A preliminary evaluation of inorganic composite profile control and oil displacement agent for high water cut oil fields

ABSTRACT

An environmentally friendly inorganic composite deep profile control and oil displacement agent was prepared for high water cut oil fields with formation water containing sodium bicarbonate. Sodium silicate and calcium chloride were used as the main agents, while polycarboxylic acid dispersant and liquid polymer were used as auxiliary agents. The effects of the type of polycarboxylic acid dispersants and the mass concentration of liquid polymer on the performance of deep profile control and oil displacement agent were studied. The results showed that when the acid-ether ratio of the polycarboxylic acid dispersant was 3.7 and its molecular weight was 23800, the electrostatic repulsion and steric hindrance from polycarboxylic acid dispersant and inorganic particles achieved an effective dispersing effect. Combined with low concentration of liquid polymer (500 mg/L), the viscosity and strength of the formulated inorganic composite deep profile control and oil displacement agent could be enhanced. Results showed that this inorganic composite deep profile control and oil displacement agent could increase the flow resistance, with a plugging rate of 82.3%. It could achieve online injection, rapid dispersion, deep profile control and oil displacement with low cost, which showed high potentials in applications in high water cut oil fields.

Keywords:
High water cut oil field; Polycarboxylic acid dispersant; Adsorption performance; Steric hindrance; Liquid polymer

VISUAL ABSTRACT

1. INTRODUCTION

The formation heterogeneity of offshore oil fields is strong. Some oil fields directly inject seawater, with salinity of 3.40 × 10⁴ mg/L, and the mass concentration of calcium and magnesium ions in the injection water is up to 1000 mg/L. The injectivity or salt resistance of existing polymer gel is difficult to meet environmental requirements, while the cost of salt-resistant modified polymer is high, which is difficult to meet economic requirements.

There are a large number of cations in the high salinity injection water of offshore oil fields. When a certain concentration of liquid sodium silicate and a low concentration of liquid polymer are injected into the reservoir, chemical reaction occurs after encountering the cations in the formation water. The generated inorganic diverting agent can form a coating on the surface of the formation pore channel, increase the flow resistance, generate the flow diversion, and realize the effect of profile adjustment. Among them, the traditional sodium silicate-calcium chloride inorganic displacement system has the problem of rapid reaction and precipitation after contact. By adding precipitation inhibitors (PBTC, ATMP, HEDP, etc.), the generation time and initial particle size of calcium silicate precipitation can be effectively controlled. The injected liquid system moves with the injected water to the deep part of the formation and settles and accumulates, achieving the effect of effectively blocking high permeability channels. It is applied in high-temperature and high-salinity offshore oil fields and has achieved good oil recovery effects [¹,²,³,⁴]. However, under conditions where the content of divalent ions in the injected water is low and the water contains a certain amount of bicarbonate ions, the added calcium chloride will quickly form a calcium carbonate precipitate with the sodium bicarbonate in the injected water. Due to the large injection water volume (500~1000 m³/d) in the Bohai Oilfield, a calcium carbonate scale coating is easily formed in the injection pipeline during the injection of the profile control agent, which can block the injection pipeline [⁵,⁶,⁷,⁸,⁹]. At the same time, as the main offshore water flooding development oil fields enter the high water cut development stage, reservoir heterogeneity and large pore development are significant. While ensuring good injectivity and deep migration of the inorganic precipitation system, it is necessary to further improve the plugging strength of the precipitation system [¹⁰,¹¹,¹²], ensure effective plugging of large pore channels between wells, and increase the swept volume of injected water.

In response to the above issues, this study first optimized polycarboxylic acid dispersants, which affect particle dispersion performance through electrostatic repulsion and steric hindrance, thereby enhancing the dispersion stability of precipitated particles. Then, liquid polymer with low hydrolysis degree and low molecular weight is selected to enhance the strength of inorganic precipitation profile control and flooding agent, and an inorganic composite profile control and flooding agent is obtained, and experiments on characterizing its dispersion, stability, and plugging performance are conducted. This inorganic composite flooding system can ensure the rapid dispersion and non precipitation of the flooding agent during injection for reservoirs with low divalent ion content and a certain amount of bicarbonate ions in the water. It is not easy to form a calcium carbonate scale coating in the water injection pipeline, which can block the water injection pipeline. At the same time, it can effectively migrate to the deep part of the formation and then precipitate and aggregate to block large pores.

