Study on Reducing Injection Pressure of Low Permeability Reservoirs Characterize

Zhao Lin; Qin Bing; Wu Xiongjun; Wang Zenglin; Jiang Jianlin

(1. SINOPEC Research Institute of Petroleum Processing, Beijing 100083;2. SINOPEC Research Institute of Petroleum Engineering, Beijing 102206;3. SINOPEC Shengli Oilfield, Dongying 257001)

Abstract: In view of the problems of high injection pressure and low water injection rate in water injection wells of low permeability reservoirs featuring high temperature and high salinity, two new surfactants were synthesized, including a quaternary ammonium surfactant and a betaine amphoteric surfactant. The composite surfactant system BYJ-1 was formed by mixing two kinds of surfactants. The minimum interfacial tension between BYJ-1 solution and the crude oil could reach 1.4×10-3 mN/m. The temperature resistance was up to 140 ℃, and the salt resistance could reach up to 120 g/L.For the low permeability core fully saturated with water phase, BYJ-1 could obviously reduce the starting pressure gradient of low permeability core. While for the core with residual oil, BYJ-1 could obviously reduce the injection pressure and improve the oil recovery. Moreover, the field test showed that BYJ-1 could effectively reduce the injection pressure of the water injection well, increase the injection volume, and increase the liquid production and oil production of the corresponding production well.

Key words: low permeability reservoir; quaternary ammonium salt; betaine surfactant; interfacial tension; reducing injection pressure; enhancing oil recovery

1 Introduction

Compared with medium and high permeability reservoirs, low permeability reservoirs have the following characteristics: small pore throat, large specific surface area, low porosity, etc[1-4]. The main problems in water flooding development of low permeability reservoirs include the following aspects. The injection pressure of water injection well is high and injection is difficult.The starting pressure gradient of fluid in core is high and seepage resistance is high[5]. The energy diffusion in the formation is slow, so the pressure is hard to release.And the porosity of reservoir is small and the water absorption capacity is poor[6-7]. High injection pressure of water injection well will increase the load of injection distribution system and energy consumption of water injection. At the same time, long-term high-pressure water injection is prone to causing the casing damage of water injection well. The continuous low production of oil well will increase the difficulty of reservoir exploitation and cause huge economic loss. Accordingly, reasonable reduction of water injection pressure in low permeability reservoirs is not only the key but also a gut issue in the development of low permeability reservoirs[8-10].

In view of the blockage caused by suspended solids,acidizing can be used to increase the fluid flow space of the formation near the wellbore. However, in view of some problems such as strong Jamin effect and the formation blocking by residual oil, the injection of surfactant is also a good idea[11-14], which can reduce the interfacial tension, make the oil drop easily deformed,and reduce the residual oil saturation, leading to reduced injection pressure[15-16]. Generally speaking, the mechanisms of surfactant include reducing the interfacial tension between oil and water, changing the wettability of rock, emulsifying and solubilizing crude oil, etc.The environment suited for application of surfactants depends on the reservoir characteristics, such as temperature, salinity, divalent ions, and complex crude oil properties, which put forward higher requirements for surfactants. For low permeability reservoir, the molecular weight of surfactant should not be too high.For the reservoir with high salinity, the surfactant should have enough salt resistance and resistance to divalent calcium and magnesium ions. For high temperature reservoir, the surfactant should have good temperature resistance. Some anionic surfactants and nonionic surfactants have been used in low permeability reservoirs. Generally, the interface performance of anionic surfactants is good, and their adsorption capacity in sandstone is small. However, anionic surfactants are easy to precipitate with calcium and magnesium ions in the formation. They have poor salt resistance and are not suitable for high salt reservoirs. Nonionic surfactants have strong salt resistance and good emulsification performance. Nevertheless, due to the existence of cloud point and poor temperature resistance, they are not suitable for high temperature formation. At present,for high temperature, high salt and low permeability reservoirs, there are very few surfactants that can significantly reduce the injection pressure of water injection wells, and improve the development effect of low permeability reservoirs. Moreover, the mechanisms of surfactant for reducing injection pressure in low permeability reservoirs are unclear.

