Oil and gas wells represent the capital expensive constructions serving many decades. It is reached by the connection of a productive stratum with daylight by the tight, strong and durable channel. However, the drilled borehole does not represent such a channel, owing to the instability of rocks, existence of the strata sated with various fluids (water, oil, gas and their mixes) which are under various pressures. Therefore, at construction of a well it is necessary to fix its borehole and to separate to (isolate) the strata containing various fluids.
In recent years the new technologies based on directional drilling, made a real revolution in practice and the oil production theory. Outputs of the wells having the directional boreholes of a big extent considerably increased. Grids of wells were as a result discharged, decreased, depressions, the time of waterless operation considerably increased, the categories of stocks which were considered earlier not taken, which now changed can be taken effectively commercially, efficiency of many old methods of impact on a stratum raised at their realization by means of horizontal wells.
Vertical, together with inclined wells, getting to a productive stratum, often open also plantar water. While a horizontal well is directed in the productive horizon there can be plantar water.
At modes with motionless contours the uniform grid of placement of wells on the area is accepted. After a choice of the scheme of placement of wells on the area possible options of development of this deposit are defined.
Drag calculations for definition of the current outputs of wells are made for each option of placement of wells on the area in time, the current and total selection of oil from a deposit, term of development of a deposit, etc. The resources of natural formation energy are thus considered, and completion of this energy from the outside is provided in case of need.
It is necessary to notice that in case of a deposit drilling-out by the directional, and even the horizontal wells the system of development of a field work better and its operation becomes cheaper and simpler.
It is known that oils in rock are filtered tens of years on hundreds feet through the minute pores of a stratum from the periphery to the boreholes of wells, meeting often natural barriers on the way. These barriers or natural, lithological or tectonic screens, or stagnant zones with low gradients of pressure in a filtration field, or the “tongues” of water broken and cutting an oil field, etc., also are the main reasons for losses of huge stocks of oil in strata. In subsoil there are billions of tons of “residual” oil.
Unfortunately, many fields with large reserves of hydrocarbons, but with low collector properties or with heavy oil where small outputs do not justify the costs of drilling, are not developed.
Owing to the huge competition in the oil and gas market now it is required to reduce sharply the prime cost of the extracted hydrocarbons by radical improvement of drainage abilities of collectors. Such a requirement is satisfied by the directional drilling. The main direction of application of directional drilling in the country should have a revival of old oil fields and extraction of the remained stocks of oil from them (which make to 60-80 % from initial stocks). In the process of expansion of works, development of equipment and technology, experience acquisition, this technology will be gradually transferred to the other objects (deposits with oil, high-viscosity oil, dense collectors, etc.).
“Over the past thirty years, moreover, oil and natural gas have accounted for about two-thirds of all the increase in the world’s consumption of fuel. They have not been the only dynamic elements in the rapid growth of the use of commercial energy during that period: the exponential curve of electricity consumption, which has continued over many years with hardly even a ripple to acknowledge the occurrence of slump or war, shows an even more dramatic rise. But electricity is a ‘secondary fuel’, one of the convenient ways that man has devised to make use of energy, not a source of energy in itself; it has to be made from some ‘primary fuel’ such as coal, oil, falling water or fissile uranium. The surge in consumption of the two petroleum primary fuels has probably also resulted partly from the fact that these are convenient to use. The specialized secondary fuels that you can refine from crude oil are convenient for many purposes and practically indispensable for some, and natural gas is a primary fuel that starts out with all the convenience that coal can only achieve rather expensively through the processes of traditional gas manufacture. But our growing dependence on petroleum has not come wholly from the fact that consumers prefer convenience. In the first decade after the Second World War, in particular, oil and natural gas supplied the lion’s share of the extra energy the world required simply because they were there to be had in abundance, which coal emphatically was not. Vast new sources of petroleum were steadily developed, and the pipelines, tankers and refineries were brought into being to deliver them to the consumer, at a time when a comparable investment of capital in some of the coal’s industries was having practically no visible effect at all on a depressingly inflexible level of coal output. Sheer availability, when the coal that people would ordinarily have used was not there, opened many markets to oil and to natural gas” (Hartshorn: 1962).
The advantages of the directional drilling are conclusive. Such drilling, without breaking a covering, passes all land and underground obstacles: regions of a dense housing estate, highways, railroad tracks, rivers, dams and channels.