2. MATERIALS AND METHODS

2.1. Materials

Sodium chloride (reagent grade, content 99.5%), calcium chloride (reagent grade, content 99.5%), Aladdin Biochemical Technology Co., Ltd; Main agent sodium silicate (industrial grade, content 99%), Nantong Mengya New Material Technology Co., Ltd; Polycarboxylate dispersant samples 1 to 7 (industrial grade, mass fraction 99%), Hunan Jinyu Huatai Co., Ltd; Liquid anionic polyacrylamide HPAM (content 29%, degree of hydrolysis 5%, molecular weight 3 million), Essen China Flocculant Co., Ltd.

DV-II viscosity meter, Brookfield Corporation, USA; Zetasizer Nano ZS Nano Laser Particle Size Analyzer, Malvern Instruments Limited, UK; BIOTECTOR B3500e Total Organic Carbon (TOC) Analyzer, Hash Corporation, USA; UF160 electric constant temperature drying oven, Meierte GmbH, Germany; LSP sand filling pipe displacement simulation device, Jiangsu Hua’an Scientific Research Instrument Co., Ltd.

2.2. Methods

2.2.1. Preparation of inorganic precipitation controlled flooding system

The preparation process of the mother liquor of main agent is as follows: prepare dispersant solution (15% mass fraction) using a certain amount of simulated formation water, and at a stirring speed of 300 r/min, add 5% mass fraction of sodium silicate powder until completely dissolved. The preparation process of additive mother liquor: prepare calcium chloride solution (15% mass fraction) using deionized water at room temperature. Mix the designated proportion of main agent mother liquor and auxiliary agent mother liquor, and place them in a colorimetric tube. The composition of simulated formation water is in Table 1.

Table 1
Ionic composition of simulated formation water in Q oilfield.

ρ(CO32−)/ (mg/L)	ρ(HCO3−)/ (mg/L)	ρ(Cl−)/ (mg/L)	ρ(SO42−)/ (mg/L)	ρ(Ca2+)/ (mg/L)	ρ(Mg2+)/ (mg/L)	ρ(K+ + Na+)/ (mg/L)	SALINITY/ (mg/L)
405.00	421.04	1178.71	19.18	8.02	4.86	1224.75	3261.00

The sample of polycarboxylate dispersant used in the experiment is a zwitterionic polycarboxylate, prepared by free radical aqueous solution polymerization using methallyl polyethylene glycol ether (HPEG), acrylic acid (AA), and methacryloyloxyethyltrimethylammonium chloride (DMC) as raw materials. The monomer mass fraction was 38%, the initiator ammonium persulfate (APS) was added at 1.8% of the total monomer mass, and the chain transfer agent mercaptoacetic acid (TGA) was added at 0.5% of the total monomer mass. The reaction temperature was 65 °C and the reaction time was 4 hours. Polycarboxylic acids with different acid ether ratios were synthesized. The acid ether ratio is the ratio of AA to HPEG (A/E value), and the conversion rate is represented by PE. The relevant characteristic parameters of the polycarboxylate dispersant samples (samples 1−7) are listed in Table 2.

Table 2
Characteristic parameters of samples of polycarboxylic acid dispersants.

SAMPLE NAME	SAMPLE 1	SAMPLE 2	SAMPLE 3	SAMPLE 4	SAMPLE 5	SAMPLE 6	SAMPLE 7
A/E	3.2	3.7	4.3	5.3	3.7	3.7	3.7
Mw	23 400	23 800	23 600	23 800	16 200	27 400	36 700
PE/%	90.5	90.6	90.7	90.5	90.8	90.8	91.0

2.2.2. Preparation of inorganic composite EOR system

On the basis of precipitation profile control and flooding agent prepared in Section 2.2.1, add 300~800 mg/L liquid polymer solution to prepare inorganic composite profile control and flooding system.

2.2.3. Performance characterization

1. Stability testing: Use a nano laser particle size analyzer to test the particle size distribution of the system at different heating times, and plot the variation curve of particle size distributions with heating time.

2. Composite system viscosity test: Use a rotary viscosimeter to test the viscosity change of the composite system when adding different proportions of liquid polymer (0, 300, 500, 800 mg/L) to the inorganic profile control agent.