Aiming at the high temperature, high salinity and low permeability reservoir, two new surfactants were synthesized, including the quaternary ammonium surfactant and the betaine amphoteric surfactant.

By optimizing the interface properties, a composite surfactant formula was formed. The temperature resistance, salt resistance and wettability of the composite surfactant system were evaluated. Moreover,the influence of composite surfactant on starting pressure and injection pressure of low permeability cores were evaluated through core displacement experiments. According to the experimental results, the mechanism of surfactant for reducing injection pressure was analyzed, and then the field test and effect tracking were carried out.

2 Experimental

2.1 Materials

The reagents, diethylamine, epichlorohydrin, sodium bisulfate, dodecyl dimethyl tertiary amine, bromophenol blue, sodium dodecyl sulfate (SDS), and dichloroethane were of analytically pure grade and were commercially available. The experimental oil was a crude oil collected from a block of Shengli Oilfield, with a density of 0.851 g/cm3, and a viscosity of 2.56 mPa·s at 70 ℃. Table 1 shows the analysis data of ion components contained in the injection water sample. The interfacial tension between the injection water and the experimental oil was 18.6 mN/m.

The experimental cores were the natural low permeability cores obtained from the formation, with the basic data of cores shown in Table 2.

Table 1 Analysis data of ion components in injection water

Table 2 Basic data of experimental cores

2.2 Synthesis of surfactants

The synthetic steps of cationic surfactant BJ-12 according to Figure 1 were as follows. Diethylamine hydrochloride was prepared by the reaction of diethylamine and hydrochloric acid. Then epichlorohydrin was added to enter into reaction with diethylamine hydrochloride for 6hours in the presence of distilled water acting as solvent.The pH value of the mixture was adjusted to about 6 by hydrochloric acid. Dodecyl dimethylamine was added and the reaction lasted for 4 hours at 85 ℃. The solvent was removed by a rotary evaporator, and a white product was obtained, which was washed withn-hexane and recrystallized with acetone, and the final product was named “BJ-12”. The yield of BJ -12 was determined by bromophenol blue reverse titration. The yield of BJ-12 could reach 83%.

The steps for synthesis of amphoteric surfactant YJ-12 according to Figure 2 were as follows. 3-Chloro-2-hydroxypropyl sulfonic acid was prepared by the reaction of sodium bisulfite and epichlorohydrin in the presence of distilled water (acting as solvent) at 85 ℃ for 2 h, then the reaction mixture was cooled and filtered. In the presence of solvent (withV(isopropanol):V(water) =1:2), sodium 3-chloro-2-hydroxypropyl sulfonate reacted with dodecyl dimethyl tertiary amine at 80 ℃ for 5 hours. The product was obtained via vacuum distillation and desalting by hot ethanol filtration, and was named “YJ-12”.The conversion of tertiary amine was determined by bromophenol blue indicator titration. The yield of YJ-12 could reach 78%.

Figure 1 Reaction equation of BJ-12

Figure 2 Reaction equation for synthesis of YJ-12

Figure 3 Real core slice model

The products were analyzed by high resolution mass spectrometry. For the purified reaction products, the infrared spectrum was tested to characterize the structure of the products.

2.3 Performance evaluation of surfactants

2.3.1 Surface tension test

For two kinds of surfactants BJ-12 and YJ-12, surfactant solutions of different concentrations were prepared with injection water. At 25 ℃, the surface tension of different surfactant solutions and experimental oil were measured by a surface tension meter. The critical micelle concentration (CMC) was calculated by the surface tension curves at different concentrations.

2.3.2 Interfacial tension test

At 70 ℃, the oil-water interfacial tension of different surfactant solutions and experimental oil was measured by a TX-500D rotating drop interfacial tension meter.The viscosity and density of the experimental oil at 70 ℃were 2.56 mPa·s and 0.851 g/cm3, respectively.