A solution of the problem of high-quality directional drilling of wells
- to investigate the hydrodynamics of a stratum of oil and gas pools of various types for the purpose of creation of the optimal systems of oil and gas development;
- to investigate a tension of the rocks opened by these wells and the mechanics of a borehole’s formation by directional tools of various types;
- to develop the system of optimal control of a trajectory of deep wells for various geological conditions and ways of drilling;
- to develop the effective technology of drilling, opening of layers and, especially, to pay attention to the development of a special drilling mud and a cementing program and hydrodynamic features of their work in these conditions;
- to develop effective means (rejecting, focusing, stabilizing and measuring) for directional drilling.
Besides, it is necessary to develop equipment and technology of drilling of horizontal wells for oil and gas pipelines under the rivers and other obstacles. It is also a problem question of horizontal drilling.
“Directional drilling, also called horizontal, deviated, or slant drilling, allows for hydrocarbon deposits that are not directly under a well pad to be accessed. Using this technology, it is possible to concentrate wells on a more limited number of well pads yet still reach the oil and gas, which reduces the environmental impacts of drilling. The technology and practicality of directional drilling is improving and at this point hydrocarbon deposits several thousand feet, and even more, from a well pad can be reached. On the Pinedale Anticline natural gas field in western Wyoming, directional drilling will allow for thirty-two wells to be drilled from a single, consolidated well pad” (Pendery: 2010).
The ecological component is also very important: untouched plantings and a land relief, the kept fertile layer of earth. When using the horizontally directed drilling the efficiency of works increases considerably: as a rule, one drilling rig and a brigade of workers out of 3-4 people are involved. All this gives a huge economy of financial means, approximately by 30 %.
Applications, terms and definitions
The beginning of a well is called a hole mouth; a lateral cylindrical surface – a wall or a borehole, a bottom – a bottomhole. The distance from the well mouth to a bottomhole on an axis of a hole determines the length of a well and by a projection of an axis 4 to a vertical – its depth.
The wells deepen, destroying the rocks on all area of a bottomhole (a continuous bottomhole) or by its peripheral part (a ring bottomhole).
A well’s diameter, as a rule, decreases from the mouth to a bottomhole step by step on certain intervals. The initial diameter of oil and gas wells usually does not exceed 900 mm, and seldom there are less than 165 mm. The depths of oil and gas wells change within several thousand feet.
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Oil and gas wells are drilled in land and by sea by means of drilling rigs. In the latter case drilling rigs are mounted on platforms, floating drilling platforms or vessels.
The wells of the purpose in il and gas branch
- Operational – for oil, gas and gas condensate production.
- Injection – for pumping of water (gas or air) into the productive horizons for the purpose of maintenance of the formation pressure and extension of the gushing period of the development of fields, increases in an output of the operational wells supplied with pumps and air lifts.
- Exploratory – for identification of the productive horizons, delineation, test and an assessment of their industrial value.
- Special – basic, parametrical, estimated, control – for studying of a geological structure of the little-known area, definition of the change of collector properties of the productive strata, supervision over a formation pressure and the front of the movement of a water-oil contact, extent of the development of separate sites of a stratum, thermal impact on a layer, ensuring of fire flooding, oil gasification, dumping of sewage in deep-laying absorbing strata, etc.
- Stratigraphic test – for the specification of position of perspective oil-and-gas structures on the top marking (defining) horizons repeating their outline, according to the drilling of small, less expensive wells of a small diameter.
Taluses, collapses etc. are possible in the course of a well drilling in unstable rocks. The design of a well is understood as a data set about the number and the sizes (diameter and length) of a casing pipe, diameters of a borehole under each string, cementation intervals, and also the ways and intervals of a well’s connection with a productive stratum. A flow string goes down to a well for oil and gas extraction or infusion of water or gas in the productive horizon for the purpose of maintenance of a formation pressure.
Hole deviation is called a deviation from a vertical or other set direction in the course of drilling owing to the natural or artificially created reasons. The natural reasons of a hole deviation are heterogeneity on hardness of the rocks destroyed by a chisel and an inclined anisotropic attitudes of bed. The artificially created reasons include the application of special “directional” configurations of a bottom of a drill string and the corresponding modes of drilling.
