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1.
The drilling fluid, or "mud" performs several functions in the drilling process. First, it serves to clean away the cuttings from the bottom of the hole, and also provides a means for transporting them to the surface. It also lubricates the bit and string and keeps them cool. The mud also controls the formation pressure and the cuttings brought to the surface provide vital information about the formations encountered. Thus a well planned mud program benefits both the drilling contractor and the operating company.
The mud engineer may service several shallow or moderate level wells, but on a deep well project, he may be assigned full time. Normally, he is employed by the mud supply company, and may also be called a drilling fluid specialist. He will test the physical and chemical properties of the fluid, prepare a report that shows the mud weight, and includes the materials, additives and chemicals used, and supervise the mud mixing and the use of the equipment. Much of the routine testing will be done by the drilling engineer, tool pusher, and other drilling personnel, however the mud engineer will work closely with them.
One of the major ingredients of drilling mud is barite, which adds weight to the mixture. Other components may include oil, asbestos, clays, mica, ground-up nut hulls, and cellophane. Provisions for the storage of bulk mud materials are made at the well site so they can be mixed as needed.
2.
A well that is drilled exactly vertically is called a straight hole. However there is almost always some deviation from the vertical. The maximum amount of deviation permissible is specified in the drilling contract. There are several causes for the bit to wander from the vertical, as there are ways of measuring the amount and methods of correction.
If heavy weight is placed on the bit to maintain a constant rate of penetration and a slanting formation is encountered, the bit may deviate. To counter this, the driller can place a stabilizer above the first collar on the string. This acts as a pivot point and when weight on the bit is reduced, the collar becomes a pendulum and the string tends to naturally swing back to the vertical. If the hole is slanted, but within the limits of the contract, a number of different bottom-hole assemblies can be utilized to keep it as straight as possible.
To determine deviation, the hole is periodically surveyed. One such instrument which can be lowered inside the dri11 string utilizes а paper disk which is punched by a device much like a bob and plumb line. The angle of drift can then be determined by how far the punched hole is from the center of the disk. Another device, working on the same principle, utilizes a backlighted disk, followed down by a special camera. The image indicates how far off-center the hole may be.
3.
Secondary recovery is the recovery of oil and gas by any method, such as artificial flowing or pumping, that may be employed through the joint use of two or more wells. Liquids or gases are injected into the common reservoir through one or more injection wells, and the oil and gas are produced through other wells by flowing or pumping.
Water flooding. Water flooding is the most efficient method of secondary recovery if structural and sand conditions are favourable. The secondary source of energy is water under pressure. The water, which is injected into the reservoir under pressure, operates essentially as a flushing agent, pushing the oil ahead of it.
Water flooding operations have been very successful in certain fields. The structure of the area should be gently dipping and without faults. Permeability should be uniform and the reservoir rock continuous.
Experiments have shown that a residual oil content of from 15 to 25 per cent will remain in the reservoir sand after it has been wetted by water, as in a water flooding operation. It is possible therefore to determine by a thorough study of cores the approximate amount of oil which can be recovered by water flooding. An additional recovery as large as that obtained during natural flow is possible if the connate water saturations are high.
4.
After the discovery of an oil and gas field the most important objective is the recovery of the maximum possible amount of oil and gas in the reservoir, the maximum recovery of oil and gas from a reservoir depends upon a number of factors, many of which are of a geological nature.
Recovery Mechanisms. Oil itself has no inherent energy; it is therefore necessary to displace oil from sand by either water or gas. Oil may be displaced from sand by any one or a combination of three mechanisms, as follow:
1. Water drive, in which the oil is displaced by water rising from below. Water drive is generally the most efficient primary or natural oil recovery mechanism.
2. Gas cap drive, in which a free gas cap is present but with no water encroachment. The displacement action of the downward expansion of gas will drive oil out of the sand. A high recovery is possible by this mechanism.
3. Dissolved gas drive, in which there is no water encroachment and no free gas present. The release of pressure will cause gas to come out of solution and expel part of the oil and most of the gas from the reservoir. A large amount of oil is left in the sand. This is the least efficient of the primary or natural recovery mechanisms.
5.