2.3.3 Temperature resistance evaluation

The interfacial tension between surfactant solutions and experimental oil was measured at different temperatures ranging from 30 ℃ to 140 ℃.

2.3.4 Salt resistance evaluation

According to the ion composition of the injection water in Table 1, salt solutions with different salinity were prepared. Surfactant solutions of different concentrations were prepared for studying these salt solutions. The interfacial tension between surfactant solutions and experimental oil was tested.

2.3.5 Wettability alteration

The effect of surfactant on the wettability of rock surface was measured by using a DSA100 optical contact angle meter made by the KRÜSS Company of Germany. The polished rock piece was installed on two supports of the contact angle meter. Water was injected into the container so that the polished rock piece was immersed in the injected water at 70 ℃ for more than 36 hours. The contact angle changed slowly when the oil drops were injected under the rock slice with a special micro syringe.Through the optical lens of the instrument, the contact angle after balancing was measured by the scale of the instrument at 70 ℃. Then the rock slice was immersed in the surfactant solution at 70 ℃ for more than 36 hours.An oil drop was injected beneath the rock again. The contact angle of rock slice after the action of surfactant solution was measured at 70 ℃.

2.3.6 Experiment of starting pressure gradient

Without considering the influence of oil sample on the premise that the core was free of oil, the experiment of starting pressure gradient of single phase fluid was carried out, including a single water flow and a single surfactant solution flow. The specific experimental steps were as follows.

(i) The core was dried and weighed, and the core was evacuated and then was saturated with formation water.The pore volume of core could be calculated. (ii) The injection water was used to displace the core at a flow rate of 0.005 mL/min at 70 ℃. When water began to flow out from the core outlet, the pump was shut down quickly.The injection pressure was observed and recorded. The injection pressure measured in the steady state was the minimum starting pressure, and the minimum starting pressure divided by the length of core was the minimum starting pressure gradient. Then the core was displaced by injection water at a constant flow rate again. The flow rate was increased from low to high, including 0.01 mL/min, 0.02 mL/min, 0.04 mL/min, 0.06 mL/min, 0.08 mL/min, 0.1 mL/min, 0.15 mL/min, and 0.2 mL/min. The injection pressure after stabilization at each flow rate was also recorded. The relationship between the velocity and the pressure gradient was described. The ratio of slope to intercept of straight section in the curve was the starting pressure gradient of core. (iii) Then according to the above steps, the formation water was replaced by the surfactant solution for experimental study, and the starting pressure gradient of the core under the action of surfactant was tested. The effect of surfactant on the starting pressure gradient of core was analyzed.

2.3.7 Experiment of reducing injection pressure

Through the displacement experiment, the changes of injection pressure and oil recovery before and after injecting surfactant into low permeability core were measured. The specific experimental steps were as follows.

(i) The core was dried and weighed, and the core was evacuated with vacuum pump and then saturated with formation water. The pore volume of core could be calculated. (ii) The core was displaced by injected water at a flow rate of 0.1 ml/min until the pressure reached equilibrium, and the water phase permeability was calculated. (iii) The core was saturated with experimental oil, and the volume of saturated oil in the core was recorded. (iv) The core was displaced by injected water at a flow rate of 0.1 ml/min until the pressure was stable and no oil could be produced at the core outlet. (v) The core was displaced by 0.5 PV of surfactant solution. (vi)The core was injected with water at a flow rate of 0.1 ml/min again until the pressure was stable and no oil was produced at the core outlet. The injection pressure and oil production at different time duration were recorded,and the oil recovery was calculated. The influence of surfactant on injection pressure and oil recovery was analyzed.

2.3.8 Micro experiment of core slice model

The process for reducing residual oil by surfactant and injection pressure could be studied by virtue of the real core slice model experiment. Upon using the micro rock displacement device, the change of residual oil could be observed directly, and the effect of surfactant on residual oil and the mechanism of reducing injection pressure could be analyzed.

