Unit 14 fishing. Lost circulation.

14.1. Remember the meaning of the following words and word-combinations:

 

fishing – ловильные работы (в скважине)

retrieval – извлечение

hamper – мешать, затруднять, препятствовать

either …. or – или … или; либо … либо

junk – металлические обломки (на забое скважины)

bit nozzle – промывочная насадка бурового долота

rock bit cone – съемная головка бура

twist off – поворот, отклонение

bottom hole assembly – компоновка низа бурильной колонны (КНБК)

logging tool – каротажный прибор

MWD tool – прибор для проведения каротажа во время бурения

assessment – оценка

lost circulation – потеря циркуляции; уход (бурового раствора); поглощение

(бурового раствора)

vuggy formation – пористый пласт, кавернозный пласт

mudcake – глинистая корка

result in – приводить к ч – л.

densely fractured interval – интервал (участок скважины) с большим

количеством трещин

low strength formation – пласт низкой прочности

consequences – последствия

open hole section – часть скважины не закрепленная обсадными трубами

lost circulation material (LCM) – материал для борьбы с поглощением

plug – закупоривать

shale shaker screen – экран вибрационного сита

impair productivity – ухудшать продуктивность

injectivity – приёмистость

squeeze cement slurry – нагнетать цементный раствор под высоким давлением

trouble horizon – проблемный, поврежденный горизонт

karstified limestone – закарстованный известняк

drill ahead – (зд.) продолжать бурить

mudlogging – каротаж

reduce the hydrostatic head – снизить гидростатический напор

exceed – превышать, быть выше

escape to the surface – выходить на поверхность

 

14.2. Read and translate the text using a dictionary:

FISHING. LOST CIRCULATION

Fishing describes the retrieval of a foreign object from the borehole. Fishing operations will be required if the object is expected to hamper further drilling progress either by jamming the string or damaging the drill bit. This ‘junk’ often consists of small non-drillable objects, e.g. bit nozzles, rock bit cones, or broken off parts of equipment. Other common causes for fishing are:

· drill pipe left in the hole (either as a result of twist off, or cementing operations)

· items that have been dropped into the hole which can cause major drilling

problems (e.g. rig floor tools, parts of the drill string).

Bottom hole assemblies and certain types of downhole equipment (e.g. logging tools, MWD tools) cost several US$ 100,000. Some logging tools will have radioactive sources which may need to be recovered or isolated for safety and legal reasons. However, prior to commencing fishing operations, a cost benefit assessment will have to be made to establish that the time and equipment attributable to the fishing job is justified by the value of the fish or the cost of sidetracking the hole. Due to the different nature of ‘junk’ a wide variety of fishing tools are employed.

Lost Circulation

During drilling operations sometimes large volumes of drilling mud are lost into a formation. In this case normal mud circulation is no longer possible and the fluid level inside the borehole will drop, creating a potentially dangerous situation as described below.

The formations in which lost circulation can be a problem are:

· A highly porous, coarse or vuggy formation which does not allow the build-

up of an effective mudcake;

· A ‘ karst structure’' i.e. a limestone formation which has been eroded resulting

in a large scale, open system comparable to a cave;

· A densely fractured interval;

· A low strength formation in which open fractures are initiated by too high

mud pressure in the borehole.

The consequences of lost circulation are dependent on the severity of the losses, i.e. how quickly mud is lost and whether the formation pressures in the open hole section are hydrostatic or above hydrostatic. Mud is expensive and losses are undesirable but they can also lead to a potentially hazardous situation. Moderate losses may be controlled by adding ‘ lost circulation material’ (LCM) to the mud system, such as mica flakes or coconut chippings. The LCM will plug the porous interval by forming a sealing layer around the borehole preventing further mud invasion. However, LCM may also plug elements of the mud circulation system, e.g. bit nozzles and shale shaker screens and may later on impair productivity or injectivity of the intervals. In severe cases the losses can be controlled by squeezing cement slurry into the trouble horizon. This is obviously not a solution if the formation is the reservoir section!

If sudden total losses occur in a hydrostatically pressured interval, e.g. in a karstified limestone, the decision may actually be taken to drill ahead without drilling mud but using large quantities of surface water to cool the bit. The fluid level in the annulus will usually stabilize at a certain depth; this type of operation is also referred to as ‘drilling with a floating mud cap’. Since no cuttings are returned to surface, mudlogging is no longer possible, therefore preventing early reservoir evaluation.

In the event of a sudden loss of mud in an interval containing overpressures the mud column in the annulus will drop, thereby reducing the hydrostatic head acting on the formation to the point where formation pressure exceeds mud pressure. Formation fluids (oil, gas or water) can now enter the borehole and travel upwards. In the process the gas will expand considerably but will maintain its initial pressure. The last line of defence left is the blowout preventer. However, although the BOP will prevent fluid or gas escape to the surface, 'closing in the well’ may lead to potentially disastrous situations.

