Study on instability mechanism of KCl/PHPA drilling waste fluid

Xiaoxuan Guo; Bin Li; Guilei Zhang; Haiyuan Yan; Wei Li; Zhenzhong Fan; Qilei Tong

doi:10.1515/phys-2023-0271

Article Open Access

Study on instability mechanism of KCl/PHPA drilling waste fluid

Xiaoxuan Guo , Bin Li , Guilei Zhang , Haiyuan Yan , Wei Li , Zhenzhong Fan and Qilei Tong

Published/Copyright: August 30, 2023

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From the journal Open Physics Volume 21 Issue 1

Abstract

The KCl/PHPA waste drilling fluid is produced during drilling operations in Bohai Oilfield, China, which cannot be directly used or discharged, it needs to be flocculated. In order to know the aggregation mechanism of colloidal particles in KCl/PHPA drilling fluid, the calculation method of zeta potential and the relationship between the stability of waste fluid and zeta potential are discussed in detail in this article. The process of decreasing the zeta potential of waste fluid due to the addition of cationic polyacrylamide is called instability. The process in which unstable particles aggregate into larger particles is called coagulation, and the process in which unstable particles aggregate into large flocs is called flocculation. According to the types of treatment agents in KCl/PHPA waste drilling fluid, the structure and energy changes formed between low-viscosity polyanionic cellulose, partially hydrolyzed polyacrylamide, xanthan gum, starch, and water molecules are constructed. The energy of the four treatment agent systems decreased by 21.4–67.5% after the cationic polyacrylamide was added. The reduction of system energy reduced the repulsive force between colloidal particles in the waste fluid and promoted the agglomeration of colloidal particles. The agglomeration mechanism of waste liquid particles was obtained.

Keywords: waste drilling fluid; cationic polyacrylamide; colloidal particles; molecular simulation

1 Introduction

In the twenty-first century, environmental protection is an increasingly important issue [1,2]. According to marine protection needs [3] and national laws [4] and regulations [5], offshore oilfield operations require full waste recovery [6]. The most common waste in offshore operations is drilling fluid, which is usually composed of liquid, solid, and chemical substances [7]. After being circulated in the wellbore, the composition becomes more complex, and the toxicity to the environment is also increased. Environmental protection has become increasingly important, among which the Bohai Oil Field and the West South China Sea Oil Field have the highest environmental protection production requirements [8]. The drilling fluids and oil-containing completion/workover fluids in the reservoir must be fully recovered [9]. In 2020, there were 511 wells drilled in Bohai Oilfield. A single well will produce about 600 m³ of completion/workover fluid and waste drilling fluid, and a total of about 300,000 m³ of waste fluid will be produced. CNOOC has packaged the full recovery of drilling/completion/workover/cementing slurry waste into the service contract [10]. However, there is no mature process plan for on-site reduction in waste drilling fluid [11]. If the completion/repair is not perfect [12] and the shortcomings of waste drilling fluid treatment technology will seriously affect subsequent business development [13], even face the risk of contract suspension [14]. Only by completing relevant technical research can the entire business line form a closed loop [15]. With the increasingly severe environmental protection situation [16], in order to implement a zero-emission policy in marine operations [17], it is urgently necessary to study technologies suitable for offshore drilling waste treatment [18]. However, the mechanism of waste flocculation is not yet clear [19], so this article carried out a molecular simulation of flocculation. The mechanism by which the cationic polyacrylamide flocculates the waste is obtained.

2 The relationship between the stability of waste liquid and zeta potential

For drilling fluid, completion fluid, and workover waste fluid, they are all a colloidal system. The colloidal system is composed of a colloidal core, an adsorption layer, and a diffusion layer. The colloidal core is a colloidal particle formed by the polymerization of colloidal particles. It adsorbs a certain ion on the surface of the colloidal core and is charged. It is a counter ion, a part of the counter ion is closely attached to the surface of the colloidal particles and moves with the particles, which is called bound counter ions, forming an adsorption layer; the other part of the counter ions does not move with the colloidal particles to form a diffusion layer, which is called free counter ions. When the colloidal particles move, most of the free counter ions in the diffusion layer will escape and diffuse into the main body of the solution. The result is to make the colloidal particles generate a residual charge, so that a potential difference is formed between the colloidal particles and the diffusion layer, which is often called the zeta potential.

