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Abstract：In this paper, the swelling, transformation and dissolution of clay minerals after steam. 7 injection in heavy oil reservoir were investiga...
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Microscopic Mechanism of Clay Minerals on Reservoir Damage during Steam Injection in Unconsolidated Sandstone Yan Zhuang, Xiangjun Liu,* Hanqiao Xiong, and Lixi Liang State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, Sichuan ABSTRACT: In this work, the swelling, transformation, and dissolution of clay minerals after steam injection in heavy oil reservoir were investigated, and the damage mechanism of steam injection was discussed to research the microscopic mechanism of clay minerals on reservoir damage during steam injection in unconsolidated sandstone. The results show that the swelling of clay minerals increases with the increase of the pH of the brine and decreases with the increase of the salinity of the brine. As we all know, the swollen clay minerals are liable to fall from the inner wall of the pass, which may cause the blockage of reservoirs. What is more, the environment of high temperature and high pH would promote the transformation of the clay minerals. Montmorillonite can be transformed into illite and analcime, and kaolinite can be transformeddue to the water sensitivity of the clay mineralto montmorillonite and analcime, whereas illite is relatively stable. For the movement of particles in the reservoir during the injection of steam, the water sensitivity of clay mineral montmorillonite and the new clay minerals analcime can easily plug the small reservoir pore, which is one of the main ways to cause damage to the reservoir during steam injection. The dissolution of clay minerals increases with the increase of temperature and increases with the increase of the pH of the brine. The dissolution of clay minerals would produce a large number of particles and make the rock matrix more loose, which may cause some reservoir damage, such as reservoir collapse, and so on.

1. INTRODUCTION Heavy oil, an unconventional oil resource, has been paid more and more attention to due to the increasing demand for oil and natural gas with the rapid development of economy.1−4 There are huge heavy oil resources in the world. The countries with rich viscous oil resources are Canada, Venezuela, the United States, the former Soviet Union, and so on.5 The viscosity of heavy oil is high, and the displacement efficiency is low during the mining process. Steam injection can reduce the viscosity of crude oil effectively and enhance the flow of crude oil.6 However, the injection of high temperature steam liquid would cause some reservoir damage. Therefore, investigation of the microscopic mechanism of clay minerals on reservoir damage during steam injection in unconsolidated sandstone can be of great importance for the development of the petroleum industry. Steam injection is always used to develop heavy oil, but the steam with high temperature and high pressure injected into a heavy oil reservoir would cause some reservoir damage.7 When clay minerals contact with steam, the clay minerals would swell, transform, or dissolve, which can lead to the decrease of the permeability of the reservoir.8 The water−rock reaction in the reservoir during steam injection was investigated by some specialists.9−14 They all think that the steam injected into the reservoir would cause the swelling, transformation, and dissolution of clay minerals, which would lead to serious reservoir damage. Some scholars15−20 investigated the reservoir damage mechanism with steam injection, and they discussed the factors that influence reservoir damage with steam injection. These researchers indicated that the high temperature and strong alkali conditions are the two most important factors for the steam injection to cause reservoir damage. Therefore, the production with steam injection in the actual development process failed to achieve the desired results.21 © XXXX American Chemical Society

In previous research, the mechanism of clay minerals on reservoir damage during steam injection in unconsolidated sandstone has been explored, but the research was not systematic and failed to reveal the damage mechanism. Steam injection is often used in shallow reservoirs with low formation temperature and poor consolidation, and clay minerals in these reservoirs are often rich in montmorillonite and kaolinite.22 The salinity of high temperature steam is far lower than that of formation water, which could result in the swelling of clay mineral and then lead to a decline in the permeability of the reservoir. Many scholars23−31 conducted a series of experiments to research the microscopic mechanism of clay minerals swelling and the influence of steam injection on swelling of the clay minerals. These researchers indicated that the swelling of clay minerals would be promoted in a strongly alkaline environment; the dispersion of clay minerals could be enhanced under the OH− environment. They also found that low salinity will increase the concentration difference between clay minerals and the external solution, which could result in the swelling of clay minerals. In this work, the effect of steam injection on the swelling of montmorillonite was studied from the two angles of static expansion rate and the change of the median particle size of the particles, which is different from previous research methods. Under the condition of high temperature, high pressure, and strong alkality, the clay minerals such as kaolinite, montmorillonite, and illite can be transformed to sensitive minerals, which can also cause reservoir damage.32 Some authors33−37 have done a lot of research on the transformation of clay minerals. Their research has clarified the conditions for Received: November 25, 2017 Revised: March 1, 2018 Published: March 5, 2018 A

