Drilling fluid formula and function

Drilling fluid formula and function

First, the main components of drilling fluid

1.1 Basic liquids

Base fluid is one of the main components of drilling fluid, usually including water, oil, emulsion, ethylene glycol, etc. The choice of base fluid depends on the geological conditions of the drilling, the operating environment and the purpose of the drilling fluid, such as controlling the wellbore pressure, lubricating and cooling the drill bit, and maintaining the stability of the wellbore.

1.2 Thickener

Thickeners are used to increase the viscosity and density of drilling fluids and typically include clay and starch. Clay is generally used to make mud, while starch is mainly used to make starch slurry. The choice of type and amount of thickener depends on factors such as the geological conditions of the well, well depth and pore size.

1.3 Viscosity agent

Viscosity agents are used to improve the fluidity and lubricity of drilling fluids, and typically include xylitol, polymers, and waxes. These substances can give drilling fluids a certain viscosity and lubricity, reduce friction between the drill bit and the wellbore, prolong the life of the drill bit, and prevent the wellbore from collapsing.

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Fig. 1 Drilling fluid tackifier

1.4 Preservatives

Preservatives are used to protect drilling fluids from microorganisms and other contaminants, usually including formaldehyde, formic acid, etc. These substances kill microorganisms and other harmful organisms and maintain the cleanliness and stability of drilling fluids.

1.5 pH regulator

PH regulators are used to regulate the pH of drilling fluids, usually including calcium oxide, sodium hydroxide, etc. These substances can adjust the pH of drilling fluids, prevent rock in the wellbore from dissolving or corroding, and also protect the surface of the drill bit.

1.6 Fungicides

Fungicides 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 maintain the cleanliness and stability of drilling fluids.

1.7 Suspension agent

Suspension agents are used to maintain the dispersion of solid particles in drilling fluid, usually including silica gel, dolomite, etc. These substances can keep the solid particles in drilling fluid in a dispersed state, prevent them from precipitating and agglomerating, and maintain the stability and fluidity of drilling fluid.

Fig. 2 Drilling fluid suspension

Second, the proportion of drilling fluid deployment

2.1 The density is 1.06~ 1.44g/cm3 disperse drilling fluid formula

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 disperse 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 technology, the use range of disperse drilling fluid is not as extensive as in the past, but the preparation method is simple, the amount of treatment agent is less, and the cost is lower. It is suitable for preparing dense drilling fluids, especially when drilling the surface.

The formulation of dispersed drilling fluid typically 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 a solid component of drilling fluid, it can increase the viscosity of drilling fluid and control the amount of filtration loss.

Iron chromium lignin phosphate: reduce the dynamic shear force and static shear force, control the amount of filtration.

Lignite (or configured as lignite lye): reduce the dynamic shear force and static shear force, and control the amount of filtration loss.

Polymer: reduces dynamic and static shear forces.

CMC: Filtration loss control, viscosity enhancement.

Polyanionic cellulose: fluid loss control, viscosity improvement.

Caustic soda: used to adjust the pH of drilling fluid.

Barite: Increases density.

Fig. 3 Water-based drilling fluid

2.2 Density greater than 1.44g/cm3 dispersion drilling fluid formulation

Water: As the main component of drilling fluid, it is used to adjust the density and viscosity of drilling fluid.

Bentonite: As a solid component of drilling fluid, it can increase the viscosity of drilling fluid and control the amount of filtration loss.

Iron chromium lignin phosphate: reduce the dynamic shear force and static shear force, control the amount of filtration.

Lignite (or configured as lignite lye): reduce the dynamic shear force and static shear force, and control the amount of 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 Calcium treatment drilling fluid formula

Calcium treated drilling fluid is a kind of drilling fluid which has good resistance to salt and calcium pollution and strong inhibitory effect on shale hydration.

The calcium treated drilling fluid system is mainly composed of inorganic flocculant containing Ca2 +, viscosity reducer and fluid loss reducer. 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.

At present, there are three main types of inorganic flocculants commonly used: lime, gypsum and calcium chloride, namely lime drilling fluid, gypsum drilling fluid and a new type of potassium lime drilling fluid. Calcium-treated drilling fluid provides an inhibitory chemical environment with Ca2 +, which converts the sodium soil in the drilling fluid into calcium soil, and converts the clay particles from highly dispersed to moderate flocculation. Calcium-treated drilling fluid has the characteristics of anti-collapse, anti-pollution and stable performance when it contains more Ca2 +.

2.4 Definition and classification of brine drilling fluids

Drilling fluids with a NaCl content of more than 1% (mass fraction, C1-content of about 6000 mg/l) are called saline drilling fluids. Generally divided into three types:

(1) General brine drilling fluid

Salt content is 1% until saturation.

(2) Saturated brine drilling fluid

The salt content reaches saturation, and the normal temperature concentration is about 3.15 * 106mg/l.

