Introduction and function of drilling fluid

• Jul 27, 2021
• Drilling fluid refers to the working fluid used in drilling. It can be liquid or gas. Therefore, drilling fluid should be exactly called drilling fluid. During drilling, the circulating flushing medium used in the hole. Also known as drilli...

Drilling fluid refers to the working fluid used in drilling. It can be liquid or gas. Therefore, drilling fluid should be exactly called drilling fluid. During drilling, the circulating flushing medium used in the hole. Also known as drilling fluid. Drilling fluid can be divided into clean water, mud, clay-free flushing fluid, emulsion, foam and compressed air according to its composition. Clear water is the earliest drilling fluid used, no treatment is required, easy to use, and suitable for intact rock formations and areas with sufficient water sources. Mud is a widely used drilling fluid, which is mainly used for unstable rock formations such as loose, well-developed fissures, easy to collapse and block, swelling and spalling in contact with water.

Types of drilling fluids: water-based mud WBM (water and chemical equipment), oil-based mud OBM (oil and chemical equipment), synthetic-based mud (SBM), synthetic-based mud (SBM) as an alternative to oil-based mud (OBM) The system developed from the 1980s and is divided into ester groups, ether groups, poly-α-olefin (PAO) groups, and chain alkyl benzene (LAB) groups. Ester-based mud (EBM) is used the most. SBM is easy to degrade and disperse in seawater, and has little environmental pollution. It can be discharged on-site at sea, and the biological organisms on the seabed are easier to restore to the original level. Compared with OBM, it has lower thermal stability, higher low-temperature viscosity, and faster viscosity decrease with increasing temperature, and it is not easy to dehydrate shale. The SBM system has a higher cost, but the total drilling cost is lower than that of water-based mud (WBM) and OBM, and it can be used to drill highly deviated wells, extension wells and horizontal wells.

Making of drilling fluid: The drilling fluid is composed of dispersion medium, dispersed phase and additives.

The drilling fluid is mainly composed of liquid phase, solid phase and chemical treatment agent. The liquid phase can be water (fresh water, salt water), oil (crude oil, diesel) or emulsion (oil mixed emulsion and inverse emulsion). The solid phase includes useful solid phase (bentonite, weighting material) and useless solid phase (rock). Chemical treatment agents include inorganic, organic and polymer compounds. That is, drilling fluid pH control agent, drilling fluid decalcifying agent, drilling fluid foaming agent, drilling fluid flocculant, anti-collapse agent, drilling fluid corrosion inhibitor, drilling fluid viscosity reducer, drilling fluid viscosity increasing agent, drilling fluid reducing agent Fluid loss agent, drilling fluid lubrication, jam release agent, temperature stabilizer, density adjustment material, leakage stoppage material. In the classification of drilling fluid treatment agents by use, the last two categories are called materials because they are used in large amounts, generally exceeding 5%.

The role of drilling fluid:

1. Flushing the bottom of the well: Drilling fluid can form a high-speed flow at the water hole of the drill bit and spray to the bottom of the well. This high-speed jet of drilling fluid can wash up the cuttings held at the bottom of the well due to the difference between the pressure of the drilling fluid and the formation pressure, and play the role of flushing the bottom of the well.

2. Carrying cuttings: When the up-and-back speed of the drilling fluid in the annulus is greater than the settling speed of the cuttings, the drilling fluid can bring the cuttings out of the well, that is, at a certain up-and-back speed, the drilling fluid will carry cuttings. (Referred to as Xieyan).

3. Balance the formation pressure: the fluid column pressure of the drilling fluid must be balanced with the formation pressure to achieve the purpose of preventing blowout or a large amount of drilling fluid leaking into the formation. The pressure of the fluid column of the drilling fluid can be controlled by adjusting the density of the drilling fluid to balance it with the formation pressure.

4. Cooling and lubricating the drill bit: The drilling fluid can transfer the heat generated by the friction between the drill tool (drill bit and drill string) and the formation to the ground during the drilling process for cooling. At the same time, the drilling fluid can effectively reduce the friction between the drilling tool and the formation and play a lubricating effect.

5. Stabilization of the borehole wall: an appropriate processor can be added to the drilling fluid to make the drilling fluid have the ability to inhibit shale expansion and/or dispersion, and at the same time produce a thin and tough filter cake, which stabilizes the borehole wall.

6. Suspended cuttings and density adjustment materials: When the circulation is stopped, the drilling fluid is in a static state, and the bentonite particles in it can be connected to each other to form a structure to suspend the cuttings. If density adjustment materials (such as barite) are added to the drilling fluid, the drilling fluid can also suspend it when the circulation is stopped. The ability of drilling fluid to suspend cuttings and density adjustment materials can make it easy to restart after the drilling fluid stops circulating.

7. Obtain formation information: Through the cuttings carried by the drilling fluid, many formation information can be obtained, such as oil and gas display, formation physical properties, etc.

8. Transmission power: The drilling fluid can transfer the power of the drilling fluid pump through the drill string from the water hole of the drill bit to the bottom of the well, improving the rock breaking ability of the drill bit and speeding up the drilling speed.
 

Purpose of recovering drilling fluid:

Cost saving: Recycling drilling fluid saves drilling costs. The cost of mud materials and drilling fluid is very high. If a well is used for oil machine mud, it can reach more than 5 million yuan.

Protect the environment: Drilling fluid can seriously pollute groundwater and vegetation on the ground. There is almost no grass growing on the ground with drilling fluid.