Natural gas is a universal accompaniment of liquid petroleum. The "fixed" hydrocarbon gases (principally methane and ethane) are probably formed as a product of the same reactions that are responsible for the formation of liquid petroleum. Furthermore the liquid hydrocarbons have a high vapour pressure, tending to enclose themselves in an atmosphere of their own vapours. This vapour pressure increases with temperature so that, at temperatures readily attainable within the earth's surface, some of the hydrocarbons constituting petroleum may at times exist only in the vapour phase. Even though subsequent condensation of these vapors should occur large quantities of methane, which is not condensable at ordinary earth temperatures and pressures, will ordinarily be present. Though these hydrocarbon vapors and gases are somewhat soluble in the liquid hydrocarbons, it is evident that the processes involved could easily account for large volumes of free natural gas in close association with deposits of liquid petroleum.
Gas moves with freedom through the interstices of porous rocks. It exerts pressure equally in all directions, and in its effort to flow from high-pressure toward low-pressure areas within the earth, liquid petroleum is carried along with it. The liquid petroleum may be carried as films surrounding gas bubbles, or it may be pushed through the rocks in relatively-large volumes ahead of a body of gas. Gas in solution in petroleum reduces its viscosity and thus indirectly assists other natural forces in bringing about its migration. Solubility of gas in petroleum increases directly as the pressure increases, so that at high pressures very large volumes of gas may thus be held in the liquid phase.
6.
One of the main and most important problems appearing in working out a development project consists in the optimal spacing of the producing wells, this determining in the long run the total number of wells in a field. This choice of well spacing is dictated by the geological features of the field and is based on hydrodynamic and economical calculations.
The choice of the system of well arrangement over the area of an oil field is also of great importance in working out a development project.
At present two systems of producing well spacing are practised: spacing in rows or in regular geometrical grids. Wells are spaced in rows in strata characterized by a good permeability and high oil content, and, as a rule, if measures for maintaining the formational pressure are taken. The rows of the producing wells are drilled parallel to the contour of the oil-bearing section of the stratum. The average spacing of the rows and wells is 400-800 metres.
A regular grid pattern of well spacing (triangular or square) is used on small oil fields, and also in developing fields in strata with a poor permeability of the rock. In the first case the wells are spaced 300-400 metres apart, and in the second case – 150-250 metres.
A regular grid pattern with the wells spaced 150-250 metres apart is characteristic of old oil fields brought into exploitation over 20 years ago.
7.
The choice of the well spacing is considerably affected by a preliminary estimation of the oil reserves and sources of formational energy in the oil field.
If an oil field is expected to be operated under a pressure drive, especially a water drive, the wells are widely spaced, since such a drive ensures the flow of the main mass of oil to the wells from the remotest sections of the stratum. If the stratum energy is limited and operation with a dissolved gas or a gravity drive is anticipated, the oil wells are spaced closer to one another so as to ensure more complete recovery of the oil.
Thus, if oil reservoirs are operated with water or gas drive, not only do the rates of oil recovery increase and the recovery factor improve, but also development of the oil field requires a smaller capital outlay.
Owing to the obvious advantages of water and gas drives, most oil field development projects provide for the artificial creation of such drives by pumping water or gas into the oil-bearing stratum, to maintain the required formational pressure and displace the oil to the bottom holes of producing wells. Wells sunk in oil fields where methods of artificial maintenance of the formational pressure are to be practised can be spaced farther apart.
8.
By the term oil field development the whole complex of work connected with the drilling of the oil field and the withdrawal of oil to the surface is meant.
Oil fields are developed in accordance with special projects which take into consideration the natural conditions of the field and the latest achievements in science and engineering. The development project is based on the data obtained during exploratory drilling and trial exploitation of the first oil wells, and also on the experience gained in operating oil fields characterized by similar natural conditions.
One of the main and most important problems appearing in working out a development project consists in the optimal spacing of the producing wells, this determining in the long run the total number of wells in a field. This choice of well spacing is dictated by the geological features of the field and is based on hydrodynamic and economical calculations.
Theoretical considerations and the experience gained in developing oil fields show that with a close spacing of producing wells, their interaction becomes more pronounced, and the average rate of production drops. On the contrary, with an excessively large spacing of wells in an oil field there may remain undeveloped sections, current oil recovery will be low owing to the insufficient number of wells sunk, and the duration of oil field development will increase. The choice of the well spacing is considerably affected by a preliminary estimation of the oil reserves and sources of formational energy in the oil field.
9.
The stratum fluid (oil, water, gas) will move from the stratum to the well bottom hole only if the formational pressure is greater than that at the bottom hole. The same condition, i.e., the presence of a pressure drop or depression, is necessary for the fluid to flow from one section of the stratum to another.
Unlike the flow of a fluid along free conduits or tubes, in a porous formation fluid does not flow in a continuous stream, but in separate fine streams which repeatedly change their direction, filtering through the passages formed by the particles of stratum rock. The process of fluid flow through porous rock is therefore usually called filtration.
