Introduction
Oil drilling is performed in a variety of geological structures such as plains, mountains, urban areas, or deep sea. Therefore, it is necessary to study the capabilities of various drilling methods, and examine the effective drilling parameters to select the best drilling method. The use of directional drilling, which is now an integral part of conventional drilling methods, is very common for the development of oil fields.
Unlike vertical drilling, where only the selection and design of the well length is important, in the directional drilling method, items such as measuring system, precision instruments, deflection tools, suitable drilling parameters, compatibility of the well architecture with its length and effective cementing method should be considered. In this article, some drilling methods, technologies and the principles of their use along with their advantages and disadvantages are examined.
Drilling methods
Hand Drill system: In this system, drilling is done with the help of manpower and without using mechanical energy.
Percussive drilling method
Percussive drilling methods also called Cable-Tool was first used by the Chinese in drilling water wells. The percussive drill system was a hard, sharp-edged steel drill attached to the end of a metal wire. This steel drill was inserted and raised by a derrick, thus piercing the ground. The most important advantage of using Cable-Tool drilling is its low cost in shallow wells. In general, the advantages of using this method are:
- Low cost of daily operations including fuel, maintenance, personnel and …
- Low cost of moving the rig
- Low time and therefore low cost of setting up the rig
- High drilling speed in shallow wells, especially in hard structures compared to rotary drilling method
- Rotary drilling systems are mainly designed for wells with a diameter of more than four inches. Therefore, for wells with small diameter and depth, the use of Percussive Drill system is more common.
- Ease of reconstruction and repair of its parts
Disadvantages of percussive drilling method
There are also major disadvantages to percussive drilling method, some are mentioned blew:
- The most important limiting factor of this method is drilling speed and well depth. In deep wells, this method is not cost effective due to the high time and cost of drilling. The deepest well drilled with this method is 11145 feet, which cost a lot of money.
- Impossibility to deal with possible well eruption due to lack of drilling fluid
- Instability of well wall in loose formations (due to lack of drilling fluid)
- cost
- Multiple cable ruptures, resulting in high time and cost
- This method was obsolete due to the above disadvantages and the impossibility of drilling deep wells, discovery of hydrocarbon reservoirs in the very deep levels.
Rotational drilling method
In rotational drilling method, drilling fluid is constantly moving drilling particles to the surface. The rotary drilling method is currently used in drilling oil and gas wells. In the Rotational Drilling system, the drill is connected to the drilling pipe and the operation of penetrating the rock and digging the well is done by rotating the drill and applying downward force.
In this method, the downward force is applied using thick and heavy pipes called weight pipes, which are located on top of the drill. It is usually powered by a steam, diesel or electric motor. The crushed particles at the bottom of the well are transported to the surface by air or mud circulation system.
Rotary table method
Advantages of rotary table method
- No need for new control systems and therefore simple operation
- Proper cost of repairs and maintenance
- Low volume and easy to move
- The possibility of using this equipment on any other rig
Disadvantages of rotary table method
- Safety: In rotary table drilling, the pipes are guided by the operator when adding and subtracting. As a result, due to the heaviness of the pipes and the pollution of the work environment, the safety of the operator is less.
- Drilling time: In this method, due to the need to remove the pipes and lower them again, a considerable amount of time is wasted in this process.
- Drilling pipe life: In this method, the connection of new pipes is done by the operator. For this reason, the lack of precise control of the pipes to maintain the direction of the pipes when closing, increases the risk of impact and damage to the pipe.
Directional drilling
Directional drilling is a type of drilling in which the well path deviates from the vertical position to reach the target area, based on a specific and pre-designed plan. This drilling method is done when, When drilling on the well site is inaccessible.
Development in Mud Motor can be considered as one of the most important factors in the development of directional drilling industry. Also, the construction of high-precision positioning devices has greatly contributed to the development of this industry.
Nowadays, advanced steering tools have replaced the old positioning devices as tools for orienting mud motors.
These devices are driven into the drill pipe by cable and after being placed at the appropriate height and passing through certain distances, they send the information to the ground surface. This information is processed by a computer and the exact position of the well and the temperature and pressure conditions of the bottom of the well are extracted from them. The new model of these devices is made with log devices, gamma rays and high resistance, which allows the identification of the formation at the same time with drilling operations.
Applications of directional drilling
Side Tracking
Side tracking was the first method of directional drilling. The purpose of this work was to bypass or pass over the remains inside the well. For example, this method is used when unforeseen changes are observed in the geological structure. Sometimes due to various factors such as material, slope and direction of the formation, imbalance weight on the drill and rotational speed in direct drilling drives deviate. Therefore, drilling vertical wells that have deviated from their path can also be considered as part of directional drilling.
