Main components of drilling fluid
1.1 Base fluid
Base fluid is one of the main components of drilling fluid, usually including water, oil, emulsion, ethylene glycol, etc. The selection of base fluid depends on the geological conditions of drilling, the operating environment and the purpose of drilling fluid, such as controlling wellbore pressure, lubricating and cooling the drill bit, and maintaining the stability of the well wall.
1.2 Thickener
Thickeners are used to increase the viscosity and density of drilling fluid, usually including clay and starch. Clay is generally used to prepare mud, and starch is mainly used to prepare starch slurry. The type and amount of thickeners selected depend on factors such as the geological conditions of drilling, well depth and aperture.
1.3 Viscosity agent
Viscosity agents are used to improve the fluidity and lubricity of drilling fluid, usually including xylitol, polymers and wax. These substances can make the drilling fluid have a certain viscosity and lubricity, reduce the friction between the drill bit and the well wall, extend the life of the drill bit, and also prevent the well wall from collapsing.

Fig.1 Drilling fluid thickener
1.4 Preservatives
Preservatives are used to protect drilling fluids from microorganisms and other contaminants, usually including formaldehyde, formic acid, etc. These substances can kill microorganisms and other harmful organisms and keep the drilling fluid clean and stable.
1.5 PH regulators
PH regulators are used to adjust the pH of drilling fluids, usually including calcium oxide, sodium hydroxide, etc. These substances can adjust the pH of drilling fluids, prevent the rocks in the wellbore from dissolving or corroding, and also protect the surface of the drill bit.
1.6 Bactericides
Bactericides are used to kill microorganisms in drilling fluids, usually including calcium peroxide, sodium hypochlorite, etc. These substances can effectively kill bacteria and other harmful organisms and keep the drilling fluid clean and stable.
1.7 Suspending agents
Suspending agents are used to maintain the dispersion of solid particles in drilling fluids, usually including silica gel, dolomite, etc. These substances can keep the solid particles in the drilling fluid dispersed, prevent them from settling and agglomerating, and maintain the stability and fluidity of the drilling fluid.

Fig.2 Drilling fluid suspension agent
II. Drilling fluid mixing ratio
2.1 Dispersed drilling fluid formula with density of 1.06~1.44g/cm3
The water-based drilling fluid prepared by fresh water, slurry bentonite and various treatment agents (referred to as dispersants) that disperse clay and drill cuttings is called dispersed drilling fluid. It is the earliest type of water-based drilling fluid used in oil and gas drilling and has been used for a long time. With the continuous development of drilling fluid process technology, the application scope of dispersed drilling fluid is not as wide as in the past, but the preparation method is simple, the amount of treatment agent is small, and the cost is low. It is suitable for preparing drilling fluid with higher density, especially when drilling the surface.
The formula of dispersed drilling fluid usually contains the following ingredients:
Water: As the main component of drilling fluid, it is used to adjust the density and viscosity of drilling fluid.
Bentonite: As the solid phase component of drilling fluid, it can increase the viscosity of drilling fluid and control the filtration loss.
Iron-chromium-lignin phosphate: reduce dynamic shear force and static shear force, control filtration loss.
Lignite (or configured into lignite alkali solution): reduce dynamic shear force and static shear force, control filtration loss.
Polymer: reduce dynamic shear force and static shear force.
CMC: control filtration loss, increase viscosity.
Polyanionic cellulose: control filtration loss, increase viscosity.
Caustic soda: used to adjust the pH of drilling fluid.
Barite: increase density.