A spatial provision of any well can be defined by its three parameters: depth of S, inclination and azimuth angles. An inclination angle in B point in a trajectory of a well is called the angle between a tangent in this point and a vertical. The angle between a tangent projection to a borehole axis in this point on the horizontal plane and the direction to the North is called an azimuth corner in this point.
Magnetic and geographical azimuths
The latter is sometimes called a directional angle. A magnetic azimuth in this point of a borehole axis is called an angle between a tangent projection in this point on the horizontal plane and the direction to the magnetic North Pole of Earth counted clockwise, geographical – when the angle is counted from the direction to the geographical North Pole.
Because of the discrepancies of magnetic and geographical poles, in each point of a terrestrial surface magnetic and geographical meridians form among themselves an angle called magnetic declination. The northern end of a magnetic needle can deviate from a geographical meridian, both to the East, and to the West. Depending on it the eastern – positive and western – negative declinations are distinguished. In various points of Earth magnetic inducement has various sizes. The depth of S of a well is a distance on a well from a hole mouth to a borehole or any point in which measurement of an antiaircraft corner and a well azimuth is made.
The vertical and horizontal plane
It is possible to represent the position of a well, having designed its axis on the vertical and horizontal plane. The length of a borehole is usually measured in the process of the chisel tool and at the control measurements which are fulfilled by geophysical parties in the process of deepening of a well. Two concepts can be meant by a hole axis:
- real axis is a geometrical place of points of the center of the destroyed bottomhole moving at deepening of a well. It represents a flat or spatial curve with the local excesses, displaying actual form of a borehole;
- approximate axis fixed by measuring (inclinometric) equipment. It displays the general borehole deviation and carries the name of a well’s route and represents the maleficiated in comparison with valid axis flat or spatial line consisting of the pieces of straight lines and simple arches, having the general tangents and adjoining to each other. The term “axis” or “route” includes an approximate axis.
The vertical plane, tangent to an axis of an inclined well in this point of an axis, is called the apsidal plane of a well. The change of only zenith angle in the course of drilling of a well cause its zenith deviation, it occurs only in one – vertical plane, and through an axis of such a well it is possible to make only one apsidal plane. The change of only azimuth angles causes an azimuth deviation of wells. Practically, at a hole deviation there is a joint change of zenith and azimuth angles which cause a spatial hole deviation. The position of its curvilinear axis in space is called a route of a well which is also called a profile. All wells drilled on a plane or spatial curvilinear axis are curved. Thus, it is necessary to understand the change of the direction of an axis of a well in space in relation to its initial position. Sometimes the term “curve of borehole” is understood as a divergence between the actual and design routes of a well. Between the valid position (axis) of a well and its project profile should be called a hole deviation from the project route.
Hole deviation is submitted to certain dependences. Many of them have a rather stable, constantly repeating character and can be considered as natural laws; the others have casual, not constant character, their repeatability and sizes are not stable; they are established only with a low degree of probability and are irregular.
Generally, any inadvertent hole deviation happening because of the inconstancy of physic-mechanical, structural and other properties of rocks (geological factors) and the technological modes of their drillings (technology factors) operating separately in common, can be called a natural hole deviation. The change of hole axis in space with application of special means (deflector), or as a result of earlier set change of technological modes of drilling and the structure of configurations of a bottom of a drilling string, etc. is called an artificial hole deviation. The change of a route of a well is characterized by its deviation which quantitatively is determined by the change of its zenith and azimuth angles. The degree of a hole deviation is characterized by the intensity of deviation , which represents an increment of the sizes of zenith or azimuth angles per unit of length of a certain interval of S of a borehole.
In practice it is accepted: for intervals where there is an intensive change of a zenith angle, the intensity is expressed in degrees per 10 feet (usually from 0.5 degrees per 10 feet and higher) and for the intervals with a low-intensive change of a zenith corner – in degrees per 100 feet (less than 0.5 degrees per 10 feet).
The deviation from a well across and down at the directed drilling (a change of an antiaircraft corner and a drilling azimuth) is carried out by the directional tools. The large number of various directional tools having essential differences in their schematic kinematic diagrams, appointment and scopes, is developed.