DOI: 10.1021/acs.energyfuels.7b03686 Energy Fuels XXXX, XXX, XXX−XXX

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after the injection of steam into the reservoir. Through the investigation of literature, it is found that the increase of temperature would promote the transformation among clay minerals, while the salinity of steam liquid has a greater influence on the swelling of clay minerals. Therefore, the swelling of clay minerals in the reservoir after steam injection from the two aspects of fluid salinity and the pH of the brine were investigated. And the transformation and dissolution of clay minerals in the reservoir after steam injection were discussed from the two aspects of temperature and pH. 2.1. Samples. The drilling mud cuttings were collected from the Jurassic Xishanyao Formation of xinjiang H block, which was buried in −615 to −715 m. The salinities of the experimental brine were determined mainly by the characteristics of the formation water in the Jurassic Xishanyao Formation of xinjiang H block. The composition of the experimental brine was determined by the “ formation damage evaluation by flow test” (SY/T 5358-2010) of China. What is more, the experimental temperature and pressure were determined by the target layer conditions of Jurassic Xishanyao Formation of xinjiang H block. Besides, the pure samples of montmorillonite, kaolinite, and illite, which were taken from the Hui Mao mineral processing plant in Lingshou county, were used for the experiment. 2.2. Experimental Methods. 2.2.1. Experiments on the Swelling of Clay Minerals. The standard method of the swelling of clay minerals was recommended by “Bentonite” (GBT 20973-2007) of China. NaCl and distilled water were used to prepare brine with a salinity of 5000 mg/L. Different amounts of NaOH solution were added to the brine to prepare different pH brines. And then, 10 mL samples of brine of different pH values were instilled into six test tubes, while the pure samples of 2 mL (about 1.2 g) of montmorillonite were added. After the tubes were kept static for 48 h, the expansion volume of each tube was observed and the expansion rate was calculated. What is more, MASTER SIZER2000 laser particle size analyzer was used to analyze the particle size of the montmorillonite after different pH reaction. In this way, a comparative study of the expansibility of montmorillonite could be made from two perspectives: macroscopic and microscopic. As we all know, the salinity of brine can also affect the swelling of montmorillonite. The salinities of 0, 500, 1000, 2500, and 5000 mg/L were prepared according to the proportion of NaCl:CaCl2:MgCl2·6H2O = 7:0.6:0.4, which is based on the water conditions of the reservoir formation and “formation damage evaluation by flow test” (SY/T 5358-2010) of China. The other steps are the same as above. 2.2.2. Experiments on the Transformation of Clay Minerals. As we all know, the amounts of Na+ and K+ have great influences on the transformation of clay minerals. In order to ensure that there is sufficient Na+ and K+ in the brine, the combination of NaOH and KHCO3 was chosen to prepare the brine. NaOH solution is added to the KHCO3 solution of 5000 mg/L to regulate the different pH values (pH = 8, 9, 10, 11, 12, and 13) of the brine. A 5 g amount of montmorillonite samples was added to the high temperature reactor, and 350 mL of brine (pH = 8) was slowly added. The temperature of the high temperature reactor was set to 150 °C. During the temperature rising process, the pressure of the high temperature reactor was kept at 6 MPa by adjusting the air intake and air outlet. After 48 h of reaction, the samples were removed into a beaker, and the supernatant was gotten rid of by a syringe. The remaining samples were dried for 24 h in the thermostat at 110 °C. The dried samples were made by X-ray diffraction (X’Pert Pro) and scanning electron microscopy (Quanta 450) to obtain sample changes. The results of the transformation experiment of montmorillonite were obtained by repeating the experiments with pH values (8, 9, 10, 11, 12, and 13) and temperatures (150, 200, and 250 °C) of brine changed in turn. The experimental results of the transformation of illite and kaolinite can be obtained by repeating the above-mentioned experiments. 2.2.3. Experiments on the Dissolution of Clay Minerals. The oil of drilling mud cuttings needs to be washed away before the experiment. And the diameter of drilling mud cuttings between 200 mesh and 10 mesh should be screened out. The drilling mud cuttings screened were dried for 24 h in the thermostat at 110 °C. Drilling mud cuttings of 5 g each were added to the high temperature reactor, and 350 mL of brine