The colloidal particles can remain dispersed in water for a long time without sinking. This characteristic is called the stability of the colloid. The higher the zeta potential, the higher the stability of the colloid. The stability of the colloidal particles is mainly caused by three reasons: the Brownian motion of the colloidal particles, the electrostatic repulsion between the colloidal particles, and the solvation of the surface of the colloidal particles. For hydrocolloids (such as protein, starch, and other organic substances), the main reason for their stability is the hydration of the colloid. The charged colloidal particles can attract polar water molecules to form a hydration film, thereby preventing the colloidal particles from contacting each other and maintaining their stability. The stability of hydrophobic colloids (such as inorganic substances such as clay and some inorganic cationic polyacrylamides in water) is mainly caused by the repulsion between colloidal particles. When they collide in Brownian motion, they are caused by zeta potential. The electrostatic repulsion will prevent them from approaching and colliding with each other, so they cannot form larger particles and sink. Although there is also a mutual attraction between the colloidal particles-van der Waals force, due to the larger thickness of the diffusion layer outside the colloidal particles, the distance between the colloidal particles is large, and the kinetic energy of the Brownian motion is not enough to push the two colloidal particles closer to each other. The distance at which van der Waals force acts, so the influence of gravity can be ignored. The magnitude of electrostatic repulsion is related to two factors: one is the magnitude of zeta potential; the other is the distance between colloidal particles. The higher the zeta potential or the closer the distance between the colloidal particles, the greater the electrostatic repulsion. It is precisely because of the existence of this electrostatic repulsion that the colloidal particles cannot coalesce and maintain a stable dispersion state for a long time.

Zeta potential is an important indicator of the stability of the colloidal system. Because the surface of the colloidal particles is charged and attracts the surrounding counter ions, these counter ions are distributed in a diffusion state at the two-phase interface to form a diffuse electric double layer.

Zeta potential is usually calculated from the data of electrophoresis or electroosmotic velocity. The calculation methods of the two zeta potentials are as follows.

2.1 Calculate the zeta potential from the electrophoresis speed

Suppose that the electric particle charged is q, in an electric field with an electric field strength of E (suppose the distance between two levels is L, and the potential difference is ΔV; then, that is, E = ΔV/L, the potential difference per unit distance), the electrostatic force acting on the colloid is:

(1) f = q E .

If the radius of the spherical colloidal particles is r, the electrophoresis speed is v, and according to Stokes’ law, its frictional resistance is:

(2) f ' = 6 π η r v .

When the colloidal particles move at a constant speed, f = f ' :

(3) q E = 6 π η r v ,

(4) v E = q 6 π η r ,

v / E ( = U ) is the electrophoresis velocity of charged particles under unit electric field intensity, and the unit is m² V⁻¹ s⁻¹.

Generally, the charged properties of colloidal particles are not often expressed by the number of charges they carried, but by the magnitude of the zeta potential. According to the law of electrostatics:

(5) ζ = q D r .

In the formula, D is the dielectric constant of the liquid between the electric double layers. Put Eq. (4) into Eq. (5) to get:

(6) ζ = 6 π η v D E .

It can be seen that when the electrophoresis speed of the colloidal particles is measured under certain conditions, the zeta potential of the colloidal particles can be calculated according to formula (6). But formula (6) only applies to spherical colloidal particles. For rod-shaped colloidal particles, a correction factor of 2/3 is usually multiplied:

(7) ζ = 4 π η v D E .