Orientator is a device for fixation of the position of a deflector on the surface of Earth in a borehole. Deflectors are subdivided into immersed, a part of a drilling tool, and taken, lowered in a boring column only in the orientation of the deflector, the orientator can serve as a sensitive element liquid level, a plumb, a ball, a pendulum, a potentiometer, etc.
It is the revolutionary development of the DCI Company.
The working frequency set by Eclipse is optimal for drilling, and it absolutely excludes hindrances. The unique configuration of the aerial of the receiver allows the device to define the location of the points of positioning and a radiator. The radiator provides three-dimensional remote control to the left – to the right and up – down.
- Radius of action of telemetry is 610 m.
- Accuracy – absolute error 5 %
- The power supply – nickel – cadmic battery Digi Trak
- Battery capacity – 8-12 hours
- Depth indicator in real time
- Range of working temperatures – from – 20’С to 60’С
- Height – 31 mm
- Width – 18.3 mm
The purpose of the directional drilling is a direct hit of a final borehole in previously set point of a productive layer. As a rule, this point is a set on a roof of a productive stratum and is the center of the admission circle. At a borehole hit in this circle the detailed design is considered executed. For various mining-and-geological conditions, purpose of a well, its depth (down), the size of a radius of the circle’s admission fluctuates within the limits 15-60 feet. The project is considered to be executed for horizontal wells if the horizontal part of a borehole did not fall outside the limits design values of the corridor limited to two vertical and two horizontal planes. Sometimes a directional drilling is made for the purpose of crossing of a borehole by emergency, flowing with well oil or gas for its muffling.
The reasons for directional drilling
- decrease in the expenses for the development of a field when drilling of obliquely directed wells from cluster platforms (cluster drilling);
- opening of a productive stratum under a certain degree for increase in the area of a filtration;
- making of several wells from platforms, located in the sea or on the lakes;
- hole making of wells to the productive strata located under the grounds with strongly crossed relief (ravines, hills, mountains);
- opening of productive layers under a bottom of oceans, seas, lakes, rivers and bogs;
- leaving aside from an emergency or unproductive well by means of pre-drilling of a lateral borehole;
- opening of the productive strata which lay under the bottom of oceans, seas, rivers and under the flat dumping or between two parallel dumpings;
- a borehole deviation from a waste zone (a gap zone) in the direction of the productive horizon;
- opening of the productive strata under the hydrochloric domes in connection with the complexity of drilling through them.
The directed drilling can be fulfilled without the application of special artificial deflectors – only on the basis of the accounting of the regularities of a curvature known for this region, but more often it is made with the use of such devices.
The main advantages of the directional drilling are: the possibility of determination of a true capacity of the inclined layers, the decontaminations of wells. However, there are the following shortcomings: the increased probability of emergency because of the use of wedges, requirements to durability, a need of frequent use of a deflector and measuring devices.
Well profiles coordinate system
In the recent past, people should describe the location by different methods. Now in a oil and gas industry, as well as in other areas of human activity, transition from the relative indication of location to the absolute one was fulfilled. It was caused by the complication of the questions connected with exact definition of a site of objects. Therefore, now it is required to know much more about the systems of coordinates and ways of definition of an exact site.
The maps and the schemes applied in directed drilling are flat. The mapping of the lines lying on and under a surface of the sphere on the flat map is impossible without compromises and introduction of the controllable mistakes.
In such sciences as geodesy and cartography it was necessary to do a great job for people who are engaged in directed drilling to have a clear way of mapping of the coordinate data on maps.
For identification of location of a point on the earth, its surface is mentally covered with a network of lines. Usually they are called meridians and parallels. For the data of the North and South poles which approximately are the axis ends concerning which the earth and some imagined line lying on the middle between poles rotates, the parallels of width are formed by the circles surrounding the globe and the planes of which are parallel. If the circles are drawn on a sphere surface through equal intervals, dividing into 90 parts distance between the equator and each pole, each such interval is called a degree of latitude. The circles are numbered from 0 on the Equator to 90 in the North. Each degree consists of 60 minutes and every minute consists of 60 seconds of an arch of a circle.
The longitude meridians are formed by a series of the imagined lines, each of which is crossed with each other both on the North and South poles. All of them are crossed with the width lines under the right angles and divide the Equator into 360 equal parts. It also leads to the division of a longitude into 360 degrees. In its turn, each degree is divided into 60 minutes, and every minute – into 60 seconds. While the length of a latitude degree on the sphere is everywhere identical, the length of a longitude degree changes depending on the width. On the sphere Equator the distance of one degree of a longitude equally to the distance of one degree of latitude, but in the other parts they are shorter.