the transformation of clay minerals and the effect of transformation on reservoir. These scholars indicated that the transformation of clay minerals would cause reservoir damage. What is more, the transport of high speed flowing steam easily causes the migration of reservoir particles, which could block the pore channel, then resulting in a significant decline in permeability.38,39 In their research, the transformation of kaolinite attracted more attention for it can be transformed to montmorillonite and analcime under certain conditions. But the conversion conditions were clear from their research. In this work, the conditions and processes for the transformation of kaolinite are clearly defined, which is very important for studying the change of clay minerals in a steam injection reservoir. Besides, kaolinite, illite, and montmorillonite were all systematically investigated. What is more, the pure samples were used for experiment, which can exclude the influence of other factors. Some scholars40−44 put forward that the particle migration in reservoirs is one of the main reservoir damaging factors. Some models for particle transport were constructed by these researchers. From their research, we know that the injection of steam would accelerate the dissolution of the reservoir and loosen the reservoir. What is more, the steam also has a great influence on the properties of clay minerals in the reservoir, which could cause some reservoir damage. In different parts of the reservoir, the influence of clay minerals’ swelling and dissolution on the permeability of the reservoir is different due to the heterogeneity of clay mineral composition and distribution. It has the greatest influence on the medium and low permeability layers with high clay mineral content, whereas it has little effect on the relatively high permeable layers with low content of clay minerals.45 Moreover, the swelling of clay minerals contacting with steam would aggravate the heterogeneity of the reservoir. The uneven decrease of the permeability of the reservoir can lead to the inadequate exploitation of the reservoir and reduce the ultimate recovery of the reservoir. In addition, the swelling and dissolution of clay could lead to the destruction of the rock structure, which could result in sand production in the reservoir during the process of steam injection for the clay minerals are heavy cement in the reservoir. Thus, the permeability of the reservoir would decrease.46,47 The objective of this work is to investigate the microscopic mechanism of clay minerals on reservoir damage during steam injection in unconsolidated sandstone. The swelling mechanism of montmorillonite caused by steam injection was studied from the point of salinity and alkalinity of the reservoir. The transformation mechanism of kaolinite, illite, and montmorillonite under steam injection was studied, and the conversion conditions were determined. The dissolution of clay minerals caused by steam injection was studied from the point of view of temperature and alkalinity. On this basis, the microscopic mechanism of clay minerals on reservoir damage during steam injection in unconsolidated sandstone was researched and the reservoir protection measures for steam injection were proposed. It was anticipated that our results might provide important theoretical and instructional significance for the exploration and development of an unconsolidated sandstone heavy oil reservoir.

2. SAMPLES AND EXPERIMENTS Three factors, the temperature of the reservoir, pH of the brine, and salinity of the brine, would change with steam injection. Therefore, the swelling, transformation, and dissolution of clay minerals were studied B

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Energy & Fuels (pH = 8) was slowly added. The temperature of the high temperature reactor was set to 150 °C. During the temperature rising process, the pressure of the high temperature reactor was kept at 6 MPa by adjusting the air intake and air outlet. After 48 h of reaction, the samples were removed into a beaker, and the supernatant was gotten rid of by a syringe. The remaining samples were dried for 24 h in the thermostat at 110 °C. Then the weight of the sample was measured, and the amount of dissolved mineral was calculated. The results of the dissolution experiment of montmorillonite were obtained by repeating the experiments with the pH (8, 9, 10, 11, 12, and 13) and temperature (150, 200, and 250 °C) of the brine being changed in turn. The experimental results of the dissolution of quartz can be obtained by repeating the above-mentioned experiments.

