred it is not easy to correct the trouble, or to overcome the difficulties arising through ii:. The drilling debris is carried from the well by the returning mud circulation, the carrying capacity being a function of the viscosity of the: fluid and the velocity of the flow-stream.
If excessive caving occurs, the area of the annul us outside the drill pipe is increased, and therefore the velosity of the circulating fluid over this zone is decreased. As soon as the speed is reduced below a critical point, the debris can no longer be carried upwards in the flow-stream, but settles around the bottom of the “funnel” caused by the cave.
In extremely bad cases the cave must be filled up with cement and a new hole drilled through it before drilling can proceed.
Sometimes it is necessary to keep reaming out the well and to insert casing to shut off the zone as soon as possible.
Caving of the walls with the dri 11 pipe on bottom will present a more serious problem. In this case, the drill column may be so thoroughly buried that it cannot be manipulated freely and is apt to become permanently frozen in the hole. This results usually from improper mudding of the walls or the swabbing effect of the drill column in a tight hole.
If freezing (or cave-in of the rock) has not stopped circulation altogether it might be possible to wash away the caved-in rock by pumping in fluid under high pressure.
The drill column may then be raised cautiously with slow rotation until the bit is above the caved interval. If the caved interval is of considerable thickness, it may be impossible to free the drill column and it must then be parted above the caved interval and the lower part sidetracked in subsequent redrilling. The drill column may be also released through shooting inside it. Hydrogen chloride solution, water and oil are employed to release frozen tools. The fluid is pumped into the freeze zone and a little above. When after a lapse of time the tool has been spudded up and down it may be raised to the surface Freezing of the drill column in the well may bring serious difficulties and should be avoided by all possible means.
When formations containing considerable heavy clay are being drilled, they may be penetrated more rapidly than the material can be hydrated or suspended in the drilling fluid and a mass of clay will accumulate on the drilling bit and about the drill collar. The drill column becomes “logy” and is difficult to manipulate in the well. In trying to lift it, the clay is perhaps further compacted and compressed until it becomes fast in the hole. The remedy for this condition is obviously a slower rate of penetration or greater volume rate of circulation of drilling fluid. In other cases, owing to use of poor quality of drilling fluid in a very permeable, low-pressure formation, thick clay cake may form on the walls of the well, restricting the annular space between the wall of the hole and the drill column.
In drawing out drill pipe from the well, the large-diameter drill collar and bit may scrape off some of this material so that it accumulates above the drill collar and freezes the column in the hole. To avoid this condition, a drilling fluid of better colloidal value should be used, forming a thinner wall sheath.
The most common cause of frozen drill pipe is drawing the drill collar up into a keyseated portion of the hole. Keyseating may result in a soft formation where drill pipe under tension is operated. To avoid this, the keyseated intervals should be reamed.
Another cause of frozen drill pipe is found in the settling of drill cuttings and heavy minerals suspended in drilling fluid when
circulation is interrupted. To avoid this difficulty, a drilling fluid of proper thixotropic properties should be used that will gel and suspend the drill cuttings when circulation fails.
If the column cannot be freed by such methods, resort must be had to fishing procedures.
3 Find in Text 1 the sentences containing modal verbs and translate them into Ukrainian.
4 Make a written translation of the following:
When formations containing considerable heavy clay are being drilled, they may be penetrated more rapidly than the material can be hydrated or suspended in the drilling fluid and a mass of clay will accumulate on the drilling bit and about the drill collar. The drill column becomes “logy” and is difficult to manipulate in the well. In trying to lift it, the clay is perhaps further compacted and compressed until it becomes fast in the hole. The remedy for this condition is obviously a slower rate of penetration or greater volume rate of circulation of drilling fluid. In other cases, owing to use of poor quality of drilling fluid in a very permeable, low-pressure formation, thick clay cake may form on the walls of the well, restricting the annular space between the wall of the hole and the drill column.
5 Speak about causes of frozen drill pipe.
6 Write a summary of Text 1.
7 Learn the meaning of the following words, word-combinations and word groups:
efflux, traverse, confine, resilient, brittle, exposure, after-effect, residual, swell, cohesion, bulk up, jointy, crumble, filter cake, ascend, descent, mounting, backwash, avert, augment, fluctuation, fatigue, overlap.
8 Read Text 2:
Text 2
Drilling is sometimes attended by crumbling and caving-in of the rock and narrowing of the well bores, this manifesting itself in rising pressure in the mud pumps during flushing, in efflux to the day surface of a large quantity of sand and of fairly big rock fragments, and also in an appreciably greater effort needed to raise the pipe string.
One of the causes responsible for this trouble is a changed stress state in the rock traversed by drilling.
In a naturally occurring rock any of its elementary volume is subjected to confining pressure, with side pressure Ps acting on its walls in the horizontal plane. On drilling in the rock the side (lateral) pressure against the borehole walls decreases from Ps down to the pressure exerted by the mud fluid column. This results in a changed stress state in the area in and around the well bore.