A local system of coordinates
In most cases, a local system of coordinates in daily work is applied in the directional drilling. This local system has a direct dependence on everything that was discussed above in this chapter. At the definition of a local coordinate system there are many not obvious, but important assumptions. It is necessary to define carefully the local coordinate system in order to keep the direct and meant relations with the “lawful” system.
The local coordinate system should have the beginning in a point which should be defined in “lawful” coordinate system. This point should serve as the Structural reference point in case of only one element (a platform) or the Reference point of a field (area) if the local coordinate system is applied to the whole field. The term “Reference point” is applied to any of these two cases. The reference point is defined in “lawful” system of coordinates and in new local system of coordinates is located in (0.0). This reference point has only the North and the East coordinates. In addition to it, it is necessary to determine the basic sizes down for determination of the true vertical depth (TVD) and a borehole depth (BD). If necessary, it is possible to define the basic size of a vertical both for BD, and TVD.
If the return is not additionally discussed, the axes of a local coordinate system are oriented parallel to the axes of the “lawful” system of coordinates in which the reference point is defined. Obviously, the unit of length should also be defined. It is usually dictated by the wishes of the customer or governmental decrees.
Coordinate grid of the ‘lawful system’
By definition, the coordinate grid of a local coordinate system should use the North of a coordinate grid of the “lawful system” in order to make it possible to put it correctly. Only in this case angles and distances can be measured correctly. If to use the True North or the Magnetic North when drawing these measurements, the interrelation between the points and lines will not be linear and, consequently, they cannot be measured directly by roulette and a compass. Very often this mistake (deviation) is small, but sometimes it happens to be rather considerable. In many cases governmental decrees dictate the need to use the North grid.
It often happens necessary to transform the coordinate of location of a point from one “local system” to another. A good example is the project of several wells drilled from one platform. In this case the platform can serve as a “local” system of coordinates; it, in its turn, is located in some “lawful local” system in which its site is defined.
The magnetic inclination
The amendment of a magnetic inclination (inducement) is the angle between the magnetic North and the true North. The sizes of a magnetic inclination change depending on the time and location.
The designer can plan the well at his discretion either in the platform system, or in “lawful local system”. Transition from one system of coordinates to another is fulfilled by the parallel transfer in the North direction, the East (+,-) and the turn of th3 axes round the beginning of coordinates. If the movement of the magnetic North is constant and predictable, the magnetic inclination can be calculated in any point and during any time.
At present, the charts of magnetic inclinations and speeds of their change (usually expressed as annually changing) are widely used. The inducement in east direction is expressed by the positive sizes, and in the western direction – negative. In spite of the fact that transition from one system of counting to another seems an easy problem, it is necessary to be very attentive to consider the relative directions of convergence and magnetic inducement
Well profiles coordinated system is represented by the inclinometric researches – the measurements of a zenith angle and a well azimuth as its depth function. Degree is a unit of measure.
Inclinometric researches are fulfilled when lifting the drilling device in vertical wells over 300 feet depth and in inclined wells over 100 feet depth for the salvation of the following tasks:
- control of the set direction of an axis of a borehole in space projected in the course of drilling;
- allocation of the sites of bends of a borehole’s axis which can cause complications when drilling;
- reception of the basic data for geological constructions, including definitions of true depths of the productive strata, for interpretation of data of magnetic logging and dip feet survey.
Researches are fulfilled by magnetic (dot and continuous) in open borehole and gyroscopic inclinometers in open and cased wells.
The requirements for inclinometers for investigation of the open wells
- a range of azimuth measurement – from 0 to 360°;
- borders of the ranges of measurement of an antiaircraft angle – from 0 to 45, 90, 135, 180 °;
- a range of the measurement of an apsidal angle – from 0 to 360°;
- an allowed main error of the measurement of an azimuth for zenith angles more than 3 ° – no more than ± 2 °;
- an allowed main error of measurement of zenith angle – no more than ± 0,5°;
- the additional error caused by the change of supply voltage – no more than 0.2 values of the main error;
- the additional error caused by the change of ambient temperature, should not exceed 0.1 values of the main error per each 10°C of rather standard value of the temperature equal to 20°C.