Fig.3 Water-based drilling fluid
2.2 Formula of dispersed drilling fluid with density greater than 1.44g/cm3
Water: as the main component of drilling fluid, used to adjust the density and viscosity of drilling fluid.
Bentonite: as the solid phase component of drilling fluid, it can increase the viscosity of drilling fluid and control filtration loss.
Iron-chromium-lignin phosphate: reduce dynamic shear force and static shear force, control filtration loss.
Lignite (or lignite alkali solution): reduce dynamic shear force and static shear force, control filtration loss.
Caustic soda: used to adjust the pH of drilling fluid.
Phosphated lignite and resin treatment agents: control HTHP filtration loss and act as stabilizers.
Barite or iron oxide powder: increase density.
2.3 Formula of calcium-treated drilling fluid
Calcium-treated drilling fluid is a type of drilling fluid that has good resistance to salt and calcium pollution and has a strong inhibitory effect on hydration of shale.
The calcium-treated drilling fluid system is mainly composed of inorganic flocculants containing Ca2+, viscosity reducers and filtration loss reducers. The clay particles in the calcium-treated drilling fluid system are in a coarse dispersion state of moderate flocculation, which is called coarse dispersion drilling fluid.
There are three main types of commonly used inorganic flocculants: lime, gypsum and calcium chloride, namely lime drilling fluid, gypsum drilling fluid and new potassium lime drilling fluid. Calcium-treated drilling fluid uses Ca2+ to provide an inhibitory chemical environment, transforming sodium soil in the drilling fluid into calcium soil and transforming clay particles from highly dispersed to moderately flocculated. Calcium-treated drilling fluid has the characteristics of anti-collapse, anti-pollution and maintaining stable performance when containing more Ca2+.
2.4 Definition and classification of brine drilling fluid
Drilling fluid with a NaCl content of more than 1% (mass fraction, C1- content of about 6000mg/l) is called brine drilling fluid. It is generally divided into three types:
(1) General brine drilling fluid
Salt content of 1% until saturation.
(2) Saturated brine drilling fluid
Drilling fluid with saturated salt content and a normal temperature concentration of about 3.15*106mg/l.
(3) Seawater drilling fluid
Salt drilling fluid prepared with seawater. The system contains about 3*104mg/l NaCl and a certain amount of Ca2+ and Mg2+. In foreign countries, salt water drilling fluid is divided into: slightly salty drilling fluid with a salt content of 1%-2%, seawater drilling fluid with a salt content of 2%-4%, unsaturated salt water drilling fluid with a salt content between 4% and near saturation, and saturated salt water drilling fluid with a salt content of 31.5% at its maximum.
2.5 Anionic polymer drilling fluid
Main treatment agents:
① Polyacrylamide

The structural formula of polyacrylamide (PAM for short) is shown in Figure 4. With the increase of the relative molecular mass of polyacrylamide, the flocculation ability, viscosity enhancement effect, plugging and anti-collapse effect are improved. The derivatives of polyacrylamide are mainly used.
② Partially hydrolyzed polyacrylamide

It is prepared by adding alkali to the aqueous solution of polyacrylamide. The molecular structural formula is shown in Figure 5. The degree of hydrolysis is an important parameter affecting the performance of PHPA. As the degree of hydrolysis increases, the molecular chain stretches, the bridging effect in the drilling fluid is enhanced, and the flocculation effect on poor quality soil is enhanced. If the degree of hydrolysis is too large, the adsorption effect on clay particles is weakened, and the electrostatic repulsion between carboxylate groups is enhanced, which reduces the flocculation effect on poor quality soil. PHPA with a hydrolysis degree of 60%-70% is used for on-site control of filtration loss and viscosity enhancement and plugging, and PHPA with a hydrolysis degree of 30%-40% is used for flocculation.
③ Hydrolyzed polyacrylonitrile (sodium salt)

The molecular formula is shown in Figure 6. Hydrolyzed polyacrylonitrile (sodium salt) is mainly used as a filtration reducer. The degree of hydrolysis and degree of polymerization are the main factors affecting the filtration reduction effect. The filtration reduction effect is best when the carboxyl content is 70%-80%. In addition to the filtration reduction effect, hydrolyzed polyacrylonitrile (sodium salt) generally has a viscosity-increasing effect on freshwater drilling fluids; it has a viscosity-reducing effect on saltwater drilling fluids. Hydrolyzed polyacrylonitrile (sodium salt) has a strong resistance to sodium salts and a weak resistance to calcium.
④ Hydrolyzed polyacrylonitrile ammonium salt