3. RESULTS AND DISCUSSION The experimental research of the change of clay minerals in a reservoir affected by temperature and pH is done to study the mechanism of damage of a steam injection reservoir. By studying the swelling of clay minerals, the transformation of clay minerals, and the dissolution of clay minerals with different temperatures and pH, we can know the change of clay minreals in the reservoir after steam injection. What is more, the damage to the reservoir can be discussed. In fact, it provides the basis for steam injection protection technology in a heavy oil reservoir. 3.1. Influence of Steam Injection on Swelling Properties of Clay Minerals. The swelling of clay minerals in a loose sandstone reservoir is mainly the swelling of montmorillonite.48 Montmorillonite easily falls off after hydration, and it may block the reservoir along with the steam flow, which could result in some reservoir damage.49−53 In view of the study of the swelling properties of clay minerals in the reservoir after steam injection, investigations are mainly carried out to focus on the two aspects of fluid salinity and pH. 3.1.1. Influence of Salinity on Swelling Properties of Montmorillonite. Low salinity and high pH steam injection would reduce the salinity of the reservoir and have a great influence on the swelling of montmorillonite in the reservoir. The experimental results of the influence of salinity on the swelling of montmorillonite were shown in Figure 1. The static swelling rate is only a macroscopic study of the swelling rate of montmorillonite. In order to further study the expansibility of montmorillonite, the experiment of laser particle size analysis of montmorillonite with different salinity brine was carried out. The change of particle size of

montmorillonite was observed carefully from the microscopic point of view. It provides reference for the study of the swelling of montmorillonite. The results were shown in Figure 1. According to the experimental results (Figure 1), it could be found that the swelling rate of montmorillonite would decrease with the increase of salinity and the low salinity environment would increase the swelling rate of montmorillonite. When the salinity is less than 1000 mg/L, the swelling rate of montmorillonite changes little, at this time the swelling rate has reached 900%; When the degree of mineralization is greater than 3000 mg/L, the swelling rate of montmorillonite decreases obviously. From Figure 1, we can also know that the change of median particle size of montmorillonite is basically the same as that of macroscopic static swelling rate and decreases with the increase of solution salinity. The reason is that the cation concentration between the montmorillonite layers is higher than the cation concentration of the external solution. When the montmorillonite crystal layer and the external solution formed the concentration difference, the water would enter the crystal layer and increase the spacing between the crystal layers, thus realizing the hydration swelling. The larger the concentration difference between the external solution and montmorillonite crystal layer, the greater the swelling rate of montmorillonite. Therefore, attention should be paid to the salinity of steam during steam injection. According to the results of reservoir salt sensitivity test and clay mineral swelling, the salinity of injected liquid should be controlled more than 3000 mg/L. The salinity of formation water is 5000 mg/L. Dropping KCL at the steam outlet of the boiler or other measures could be taken to increase the salinity of injected steam. Meanwhile, K+ can also inhibit the swelling of montmorillonite and effectively reduce the reservoir damage caused by steam injection. 3.1.2. Influence of pH of Brine on Swelling Properties of Montmorillonite. The steam injection with high pH would increase the pH of the reservoir. The swelling rate of clay minerals is also different with different pH environment. Experimental results of swelling of montmorillonite by pH of brine were shown in Figure 2. The median particle sizes obtained by laser particle size analyzer at the microscopic level were shown in Figure 2. From Figure 2, it could be found that when the pH value is less than 11, the swelling rate of montmorillonite increases slowly, but when the pH value is greater than 11, the swelling

Figure 1. Influence of salinity on swelling curves of montmorillonite.

Figure 2. Swelling curves of montmorillonite in different pH brine. C

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Figure 3. Transformation process of montmorillonite to analcime (a) original state of montmorillonite; (b) monmorillonite beginning to arrange in orderly fashion; (c) intermediate form of montmorillonite transformed into analcime; (d) analcime beginning to appear; (e) single analcime; (f) block analcime.

From Figure 1 and Figure 2, it could be found that the changes of microscopic median particle diameter of montmorillonite were the same as the changes of the macroscopic static swelling rate of montmorillonite, which indicates the median grain size changes to determine the swelling properties of montmorillonite is feasible. At the same time, the two groups of experiments showed that the swelling properties of montmorillonite increased with the increase of the pH of brine and decreased with the increase of the salinity of brine. In order to reduce the reservoir damage caused by the hydration swelling of montmorillonite, the pH of injected steam should be reduced as much as possible. Through the swelling experiment of montmorillonite, it could be found that the swelling rate of montmorillonite increased with the increases of the pH of brine, especially when the pH of brine reached 11. The swelling rate of montmorillonite decreased with the increases of the salinity of brine. The injection steam of the reservoir was high pH and

rate of montmorillonite would rise rapidly. What is more, we can also find that the median particle size (D50) of montmorillonite increases with the increase of the pH of the brine. When NaOH was added, the median particle size of montmorillonite changed greatly, and the median particle size of montmorillonite increased faster when the pH was greater than 11. This is because montmorillonite is more easily dispersed in an alkaline environment. With the growth of pH, the OH− in solution increased, which would promote the dispersion of montmorillonite. As a result, the swelling rate of montmorillonite increased with pH increases of the brine, and the median particle size of montmorillonite was the same. Therefore, it is more important to adjust the pH of steam. In order to protect the reservoir effectively, we should try to reduce the pH of steam as much as possible to weaken the dispersion of montmorillonite and inhibit the swelling of montmorillonite. D

