In the process of oil and gas exploration and development, drilling operation is the key link to obtain underground resources. The drilling fluid circulation system is the core component of oil drilling operations, and the mud tank, as the key equipment of the system, undertakes many indispensable functions. This paper discusses the specific role of mud tanks in the drilling fluid circulation system, including the storage, purification and deployment of drilling fluid, as well as providing a stable guarantee for the operation of the system, etc., and elaborates on the significance of the mud tanks to ensure the safety and efficiency of drilling operations.
一.Composition of Mud Tank
Mud tank is usually composed of tank body, mixing device, level control system, inlet and outlet pipelines and various connecting accessories. The tank is generally made of welded steel plate with sufficient strength and volume to meet the demand for drilling fluid storage in the drilling process. The mixing device is installed inside the tank, which prevents solid-phase particles in the drilling fluid from settling and maintains the uniformity and stability of the drilling fluid. The level control system monitors the level of the drilling fluid in the tank in real time to ensure that the level is in the right range to avoid overflowing or pumping out. The inlet and outlet pipelines are used to realise the inflow and outflow of drilling fluid, and are connected with other equipment of the whole drilling fluid circulation system. These structural components cooperate with each other, and together ensure the effective performance of the mud tank function.
二.Working Principle of Mud Tank
Mud tank is an important equipment for storing, treating and circulating mud in the oil drilling process, and its working principle is as follows:
Mud storage: the mud tank has a certain volume for storing a large amount of mud. During the drilling process, mud needs to be injected into the well continuously, and the mud tank provides a stable storage place for the mud to ensure the continuous drilling operation.
Mud circulation: The mud in the mud tank is pumped out by mud pumps, transported to the drill pipe through high-pressure pipelines, and then sprayed out from the nozzle of the drill bit, and then carries the drill cuttings from the annular space between the well wall and the drill pipe to return to the ground and flow back to the mud tank, forming a complete mud circulation system.
Solid-phase separation: Mud tanks are usually equipped with various solid-phase separation equipment, such as vibrating screens, desanders, mud removers and so on. Vibrating screen through high-frequency vibration, the mud in the larger particles of drill cuttings and other solid-phase substances separated out, so that the initial purification of the mud. Desander and mud remover, on the other hand, utilise the principle of centrifugal force to further separate the smaller particles of sand and mud in the mud and improve the purity of the mud.
Mud treatment: According to the requirements of the drilling process, the performance of the mud needs to be adjusted and treated. For example, the viscosity, density, water loss and other performance parameters of the mud are adjusted by adding chemical treatment agents to meet the needs of different geological conditions and drilling operations. In addition, the temperature of mud can be controlled by heating or cooling device to ensure the stability of mud performance.
Liquid level control: Liquid level sensors are installed on the mud tank for real-time monitoring of the liquid level height of the mud. When the liquid level is too high or too low, the level sensor will send a signal to control the starting and stopping of the mud pump or the replenishment of the mud, in order to maintain the stability of the liquid level in the mud tank, and to prevent the mud from overflowing or insufficient supply.
三.The core role of mud tank in drilling fluid circulation system
1.Drilling fluid storage function
Satisfying the need for continuous drilling
The primary role of the mud tank is to store a large amount of drilling fluid to meet the needs of continuous circulation during the drilling process. During drilling operations, drilling fluid is continuously pumped into the well from the wellhead, carries rock chips back to the surface, and returns to the mud tank for storage after purification. As the depth of the well increases, the amount of drilling fluid required increases accordingly, and the large-capacity storage function of the mud tank ensures that the supply of drilling fluid will not be interrupted during the entire drilling cycle. For example, in some ultra-deep well drilling operations, the storage capacity of mud tanks can reach thousands of cubic metres, providing a solid material foundation for long and uninterrupted drilling operations.
Reserve for emergency situations
In addition to meeting the continuous needs of normal drilling operations, the mud tanks also provide the necessary reserves of drilling fluids to cope with emergencies. During the drilling process, complex situations such as well surges and well leaks may be encountered. When a well surge occurs, a large amount of high-density drilling fluid needs to be pumped into the well in a timely manner to balance the formation pressure and prevent a blowout; and when a well leak occurs, sufficient drilling fluid needs to be replenished quickly to maintain the pressure in the well’s fluid column and prevent the well wall from collapsing. At this time, the drilling fluid stored in the mud tank becomes an important resource guarantee to cope with these emergencies, and can be quickly put into use at the critical moment to effectively control the complex situation downhole and guarantee the safety of drilling operations.
