Preparation and Performance Evaluation of Quaternary Ammonium Cationic Clay Stabilizer for Oilfield Use

Acidizing is a crucial technique in the development of low permeability reservoirs in offshore oil fields. The introduction of acid into the formation can lead to the degradation of the clay mineral structure, causing the expansion and migration of clay particles that obstruct the flow of acid channels. The addition of a clay stabilizer with excellent properties can prevent the hydration and expansion of clay, as well as recover damage caused by long-term acidification, such as expansion, dispersion, and migration. To achieve this, the dendritic quaternary ammonium salt ZS-A was synthesized through a quaternization reaction, and subsequently used to develop the clay stabilizer ZSFP. In this study, the properties of clay stabilizer were investigated, including anti-swelling, acid and alkali resistance, water washing resistance, mudstone loss rate, and permeability retention. The results demonstrate that the ZSFP clay stabilizer performs well in terms of anti-swelling, acid-base compatibility, water washing resistance. This clay stabilizer can effectively reduce water-sensitive damage to the reservoir and protect it during acidizing operations.


Introduction
During the development of offshore oilfield reservoirs, the presence of clay minerals, such as montmorillonite and kaolinite, can have a significant impact on the severity of reservoir damage.These minerals have strong hydration expansion characteristics, making them highly water-sensitive.Acid fluids and injection water can easily cause serious damage to such reservoirs over the long-term.In the process of acidification, it is crucial to carefully choose clay stabilizers that are appropriate for the unique characteristics of offshore oil fields.This helps to minimize water sensitive damage to the reservoir, and ultimately improve the development of high water cut later oil reservoirs, leading to an increase in the recovery rate of old oil fields [1][2][3][4].
The clay stabilizer industry has experienced rapid changes due to the continuous development of oil field industries both domestically and internationally.In the early days of oil fields, inorganic salts were commonly used to prevent swelling.This was achieved by the ionization of cations after the dissolution of inorganic salts, which then neutralized negative charges in the clay.This eliminated the repulsive force between the layers of clay minerals, making it difficult for water molecules to penetrate the crystal layers of clay minerals [5][6][7][8][9].Cationic surfactants have been increasingly utilized in the industry for anti-swelling treatment.These surfactants are soluble in organic cationic groups that are ionized by water, replacing K + and Ca 2+ plasma on the surface of clay layers.They can adsorb on the surface of clay particles, thereby weakening interlayer repulsion.Furthermore, their clay stability is better than that of inorganic salts.The organic cationic surfactant, which is a quaternary ammonium salt, has been developed and extensively used in oil fields due to its superior anti-swelling performance.It possesses stronger electrostatic adsorption and multipoint adsorption characteristics compared to inorganic salts and general cationic surfactants.Additionally, it has the advantages of low consumption, strong adsorption, minimal impact from acidity and alkalinity, and high compatibility.As a result, it is considered a key anti-swelling product in the industry [10][11][12][13][14][15][16].
In this study, the authors synthesized a cationic surfactant in the form of quaternary ammonium salt through indoor experiments.The reaction conditions for temperature and raw material molar ratio were optimized to develop a clay stabilizer that is suitable for offshore oilfield reservoirs.The clay stabilizer was evaluated for its anti-swelling effect, acid and alkali resistance, water washing resistance, mudstone loss rate, and dynamic anti-swelling effect.The use of this clay stabilizer during the acidification process can significantly reduce water sensitive damage to reservoirs.This is highly beneficial in improving the development of reservoirs during the later stages of ultra-high water cut and increasing the recovery rate of old oil fields.
The equipment used in the experiment included a four-mouth flask, a thermostat water bath, a drip funnel, a 3K15 desktop centrifuge from SIGMA in Germany, a core displacement testing device from Yangzhou Huabao Instrument Co., Ltd., and an electronic balance from Lichen Technology.

Preparation Method.
To synthesize the quaternary ammonium cation reaction product ZS-A, start by adding p-phenyldimethylamine to a four necked flask.Connect the flask to a condensing tube, thermometer, drip funnel, and stirrer, and heat it through a water bath.While stirring, add N, N, Ntrimethyl-N-3-chloro-2-hydroxypropylammonium chloride from the drip funnel during a constant temperature process in the water bath.Allow the reaction to continue at a constant temperature for a period of time.To obtain the clay stabilizer ZSFP, mix ZS-A and KCl in a mass ratio of 7:1.

Evaluation Method for Anti Swelling.
The anti-swelling rate and water washing resistance rate are evaluated according to the SY/T 5971-2016 standard, which specifies the performance evaluation method for clay stabilizers used in oil and gas field fracturing, acidification, and water injection.The anti-swelling rate is determined through the centrifugation method, and its calculation formula is as follows: In the formula, B: anti-swelling rate,%; V0: Volume of bentonite in kerosene, mL; V1: Volume of bentonite in clay stabilizer solution, mL; V2: Volume of bentonite in distilled water, mL.