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Figure 4. Transformation process of montmorillonite to illite: (a) original state of illite mixed layer; (b) illite mixed layer; (c) illite beginning to appear; (d) formed illite.

low salinity, which would promote the hydration swelling of montmorillonite. From the changes of median particle size of montmorillonite, this law was also verified. Therefore, in the steam injection, we should consider adding KCl into the outlet of the steam furnace to reduce the injection steam and increase the salinity of the injected steam. In addition, the positive charge density of chloride free cationic clay stabilizer is high. When it was added to the water, the ionized organic cations could be adsorbed on the surface of the clay particles by electrostatic action, due to the organic cation polymer. And an organic cationic polymer adsorption protective film was formed on the surface of clay particles, which protected clay particles and prevented clay particles from hydration, swelling, and dispersion. 3.2. Influence of Steam Injection on Transformation of Clay Minerals. As we all know, clay minerals can be transformed under certain conditions, which may lead to some reservoir damage, especially the formation of water sensitive clay minerals.54−59 In this work, montmorillonite, illite, and kaolinite were used as experimental samples to study the influence of temperature and pH on the transformation of clay minerals under the condition of sufficient Na+ and K+. What is more, the conditions for the transformation of clay minerals were also clear, which is of great significance to clear the mechanism of reservoir damage caused by steam injection and put forward protection measures in the process of steam injection. 3.2.1. Transformation of Montmorillonite Caused by Steam Injection. Montmorillonite is a relatively common water sensitive clay mineral, and its change will have a great influence on the reservoir. In this work, the transformation of montmorillonite caused by steam injection was investigated through the experiments of scanning electron microscopy

(SEM) and X-ray diffraction (XRD). The experimental results were shown in Figure 3 and Figure 4. From Figure 3 and Figure 4, it could be found that the montmorillonite was transformed into illite and analcime with the increase of temperature in the environment rich in Na+ and K+. When the temperature reached 150 °C, the montmorillonite changed from the initial disordered state to the ordered state. The clay minerals in brine were mainly illite mixed layer, and no illite and analcime were found in the brine with different alkalinity. When the temperature reached 200 °C, illite appeared in the brine of different pH. When the pH of the brine reached 11, the trace of the transformation from montmorillonite to analcime could be found clearly. When the pH of the brine reached 13, the crystal form of the analcime could be found. When the temperature reached 250 °C, the transformation traces of illite mixed layer to illite in brine could be seen clearly. The illite and analcime increased with the increase of alkalinity, while the morphology of the analcime also changed from the original single crystal to multiple crystals and the ordered square analcime could be seen when the pH of the brine reached 13. There was no transformation of montmorillonite until the temperature rearched 200 °C for the temperature is a very important factor for the tansformation of montmorillonite in the environment rich in Na+ and K+. When the temperature rearched 200 °C, the montmorillonite was transformed to illite and analcime, and it was becoming more and more obvious with the increase of pH of the brine. The reason for the transformation of montmorillonite is montmorillonite + Me (Na+, K+) → illite mixed layer → illite. When the temperature rearched 250 °C, the morphology of the analcime changed from the original single crystal to multiple crystals with the pH of brine reaching 13, which indicated that E

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Figure 5. Original state of illite: (a) macroscopic; (b) microscopic.

Figure 6. State of illite at 150 °C: (a) illite panorama at 150 °C; (b) flake illite; (c) block illite.

Figure 7. State of illite at 250 °C: (a) intermediate form of transition of filamentous illite; (b) filamentous illite.

the pH of brine would promote the transformation of clay minerals.