2.Purification function of drilling fluid
Solid-phase particle separation
The drilling fluid will carry a large number of solid-phase particles such as rock cuttings taken out from the bottom of the well during the circulation process. If these solid-phase particles are not separated from the drilling fluid in a timely manner, it will have many adverse effects on drilling operations, such as abrasion of drilling equipment, lowering the performance of the drilling fluid, and affecting the stability of the well wall. As an important part of the drilling fluid purification system, the mud tank works with the vibrating screen, desander, mud remover and other purification equipment to achieve effective separation of solid-phase particles from the drilling fluid. After the drilling fluid returns from the wellhead, it first enters the mud tank, where it undergoes preliminary precipitation, and the larger particles of rock cuttings will settle to the bottom of the tank. Then, the drilling fluid in the tank is transported by the mud pump to the vibrating screen, which uses the vibration of the screen mesh to sieve out the solid-phase particles with large particle size. Then, the drilling fluid processed by the vibrating screen enters the sand remover and mud remover in turn, using centrifugal force and other principles to further separate out the smaller sand and mud particles. Through such a series of purification and treatment processes, the mud tank can effectively remove the solid-phase particles in the drilling fluid, ensure the cleanliness of the drilling fluid, and improve the performance and service life of the drilling fluid.
Harmful Gas Removal
During the drilling process, some harmful gases in the formation, such as hydrogen sulphide (H₂S), methane (CH₄), etc., may return to the surface together with the drilling fluid and dissolve in the drilling fluid. These harmful gases not only pose a serious threat to the health of drilling operators, but may also cause explosions and other safety accidents. To a certain extent, mud tanks have the function of removing these harmful gases. Usually, the design of mud tanks will consider the installation of gas discharge devices, such as breathing valves, exhaust pipes and so on. When the drilling fluid enters the mud tank, the harmful gases dissolved in the drilling fluid will gradually escape due to pressure reduction and agitation. Through the breathing valve and exhaust pipe and other devices, these harmful gases will be discharged to a safe area to avoid accumulation in the tank. In addition, some advanced mud tanks are also equipped with special gas treatment equipment, such as desulphurisation devices, degassers, etc., which can more effectively remove and treat harmful gases in the drilling fluid, further guaranteeing a safe environment for drilling operations.
3.Drilling fluid blending function
Performance adjustment
During the drilling process, the performance of the drilling fluid needs to be adjusted in real time according to different stratigraphic conditions and drilling operation requirements. The mud tank provides an operation platform for drilling fluid performance adjustment. For example, in order to increase the density of the drilling fluid to balance the high-pressure formation pressure, it is necessary to add barite and other weighting materials to the mud tank; in order to improve the rheological properties of the drilling fluid, such as reducing the viscosity and improving the fluidity, it may be necessary to add chemical treatment agents such as diluent and viscosity reducer; and when it is necessary to enhance the wall protection performance of the drilling fluid, it is necessary to add some additives with film-forming effect. Through the accurate addition of various additives and full mixing and blending in the mud tank, the performance of the drilling fluid can be made to meet the needs of different drilling stages, ensuring the smooth progress of drilling operations.
4.Provide stable guarantee for system operation
Stabilising pressure and flow rate
The mud tank plays an important role in stabilising the pressure and flow rate in the drilling fluid circulation system. As the drilling fluid will be affected by various factors in the process of circulation, such as the start and stop of pumps and changes in the resistance of equipment, resulting in fluctuations in pressure and flow rate. The large-capacity storage characteristics of the mud tank can buffer these fluctuations in pressure and flow rate, providing a relatively stable working environment for the entire circulation system. When the output flow rate of the pump suddenly increases, the mud tank can temporarily store excess drilling fluid to avoid excessive system pressure; and when the output flow rate of the pump decreases, the mud tank can timely replenish the drilling fluid to maintain the normal flow rate of the system and prevent pumping out. This function of stabilising pressure and flow rate helps to protect the drilling equipment and prolong the service life of the equipment, and also ensures that the drilling fluid can circulate evenly and stably to improve the quality of drilling operations.
Connection and Transition Role
Mud tank is the connecting hub and transition link between various equipments in the drilling fluid circulation system. It collects the returned drilling fluid from the wellhead and connects it with other equipment such as vibrating screen, desander, mud remover, mud pump and so on through the import and export pipelines, which realises the orderly flow of drilling fluid in the whole circulation system. In the process of drilling fluid circulation, the mud tank plays the role of the top and bottom, so that each equipment can work together to form a complete and efficient drilling fluid circulation system. For example, the drilling fluid returned from the wellhead first enters the mud tank, and after preliminary treatment, it is then transported through the mud pump to the subsequent purification equipment for further treatment, and the clean drilling fluid after treatment is returned to the mud tank for storage, and then pumped into the well for recycling again. This connection and transition ensures the smooth operation of the drilling fluid circulation system, which is one of the key factors in realising efficient drilling operations.
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