 

14.3. Give English equivalents of the following expressions. Use the text:

Сдавить бурильную колонну; повредить долото; состоит из маленьких предметов, не поддающихся разбуриванию; из экологических и юридических соображений; прежде чем начать ловильные работы; стоимость ухода в сторону боковым стволом; создавая потенциально опасную ситуацию (3); сравнимой с котловиной; зависит от серьезности потерь; умеренные потери; хлопья слюды или скорлупа орехов; используя большое количество воды для охлаждения долота; уровень флюида в затрубном пространстве; называется ‘бурение с плавающей буровой шапкой’; ранняя оценка коллектора; пластовые флюиды; попадать в скважину и подниматься вверх.

 

14.4. Insert the missing prepositions:

 

1) Fishing is retrieval …….. a foreign object …….. the borehole.

2) The ‘junk’ often consists …….. small non-drillable objects.

3) Common causes ……. fishing are drill pipe left ……. the hole and items that

have been dropped …….. the hole.

4) Due …. different nature … ‘junk’ a wide variety of fishing tools are employed.

5) Sometimes large volumes ….. drilling mud are lost …… a formation during

drilling operations.

6) A ‘karst structure’ may result...... a large scale, open system comparable to a

cave.

7) The consequences of lost circulation depend ……. the severity of losses.

8) Moderate losses may be controlled ……. adding LCM …. the mud system.

9) The fluid level …… the annulus will usually stabilize ……. a certain depth.

10) Since no cuttings are returned …the surface, mudlogging is no longer possible.

11) Although the BOP will prevent fluid or gas escape …… the surface, ‘closing

in the well’ may lead …… potentially disastrous situations.

 

14.5. Complete these sentences with a suitable word or phrase from the text:

 

1) Fishing operations are necessary if the object may _________ further drilling progress.

2) _________ consists of broken off parts of equipment, bit nozzles, or rock bit cones.

3) _____________ like logging tools or MWD tools may cost several US $100,000.

4) Some logging tools should be isolated for safety and legal reasons because they have _______________.

5) During drilling operations large volumes of ___________ may be lost into a formation.

6) A ___________ formation doesn’t allow to build-up an effective mudcake.

7) _____________ is expensive and losses are undesirable.

8) Mica flakes and coconut chippings are examples of _________________ which is added to control moderate losses.

9) LCM may plug elements of _______________ and later impair productivity and injectivity of the objective intervals.

10) In a karstified limestone drilling may go ahead without drilling mud but using

large quantities of _______________ to cool the bit.

11) ________________ (oil, gas or water) may enter the borehole and travel upwards.

12) Although the _________ will not allow fluid or gas to escape to the surface, ‘closing the well’ may lead to disastrous situations.

 

14.6. Answer the following questions on the text:

1) What is fishing? 2) When is fishing required? 3) What does ‘junk’ consist of? 4) Why do bottom hole assemblies and downhole equipment need to be recovered? 5) What is the alternative for fishing? 6) What is ‘lost circulation’? 7) In what formations can lost circulation be a problem? 8) How can moderate losses be controlled? 9) What is the action of LCM? 10) What are the negative consequences of LCM usage? 11) How can losses be controlled in severe cases? 12) What is done if sudden losses occur in a hydrostatically pressured interval? 13) What is the last line of defence?

14.7. Translate the last two paragraphs of the text in writing.

 

14.8. Make up sentences using the following words:

 

a. pipe for are items in the dropped Among fishing common into causes the hole drill and the left hole.

 

б. may by elements adding ‘lost circulation material’circulationModerate be losses which controlled plugs of the mud system.

 

 

14.9. Render the text “Fishing. Lost circulation” in English or in Russian.

 

APPENDIX

Translate the following texts in writing using a dictionary:

Drilling fluids (muds)

The progress and efficiency of drilling depend largely on the use of the right drilling fluid for the rock being drilled. Normal drilling fluids usually consist of colloidal suspensions of clays in water, with chemical additives to control viscosity and other properties. Under many conditions the use of an oil-in-water emulsion, rather than water alone, is advantage­ous. When drilling in low pressure oil-bearing formations, a non-aqueous oil-base mud may be preferable. Air, gas or aerated liquids can also be used in certain circumstances and result in very rapid drilling. In extremely deep wells the mud must remain fluid at temperatures of up to 400 °F (205°C).

When highly permeable formations are penetrated, drill­ing fluid may escape into them and either part or all of the fluid stream may be lost. To combat this, fibrous, flaky or granular 'lost circulation' materials (e.g. asbestos fibres, cellophane flakes or walnut shells) may be added to the drilling fluid. If losses cannot be stopped by this means, a slurry of cement, bentonite and water or diesel oil can be forced into the rock to plug the pores. For very large losses in vugular limestone a slurry of sand in water can be pump­ed down to fill the crevices. Alternatively in certain circum­stances drilling can be continued without any return of the drilling fluid to the surface, allowing it all to disappear into the formation and maintaining a 'floating mudcap' above the loss of circulation zone to prevent the well from blowing out.

