# Directional drilling

Directional drilling

With directional drilling (English "directional drilling") refers to processes which enable the direction of a deep hole to influence. In the simplest case, directional drilling ensures that the drill is exactly perpendicular. With more complex systems it is possible to change and determine the drilling path in any direction.

## background

In deep boreholes, there is always an unwanted directional influence on the borehole through cavities, layers of different densities and hardnesses or other circumstances. A drill string is often much longer than 1000 m, but becomes so flexible over a fraction of this length that it is not able to keep the direction due to its rigidity. It is essential to record the drilling path. The next step is then to correct the course according to the objectives. In addition, the entire drill string then does not have to absorb the forces for transmitting the drive force (in contrast to the rotary process ), so that the drill string can be manufactured from a lighter, but less torsion-proof material.

## history

The historically oldest form of influencing direction works as follows: A conventional, rotating drill string is supplemented by a hydraulic motor . This sits directly on the drill bit and is tilted by about 3 ° in relation to the drill rod. During normal drilling advance, the drill bit is driven by a hydraulic motor both through the drill string and via the drilling fluid. The drill bit thus circles the center axis of the bore with a deviation of 3 °. In this way, there is no deviation on average and the drilling is conventional. Should a change of direction or correction be necessary, the drill string is stopped in its rotational movement. Now the drilling follows the inclination of the hydraulic motor of 3 °. The challenge with this method is to record the drilling path and the tool face . The former can be determined with measuring probes that are lowered inside the drill string. The drill master determines the tool face from the rotational position of the drill rod, which requires a lot of experience. No electronic components are required underground and the course of the drilling is recorded by photographing plumb bob and compass at a specified depth. The procedure is tried and tested and also cheap and is mastered by surgeons all over the world. It is still used today.

While the USA has always set the tone in the technology of oil production, the technical principles of directional drilling go back to the Soviet drilling engineer Kabeljuschnikow immediately after the Second World War . So-called turbine drilling dominated drilling technology in the Soviet Union as early as the mid-1950s.

With the withdrawal of the Soviets in 1955, the technology came to the West via Austria , whose oil region in the Weinviertel north-east of Vienna came to lie in the Soviet occupation zone from 1945 and was exploited by the "Soviet Mineral Oil Administration" (which resulted in the OMV ).

Turbine drilling technology was first used outside the Soviet Union in 1952/53, when the entire drilling rig sank into a crater after a gas eruption from the Zwerndorf 1 well, which meant that the outbreak could no longer be brought under control with conventional means. The natural gas eruption was ended after 11 months, in which an estimated billion cubic meters of natural gas escaped in an uncontrolled manner , by means of three directional boreholes sunk around the eruption site by Soviet turbine drilling brigades, one of which hit the unfortunate borehole at a depth of 1,295 m, and subsequent so-called pumping .

The drilling turbines of the Soviet design consisted of a system of about 100 pairs of impellers and idlers, which were driven by the flushing flow. With a flushing flow of around 45 liters per second, the chisel achieved an output of around 250 kW and a speed of around 550 revolutions per minute.

The most frequently used design of a drilling turbine today is called the "Positive Displacement Motor" (PDM). The function is practically the same as an eccentric screw pump , except that the eccentric screw is not driven by a motor, but is set in rotation by the flushing liquid flowing through it and drives the drill bit.

## present

Much more modern, but also technically more demanding, are methods that continuously record the drilling process during drilling ( measurement while drilling ), transfer these measured values ​​to the surface and, if necessary, control them automatically.

In the simplest case, this is a tool for creating vertical holes. These are often required in mining, e.g. B. to insert new elevators, ventilation or supply shafts into existing shaft systems. For this, tunnels must be hit with pinpoint accuracy over several 1000 m.

Vertical drilling systems are easiest because the sensory effort is limited to determining the perpendicular. Any movement out of the perpendicular must be corrected automatically. To achieve this, two sensors / inclinometers , which are arranged horizontally offset by 90 °, are sufficient . If both sensors show a "zero signal", the tool is exactly vertical. If the tool tilts in any direction, one or more sensors will show a positive or negative change from the zero position. Such a zero crossing is particularly easy and precise to detect in terms of measurement technology, since these values ​​can be highly amplified and the rate of change around the zero point has its maximum.

The sensors and the sub-assemblies for influencing the direction, the so-called ribs, are located directly behind the drill bit in a cylindrical component carrier, the control sub. This is freely rotatable mounted on the drill string. The control electronics cause hydraulically operated ribs to press against the wall of the bore, thus allowing a force to act that directs the course of the bore in the opposite direction. By extending at least one control rib, the control sub is wedged and rotatable in the borehole. Only the drill string rotates within the control sub. Otherwise it would not be possible to determine the plumb bob with the required accuracy. The rotating part of the drilling tool contains turbines and generators to provide electrical and hydraulic energy. In addition, at least one pulser is also included, which transmits important parameters of the tool and the drilling process via pressure fluctuations in the rinsing solution above days.

## Freely controlled drilling

The fine art of directional drilling is freely controlled drilling. This requires three sensors to determine the inclination and three magnetometers to calculate the position in the earth's magnetic field. The gravimetric and magnetic tool face can also be calculated from this. The principle of influencing the direction is the same as in the vertical drilling system described above. It consists of 3 to 4 control ribs, which press against the wall according to the specifications of the control electronics and thus generate a vector that influences the drilling process accordingly. However, proportionally controllable ribs are also often used. This allows the influencing vector to be set and aligned more precisely.
The electronic and mechanical engineering effort is many times higher compared to tools that control purely vertically, as they have to process significantly more data and the need for communication is considerably greater. Not only do directional data have to be transmitted from underground to surface, but also correction and control commands in the opposite direction. The ability to control freely means that it is possible to react quickly to geological measurement data and to adapt the drilling. For this reason, such drilling tools are often combined with physical and chemical sensors, which are supposed to provide information about the surrounding rock. These data also have to be transmitted or saved for days.
The mechanical challenges are also considerable, as the entire technology cannot be optimized to a single position in relation to the earth's gravitational field. This mainly affects the design of the hydraulic system. Tools that can drill upwards are even more complex. In addition, there are gyro-controlled systems in which a complete, hardened inertial navigation system is housed in the drill head. This tells the machine operator whether he has to steer higher, lower, right or left, and guides the drill head precisely with a deviation of around 1 m over a distance of 2000 m along the predefined trajectory. The use of these devices leads to the most accurate and environmentally independent results in directional drilling today.

## use

This makes it possible to develop a deposit of crude oil or natural gas from the side, for example in the case of deposits under populated, difficult terrain that is to be protected or used for military purposes. Directional drilling can also be used to correct bores and to bypass borehole sections that have become unusable.

The main application today is drilling in several directions from one point. Directional drilling is therefore the usual form of development in offshore production, because the often extensive deposit can be reached from a few (expensive) drilling rigs .

Directional drilling has now also established itself on land. The aim here is to concentrate the production sites on a few drilling sites, mainly for reasons of environmental protection and the protection of residents. In extreme cases, more than 200 wells (e.g. oil field , Long Beach (California) ) are drilled from one drilling site .