According to the natural cores in this formation, the experimental models were made by sticking core slice between two pieces of glass after being treated by oil washing, drying, slicing and grinding. The properties and pore structure of cores were basically maintained.A model photo is shown in Figure 3, the bearing pressure was 0.4 MPa, and the maximum experimental temperature was about 80 ℃. The experimental procedure was the same as the experiment of reducing injection pressure (see Section 2.3.6). For the convenience of observation, the experimental oil was kerosene dyed red with Sudan red.

3 Results and Discussion

3.1 Structure characterization

The infrared spectra of two surfactants including BJ-12 and YJ-12 are shown in Figure 4. It can be seen from the infrared spectrum of BJ-12 that the absorption peak at a wavenumber of 3418 cm-1was attributed to the characteristic vibration peak of –OH. Two strong absorption peaks at 2 921 cm-1and 2 854 cm-1showed the characteristic vibration peaks of long carbon chain.However, there was no absorption peak in the range of 1 360―1 310 cm-1, indicating that dodecyl dimethyl tertiary amine had been completely consumed in reactions. Furthermore, the absorption peaks between 1 050―1 200 cm-1and 500―700 cm-1indicated the characteristic peaks of C-C, C-N, and C-Cl in the synthetic product. These results showed that the synthetic product was surfactant BJ-12.

It can be seen from the infrared spectrum of YJ-12 that there were O-H stretching vibration peak at 3 378.7 cm-1,CH2asymmetric stretching vibration peak at 2 917.8 cm-1,CH2shear vibration peak or CH3asymmetric vibration peak at 1 469.5 cm-1, and C-N stretching vibration peak at 1 205.3 cm-1. In addition, a (CH2)n(n＞ 4) plane swing vibration peak appeared at 721.3 cm-1. It could be proved that this substance was the target product of betaine amphoteric surfactant YJ-12.

The high resolution mass spectrometry analyses of two surfactants including BJ-12 and YJ-12 are shown in Figure 5 and Figure 6, respectively. The results of mass spectrometry analyses were consistent with those of the target products, indicating that the expected target products were obtained.

3.2 Surface tension

Surfactant solutions of different concentration were prepared with surfactant and the injection water. The surface tension of surfactant solutions was measured at 25 ℃, with the results shown in Figure 7.

It can be seen from Figure 5 that the surface tension of surfactant solution decreased sharply with the increase of its concentration. When the concentration reached a certain value, the surface tension was basically unchanged. The minimum surface tension of BJ-12 and YJ-12 was 28.4 mN/m and 29.5 mN/m, respectively.

Figure 4 Infrared spectra of surfactants

Figure 5 Mass spectrogram of surfactant BJ-12

Figure 6 Mass spectrogram of surfactant YJ-12

Figure 7 Surface tension of different surfactant solutions

And the critical micelle concentration (CMC) of BJ-12 and YJ-12 was around 0.2 mmol/L and 2 mmol/L,respectively.

3.3 Interfacial tension

Surfactant solutions of different concentration were prepared with surfactant and the injection water. In addition, the composite surfactant system was formed by adding different concentration of surfactant YJ-12 into 0.006% of surfactant BJ-12. The interfacial tension between these solutions and the experimental oil was measured at 70 ℃, with the results shown in Figure 8.

Figure 8 Interfacial tension of different surfactant solutions

It can be seen from Figure 8 that with the increase of surfactant mass fraction, the interfacial tension first decreased and then slightly rose. The minimum interfacial tension of YJ-12 surfactant solution was 0.32 mN/m. When the mass fraction of BJ-12 reached 0.006%, the interfacial tension reached a minimum of 0.028 mN/m. The lowest interfacial tension of YJ-12 surfactant solution was one or two orders of magnitude lower than that of ordinary surfactants.In order to further reduce the interfacial tension, a 0.006%BJ-12 solution was added with different content of YJ-12 to form composite surfactant systems. The lowest interfacial tension was 1.9×10-3mN/m, when the mass fraction of YJ-12 was 0.02% and the mass fraction of BJ-12 was 0.006%. The interfacial tension of composite surfactant was lower than that of single surfactant BJ-12 or YJ-12. This composite surfactant solution with a BJ-12 mass fraction of 0.006% and a YJ-12 mass fraction of 0.02% was called BYJ-1 for short.