2.2 Calculate zeta potential from electroosmotic velocity data

The zeta potential can also be calculated from the electroosmotic velocity data. This method is similar to the method of deriving the electrophoresis formula, but the more convenient method is to directly apply formula (7) to electro-osmosis, where the formula refers to the linear velocity of liquid flow. In the experiment, the volume of fluid flowing through the capillary per unit time is V ′ ( ml ) , the cross-sectional area of the capillary is A(cm²), v = V ′ A . According to Ohm’s law and the conductance formula, we can get:

(8) ζ = 4 π η v D E = 4 π η ( V ′ / A ) D ( I / K A ) = 4 π η K V ′ D I .

where K is the conductivity of the liquid, and the unit is S/m; I is the current, and the unit is A.

It can be seen from Eqs. (6) and (8) that the speed of electrophoresis or electro-osmosis is directly proportional to the applied electric field strength, zeta potential, and the dielectric constant of the liquid, inversely proportional to the viscosity of the liquid, and irrelevant to the particle size and the capillary length. The zeta potential is the potential difference between the continuous phase and the fluid stabilization layer attached to the dispersed particles. The important significance of zeta potential is that its value is related to the stability of colloidal dispersion. Zeta potential is a measure of the strength of mutual repulsion or attraction between particles. The smaller the molecule or dispersed particles, the higher the absolute value (positive or negative) of the zeta potential, and the more stable the system. On the contrary, the lower the zeta potential, the more likely it is to condense or agglomerate; that is, the attractive force exceeds the repulsive force, and the dispersion is destroyed, condensation or agglomeration occurs. The approximate relationship between zeta potential and system stability is shown in Table 1.

Table 1

The relationship between the zeta potential and the stability of the colloidal system

Zeta potential (mV)	0 to ±5	±10 to ±30	±30 to ±40	±40 to ±60	More than ±61
Colloidal stability	Fast setting or cohesion	Start to become unstable	General stability	Good stability	Excellent stability

In the research of this project, the zeta potential before and after cationic polyacrylamide ion of water-based drilling fluid was measured to analyze the relationship between zeta potential and the stability of the drilling fluid system.

The changes in zeta potential before and after drilling fluid system treatment are shown in the following Table 2.

Table 2

Changes of zeta potential before and after drilling fluid system treatment

Before processing	After treatment
Sample name: Drilling waste fluid	Sample name: Drilling waste fluid supernatant
Temperature: 25°C	Temperature: 25.2°C
Zeta potential (mV): −49.1	Zeta potential (mV): −17.7

It can be seen that the zeta potential of the waste drilling fluid before treatment is −49.1 mV, the drilling fluid system has good stability, and after the treatment agent is added, the solid phase in the waste drilling fluid is basically separated. After going out, the zeta potential of the separated supernatant is −17.7 mV, which is in an unstable state, which also explains that the zeta potential is related to the stability of the colloidal system.

To test the change in zeta potential before and after treatment, the following experiments are required: take 1,800 mL KCl/PHPA waste drilling fluid, with an average of three 600 mL each, and add 10 mL of cationic polyacrylamide solution with a concentration of 5% (the cationic polyacrylamide is a laboratory-made cationic polymer with a molecular weight of 200 × 10⁴, and a cationic degree is 25%). The rotation speed of the agitator is 300 rpm, and the mixing time is 10 min. It can be clearly observed that the drilling fluid starts to flocculate. After stopping the mixing for 30 min, the solid–liquid separation of waste drilling fluid can be observed, as shown in Figure 1.

Figure 1

Flocculation Test of KCl/PHPA Drilling Waste Fluid. (a) Add cationic polyacrylamide solution to start flocculation. (b) After adding cationic polyacrylamide and standing for 30 min, the KCl/PHPA waste drilling fluid system is solid–liquid separated.

3 Research on destabilization mechanism of KCl/PHPA drilling waste fluid

From the above-mentioned analysis, it can be seen that the stability of the colloidal particles is mainly related to the zeta potential, and the zeta potential represents the mutual repulsion between the two colloidal particles. Therefore, it can be calculated by calculating the difference between the different colloidal particles in the KCl/PHPA waste drilling fluid system. Energy changes to determine the mutual repulsion between two colloidal particles.

3.1 KCl/PHPA drilling fluid

KCl/PHPA drilling fluid is widely used in drilling operations in Bohai Oilfield, China, and the basic components are shown in Table 3.