If the stresses iri the walls and in the area around the well bore turn out to be greater than the strength of the resilient-brittle rock, this will begin disintegrating and crumbling into the well bore. In case of a plastic-brittle or a highly plastic rock, it will undergo plastic deformation and be squeezed into the well bore as soon as the stresses in the walls and around the well bore area surpass the yield limit. The after-effects of tectonic processes (crumbling of rocks, jointing, residual stresses) reduce the resistance of rocks to disintegration and thus act as a contributing factor to the emergence of crumbling, caving, and also to the narrowing of the well bores.
The diminished strength and, consequently, stability can be due to swelling of some, chiefly argillaceous, rocks following penetration therein of the mud filtrate and weakening of the cohesion forces between partic les of the rock skeleton. The bulking up rate depends upon the mineralogical composition of the rock and chemical composition of the filtrate.
In traversing jointy and crumpled rocks the well bore not infrequently gets narrowed owing to the mud wetted fragments of crumbling and drilled-out rocks sticking to the walls. Within intervals composed of permeable rocks the narrowing is caused by the formation of a thick fi lter cake on the walls, especially in cases of long delayed flushing and utilization of a high filter-loss mud.
Because of crumbling and cavmg-in of the rocks the well bore gets enlarged locally (through formation of caverns); the transport of crumbled and drilled-out rock to the: ground surface becomes rather difficult, since the speed of the ascending current (up-hole velocity) and its lifting force diminish; drill pipe failures, particularly in rotary drilling, become more frequent, for in the cavernous zone the deflection and, consequently, the bending stress are on the increase. In view of the danger of the drill pipes breakdown the drilling weight has to be reduced and this brings on the diminishment of the drilling rate. The time and means spent in flushing the well prior to withdrawing the worn-out bit and in its working and flushing during descent of the new bit go up. Caving-in of the rock and narrowing of the well bore bring forth a mounting pressure in the pumps at the time of flushing due to an increase of hydraulic resistances at the sites of contraction and, occasionally, the backwash of the well becomes infeasible altogether.
In case of crumbling rocks and narrowing of the well bore the magnitude of the force necessary to move the drill pipe string may have to be increased quite impressively. Occasionally,' the movement of the string is made fully impossible, for the arising stresses exceed the strength of the pipes' material.
In many instances a trouble: of this kind can be averted by augmenting the pressure acting against the borehole walls, i.e. practically through increasing the density of the mud fluid.
Therefore, to preclude the occurrence of this kind of troubles one should utilize drilling fluids that do not wet the rock, or, in an extreme case, muds with a very low water loss and of such composition that their filtrate would not produce any appreciable and rapidly diminishing strength and stability of the rocks. It is, for example, expedient to make use of lime-bituminous drilling fluids and also of some inverted emulsions arid suspensions with hydrogels as their base, as the dispersion medium practically does not filter out from them and fails to humidify the rock.
Moreover, an effective measure would be to lower the frequency and amplitude of the hydrodynamic pressure fluctuations. When the fatigue failure of the borehole walls appears to be the principal cause of troubles, it is sometimes advisable to change the casing program of the well and to overlap the danger zone with a casing column.
9 Translate the following sentences into English using the words and expressions from Text 2.
1) Після розкриття породи бічний тиск на стінки стовбура
свердловини зменшується.
2) Наслідки тектонічних процесів знижують опірність породи руйнуванню.
3) Величина і швидкість набухання породи залежать від мінералогічного складу породи і хімічного складу фільтрату.
4) Через небезпеку поломки бурильних груб доводиться зменшувати навантаження на долото, а це призводить до зменшення швидкості буріння.
5) Для запобігання ускладнень потрібно використовувати промивальні рідини тільки з малою водовіддачею і такого складу, щоб їхній фільтрат не викликав помітного зменшення міцності і стійкості порід.
10 Supply a heading for Тext 2.
11 Divide Text 2 into logically complete parts and give each a subtitle.
12 Working in pairs test each other’s ability to describe the principal causes of crumbling and caving-in of rocks and narrowing of the well bore.
Unit 22
Sticking of Drilling and Casing Strings
1 Learn the meaning of the following words, word-combinations and word groups:
hoist, tight pull, tightening, sticking, elaborate, eliminate, exorbital, asunder, aggravate, breakdown, emergence, furrow, deflect, tool joint, balling, filter cake, ascending, abrupt, disrupt, scour, threaded pipe joint, adhesiveness, coarse-dispersion, curtail, sludge trap, lubricating additive, surfactant.
2 Read and translate Text 1:
Text 1
To hoist a pipe string out of the well, it becomes necessary on many occasions to apply a force that is appreciably (10-20 per cent and more) in excess of the weight of the string itself. Such phenomena are known by the name of tight pulls or tightenings.
Sometimes, to start off the string and bring it up to the ground surface an effort closely approaching the ultimate one that the strength of the pipes permits and even surpassing it has to be applied. Such tight pulls are called stickings. A sticking is a [rouble arising out of non-observance of the drilling technology or nf an improper consideration of features specific for the geological structure of the deposit, lithological composition and properties of [he rocks, while elaborating the technology of drilling. At times, in in attempt at eliminating sticking exorbitant forces are applied and the string breaks asunder. In this case the sticking is aggravated by a breakdown. For this reason, stickings are often classified as breakdowns.
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