The interfacial tension between composite surfactant BYJ-1 and crude oil at different temperatures was measured, and the temperature resistance of the formula was evaluated.The experimental results are shown in Figure 9.

Figure 9 Temperature resistance of BYJ-1

Figure 10 Salt resistance of BYJ-1

According to the ion composition of the injection water,it could be expanded or reduced in proportion to form water samples with different mineralization degrees. The interfacial tension between BYJ-1 solution with different salinity and crude oil was measured at 70 ℃. The experimental results are shown in Figure 10. In Figure 10, when the salinity was 60 g/L, the interfacial tension reached 1.6×10-3mN/m. When the salinity reached 120 g/L, the surfactant still had good water solubility, and the interfacial tension was 1.32×10-2mN/m, indicating that the composite surfactant BYJ-1 had very good salt resistance and was suitable for high temperature and high salt formation conditions.

3.4 Wettability alteration

The contact angle of the rock slice was measured after immersion in injection water for 36 hours at 70 ℃.Next, the contact angle of the rock slice was measured after being immersed in the BYJ-1 surfactant solution for 36 hours. The experimental results are shown in Figure 11.

Figure 11 Measurement of contact angle

In Figure 11, the contact angle of rock slice after being immersed in injection water was 36.3°. However, the contact angle of rock slice after being immersed in BYJ-1 surfactant solution increased to 82°. This result showed that the composite surfactant BYJ-1 could transform the rock surface from hydrophilic to weakly hydrophilic or neutral. Small pores could continue to play the role of imbibition, so that a lot of oil was gradually displaced into large pores. The residual oil in the large pores could form oil film on the pore wall, and the Jamin effect and flow resistance would decrease, so that a large amount of residual oil could be transported out in the form of oil flow. The injection pressure of low permeability reservoir could be greatly reduced by the decrease of Jamin effect and residual oil.

3.5 Experiment results of starting pressure gradient

At 70 ℃, Core 1 was displaced by injection water, and the minimum starting pressure gradient of core was 0.0578 MPa/m. When Core 1 was displaced by BYJ-1 surfactant solution again, the minimum starting pressure gradient of core was 0.0487 MPa/m. It showed that the minimum starting pressure gradient measured in the presence of surfactant was obviously smaller than that measured in the presence of injection water.

The relation curve between velocity and pressure gradient for two fluids is shown in Figure 12. The intersection of the curve and X-axis in Figure 12 is the minimum starting pressure gradient. The relationship between velocity and pressure gradient was not a straight line. When the pressure gradient was greater than a certain value, it became an approximate straight line. Under the same flow rate, the pressure gradient of core displaced by surfactant was smaller than that of core displaced by injection water.The ratio of intercept to slope of straight section was the starting pressure gradient of core. The starting pressure gradient of core displaced by injection water is 0.445 MPa/m. The starting pressure gradient of core displaced by surfactant was 0.352 MPa/m. The starting pressure gradient of surfactant was obviously smaller than that of injection water, and the reduction ratio could reach 20.9%.

Figure 12 Relation curve between seepage velocity and pressure gradient

The analysis shows that there are complex interface reactions between solid and liquid in the pores of low permeability cores, because the pores are very small and the specific surface area is very large. When the fluid starts flowing in the pore, the adsorbed layer, namely the boundary layer, will be formed on the pore surface.Because of the existence of solid-liquid interfacial tension, the adhesion between the boundary layer and the pore surface is large, which makes the fluid incapable of easily flowing. Only when the displacement pressure exceeds a certain value, the fluid can overcome the adhesion and start to flow. This is also an important reason why the percolation law of low permeability reservoirs does not conform to the Darcy’s law. On the other hand, clay minerals in the pores have certain water absorption capacity. When the fluid percolates in the clay, it will form a solid hydration membrane, which can narrow the actual flow channels.