3. Adsorption capacity experiment: Referring to GB/T 32116-2015 “Determination of Total Organic Carbon (TOC) in Circulating Cooling Water”, the total organic carbon analysis method is used to test the adsorption performance of polycarboxylate dispersants.

4. Zeta potential analysis: Use a zeta potential meter to test the zeta potential of the system.

5. Injection and plugging performance testing: A sandpack displacement experimental device is used, with pipe length of 100 cm, pipe diameter of 2.54 cm, one pressure measuring point installed every 20 cm, and one pressure measuring point installed at the inlet end, for a total of 5 pressure measuring points.

3. RESULTS AND DISCUSSION

3.1. Influence of dispersant acid ether ratio on stability

After mixing the mother liquor of main agent and the mother liquor of auxiliary agent, an inorganic flooding agent can be formed. The molecular structure of prepared dispersant is shown in Figure 1. At simulated reservoir temperature (65 °C), the sample formed by organic phosphorus precipitation inhibitor precipitates and adsorbs at the bottom of the colorimetric tube within 30 minutes of mixing. The other four samples (using polycarboxylate dispersants) showed good stability of the system at 65 °C.

Figure 1
Molecular structure of prepared dispersant.

For polycarboxylate dispersants, under the same polymer molecular weight (Mw) and polyether conversion rate (PE), the effect of acid ether ratio on the stability of the samples was tested. The parameters of dispersant samples 1 to 4 are listed in Table 2. After mixing a main agent solution with an effective content of 1000 mg/L sodium silicate (containing a dispersant mass concentration of 3000 mg/L) and an auxiliary agent solution with an effective mass concentration of 1000 mg/L calcium chloride in a continuous slow stirring manner, the experimental results of the particle size variation of the system with heating time are shown in Figure 2, and the particle size distribution after heating for 120 hours is shown in Figure 3.

Figure 2
Effects of acid-ether ratios (A/E values) of the dispersants on the median particle size.

Figure 3
The particle size distribution for system of dispersants with different A/E values after heating for 120 hours.

By analyzing the dispersibility of samples with different dispersants, it can be seen that under the same molecular weight of dispersants, the acid ether ratio of dispersants has a significant impact on the ability to disperse and precipitate particles. When the ratio of acid to ether is higher, it indicates that more acid is used in the synthesis process of the dispersant, that is, the content of groups that adsorb and precipitate particles is higher, the adsorption capacity is relatively strong, and it is easier to form smaller particles in the initial stage. Therefore, sample 4 has the smallest initial particle size. However, when the acid ether ratio exceeded 4.3 (samples 3 and 4), the particle size of the system increased significantly after 48 hours of heating, and aggregation and precipitation phenomena appeared. It is speculated that when the acid ether ratio increases to a certain value, under a certain molecular weight, as the acid ether ratio increases, the density of side chains in the polycarboxylate dispersant decreases, the spacing between side chains increases, and side chain curling occurs. An important mechanism for dispersing precipitation particles with polycarboxylate dispersants is the extension of the side chains of the polycarboxylate dispersants into the liquid phase, forming a polymer molecule adsorption layer on the surface of the precipitation particles. When the precipitation particles approach each other and overlap with the adsorption layer, a steric hindrance effect is generated, achieving effective dispersion of the precipitation particles [¹³]. For samples 3 and 4, due to the high acid ether ratio of the polycarboxylate dispersant, although it initially had a good effect on dispersing and precipitating particles, with the increase of heating time, the low side chain density caused side chain curling, and the steric hindrance effect of the dispersant weakened, resulting in a decrease in the stability of the inorganic precipitation displacement control agent. Therefore, through optimization, sample 2 (with an acid ether ratio of 3.7) has the most suitable acid ether ratio.

3.2. Influence of dispersant molecular weight on stability

The molecular weight of polycarboxylate dispersants has a significant impact on the performance of dispersing inorganic precipitate particles. Select dispersant products with an acid ether ratio of 3.7 (samples 2, 5, 6, and 7), and investigate the effect of different molecular weights of polycarboxylates on the dispersion and precipitation of particles. The change in system particle size with heating time is shown in Figure 4 after mixing a main agent solution of sodium silicate with a mass concentration of 1000 mg/L (containing a dispersant mass concentration of 3000 mg/L) and an auxiliary agent of calcium chloride with a mass concentration of 1000 mg/L in a continuous slow stirring manner. The particle size distribution for system of dispersants with different molecular weights after heating for 120 hours is shown in Figure 5.