The inflow of fluid to the bottom hole of a well and, consequently, its yield depends on many factors: on the depression – the difference between the formational and bottom hole pressures, on the permeability of the bottom hole zone, the thickness of the oil-bearing stratum, the oil viscosity, etc.
The dependence of the rate of production on the depression of the bottom hole is the most obvious one. Within certain limits it is close to linear, i.e., every increase in the depression is accompanied by a similar increase in the production. The linear relation becomes violated, however, at high rates of production owing to the change in the nature of fluid filtration in the zone close to the bottom hole.
The difference between the bottom hole and formational pressure is distributed around the wells of a stratum according to a definite law.
10.
Upon completion of drilling, or upon drilling through a certain amount of rock, a casing string made up of high-strength threaded steel tubes is lowered into the well.
The string is fastened in the well by pouring cement mortar into the annular space between the walls of the well and the tubes. The casing string and the cement ring formed around it protect the walls of the well from collapsing, prevent the flow of water or oil from one stratum to another, and make it possible to produce oil from a given stratum for a long time.
Depending on the geological conditions, the available equipment and the drilling technology, wells can be reinforced with one or several casing strings, making cement rings of different thickness.
By the construction of a well is meant the set of data characterizing the diameter of the well at different depths; the number, diameter and length of the casing strings lowered into the well, and also the dimensions of the casing clearance filled with cement.
The simplest and cheapest is a single-string well, but it is not always possible to construct such wells for various reasons. Wells are more often lined with two, and sometimes three casing strings.
Drilling of a well is started by making a cellar up to three metres deep. It protects the well head from destruction by the stream of drilling liquid in drilling. The cellar can be made of a large-diameter steel tube, or sometimes in the form of a well fastened with the aid of a wooden frame, quarrystone or cement mortar.
11.
By flowing or gushing of an oil well the process of oil motion from the bottom hole to the head of the well under the action of the formational energy is meant.
Natural flowing of an oil well is possible only if its bottom hole pressure is greater than the hydrostatic pressure exerted on the bottom hole by the weight of the column of gas-oil mixture rising to the well head. The gas which evolves from the oil when it approaches the head of the well is exceedingly important for flowing. The numerous bubbles of gas dispersed in the oil not only reduce its specific gravity, but also actively participate in lifting the oil, since they entrain particles of the surrounding liquid when they float to the surface.
The energy of the expanding gas is best used by fitting flowing wells with small diameter tubings which are usually lowered almost to the bottom of the well, to the perforated interval. The gas disperses uniformly over the entire cross section of the tubing as it rises and also carries along the oil near the walls of the tubes. In tubings with a great diameter partial separation of the oil and gas occurs: the oil is more saturated with gas bubbles at the central section of the tube and it moves with a greater velocity, while the oil is much less mobile at the walls of the tube, it becomes gradually degassed, and, mixing with fresh oil, increases its specific gravity, thus creating additional back pressure г on the stratum. This is why flowing wells are no longer operated by withdrawing oil directly through the casing string.
12.
A well is a cylindrical hole sunk in rock with a diameter much smaller than its length.
The beginning of a well is called its head, and the end – its bottom hole.
At present all oil and gas wells are sunk by rotary drilling consisting in the rock being crushed at the bottom hole during the continuous rotation of a special tool – a bit.
The bit is lowered into the borehole at the end of a drill column, whose tubes are used for pumping in drilling liquid or mud which carries the broken rock from the bottom hole to the surface, via the annular space formed between the drill column and the walls of the well. The mud filling the borehole prevents gas and oil blowout from the already penetrated gas- and oil-bearing strata, and protects the borehole from destruction and collapsing. The thin crust of clay forming on the walls of the well prevents the penetration of fluid from the well into the stratum and vice versa.
Water is used as a drilling fluid in some regions where the wells penetrate and run through consolidated rock, while drilling mud is used in loose rock or when penetrating into an oil-bearing formation.
The bit can be rotated by means of mechanisms arranged at the surface, or of special motors installed above the bit directly at the well bottom hole.
13.
In order to evaluate the potential of the reservoir, the petroleum geologist must have the following data: 1) the capacity of the rock to contain fluid, 2) the relative amount of fluid present, and 3) the ability of the fluid to flow through the rock to the well. This last is determined by two factors, porosity and permeability.