After the injection of BYJ-1 surfactant into the core, the BYJ-1 surfactant molecules can penetrate into the pores and can be adsorbed on the pore surface. BYJ-1 can reduce the interfacial tension between solid and liquid,and then reduce the flow resistance of the fluid in the boundary layer. So the fluid in the small pores can flow under a smaller displacement pressure. Furthermore,ionic surfactants can dissociate a certain amount of charges, which can be adsorbed on the surface of the boundary layer. The adsorbed charge can compress the diffused electric double layer. The thickness of the boundary layer will be decreased. In addition,the adsorbed charge can also inhibit the expansion and migration of clay particles. Therefore, the BYJ-1 surfactant can reduce the starting pressure gradient of low permeability cores.

Sixteen people dead on the job, and the seventeenth, in precisely6 the same situation, figures out a way to live. That man was having a party where you and I would probably not last three days. The boredom7! He and I did have lunch later, and he said, I don t understand why anybody would think my job is boring. I have a corner office, glass on all sides. I can see the Golden Gate, San Francisco, the Berkeley hills; half the Western world vacations here and I just stroll in every day and practice dancing.

3.6 Experimental results of reducing injection pressure

For Core 2 and Core 3, the changes of injection pressure and oil recovery before and after injection of 0.5PV BYJ-1 surfactant were measured through the displacement experiments. The experimental results are shown in Figure 13.

It can be seen from Figure 13 that after the injection of 0.5PV BYJ-1 surfactant, the recovery of Core 2 (with a gas permeability of 12.1×10-3μm2) increased by 7.57%, and the injection pressure decreased by 10.17%;the recovery of Core 3 (with a gas permeability of 38.7×10-3μm2) increased by 14.78%, and the injection pressure decreased by 27.39%. Therefore, the composite surfactant system has good effect on reducing pressure and increasing injection and enhancing oil recovery from cores, and the core with larger permeability has more obvious effect on reducing pressure and increasing oil recovery.

3.7 Micro experiment of core slice model

Under the optical microscope, the core slice was observed. The size distribution of sand particles was uneven, the particle size was mainly distributed in the range of 100 —600 μm. The pore connectivity was poor.The pore diameter was mainly in the range of 20—70 μm and the throat diameter ranged mainly from 0.2 μm to 10 μm.

In the early stage of water flooding, water first penetrated along the low resistance channels, and the oil flow was continuous. Even if it was stuck, the oil flow could quickly coalesce. At this time, the Jamin effect was weak.When there was water flowing out of the outlet or the oil saturation in the pore medium decreased, the continuous oil flow often got stuck and blocked the small pore throat due to the large pore throat ratio of the core (Figure 14(a)), or the dispersed oil drops were formed in the large pores. At this time, the Jamin effect became the invisible seepage resistance in the pore medium, which greatly increased the pressure of water drive oil. Residual oil mainly existed in the form of columnar, clustered, flaky or stuck oil drops.

Figure 13 Core displacement test results

Figure 14(a) shows a picture of residual oil after water flooding, and the red mark represented the residual oil.The injection direction was from left to right. The residual oil was stuck at the pore throat as pointed by the arrow,resulting in the red flake residual oil on the left side of the pore throat.

Figure 14(b—i) shows pictures of BYJ-1 surfactant flooding. In the process of BYJ-1 surfactant flooding,the red color of the pore throat was deepened, indicating that the oil accumulated at the pore throat (Figure 14-b). Because BYJ-1 could reduce the interfacial tension between oil and water, it could also reduce the capillary force at the pore throat. The residual oil front was gradually softened and elongated, and it broke into oil drops under the emulsification action by BYJ-1 (Figure 14-c). The oil drop deformed under the action of BYJ-1, and then moved forward through the next pore throat(Figure 14-d,e,f). Repeatedly, the residual oil at the pore throat continuously accumulated, peeled, deformed and finally flowed away (Figure 14-g,h,i,j,k,I). The color of residual oil became lighter, and the injection pressure also decreased.