Table 3

Basic proportions of KCl/PHPA drilling fluid

Material name	Material composition	Content (kg/m³)
Seawater
Bentonite	Sodium bentonite	30
Sodium hydroxide	NaOH	3
Sodium carbonate	Na₂CO₃	3
PF-PAC LV	Low viscosity polyanionic cellulose (PAC)	5
PLH-HPAM	Partially hydrolyzed polyacrylamide	5
PF-XC-H	Xanthan gum	0.5
PF-FLOTROL	Starch	10
Sodium chloride	NaCl	120
Potassium chloride	KCl	50
Barite	BaSO₄	Adjust the dosage according to the drilling fluid density

It can be seen that the main components in the KCl/PHPA drilling fluid are low-viscosity polyanion cellulose, partially hydrolyzed polyacrylamide (PHPA), Xanthan gum, and starch, which are generally anionic polymers (low-viscosity polyanion cellulose, PHPA) or non-ionic polymers (xanthan gum (XC) and starch). The cationic polyacrylamides used are cationic polymers, which generate electrostatic adsorption with anionic polymers, reducing the zeta potential of the system.

Therefore, when simulating the energy between two colloidal particles, it mainly simulates the energy change between low-viscosity PAC, PHPA, XC, starch, and cationic polyacrylamide (cationic polyacrylamide).

3.2 Component analysis of KCl/PHPA drilling waste fluid

The oil content, suspended solids content, median particle size, and zeta potential in the KCl/PHPA drilling waste fluid are analyzed, as shown in Table 4.

Table 4

Component analysis of KCl/PHPA drilling waste fluid

Number	Test items	Test results
1	Oil content, mg/L	753.01
2	Suspended matter content, mg/L	75,000
3	Median particle size, μm	1.016
4	Viscosity, mPa·s	71
5	Zeta potential, mV	−47.4

It can be seen that the KCl/PHAP waste drilling fluid has certain oil content, high viscosity, and a large amount of suspended solids. Among them, there are large-diameter particles exceeding 5 μm, and the zeta potential is high and stable.

Through the earlier analysis, there are bentonite particles and anionic treatment agents in the KCl/PHPA waste drilling fluid. The treatment agent is adsorbed on the bentonite particles to increase the zeta potential of the colloidal particles. At the same time, the oil content in the waste fluid is relatively high. Therefore, destabilization mechanism includes three aspects: the sedimentation mechanism of bentonite particles, the viscosity reduction mechanism of anionic treatment agents, and the demulsification mechanism of crude oil.

3.3 Energy simulation of KCl/PHPA drilling waste fluid

Molecular simulation can well display the energy image of various additives, and the calculation module is relatively reasonable, which can not only provide good guidance for this study, but also be used for the drilling fluid with nano-particles, such as conduct simulation.

Materials Studio 2017R2 software is used to simulate the structure of low-viscosity PAC, XC, PHPA, starch, and cationic polyacrylamide using the Visualizer module, and use the Geometry Optimization tool in the forcite module to perform the single-molecule model structure optimization and select Compass (Version2.8) force field, electrostatic interaction, and van der Waals interaction using Ewald and Atom-based summation methods, respectively, using Smart Minimization algorithm to make the molecules reach the energy minimization model.

Construction (Legacy) in the Amorphous Cell module is selected, and optimized low-viscosity PAC, cationic polyacrylamide, and water molecule models are introduced in Constant Molecules, and the molecules number of low-viscosity anionic cellulose, cationic polyacrylamide, and water is 2, 1, and 100, respectively.

The Dynamics tool is used in the Forcite module to calculate the optimized layer. Ensemble is selected as NVT (Regular Ensemble), the temperature is 278 K, the time step is 1 fs, total simulation time is 500 ps, the number of steps is 5,000, and molecular dynamics simulation is conducted in Compass force field. Calculations for each model are repeated many times, so that the deviation of each data is within 5%.

3.3.1 Energy changes of low viscosity PAC and coagulant

The molecular structure model of low viscosity PAC and cationic polyacrylamide monomer is shown in Figure 2.