The step of measurements to an open borehole should be equal to 25 feet in vertical wells with zenith angle to 5°; 10 feet – in wells with angles higher 5 °; 5 feet – in the wells with the intensity of deviation to 0,5 °/foot; 2 feet – on the sites with intensity of a deviation 0.5 °/foot and more.
The processing and registration of the results of measurements are various for dot and continuous magnetic and gyroscopic inclinometers. The algorithms of processing are defined by the software. The regulated documents are:
- a summary table of the results of inclinometric measurements (the value of zenith and azimuth angles) with the set step on depth. For the points with repeated measurements the average values from the results of all measurements;
- coordinates X, Y and Z of a borehole’s axis in the coordinate system with the beginning in the center of a rotor and the axes parallel to the axes of a geodetic network, the plan and a profile of a well. The positive directions of coordinate axes accept the following: X axis – northern; Y axis – eastern; Z axis – down. The points of coordinates are calculated by the directional angles for what into the measured magnetic azimuths enter amendments on magnetic inducement and rapprochement of meridians.
Materials of well’s owner
The materials for a well’s owner should contain: the summary table of the results of inclinometric measurements, and for the directional wells – an additional plan and a well profile.
The well’s plan includes: the direction of coordinate axes; scale; position of a hole mouth; project and actual position of a bottomhole; bottomhole displacement; a directional angle; the distance between actual and project positions of the bottomhole in the plan.
A well’s profile may also contain: the direction of Z coordinate axis; scale; a directional angle or an azimuth of a vertical plane on which the well axis is projected.
Magnetometers measure a resultant vector of a magnetic field of the Earth and a drilling string. As it acts like a dipole and power lines of a magnetic field are directed along a string, on an axis Z the increased intensity is created. The size of this mistake depends on the magnetization a component of a drilling string and their remoteness from the measuring equipment. Usually the mistake in measurements of coordinates appears in the process of increase of a total intensity of a magnetic field (total intensity of a magnetic field appears to be higher than intensity of the Earth fields). It is caused by the fact that of the indications of the magnetometer appear overestimated along an axis Z. At the same time, the size of a total intensity of a magnetic field should remain the same if neither orientation, nor depth changes.
When magnetization of a drilling string is an error of the measurements of Z-components by the magnetometer, a component of a magnetic field of the Earth is defined wrong. The horizontal mistake component on an axis Z is equal to the mistake Z component increased by a sine of the angle of a borehole inclination. That is why it shows that, with the increase in a tilt angle of a borehole the accuracy of measurements worsens (especially in case of magnetization of a string). As the horizontal component of a magnetic field of the Earth on Alaska is less, the mistake connected with magnetization of a string appears more than on lower latitudes.
Thus, the mistake in 50 gamma has a bigger effect at smaller value of the horizontal components (0.53 % on Alaska in comparison with 0.20 % in the area of Golf Mexico). The overestimated Z-values components because of magnetization of a string usually are at the reasons of erroneous calculations of the azimuth the values of which are close to the direction to the North. It is well visible at a gyroscopic method of determination of coordinates.
Moreover, it is necessary to remember the following points:
- If a drill string creates magnetic hindrances, the full intensity of a magnetic field remains constant, irrespective of orientation, depth and an azimuth;
- Horizontal component on an axis Z equals (a mistake on Z). That is why the accuracy of a magnetometer worsens at the increase in an inclination (especially at magnetic disturbances from the side of a drilling string);
- Magnetic disturbances from a string are more strongly expressed in the areas with a big inclination.
- The best results are at the use of Shell algorithm.
For the solution of a task in a known way of determination of coordinates of a bottom hole by the time registration of distribution of the acoustic signals raised by a pulse source, to the seismograph, the acoustic signals raise on a day surface near the hole mouth of at least four points with the set coordinates. The seismograph is installed in a bottomhole and the time of distribution of acoustic signals from each point of excitement to a bottomhole is registered. The measurement results are presented in the form of an approximating hyperboloid.