Evaluation Method for Mudstone Loss Rate.
In accordance with SY/T 5971-2016, the standard industry practice for evaluating the performance of clay stabilizers used in oil and gas field fracturing acidification and water injection involves immersing natural rock cores in varying concentrations of clay stabilizer solutions and subsequently sieving, drying, and weighing them.The mudstone loss rate is then calculated based on the obtained measurements.

Evaluation Method for Permeability Retention Rate.
In accordance with the Q/HS 2044-2019 enterprise standard of CNOOC, the core of Bohai Oilfield was extracted, washed, and dried before being loaded into the core tube using the compaction method.The prepared core tube was then vacuumed, saturated with simulated formation water, and had its initial permeability K0 measured.Subsequently, 5PV clay stabilizer was injected, and the permeability Kf of the core tube was measured.The core permeability retention rate Df can be calculated using the following formula:  The results presented in figure 2 demonstrate that increasing the molar ratio from 4:1 to 6:1 resulted in a significant improvement in the anti-swelling and water-washing properties of the reaction product ZS-A.However, further increasing the molar ratio from 6:1 to 8:1 resulted in a slight decrease in these properties, although they still remained above 90%.These findings suggest that the optimal molar ratio of the feed is 6:1.The strong reaction activity of the amino group in the molecular structure of p-phenylenedimethylamine allows it to react with multiple N, N, N-trimethyl-N-3-chloro-2-hydroxypropyl ammonium chloride simultaneously, enhancing its cationic characteristics.As a result, the cationic reaction products combine with anions on the surface of clay layers, forming a diffusion double layer.The stable properties of the diffusion double layer make it challenging for water molecules in the environment to penetrate between the clay layers.However, when the molar ratio of reactants exceeds 6:1, the excess reactant N, N, N-trimethyl-N-3-chloro-2hydroxypropylammonium chloride binds easily to water molecules, resulting in lower cationic properties compared to the reaction products.Consequently, the anti-swelling and water-washing properties exhibit a certain degree of decline.

Effect of Reaction
Temperature.The reaction product ZS-A was studied for its anti-swelling rate and water washing resistance at different reaction temperatures (50 ℃, 60 ℃, 70 ℃, 80 ℃ and 90 ℃) using N, N, N-trimethyl-N-3-chloro-2-hydroxypropylammonium chloride and p-phenyldimethylamine as reactants in a molar ratio of 6:1.The results are presented in figure 3.According to figure 3, the quaternization reaction is significantly affected by the reaction temperature.The reaction product ZS-A exhibits improved anti-swelling and water-washing properties as the temperature increases.However, at temperatures of 70℃ or higher, these properties tend to plateau and even slightly decrease.Therefore, the optimal reaction temperature is 70℃.As the reaction temperature increases from 50 ℃ to 70 ℃, there is an increase in temperature which provides sufficient activation energy for the reaction.This makes it easier for the reaction to occur and obtain the target product, resulting in an increase in yield.Additionally, the anti-swelling and water-washing properties will be greatly improved.When the temperature exceeds 70 ℃, the activation energy for the reaction is high.However, due to increased molecular motion, the reaction product may become more active but less stable, resulting in incomplete reaction.Consequently, the anti-swelling and waterwashing properties are significantly reduced.

Anti Swelling Effect of Different Mass Concentrations.
In this study, the clay stabilizer ZSFP was synthesized using a combination of quaternary ammonium salt cationic reaction product ZS-A and composite inorganic potassium salt.To evaluate its effectiveness, the anti-swelling rate of ZSFP was measured through the centrifugation method across a range of mass concentrations from 0.5% to 4%.According to figure 4, the anti-swelling rate of clay stabilizer ZSFP increases as the mass concentration gradually increases.The rate reaches over 90% after reaching a mass concentration of 1% and reaches 94.5% at a concentration of 1.5%.However, as the concentration continues to increase, the increase in anti-swelling rate becomes more gradual.Therefore, the optimal concentration for using clay stabilizer ZSFP is 1.5%.

Evaluation of Acid and Alkali Resistance Performance.
To ensure the effective use of clay stabilizer in acidification measures, it is crucial to assess its applicability under varying operating conditions.In this regard, the compatibility of ZSFP, a clay stabilizer with a 1.5% mass concentration, with formation fluid, injection fluid, and acid-base conditions was evaluated.The results of this assessment are presented in table 1.The pH of the solution was modified with hydrochloric acid and sodium hydroxide.Figure 5 displays the anti-swelling rate of clay stabilizer ZSFP under various pH conditions.According to figure 5, at neutral pH levels, the anti-swelling rate and water washing resistance rate are relatively low, measuring at 94.5% and 96.4%, respectively.However, as the solution's acidity and alkalinity increase, the anti-swelling rate and water washing resistance rate also increase gradually.The highest anti-swelling rate of 97.3% and water washing resistance rate of 98.7% were observed at pH=1.On the other hand, at pH=13, the anti-swelling rate was 96.1%, while the water washing resistance rate was 97.2%.The addition of new ions during pH adjustment can help prevent swelling to some extent.Therefore, compared to neutral solutions, the anti-swelling rate and water washing resistance have been significantly enhanced.