The transformation of montmorillonite into illite can effectively reduce the water sensitive damage of the reservoir, F

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Figure 8. Transformation process of kaolinite with high temperature and high alkali conditions: (a) original state of kaolinite; (b) kaolinite panorama at 150 °C; (c) kaolinite beginning to transform into montmorillonite; (d) transformation process of kaolinite to montmorillonite; (e) illite mixed layer; (f) analcime and a small amount of illite; (g) large number of monomer shaped square analcimes.

but the newly formed analcime would stagnate with the high temperature steam flow, which could block up the reservoir in the narrow channel and cause damage to the reservoir. The transformation of montmorillonite could reduce the reservoir

damage caused by steam injection to a certain extent, but the production of analcime could also cause a certain reservoir damage. Therefore, we could take measures to promote the transformation of montmorillonite into illite and reduce the G

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Through the transformation experiment of clay minerals, it could be found that the steam injection would promote the transformation of montmorillonite into illite and analcime, and the analcime easily blocked the pores. Besides, steam injection could also promote the dispersion of illite and accelerate the dissolution of clay minerals, which made the reservoir rock skeleton more loose and led to sand production, wall collapse, and so on. The resulting particles could also block the reservoir and reduce the permeability of the reservoir, which made great damage to the reservoir. What is more, the steam injection could also promote the transformation of kaolinite into montmorillonite and analcime. There was a part of the kaolinite in the reservoir attached to the reservoir channel. It easily caused reservoir damage for hydration swelling after transformation into montmorillonite. Kaolinite could also be transformed into analcime, which easily plugged the pores and caused damage to the reservoir. Therefore, reasonable temperature should be selected in steam injection mining, not only to meet the needs of production but also to minimize the damage of the reservoir. At the same time, measures such as adding acid liquid and KCl at the outlet of the steam boiler should be taken to reduce the damage caused by steam injection. 3.3. Influence of Steam Injection on Dissolution of Clay Minerals. Steam injection is the main way for heavy oil reservoir exploitation, and the steam temperature of the injection reservoir is about 300 °C. Moreover, the salinity of injected steam is lower than that of the reservoir, and the pH of injected steam is higher than that of the reservoir. The injected steam can easily react with the reservoir rock to promote the dissolution of clay minerals with the condition of high temperature and high pH of the steam. The dissolution of clay minerals would cause the rock skeleton to loosen.63 What is more, it easily leads to sand production, collapse, and so on, resulting in reservoir damage.64,65 Reservoir damage caused by mineral dissolution is also one of the important ways for steam injection reservoir damage. Through the investigation of relevant literature, it was found that the dissolution of minerals is mainly the dissolution of silicate. In this work, the experimental samples of pure quartz sand and drilling mud cuttings were analyzed to study the effect of steam injection on the dissolution of clay minerals. The experimental results were shown in Figure 9 and Figure 10.

production of the analcime by controlling the reservoir temperature of 200 °C, reducing the pH of the reservoir as much as possible. We can realize it by controlling the steam temperature and adding acid at the outlet of the boiler. 3.2.2. Transformation of Illite Caused by Steam Injection. According to the data investigation, illite is a relatively stable mineral and difficult to dissolve and change in the rich Na+ brine with temperature less than 250 °C. In this work, the experiment also used the combination of NaOH + KHCO3 to ensure sufficient Na+ and K+. The experimental results were shown in Figure 5, Figure 6, and Figure 7. From scanning electron microscopy and diffraction experiments (Figure 5, Figure 6, and Figure 7), it could be found that the illite did not change with the increase of the temperature or the pH of brine in the case of Na+ and K+ rich environment, but the morphology of illite changed. When the temperature reached 150 °C, the illite began to aggregate from the initial sheet state and formed a laminated or blocky structure; When the temperature reached 250 °C, illite changed from massive to filamentous, crisscrossed, almost all of the illite linked together. For loose sandstone reservoirs, filamentous illite is easier to dissolve. What is more, it is easily leads to sand production, collapse, and so on, resulting in reservoir damage. As we all know, the velocity sensitivity index of the reservoir increases with the increase of illite content. Illite with fibrous distribution increased the contact area with brine, which would produce a large number of particles and cause certain damage to the reservoir. We should control the temperature or alkalinity of the brine to decrease the dissolution of the clay minerals caused by steam injection. 3.2.3. Transformation of Kaolinite Caused by Steam Injection. Through literature investigation,60−62 it was found that kaolinite is unstable under alkaline conditions, generally dissolves at pH = 9 and T = 150 °C, and disappears at pH = 11 and T = 250 °C. Moreover, kaolinite can be transformed into montmorillonite, resulting in reservoir damage. In this work, the transformation was investigated by scanning electron microscopy and X-ray diffraction with the combination of NaOH + KHCO3 that could ensure sufficient Na+ and K+. The experimental results were shown in Figure 8. From Figure 8, it could be found that kaolinite was transformed to illite and analcime with the increase of temperature in the environment rich with Na+ and K+. When the temperature reached 150 °C, kaolinite began to aggregate into blocks and convert into montmorillonite. There were mainly illite mixed layers in the reservoir for the montmorillonite was very unstable. And illite and analcime were not found in the brine. When the temperature reached 250 °C, the transformation traced from illite mixed layer to illite could be seen clearly. Additionally, more and more illite and analcime could be seen with the increase of the pH of the brine. It was found that kaolinite could be transformed into water sensitive clay minerals montmorillonite and analcime with high temperature and high pH of the brine. There was a part of the kaolinite in the reservoir attached to the reservoir channel. It was easy to cause reservoir damage for hydration swelling after transformation into montmorillonite. Kaolinite could also be converted into square analcime, which easily plugged the pores and caused damage to the reservoir. The transformation of kaolinite would cause great damage to the reservoir. The swelling of clay minerals and the transformations of clay minerals were the main source of reservoir damage caused by steam injection in the loose sandstone heavy oil reservoir.