Hydrolyzed polyacrylonitrile ammonium salt (abbreviated as NPAN or NH4-HPAN) NPAN, with a structural formula shown in Figure 7, is a high-temperature resistant filtration reducer. It can provide NH4+, has a strong ability to inhibit clay dispersion, and is a good anti-collapse agent. The usage concentration is generally 0.3%~0.4%.
2.6 Cationic polymer drilling fluid
Cationic polymer molecules carry a large amount of positive charge. In addition to hydrogen bonds, the adsorption on clay or rock mainly relies on electrostatic action, which is stronger than the adsorption force of anionic polymers. Cationic polymers can neutralize the negative charge on the surface of clay or rock, and their flocculation ability and ability to inhibit rock dispersion are also stronger than anionic polymers. Cationic polymer drilling fluid has excellent rheological properties, inhibition, well wall stability, cuttings carrying ability, anti-stuck, anti-mud packing and other properties.
The main function of small cations is to inhibit the dispersion of drill cuttings, and the main function of large cations is to flocculate drill cuttings. The adsorption rate of small cations on drill cuttings is generally faster than that of large cations. During the drilling process, small cations are first adsorbed on newly generated drill cuttings to inhibit dispersion, and large cations are then adsorbed on the drill cuttings to form floccules by bridging action, and the drill cuttings floccules are removed using solid control equipment. For the useful solid bentonite particles with strong negative charge, more small cations are adsorbed, which weakens the adsorption of large cations. The flocculation effect of large cations on bentonite is relatively weak, and an appropriate amount of useful solid phase is maintained in the drilling fluid. The synergistic cooperation of large and small cations produces a certain "selective" flocculation effect.
III. Drilling fluid effect
3.1 Maintaining wellbore stability
The drilling fluid protects the wellbore from collapse and collapse by controlling the pressure in the wellbore and stabilizing the wellbore. When the drilling fluid is injected into the wellbore, it will generate a certain pressure, thereby supporting the wellbore wall and preventing the wellbore wall from collapsing due to force. At the same time, the solid particles in the drilling fluid can also fill the cracks in the wellbore wall to maintain the stability of the wellbore wall.
3.2 Cooling and lubrication
The drilling fluid can cool and lubricate the drill bit during the drilling process, reduce friction, and extend the life of the drill bit. When the drill bit drills into the rock, it will generate high temperature and friction. The drilling fluid can reduce damage and wear of the drill bit through cooling and lubrication, and extend the life of the drill bit.
3.3 Cleaning the Wellbore
Drilling fluid can remove cuttings from the drill bit and the wellbore wall to keep the wellbore clean. During the drilling process, the drill bit will bring cuttings to the inside of the wellbore, and the drilling fluid can bring these cuttings to the wellhead to keep the wellbore clean, prevent the cuttings from blocking the wellbore and affecting the drilling efficiency.
3.4 Controlling Wellbore Pressure
Drilling fluid can control the pressure in the wellbore, prevent gas and liquid in the reservoir from penetrating into the wellbore, and ensure drilling safety. During the drilling process, if the pressure in the wellbore is not controlled in time, a large amount of gas and liquid may flow into the wellbore, affecting the drilling efficiency and even endangering the drilling safety.
3.5 Supporting the Drill String
Drilling fluid can support the drill string to prevent it from falling or breaking. During the drilling process, the drill string needs to be continuously pushed downward. If there is not enough support, the drill string may fall or break, resulting in a drilling accident. Drilling fluid can support the drill string by increasing density and viscosity to ensure the stability and safety of the drill string.
3.6 Transmit information
Drilling fluid can transmit information, such as the properties of the wellbore rock and the content of the oil reservoir, to help drillers make drilling decisions. By analyzing the solid and liquid components in the drilling fluid, we can understand the rock composition, fracture conditions and properties of the oil and gas-bearing layers of the wellbore, and provide a reference for drilling decisions for drillers.
3.7 Prevent pollution
Drilling fluid can prevent wellbore contamination and protect groundwater resources. During the drilling process, if appropriate measures are not taken, the wellbore may be contaminated, which will affect groundwater resources. Drilling fluid can be kept clean and stable by fungicides, preservatives and other substances to prevent it from polluting groundwater resources.

In short, drilling fluid plays a vital role in the drilling process and is a necessary condition to ensure drilling safety and drilling efficiency. The formulation and function of drilling fluid depend on factors such as the geological conditions of drilling, well depth, operating environment and the purpose of drilling fluid. It needs to be selected and adjusted according to the specific situation to achieve the best drilling effect.