Evaluation of Water Washability Performance.
Clay minerals, such as montmorillonite and kaolinite, readily expand when exposed to water.However, the connection between layers is relatively loose, which can cause migration and result in pore blockages during water scouring.When the clay stabilizer solution is injected into the formation, its effective concentration gradually decreases due to the continuous erosion of water injection.To ensure effective clay stabilization, it is imperative that the chosen stabilizer exhibits robust resistance to water washing.Figure 6 provides a comparison of the water washing resistance and anti-swelling rate of ZSFP clay stabilizer with three other commercially available clay stabilizers after undergoing five washes.The results presented in figure 6 demonstrate that all four clay stabilizers have achieved an antiswelling rate of over 90% and a water washing resistance rate of over 90%.Notably, ZSFP exhibited the highest water washing resistance rate of 96.4%.Even after five washes, BSA101 and ZSFP maintained their anti-swelling rates above 90%, while OFC-610 and HJZ-100 decreased to 82.4% and 84.2%, respectively.Compared to three other commercial products, ZSFP, a clay stabilizer, exhibits superior anti-swelling and water washing resistance.Its high resistance to water washing indicates a strong ability to inhibit clay hydration and expansion even at low concentrations after washing.Additionally, ZSFP's anti-swelling effect is relatively long-lasting.It is mainly due to the multi site positive charge cation on the molecular structure of ZS-A in the clay stabilizer, which can reduce the expansion of the injected water on the clay washing.

Evaluation of Mudstone Loss Rate.
The evaluation of mudstone loss rate involves measuring the loss rate of mudstone and assessing the effectiveness of clay stabilizers in preventing clay expansion and dispersion.The results of this evaluation are presented in figure 7, which shows that the loss rate of mudstone is relatively low when using the clay stabilizer ZSFP in the Bohai V oilfield core.The loss rate of mudstone decreases as the mass concentration of the clay stabilizer increases.When the mass concentration is between 1.5% and 4%, the loss rate remains below 0.05%.For the clay stabilizer ZSFP with a mass concentration of 4%, the loss rate of mudstone is 0.022%.Considering the antiswelling effect of clay and economic cost control, the optimal concentration for using clay stabilizer is 1.5% mass concentration.

Evaluation of Permeability Retention Rate.
In this study, we compared and evaluated the core permeability retention rate of clay stabilizer ZSFP with three other commercial clay stabilizers to determine the advantages and disadvantages of using clay stabilizers in protecting reservoir permeability.The concentration of the clay stabilizer used was 1.5%, and the test was conducted at a temperature of 60 ℃.The results of the evaluation are presented in table 2. Table 2 shows that ZSFP, a clay stabilizer, has a high permeability retention rate for both types of core pipes.The retention rates for 1# and 2# core pipes are 98.6% and 94.0%, respectively, which are higher than the other three commercial clay stabilizers.The test results for core permeability retention of ZSFP are consistent with the evaluation results for water washing resistance.The technical characteristics of the clay stabilizer ZSFP have been shown to meet the standards of commercial clay stabilizers, both in terms of static and dynamic anti-swelling performance.Furthermore, ZSFP possesses certain performance advantages.
Clay Stabilizer 3.1.1.Effect of Molar Ratio of Reaction Raw Materials.In this study, we investigated the anti swelling rate and water washing resistance of the reaction product ZS-A using N, N, N-trimethyl-N-3-chloro-2hydroxypropylammonium chloride and p-phenyldimethylamine as reactants.The experiments were conducted under five different conditions of molar ratios (4:1, 5:1, 6:1, 7:1 and 8:1) at a water bath temperature of 70 ℃.The results of the experiments are presented in figure 2.

Figure 2 .
Figure 2. Effect of reactant molar ratio on anti swelling rate and water washing resistance of ZS-A.

Figure 3 .
Figure 3.Effect of reaction temperature on anti swelling rate and water washing resistance of ZS-A.

Figure 4 .
Figure 4. Anti swelling rate of ZSFP under different mass concentrations.

Figure 5 .
Figure 5.Effect of pH on anti swelling rate and water washing resistance of ZSFP.

Figure 6 .
Figure 6.Water washing resistance of four clay stabilizers.