(3) Seawater drilling fluid

Salt-containing drilling fluid prepared from seawater. The system contains about 3 * 104mg/l of NaCl, and also contains a certain amount of Ca2 + and Mg2 +. Foreign saline drilling fluids are divided into: saline drilling fluid with a salinity of 1% -2%, seawater drilling fluid with a salinity of 2% -4%, unsaturated saline drilling fluid between 4% and near saturation, and saturated saline drilling fluid with a salinity of 31.5%.

2.5 Anionic polymer drilling fluid

Main treatment agent:

① Polyacrylamide

The structural formula of polyacrylamide (PAM) is shown in Fig. 4. With the increase of the relative molecular weight of polyacrylamide, the flocculation ability, viscosity-enhancing effect, blocking and anti-collapse effect are improved. Mainly use polyacrylamide derivatives.

② Partially hydrolyzed polyacrylamide

It is prepared by hydrolysis of polyacrylamide aqueous solution with alkali. The molecular structure formula is Fig. 5. The degree of hydrolysis is an important parameter affecting the performance of PHPA. The degree of hydrolysis increases, the molecular chain stretches, the bridging effect in the drilling fluid is enhanced, and the flocculation effect on inferior soil is enhanced. If the hydrolysis is too large, the adsorption effect on clay particles is weakened, and the electrostatic repulsion between carboxylic acid groups is enhanced, and the flocculation effect on inferior soil is reduced. PHPA with a degree of hydrolysis of 60% -70% and 30% -40% are used for flocculation for on-site control of filtration loss and viscosity plugging.

③ Hydrolyzed polyacrylonitrile (sodium salt)

The molecular formula is Fig. 6. Hydrolyzed polyacrylonitrile (sodium salt) is mainly used as a fluid loss reducing agent. The degree of hydrolysis and degree of polymerization are the main factors affecting the fluid loss reduction effect. The best fluid loss reduction effect is when the carboxyl group content is 70% -80%. In addition to the fluid loss reduction effect of hydrolyzed polyacrylonitrile (sodium salt), it generally has a viscosity-increasing effect on freshwater drilling fluid; it has a viscosity-reducing effect on saline drilling fluid. 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 (referred to as NPAN or NH4-HPAN) NPAN, the structural formula is Fig. 7, hydrolyzed polyacrylonitrile ammonium salt is an anti-high temperature fluid loss agent. It can provide NH4 +, has strong ability to inhibit clay dispersion, and is a good anti-collapse agent. The concentration used is generally 0.3% to 0.4%.

2.6 Cationic polymer drilling fluid

Cationic polymer molecules have a large number of positive charges. In addition to taking hydrogen bonds, the adsorption on clay or rock mainly relies on electrostatic action, which is stronger than that of anionic polymers. Cationic polymers can neutralize negative charges on the surface of clay or rock, and have stronger flocculation and inhibition of rock dispersion than anionic polymers. Cationic polymer drilling fluids have excellent rheology, inhibition, stability of well walls, ability to carry drill cuttings, anti-sticking, anti-mud bags 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 the newly generated drill cuttings to inhibit dispersion, and large cations are then adsorbed on the drill cuttings by bridging to form a flocculant. Solid control equipment is used to remove the drill cuttings flocculate.

For useful solid-phase bentonite particles with strong negative electricity, 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.

III. Drilling fluid action

3.1 Maintain borehole stability

Drilling fluid protects the wellbore from collapse and collapse by controlling the pressure in the wellbore and stabilizing the wellbore. When drilling fluid is injected into the wellbore, it generates a certain pressure to support the wellbore and prevent the wellbore from collapsing due to force. At the same time, solid particles in the drilling fluid can also fill cracks in the wellbore and maintain the stability of the wellbore wall.

3.2 Cooling and lubrication

Drilling fluid can cool and lubricate the drill bit during the drilling process, reducing friction and extending the life of the drill bit. When the drill bit drills into the rock, high temperature and friction are generated. Drilling fluid can reduce bit damage and wear by cooling and lubrication, prolonging the life of the drill bit.

3.3 Cleaning the wellbore

Drilling fluid can remove cuttings from the drill bit and well wall to keep the wellbore clean. During drilling, the drill bit will carry cuttings inside the wellbore, and the drilling fluid can bring these cuttings to the wellhead to keep the wellbore clean and prevent cuttings from blocking the wellbore and affecting drilling efficiency.

3.4 Control wellbore pressure

Drilling fluid can control the pressure in the wellbore to 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 drilling efficiency and even endangering drilling safety.

3.5 Support drill string

Drilling fluid can support the drill string and prevent the drill string from falling or breaking. During the drilling process, the drill string needs to be continuously pushed downward. Without sufficient support, the drill string may fall or break, resulting in drilling accidents. Drilling fluid can support the drill string by increasing the density and viscosity, ensuring the stability and safety of the drill string.