Figure 2

Molecular structure model of low viscosity PAC and cationic polyacrylamide monomer.

For the KCl/PHPA drilling fluid waste, the low-viscosity PAC adsorbs water molecules to form a colloidal particle. In the numerical simulation, two low-viscosity polyanionic celluloses are generally combined with 100 water molecules, and the conformation model is shown in Figure 3.

Figure 3

Conformation model of low viscosity PAC and 100 water molecules.

Materials Studio2017R2 software is used to simulate the energy in the above-mentioned molecular conformation, and the data is shown in Table 5.

Table 5

Conformational model energy of low viscosity PAC + water molecule

Simulation projects	Simulation results
Low viscosity PAC + water molecules	2:100
Intermolecular force, kcal/mol	288.641
Electrostatic force, kcal/mol	−1672.362
Correction value, kcal/mol	−9.201
Total energy, kcal/mol	−1392.923

After adding cationic polyacrylamide to the KCl/PHPA waste drilling fluid, the low-viscosity PAC and the cationic polyacrylamide are electrostatically adsorbed to form a macromolecule, which combines with 100 water molecules. The concentration of low-viscosity PAC is generally greater than the concentration of the added cationic polyacrylamide. The low-viscosity PAC, cationic polyacrylamide, and water molecules are combined in a ratio of 2:1:100. The conformation model is shown in Figure 4.

Figure 4

The conformational model of low viscosity PAC, cationic polyacrylamide, and water molecules in 2:1:100.

Materials Studio2017R2 software is used to simulate the energy in the above-mentioned molecular conformation, and the data are shown in Table 6.

Table 6

Conformational model energy of low viscosity PAC + cationic polyacrylamide + water molecule

Simulation projects	Simulation results
Low viscosity PAC + coagulant + water molecule	2:1:100
Intermolecular force (kcal/mol)	283.424
Electrostatic force (kcal/mol)	−736.698
Correction value (kcal/mol)	0
Total energy (kcal/mol)	−453.274

Comparing the energy changes in the Tables 5 and 6, before adding the cationic polyacrylamide, the total energy of the low viscosity PAC and water molecule system is −1392.923 kcal/mol, when the cationic polyacrylamide is added, the total energy of the mixed system is −453.274 kcal/mol, the absolute value of energy in the system dropped by 939.649 kcal/mol, and the rate of decrease was 67.5%. The decrease in the energy of the colloidal system is mainly due to the decrease of the electrostatic attraction in the system, which is mainly manifested by the decrease of the negative charge. It can also be seen that when the low viscosity PAC is adsorbed with the cationic polyacrylamide, because the low viscosity PAC is negatively charged, and the cationic polyacrylamide is positively charged, when the two are adsorbed, part of the positive and negative charges occurs. Electricity neutralization reduces the negative charge of the low viscosity PAC, reduces the zeta potential, and reduces the repulsive force between the colloidal particles.

The changes in energy and zeta potential between the two colloidal particles show a decreasing trend. Therefore, the two colloidal particles are close to each other and tend to aggregate.

3.3.2 Energy changes of PHPA and coagulant

a PHPA model is established, as shown in Figure 5.

Figure 5

Partially hydrolyzed polyacrylamide model.

Two PHPA molecules are combined with 100 water molecules, and the conformation model is shown in Figure 6.

Figure 6

Two PHPA models and 100 water molecules.

Materials Studio2017R2 software is used to simulate the energy in the above-mentioned molecular conformation, and the data are shown in Table 7.

Table 7

Conformational model energy of PHPA + water molecule

Simulation projects	Simulation results
Partially hydrolyzed polyacrylamide molecule + water molecule	2:100
Intermolecular force (kcal/mol)	251.077
Electrostatic force (kcal/mol)	−1179.954
Correction value (kcal/mol)	−8.856
Total energy (kcal/mol)	−937.733

Partially hydrolyzed polyacrylamide molecules, cationic polyacrylamide molecule and water molecules are combined in a ratio of 2:1:100. The conformation model is shown in Figure 7.