Porosity is the capacity of the rock to hold fluids. Or, it is the volume of the non-solid or fluid portion of the reservoir divided by the total volume. Thus porosity is always expressed in percentages. To visualize the concept of porosity, imagine a box full of balls of equal size stacked on the top of each other so that only the most outward points of each ball touch the ones above, below, and to the sides. The spaces in between the balls would be the pore spaces and would represent a porosity of 47,6%, the highest that can be expected.
If the same balls were arranged into layers so that the upper layers nested into the ones below, the porosity would be reduced to 25,9%. The size of the balls in either case would make no difference as long as they were all the same size. Since in reservoirs the rocks are never all the same size, nor stacked in neat columns, actual porosity may range from 3% to 40% (very rare) with a usual porosity in the area of 20%.
Porosity as high as 20% usually occurs only in the “younger” layers near the surface, as porosity tends to decrease in the deeper and older layers. This decrease is caused by the weight of the succeeding layers, the effect of time on the rock, and by particles becoming cemented together. This pattern of depth affecting porosity is apparent in shale as well as sandstone, although porosity is generally lower in shale since it is more compacted, and old shales at great depth have been compressed much more than sandstone at a similar level.
14.
The permeability of a reservoir is that factor which determines how hard, or easy, it is for a fluid to flow through the formation. It is not enough for the geologist to know that oil is present, he must also be able to determine how easy it will be for the oil to flow from the reservoir into the well. This will be based on several factors: the property of the fluid itself, expressed as viscosity (thickness: a thin liquid can be pushed through rock more readily than a thick one), the size and shape of the formation, the pressure and the flow (the greater the pressure on the fluid, the greater the flow).
Permeability is usually expressed in units called darcies, after Henry d’Arcy, the French engineer who in 1856 found a way to measure the relative permeability of porous rock. In most reservoirs, the average permeability is less than one darcy, so the usual figures are in thousandths of a darcy or millidarcies (md). Permeability for a fine grained sand may be 5 md. or a coarse sand that is highly porous and well-sorted may run to 475 md. However, if the coarse sand happens to be poorly sorted it may run only to 10 md.
Another factor which must be taken into consideration is that at great depth the weight of the overlaying layers may compact the sand grains closer together. Not only do smaller pores and lower porosity result, there is also a tremendous decrease in permeability. Cementation, which tends to fill the pore space also increases with depth. A reservoir that may be a good producer at one depth then, may be of no economic value at all at a lower depth if the petroleum cannon flow through the rock to the well.
15.
To continue exploration for new sources of oil and to extend knowledge of known oil-bearing formations is essential to ensure future supplies to meet increasing demands. It is a long-term operation; from five to ten or even twenty years may elapse between initial examination and commercial production. This time is spent in surface and sub-surface exploration, wildcat drilling, proving the extent of discovered accumulations, and the building of pipelines and other facilities for the movement of the crude.
Exploration is an extremely costly and uncertain business and costs are rising as remoter areas and under-sea formations come to be investigated. Modern techniques are improving in speed and accuracy but there is still no certain way of predicting oil in a new locality. The petroleum geologist can do no more than indicate the likely existence of a structure that may bear oil; drilling is necessary to prove it.
Actual drilling costs vary widely between the extremes of a deep exploratory well in a remote area and a development well in a known formation. Underwater exploration may be less expensive than land surveys owing to easier movement but drilling and development under water may cost three or four time more than on land.
As a result of continual exploration more oil has been discovered world-wide, year after year, than has been taken out of the ground.
СПИСОК ИСПОЛЬЗОВАННЫХ ИСТОЧНИКОВ:
1. www.spe.org
2. http: //www.spe.org./spe/jsp
3. www.world_oil.com
4. www.oilonline.com/news/archives
5. Баракова М.Я., Журавлева Р.И. Английский язык для горных инженеров. М.: Высшая школа, 2002. – 288 с.
CONTENTS:
Unit 1 Why do we need oil and gas?________________________________ 3
Unit 2 Oil and gas reserves_______________________________________ 6
Unit 3 How does the industry find oil and gas ________________________10
Unit 4 What is oil? _____________________________________________ 13
Unit 5 Origin, migration and accumulation of oil _____________________ 17
Unit 6 Geological features. _______________________________________21
Unit 7 Oil traps. ________________________________________________25
Unit 8 What is natural gas? _______________________________________28
Unit 9 The formation of natural gas. ________________________________32
Unit 10 What is an oil and gas reservoir? ____________________________36
Unit 11 Exploration methods and techniques. ________________________ 40
Unit 12 Drilling the well. ________________________________________ 43
Unit 13 How are oil and gas produced? _____________________________ 46
Appendix
Part I ________________________________________________________51
Part II _______________________________________________________ 59
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