3.6 Transfer of information

Drilling fluid can transmit information, such as the nature of the rock on the wellbore wall, the content of the oil reservoir, etc., to help drillers make drilling decisions. Through the analysis of the solid and liquid components in the drilling fluid, the rock composition of the wellbore wall, the fracture situation and the properties of the oil-bearing gas layer can be understood, providing drillers with a reference for drilling decisions.

3.7 Prevention of pollution

Drilling fluid can prevent the wellbore from being polluted and protect the groundwater resources. During the drilling process, if proper measures are not taken, the wellbore may be polluted, which will affect the groundwater resources. Drilling fluid can maintain cleanliness and stability through fungicides, preservatives and other substances to prevent it from polluting the groundwater resources.

In conclusion, drilling fluid plays a crucial role in the drilling process and is a necessary condition to ensure drilling safety and drilling efficiency. The formulation and role of drilling fluid depends 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.

Basic knowledge of drilling fluid

Drilling fluids serve a variety of functions: controlling formation pressure, removing cuttings from the wellbore, sealing permeable formations encountered during drilling, cooling and lubricating drill bits, transferring hydraulic energy to downhole tools and bits, and perhaps most importantly, maintaining wellbore stability and well control capability. Drilling fluids, commonly referred to as mud, were first introduced around 1913 to control underground pressure. The 1920s and 1930s saw the birth of the first companies in the United States that specialized in the distribution, development, and engineering of drilling fluids and components.

In the following decades, drilling fluid companies introduced advances in chemistry, surveying, and process engineering, which significantly improved drilling efficiency and well productivity.

Drilling fluid composition varies based on wellbore requirements, rig performance, and environmental factors. Engineers design drilling fluids to control subsurface pressure, reduce formation damage, reduce the potential for loss, control wellbore erosion, and optimize drilling parameters such as RP and wellbore cleaning. In addition, since most modern wells are highly inclined wells, drilling fluid systems must help address the well cleaning and stability issues unique to these wells.

Drilling fluid system

Drilling fluid systems have a continuous phase, which is a liquid, and a discontinuous phase consisting of solids. Sometimes, they also have a gas phase, either by design or as a result of gas entrainment in the formation. The continuous phase can be used to classify drilling fluid types into gas, aqueous fluid, or non-aqueous systems. These drilling fluids are mixtures of liquid and solid components, each of which can alter specific properties of the drilling fluid, such as viscosity and density.

Aqueous drilling fluids, commonly referred to as water-based slurries, are the most common and diverse of the three drilling fluid types (Figure 1). Their compositions range from simple water and clay mixtures to complex inhibitory or clay-stabilized drilling fluid systems, which include many components. In recent years, engineers and scientists have been working to improve the inhibitory and thermal properties of water-based systems to compete with non-aqueous fluids commonly used in challenging drilling environments.

In non-aqueous drilling fluids, often referred to as synthetic-based slurries, the continuous phase may consist of mineral oil, biodegradable esters, olefins, or other variants. Although these systems are typically more expensive than aqueous drilling fluids, they tend to have excellent wellbore control, thermal stability, lubricity, and RBR, which may help reduce the overall cost to the operator.

In environments where fractured rock or boreholes cannot support the water column, drillers can use air, fog, or foam systems to help remove cuttings from the well and maintain wellbore integrity without significant fluid loss into the formation.

basic function

Drilling fluids are formulated to have a wide range of functions. While this list is long and highly variable, the main performance characteristics are as follows:

Controlling Formation Pressure - Drilling fluid is essential to maintaining control of the oil well. Mud is pumped into the drill string, through the drill bit, and back into the annulus. In naked eye wells, the hydrostatic pressure exerted by the mud column is used to counteract the increase in formation pressure that would otherwise force formation fluids into the wellbore, potentially leading to runaway well control. However, the pressure exerted by the drilling fluid must not exceed the rupture pressure of the rock itself; otherwise the mud will leak into the formation, a condition known as leakage.

Removal of cuttings from the wellbore - Circulating drilling fluid brings cuttings (rock fragments) produced by the drill bit to the surface. Maintaining the drilling fluid's ability to carry these solid fragments is key to improving drilling efficiency and reducing the likelihood of getting stuck. To achieve this goal, drilling fluid experts work with drillers to carefully balance mud rheology and flow rates to adjust load-bearing capacity while avoiding excessive equivalent circulating density (EPS) - the actual mud density plus the pressure drop in the annulus above a given point in the wellbore. If left unchecked, high EPS can lead to leakage.

Cooling and lubricating drill bits - It helps to cool and lubricate the drill bits as the drilling fluid passes through and rotates around a rotating assembly of drilling tools. Thermal energy is transferred to the drilling fluid, which