Figure 7

2:1:100 Model of PHPA, cationic polyacrylamide, and water molecules.

Materials Studio2017R2 software is used to simulate the energy in the above-mentioned molecular conformation, and the data are shown in Table 8.

Table 8

Conformational model energy of PHPA + cationic polyacrylamide + water molecule

Simulation projects	Simulation results
Partially hydrolyzed polyacrylamide + coagulant + water molecule	2:1:100
Intermolecular force (kcal/mol)	249.936
Electrostatic force (kcal/mol)	−640.454
Correction value (kcal/mol)	0
Total energy (kcal/mol)	−390.518

Comparing the energy changes in Tables 7 and 8, before adding the cationic polyacrylamide, the total energy of the PHPA and water molecule system is −937.733 kcal/mol; after adding the cationic polyacrylamide, the total energy of the mixed system is −390.518 kcal/mol, the absolute value of energy in the system dropped by 547.215 kcal/mol, and the rate of decrease was 58.4%. The decrease in the energy of the colloidal system is mainly due to the decrease in the electrostatic attraction in the system, which is mainly manifested by the decrease in the negative charge. It can also be seen that when PHPA adsorbs the cationic polyacrylamide, because the PHPA is negatively charged, and the cationic polyacrylamide is positively charged, when the two are adsorbed, part of the positive and negative charges are electrically neutralized. So that the negative charge of PHPA is reduced, the zeta potential is reduced, and the repulsion between the colloidal particles is reduced.

The changes in energy and zeta potential between the two colloidal particles show a decreasing trend. Therefore, the two colloidal particles are close to each other and tend to aggregate.

3.3.3 Energy changes of XC and coagulant

The establishment of the XC model is shown in Figure 8.

Figure 8

XC model.

Two XC molecules are combined with 100 water molecules, and the conformational model is shown in Figure 9.

Figure 9

XC molecule and 100 water molecule model.

Materials Studio2017R2 software is used to simulate the energy in the above mentioned molecular conformation, and the data are shown in Table 9.

Table 9

The conformational model energy of XC + water molecule

Simulation projects	Simulation results
XC + water molecules	2:100
Intermolecular force (kcal/mol)	432.509
Electrostatic force (kcal/mol)	−844.298
Correction value (kcal/mol)	0
Total energy (kcal/mol)	−411.789

Two XC molecules, one cationic polyacrylamide molecule, and 100 water molecules construct a model of 2:1:100, as shown in Figure 10.

Figure 10

XC, cationic polyacrylamide and water molecules in a 2:1:100 model.

Materials Studio2017R2 software is used to simulate the energy in the above-mentioned molecular conformation, and the data are shown in Table 10.

Table 10

The conformational model energy of XC + cationic polyacrylamide + water molecule

Simulation projects	Simulation results
XC + coagulant + water molecules	2:1:100
Intermolecular force (kcal/mol)	397.703
Electrostatic force (kcal/mol)	−673.335
Correction value (kcal/mol)	−12.053
Total energy (kcal/mol)	−287.686

Comparing the energy changes in Tables 9 and 10, before adding the cationic polyacrylamide, the total energy of the XC and water molecule system is −411.789 kcal/mol, when the cationic polyacrylamide is added, the total energy of the mixed system is −287.686 kcal/mol, the absolute value of energy in the system decreased by 124.103 kcal/mol, and the rate of decrease was 30.1%. XC is generally a non-ionic polymer with no negative charge. When XC is adsorbed with clay particles in the drilling fluid, it has a partial negative charge. Therefore, the colloidal particles formed by XC are also negatively charged and exhibit electrostatic repulsion. When the cationic polyacrylamide with cations is added, the positive and negative charges will be neutralized and the electrostatic repulsion will decrease. It can also be seen that when the negatively charged xanthan colloidal particles adsorb the cationic polyacrylamide, part of the positive charge and the negative charge are electrically neutralized, so that the negative charge of the xanthan colloidal particles is reduced, and the zeta potential is reduced, the repulsive force between decreases.