Figure 4
Effects of dispersant molecular weights on median particle size.

Figure 5
The particle size distribution for system of dispersants with different molecular weights after heating for 120 hours.

As shown in Figure 4 and Figure 5, under the constant acid ether ratio of polycarboxylate dispersant (A/E value = 3.7), different molecular weights of dispersants have a significant impact on the dispersion stability of precipitated particles. As the molecular weight increases, the stability of the inorganic precipitation system first increases and then decreases. The speculated reason is that due to the fixed acid ether ratio, the proportion of adsorption groups is constant. Increasing the molecular weight can increase the absolute content of adsorption groups, enhance the adsorption capacity, and improve the dispersibility of dispersants on precipitated particles. However, excessively high molecular weight will increase the curling degree of the dispersant, which is not conducive to the spatial hindrance effect and thus reduces the dispersibility of the dispersant [¹⁴, ¹⁵]. The selected molecular weight is 23800 (sample 2), which ensures good dispersibility for calcium silicate and calcium carbonate precipitation particles.

3.3. Effect of liquid polymer on viscosity increasing of composite system

In order to further increase the strength of inorganic precipitation profile control and flooding system, low mass concentration liquid polymer is added to the inorganic profile control and flooding system to increase the viscosity of the system, thus enhancing the flow resistance of the water phase. Since the dispersant is a polymer system containing anions, the purpose of effectively dispersing precipitated particles is achieved through electrostatic repulsion and steric hindrance, while the liquid polymer has a charged polymer long chain. In order to reduce the influence of liquid polymer on the dispersion performance of the composite system, liquid polymer products with low hydrolysis degree (5%) and low molecular weight (3 million) are selected. The preferred formula for inorganic displacement control agent is: a main agent solution with a mass concentration of 1000 mg/L sodium silicate (dispersant is sample 2, mass concentration is 3000 mg/L), mixed evenly with an auxiliary agent solution with a mass concentration of 1000 mg/L calcium chloride. Analyze the viscosity change of the composite system when adding liquid polymer of different mass concentrations (0, 300, 500, 800 mg/L) to the inorganic profile control agent (Figure 6).

Figure 6
Variation of the viscosity of composite system with the addition of liquid polymer.

It can be seen from Figure 6 that the viscosity of the composite system increases with the increase of the mass concentration of liquid polymer. And compared with the viscosity of liquid polymer with the same mass concentration, the viscosity increase of the composite system is far higher than the viscosity of liquid polymer with the same mass concentration, which indicates that there is a synergistic viscosity increase effect between the inorganic precipitation system and liquid polymer [¹⁶]. Adding a certain amount of liquid polymer with low mass concentration (300 ~ 800 mg/L) can increase the viscosity of the aqueous phase of the composite system and increase the flow resistance of the system.

3.4. Effect of liquid polymer on adsorption and surface/interface properties of composite system

The prerequisite for effective dispersion of polycarboxylate dispersants is effective adsorption on the surface of inorganic precipitation particles, therefore the adsorption performance of polycarboxylate dispersants has a significant impact on the dispersion of inorganic precipitation particles. In order to characterize the dispersion performance of different composite systems, the adsorption capacity of sample 2 (mass concentration 3000 mg/L) on the surface of inorganic composite particles containing liquid polymer (0, 300, 400, 500, 600, 700, 800 mg/L) with different mass concentration was tested. The corresponding relationship between liquid polymer mass concentration and adsorption capacity is shown in Figure 7.

Figure 7
Relationship between the mass concentration of liquid polymer and the adsorption capacity.

After adding low mass concentration liquid polymer, the adsorption capacity of polycarboxylic acid dispersant on the particle surface is higher than that without liquid polymer. When the mass concentration of liquid polymer is 500 mg/L, the adsorption capacity of dispersant on the particle surface is optimal.