 

2. The main functions of the drilling fluid are as follows:

To assist in maintaining maximum drilling rate com­patible with safety. To achieve this the drilling fluid must be of such a 'weight' (density) that it will only just prevent uncontrolled influx of gas, oil or water from the formations encountered. In addition, the solid content and viscosity of the fluid must be kept to a minimum and the 'filtrate loss' to a maximum for the formations drilled.

To remove drill cuttings from the bottom of the hole and the face of the drilling bit.

To carry the drill cuttings out pf the borehole. The ability to do this increases with the velocity, viscosity and density of the ascending fluid. For efficient removal of the cuttings the velocity should not fall below a critical level (usually 120-180 ft/min).

To support and protect the wall of the hole. Caving or collapse of the hole may be prevented by the pressure of the fluid column against the wall of the hole. In addition a protective sheath (or mud cake) is deposited on porous formations by the ascending fluid. This mud cake must be thin and should restrict the entry of water (nitrate loss) from the drilling fluid into the formation.

To keep the drill cuttings in suspension when circulation is stopped, for instance when replacing a worn bit. To do this effectively the drilling fluid should stiffen or gel when at rest and become fluid again when put in motion.

To cool the bit. Considerable heat is generated by a bit drilling under heavy load.

 

Bit and drilling string

The spearhead of the drilling rig is the bit, which cuts its way far beneath the surface through formations sometimes harder than granite. The bit is screwed to the bottom of the drilling string, made up of lengths of special steel pipe (drill pipe), 30 ft long and usually 5 in. diameter but sometimes 4½ in. or 3½ in. or even smaller. Each length or joint is equipped with special steel couplings (tool joints) having' a coarse tapered thread and square shoulder to ensure' lek proof connections that can transmit torque and yet be made up and broken repeatedly, rapidly and safely.

The top joint, or kelly, passes through the rotary table and is used to transmit the torque or twisting moment from the rotary machinery to the drilling string and thus to the bit. This kelly is made square or hexagonal in cross section and sets of rollers are attached to the rotary table by means of a special housing known as the kelly bushing. The sets of rollers within this bushing form a square or hexagonal aperture through which the kelly passes and is driven by the table, thus giving an almost frictionless drive as the kelly is lowered and drilling proceeds.

Heavy, thick-walled tubes (drill collars) are used at the base of the drilling string, just above the bit. As the drilling string is lowered, the bit touches bottom and starts to take load, thus throwing the bottom of the string into compres­sion. Ordinary drill pipe, which is designed only for tension loading, would soon fail under the buckling and fatigue stresses which occur at the bottom of the string.

 

Drill string

In fact, all components of the drilling string must be of the highest quality and finish to withstand the high stresses imposed while drilling. Ordinarily several hundred horse­power may be transmitted mechanically to the bit by means of the drilling string while rotating. The high stresses are due partly to the extraordinary proportions of this string. If a scale model were made of a string 15000 ft long and 5 in. diameter, and a knitting needle of normal cross section (5/64 in. or 2 mm) were used to represent the string, the length of the needle would be 234 ft.

The drilling string with bit attached is rotated at be­tween 75 and 25orev/min, with loads (on an 8f in. dia­meter bit) as high as 80 000 lb bearing down on the bit from the drill collars. The higher loads and slower rotating speeds are used for the harder rocks, and the higher speeds, often with lighter loads, for the softer formations, although recently higher loads have been used on occasions also for the softer rocks.

The ability to apply higher loads on fast-drilling forma­tions depends on the flushing action at the bit, since drilling will slow down rapidly unless the hole bottom is properly scavenged. To achieve the proper scavenging action, the bit is provided with hardened steel nozzles through which the drilling fluid is ejected downwards at a high velocity (usual­ly 300-400 ft/sec), just ahead of the rotating cones of the bit. Some 400 to 600 hydraulic horsepower may be used up in achieving this jetting action on bottom, in addition to the horsepower required to circulate to and from the bit.

 

5. Bit types. Examination of drilling fluid.

 

The type of bit normally used is known as the three-cone rolling cutter bit. Fewer and longer teeth are used on the cones of bits intended for softer formations, while shorter teeth are used where formations are hard, the shorter teeth allowing for a larger and heavier bearing. For the softer formations it is also customary to set the cones with their axes to the left of centre, as shown in

For very hard, abrasive formations, a special bit studded with inserts of tungsten carbide may be used. After about 10 to 12 hours use, the bits must be pulled out of the hole and replaced, because of either bearing or tooth wear.

The presence of oil, gas or salt water in the formation may be revealed by traces in the mud flush returning to the sur­face. There may not be enough to be seen by visual in­spection but certain tests will reveal even small traces of oil or gas. Oil is fluorescent so that when viewed under ultra­violet light even small traces emit a distinct glow. Gas may be detected by blowing air through the fluid and passing it through a 'gas detector' in which any inflammable gas is burnt over an electrically heated platinum filament and is detected by the resultant rise in temperature.