(X-x0) 2/q2 + (y-y0) 2/q2 + 1 = t2/t20, which is transformed into the equation:
x2 + y2 + a1x + a2y + a3t2 + a4 = 0,
from which the a bottom hole’s coordinates are calculated according to a formula:
x0 = -a1/2; y0 = -a2/2,
where x, y are coordinates of the driving points of the acoustic signals;
t – the time of coming of the acoustic signal from the driving point till a hole bottom;
q – a scale factor;
a1, a2, a3, a4 – hyperboloid’s parameters ;
a1= -x0/2, a2= -y0/2, a3= -q2/t20, a4=q2+x20+y20
(x0,y0) – unknown coordinates of a hole bottom;
t0 – minimal time of spreading of acoustic signal from the daylight till bottomhole.
The method is based on the idea that there is a dependence between the depth of a bottomhole, a shift of a swab-in point of seismic signals from a projection of a bottomhole point to a daylight, the speed of distribution of acoustic signals and the time of distribution of the latter from an excitement point on a day surface to a bottomhole:
x2 = (vt) 2 – h2, where
x – a swab-in point of seismic signals from a projection of a bottomhole point to a daylight;
v – a speed of distribution of acoustic signals;
t – time of distribution of acoustic signals from a swab-in point to a bottomhole;
h – depth of bottomhole.
Because of the fact that at determination of a bottomhole location it is rather difficult to lay a profile passing through a projection of a bottomhole on a daylight, the way is considered for three-dimensional space and the function of time distribution of seismic signals from the set points on a daylight with known and required coordinates of a bottomhole is approximated by a two-way hyperboloid
x, y – coordinates of a swab-in point of seismic signals on a daylight;
x0, y0 – a bottomhole’s unknown quantities;
q – a scale factor.
t0 – a minimal time of spread of acoustic signal from daylight till a bottomhole.
Thus, the calculation of a bottomhole’s coordinates is reduced to the calculation of parameters of an approximating hyperboloid; four points not lying on one straight line are necessary for it.
Careful development of well projects in the directed drilling prior to the beginning of real works is the most important factor of success. Each directional well is unique from the point of view of its specific characteristics. At the stage of planning it is necessary to coordinate carefully all the aspects of work taking into account specific conditions. The problems of the directional drilling include well drilling from one point (located on a surface) into other (purpose) in such a manner that it could further be used on initial purpose. First of all, it is necessary to decide the location of a well on surfaces and its purpose.
Well planning includes several stages
- Definition of productive stratum conditions:
- Stratum thickness;
- Gas-oil contact (GOC);
- Oil-water contact (OWC);
- Interstices and their orientation;
- Impermeable barriers on the fluids’ walkway;
- Relative permeability;
- Necessity of a pilot borehole;
- Borehole’s stability;
- Definition of well completion scheme:
- Depth of installation of boring casing and their diameter;
- Determination of well completion scheme compliance to the conditions of a productive layer;
- 4. Definition of the requirements imposed by the opened object
- Hole depth down;
- Horizontal site;
- Entry point into a productive object;
- Exit point from a productive object;
- Necessary geological reference points;
- Parameters of a borehole.
- Project well profile:
- A deviation point from a vertical (as deep as possible);
- A site of a zenith angle (chosen for ensuring hit in the set point);
- A site of stabilization of a zenith angle (if needed);
- Direction control;
- Well completion (installation of a deep pump in a direct site);
- Horizontal site (big extent);
- The analysis of data on the drilled wells:
- Definition of possible complications and preventive and eliminating measures;
- Definition of an order of modification of the well construction project.
- Drilling mud project:
- Pollution of the productive layers;
- Well cleaning;
- Stability of a well’s walls;
- Reduction of the resistance forces;
- Drilling string design;
- Hydraulic calculations of a flushing-out of a well;
- A choice of components of drilling string bottom;
The project group on a well construction should include a drilling engineer, the engineer on deposits development, a field engineer on a well’s completion and a company’s representatives on drilling and on the directed drilling. A drilling engineer is the project manager and provides the coordination of all group members.
The first that it is necessary to make is to define the local system of coordinates and to choose a reference point on a terrestrial surface. The coordinates of the purpose “become attached” to this point.
When drilling the directional well, there is a continuous tracking of a trajectory of a borehole and a check of the compliance of its parameters of an ultimate goal. Often it is necessary to conducts expensive researches to be convinced that all necessary parameters are taken into consideration. The technology available today allows drilling wells with a very high precision. The cost in many respects depends on the accuracy of keeping of the planned parameters of a well within the limits necessary for an entrance in the set point.