Zeta potential of inorganic composite system was measured when different liquid polymer mass concentrations were added (Figure 8). It can be seen from Figure 8 that with the addition of a small amount of liquid polymer, the liquid polymer dissociates into anions in water, the increasing anions improve the electrostatic repulsion between the particles of the composite, and the Zeta potential of the composite system increases, increasing the stability of the system. However, with the continuous increase of liquid polymer dosage, the potential gradually decreased. This is because the ion strength in solution is related to the mass concentration. The higher the mass concentration, the higher the ion strength, and the stronger the shielding effect on the surface potential of particles, resulting in a corresponding decrease in the Zeta potential of particles. The added mass concentration of liquid polymer optimized by Zeta potential results is 500 mg/L. In addition, results in Figure 8 show that the absolute value of the Zeta potential is no more than 10 mV, indicating relatively weak electrostatic repulsion effect between particles in the samples. Therefore, we assume that the electrostatic repulsion is not the main reason for enhancing the stability of the inorganic composite system, and the spatial hindrance effect of polycarboxylic acid dispersants and inorganic particle surfaces contributes more to enhancing the dispersion stability of the system.

Figure 8
Relationship between the concentration of liquid polymer and Zeta potential of the composite system.

3.5. Plugging performance of composite system

Inject simulated water using a sandpack displacement device until the pressure at each pressure measuring point is basically stable, and calculate the water permeability to be 4500 mD. Inject liquid polymer/inorganic profile control and flooding composite system with a total volume of 0.5 times of the pore volume (PV) (the mass concentration of main agent silicate is 1000 mg/L, the mass concentration of auxiliary agent is 1000 mg/L, and the mass concentration of dispersant is 3000 mg/L). The simulated reservoir temperature was 65 °C and heated for 7 days before switching to subsequent water flooding, and continuously recorded the pressure changes at each pressure measuring point throughout the process (Figure 9).

Figure 9
Plots of pressure versus PV of water injection (liquid polymer/inorganic composite deep profile control and oil displacement agent).

The liquid polymer and inorganic precipitation profile control system are injected after they are combined. The pressure response in the subsequent water flooding stage is obvious, mainly because the injected liquid polymer will reduce the permeability of the pores to some extent. In addition to effectively reducing the permeability of large pores, the precipitation particles generated after 7 days of heating will also interact with the liquid polymer remaining in the pores to form larger aggregates, which will further increase the pressure of subsequent water flooding and further reduce the permeability. The test showed a plugging rate of 82.3% and a residual resistance coefficient of 7.1, demonstrating good effect in blocking advantageous flow channels.

4. CONCLUSION

1. For the high water cut oil field with formation water containing sodium bicarbonate, the environment-friendly inorganic composite profile control and flooding agent was prepared with sodium silicate and calcium chloride as main agents, polycarboxylic acid dispersant and liquid polymer as auxiliaries. In order to improve the dispersion performance of the system, the results showed that appropriately increasing the acid ether ratio or molecular weight of the polycarboxylate dispersant can accelerate the adsorption performance of the dispersant on the surface of precipitated particles. However, excessively high acid ether ratio or molecular weight has adverse effects on the dispersion performance of the dispersant in the inorganic particle system. When the acid ether ratio of the polycarboxylate dispersant is 3.7 and the molecular weight is 23800, the electrostatic repulsion and steric hindrance between the polycarboxylate dispersant and the inorganic particle surface are balanced, and the system can achieve good dispersion stability.

2. In order to enhance the plugging performance of the system for high permeability reservoirs, through characterizing the viscosity, adsorption, surface interface properties, and plugging performance of the composite system, it is preferred to add a low degree of hydrolysis, low molecular weight (degree of hydrolysis 5%, molecular weight 3 million) liquid polymer solution with a mass concentration of 500 mg/L, which can further enhance the aqueous phase viscosity of the composite system, the adsorption capacity of dispersant on the particle surface and stability. At a simulated permeability of 4500 mD, the plugging efficiency reached 82.3% and the residual resistance coefficient reached 7.1, achieving effective plugging of the high permeability channel.

5. ACKNOWLEDGMENTS

The authors wish to express their appreciation for the funding provided by National Science and Technology Major Project of China (51704035); Overseas Returned Scholars Foundation of Tianjin; Important Science & Technology Foundation of China Oilfield Service Limited, CNOOC (E-23247009).

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Publication Dates

• Publication in this collection
  25 July 2025
• Date of issue
  2025

History

• Received
  03 Mar 2025
• Accepted
  25 June 2025