Figure 14 Effect of surfactant on residual oil

Furthermore, micro experiments of core slice model showed that the BYJ-1 surfactant could significantly reduce all kinds of residual oil. The main flooding mechanisms included reducing the interfacial tension between oil and water, reducing the capillary force,changing the wettability of pore wall, reducing the adhesion work of residual oil on the pore wall,emulsifying and stripping residual oil, enhancing micro profile control of emulsified oil drop, solubilizing residual oil, coalescing, and producing an oil belt mechanism.

4 Field Application of Water Injection Well

In April 2016, the injection pressure of well SH-1 in the oil field at the initial stage of well opening was 18 MPa,and the average injection volume per day was 30 m3. Two years later, the average injection pressure of well SH-1 went up to 28 MPa before additional injection measure,and water injection volume per day decreased to only 1 m3. In October 2018, acidizing measures were taken for water injection wells. After acidification, the injection pressure was still 28 MPa. In less than one month, the daily water injection volume was restored to 1 m3.Acidification measure was considered ineffective. It was suspected that the oil in the reservoir near the wellbore blocked the pores, resulting in high injection pressure and small water injection volume.

In February 2019, the BYJ-1 surfactant formulation had been tested in the water injection well SH-1. 80 m3of surfactant solution were injected into the well SH-1. After surfactant injection, the injection pressure of well SH-1 dropped to 19 MPa. The daily water injection volume reached 33 m3. The depressurization rate of surfactant reached 32.14%. The daily water injection volume was stabilized at about 28 m3, and the validity exceeded period had reached 13 months.

In July 2019, two production wells SY-3 and SY-7 corresponding to well SH-1 showed oil increasing effect. The average daily liquid production of Well SY-3 increased from 8.1 m3to 11.5 m3. And the average daily oil production of well SH-3 rose from 5.7 m3to 8.2 m3.The liquid level of the well SY-3 had risen 313 m. 675 m3of oil had been additionally produced accumulatively. In addition, the average daily liquid production of well SY-7 increased from 7.2 m3to 10.5 m3. And the average daily oil production of well SH-7 rose from 4.4 m3to 6.8 m3.The liquid level of the well SY-7 had risen 322 m. 661 m3of oil had been added accumulatively.It can be seen that the BYJ-1 surfactant not only can reduce the injection pressure and increase the water injection, but also increase the liquid production and oil production of corresponding wells. So the BYJ-1 surfactant formula has a good prospect of application and dissemination.

5 Conclusions

(1) Two new surfactants were synthesized, including the quaternary ammonium surfactant BJ-12 and the betaine amphoteric surfactant YJ-12. The minimum interfacial tension between BJ-12 and crude oil can reach 0.028 mN/m, and the minimum interfacial tension between BJ-12 and crude oil can reach 0.32 mN/m.

(2) Composite surfactant solution BYJ-1 is a mixture composed of 0.006% of BJ-12 and 0.02% of YJ-12.The lowest interfacial tension between BYJ-1 and crude oil is 1.4×10-3mN/m. The temperature resistance is up to 140 ℃, and the salt resistance is up to 120 g/L. In addition, BYJ-1 can transform the rock surface from hydrophilic to weak hydrophilic or neutral.

(3) When the core does not contain oil, the composite surfactant BYJ-1 can obviously reduce the starting pressure gradient of low permeability core. When there is residual oil in the core, BYJ-1 can obviously reduce the injection pressure and improve the oil recovery of low permeability core. The micro slice experiments show that BYJ-1 can reduce the residual oil obviously.

(4) The field test shows that BYJ-1 can obviously reduce the injection pressure of the water injection well, increase the injection volume, and increase the liquid production and oil production of the corresponding production well.