A good interaction with geology and investigation departments prior to the beginning of works on drilling will help to avoid many mistakes and complications. It is important at the emergence of a question of a borehole correction. The first thing that it is necessary to do at azimuth change is consultation with the geology department.
Knowing an arrangement of a well on the surface and provision of a final point of a borehole, it is possible to define the best geometrical profile of a well.
Generally, all the directed wells can be divided into the following types:
The choice of a well profile is defined by geological parameters and the mechanism of a well efficiency. After a choice of a well profile, it is possible to start design. From the point of view of the directional drilling, first of all, it is necessary to define the following points:
- Kickoff is a borehole point on this depth from a surface where a well should be inclined from a vertical in this direction at this set of an angle. The choice of a kickoff is made becomes proceeding from the geometrical characteristics of a well profile and geological features.
- Determination of intensity of a set and angle falling. The most admissible intensity of set/falling of an angle is usually defined taking into consideration the following circumstances:
- Depth of a well;
- Restrictions on the maximum values of a torque;
- Big curvature of a borehole on any site leads to the increase of a torque and inhaling at a driving of the rest of a well. It can be a limiting factor for penetration into deeper horizons;
- Geological properties of a formation, through which this site should pass. In soft formations it is often impossible to achieve the high speeds of a driving at big tilt angles of a borehole.
- Mechanical restrictions of boring and casing pipes. The mechanical restrictions of the logging equipment and operational columns.
- Optimum intensity of set/falling of an angle in usual wells changes depending on the place, but it is usually in the range 1,5-3 degrees/100feet.
- After the sets / falling of an angle are defined, the kickoff should be determined. From the mathematical point of view, a well can be divided into two categories depending on the fact whether a curve degree is less than a radius on a site than a full withdrawal from a vertical or not.
“Comparing the depths at which different marker horizons have been encountered in other boreholes and at outcrop, it appears that the pre-Carboniferous surface was very flat (8 m or so relief over the 10 km distance between Rookhope and Eastgate). This suggests that future boreholes can be planned with greater confidence using existing deep borehole data” (Manning: 2007).
The most important factors which are necessary for considering at project design are:
- Stratum thickness;
- Gas-oil contact (GOC);
- Oil-water contact (OWC);
- Interstices and their orientation;
- Impermeable barriers on the fluids’ walkway;
- Relative permeability;
Depending on a definite scope and complexity of a deposit’s structure, there are also other factors which are necessary for considering.
As the well profile with small radius of a curvature is used for multidirectional drilling, the majority of wells with small radius of a curvature are finished with an open hole. Sometimes a shaft with holes is lowered.
Four main systems of drilling of a borehole of the directionally drilled wells
- The technology of wells drilling on midget radius by means of a stream of a high pressure;
- The system of drilling of wells with small radiuses of the curvature, based on the application of a rotor configuration;
- The system of drilling of wells with small radiuses of the curvature, based on use of mud motors;
- Drilling of wells on average radius of a curvature.
All four systems are suitable or will be suitable for drilling of a borehole. The first three systems demand the use of the special drilling tools and special methods of researches in wells. Small radiuses of a curvature of wells impose the restrictions on possibility of an assessment of a productive stratum and the methods of wells completion.
Unlike them at average radiuses of a curvature the ordinary boring tool, including measurement system in the course of drilling is applied to inclinometer and orientation of a deflector. The unique exception includes the restrictions of an assessment of a productive stratum and a well completion on a radial gap, connected with the restrictions on a well’s diameter.
The market of technologies for drilling of a borehole will develop, if only wells with lateral boreholes provide economic production of hydrocarbons. Wells with boreholes are of interest, as they allow reducing the cost of development projects of. The pipelines and the equipment for production are installed; the permission to use additional boreholes and transfer to operation can be received in the shortest terms. There are also the possibilities of decrease of the expenses on drilling. It will occur in the process of development by the industry of technology of a curvature of wells, and then in many cases the expenses on a driving of horizontal wells will decrease by 25-50 %. The improvement of characteristics of the equipment and encouragement of chisel contracts on such types of works will lead to even bigger decrease in the general expenses on drilling. On the other hand, these wells should increase an output of wells, stocks of oil or of extraction oil factor (EOF). These advantages should prove to be true.