Inaccessible areas
There are always some oil reservoirs under existing cities, under mountains, non-transferable facilities or generally inaccessible areas . In these cases using vertical drilling is not possible. When the reservoir or drilling target is located below a city, river, mountain or environmentally sensitive route, it is necessary to place the drilling rig further away. In these cases, using directional drilling method can be the best solution.
Some oil and gas reservoirs are geographically located in mountainous and highlands. Therefore, road construction to transport equipment to the desired location is very expensive and the use of Directional Drilling is cost-effective. Also using vertical drilling method in mountainous areas, due to the high difference in altitude, the reservoir pressure will be greatly reduced.
Fault
Sometimes vertically drilled wells deviate from their original path. This phenomenon is usually due to the presence of faults in the subsurface layers that have a large angle coefficient of the vertical line. In these conditions, directional drilling and prevention of collision with the fault is the best solution. Sometimes, to prevent the well from colliding with the fault plate, the drilling is diverted with a slope less than the slope of the fault plate. This avoids the dangers of earthquakes that can cause the earth’s crust to move over a fault.
Multiple wells from offshore structure
Today, the main application of directional drilling is offshore drilling. In many countries, a large part of the gas and oil reservoirs are under the sea. Making a Platform for any ring at sea is impractical and uneconomical. In this method, by fixing a Platform on the seabed, a large number of directional wells in different directions are drilled on a Platform. Another advantage of the above method is the optimal use of the offshore rig.
Directional drilling methods and tools
In general, equipment used in vertical drilling is also required in directional drilling. Bending tools are used at the beginning of the well deflection or hole directional correction. After obtaining the required slope and direction, the bending tools are usually discarded and drilling continues normally. Some of the additional equipment required for directional drilling are:
- Use of Steerable Down hole systems and Measuring-While-Drilling (MWD) tools
- Use of rubber protectors in the drilling string to protect pipe wear and casing in severe well deflection
- Add special lubricants to the drilling mud to reduce torque
- Using heavier mud to effectively clean the Annular Hole
- Use of articulated drilling pipes, especially in short radius drilling
- Special engineering calculations for well deflection
Under Balanced Drilling method (UBD)
Undoubtedly, the pressure drop due to high extraction from oil reservoirs causes traditional Over Balanced Drilling methods to gradually lose their efficiency. Because over balanced drilling causes irreparable damage and increases costs. In the traditional over balanced drilling method, due to higher pressure of the fluid at bottom than the pressure of the formation, there is always an influx of drilling fluid into the formation.
By adjusting the weight of the mud, the hydrostatic pressure of the mud can be adjusted to control the high pressures of the formation. The difference between the hydrostatic pressure of the mud and the pressure of the formation is called Differential Pressure. Based on this Differential Pressure, three situations can be considered for drilling:
Positive differential pressure: Drilling in this case is called over balanced drilling. In this case, there is no possibility of well eruption. But the well wall is broken and therefore lost circulation occurs. In severe cases, the hydrostatic pressure of the mud may suddenly reduce and there is a possibility of well eruption.
Zero Differential Pressure: Drilling type will be balanced drilling. In this case, there won’t be eruption and a lost circulation. A pressure difference of about 200 Psi is usually considered as reliability coefficient.
Negative Differential Pressure: Drilling type will be under balanced drilling (UBP). In other words, UBD drilling is a method in which the hydrostatic pressure applied to the bottom of the well is mainly kept less than the pressure of the formation. Under these conditions, when drilling permeable layers, formation fluids flow into the well. In this method, the pressure of the formation is controlled with special equipment. This type of drilling is typically used in fields where the reservoir pressure is low or pressure has been reduced by production and in which the pressure under proper balanced cannot be generated naturally. In the following conditions, we face many problems such as severe loss circulation, clogging of pipes(stuck pipe), etc.:
- When the reservoir rock has a high permeability due to the presence of many cracks and crevices
- When drilling with ordinary fluids such as water and even diesel emulsion mud
- When drilling horizontally
- When using conventional drilling methods
Proper selection of drilling fluids
To further reduction in weight of drilling fluid and to use under balanced drilling method in drilling hydrocarbon reservoirs, gas-based mud is used as the second phase of drilling fluid. Usually the gaseous phase of the fluid used in this type of drilling cannot be air; Because of the high percentage of oxygen in the air, the risk of fire is very high. In this regard, nitrogen gas, which is a neutral gas, is often used.
However, UBD drilling uses a wide range of gases and light fluids, including:
- natural gas
- Nitrogen
- MIST
- Foam
- Dusting
- Combustion gases from internal combustion engines
- Aerated mud
When using foam or MIST, lower drilling speed than other fluids is needed.
In wet formations, drilling particles cause the drill to mud up and create a mud ring in the well wall. In this case, some water and detergent along with air or dry gas should be used for drilling. This composition forms a two-phase fluid called Mist, which cleans the bit after entering the well.