The changes in energy and zeta potential between the two colloidal particles show a decreasing trend. Therefore, the two colloidal particles are close to each other and tend to aggregate.

3.3.4 Energy changes of starch and coagulant

The starch model is shown in Figure 11.

Figure 11

Starch model.

Two starch molecules are combined with 100 water molecules, and the conformational model is shown in Figure 12.

Figure 12

Model of 2 starch molecules and 100 water molecules.

Materials Studio2017R2 software is used to simulate the energy in the above-mentioned molecular conformation, and the data are shown in Table 11.

Table 11

Conformational model energy of starch + water molecule

Simulation project	Simulation result
Starch + water molecule	2:100
Intermolecular force (kcal/mol)	291.654
Electrostatic force (kcal/mol)	−581.872
Correction value (kcal/mol)	−12.425
Total energy (kcal/mol)	−302.643

The 2:1:100 model of starch, cationic polyacrylamide, and water molecules is shown in Figure 13.

Figure 13

The 2:1:100 model of starch and cationic polyacrylamide and water molecules.

Materials Studio2017R2 software is used to simulate the energy in the above-mentioned molecular conformation, and the data are shown in Table 12.

Table 12

Energy of conformational model of starch + cationic polyacrylamide + water molecule

Simulation projects	Simulation results
Starch + coagulant + water molecules	2:1:100
Intermolecular force (kcal/mol)	298.978
Electrostatic force (kcal/mol)	−524.764
Correction value (kcal/mol)	−12.012
Total energy (kcal/mol)	−237.799

Comparing the energy changes in Tables 11 and 12, before adding the cationic polyacrylamide, the total energy of the starch and water molecule system is −302.643 kcal/mol; after adding the cationic polyacrylamide, the total energy of the mixed system is −237.799 kcal/mol. The absolute value of energy in the system dropped by 64.844 kcal/mol, and the rate of decrease was 21.4%. Starch is generally a non-ionic polymer with no negative charge. When starch is adsorbed with clay particles in the drilling fluid, it is partially negatively charged. Therefore, the colloidal particles formed by starch also exhibit negative electrostatic repulsion. When the cationic polyacrylamide with cations is added, the positive and negative charges will be neutralized and the electrostatic repulsion will decrease. It can also be seen that when the starch colloidal particles with negative charges adsorb the cationic polyacrylamide, part of the positive and negative charges are electrically neutralized, so that the negative charge of the starch colloidal particles is reduced, the zeta potential is reduced, and the repulsion between the colloidal particles is reduced. small. The changes in energy and zeta potential between the two colloidal particles show a decreasing trend. Therefore, the two colloidal particles are close to each other and tend to aggregate.

4 Conclusion

The energy reduction rate of the low-viscosity PAC and water molecule system is 67.5%. The total energy reduction rate of the PHPA and water molecule system is 58.4%. The total energy reduction rate of the system of XC and water molecules is 30.1%. The total energy reduction rate of the starch and water molecule system is 21.4%. The decrease of energy in the four systems is mainly due to the decrease of the electrostatic attraction in the system, which is mainly manifested in the decrease of the negative charge of the whole system due to the positive charge of the coagulant. When the coagulant is adsorbed with the substances in the system, part of the positive and negative charges are electrically neutralized, the zeta potential is reduced, and the repulsion between the colloidal particles is reduced. For anionic drilling fluid treatment agents, after adding a coagulant, the system energy is greatly reduced, while for ionic treatment agents, after adding a coagulant, the system energy is reduced. For the KCl/PHPA drilling fluid contained anionic and ionic treatment agents, after adding the coagulant, the energy of the system is reduced by 44.4%, which reduces the repulsive force between colloidal particles and promotes the aggregation of colloidal particles.

Funding information: The authors state no funding involved.
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.

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Received: 2022-04-10

Revised: 2023-03-13

Accepted: 2023-07-08

Published Online: 2023-08-30

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/phys-2023-0271

Keywords for this article

waste drilling fluid; cationic polyacrylamide; colloidal particles; molecular simulation

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