Figure 9. Dissolution curves of quartz sand at different temperatures and different alkalinities. H

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Figure 10. Dissolution curves of drilling mud cuttings at different temperatures and different alkalinities.

Figure 11. Rock matrix dissolution.

From Figure 9 and Figure 10, it could be found that the dissolution of quartz sand or drilling mud cuttings at high temperature and high pH value was much higher than that at low temperature and low pH value. It was obvious that the dissolution of quartz sand increased with the increase of temperature of formation at a constant temperature. When the temperature of formation was constant, the dissolution of quartz sand increased with the increase of pH of brine. Moreover, when the pH was more than 9, the amount of quartz sand dissolution increased rapidly, so the pH of the steam should be as low as possible. Both temperature and pH of brine had a significant effect on the dissolution of quartz, which was the same as the drillling mud cuttings. The dissolution of minerals had a great influence on the physical properties and pore structure of reservoirs, which mainly displayed in two aspects: On the one hand, the dissolution mainly occurred near the steam injection wellbore, and the near wellbore zone made the reservoir rock more loose due to the dissolution of high temperature and high pH value, resulting in a large amount of sand production or formation collapse; on the other hand, the solute produced by dissolution caused the crystallization of crystals or the formation of new minerals associated with other minerals in the wellbore far away from the wellbore, resulting in a blockage in the throat. What is more, the particle produced by the dissolution of clay minerals would move with the injection of steam. When the temperature was reduced or the power was insufficient, the particles would stagnate, which would cause serious reservoir damage. By controlling the injection speed of steam, the movement of particles caused by steam injection could be reduced, which could ruduce the reservoir damage caused by steam injection. A large number of corrosion phenomena had also been found in the reservoir through the high temperature core flow experiment of loose sandstone reservoir core by scanning electron microscopy experiment. The experimental results were shown in Figure 11 and Figure 12. From Figure 11 and Figure 12, it could be found that there was a large number of dissolution phenomena in the reservoir after steam injection in unconsolidated sandstone reservoir, which indicated that the dissolution of minerals in the loose sandstone reservoir had occurred during steam injection. The dissolution of rock skeleton and clay minerals would make the reservoir more loose and easily lead to sand production, wall collapse, and other damages. In addition, the movement of

Figure 12. Corrosion.

particles caused by steam injection would also clog pores and damage reservoirs. The measures of controlling the pH of the steam within 9 should be taken to reduce the reservoir damage caused by the dissolution of clay minerals. What is more, the reservoir damage caused by steam injection could also be effectively reduced by controlling the injection rate of steam.

4. CONCLUSIONS In this work, the influence of steam injection on clay in loose sandstone reservoir is studied from the swelling, transformation, and dissolution of clay minerals. The damage mechanism of steam injection to unconsolidated sandstone reservoir was analyzed, and the protective measures of response were put forward. (1) The swelling rate of montmorillonite increased with the increase of the pH of brine and decreased with the increase of the salinity of brine. The median particle size of montmorillonite increased with the increase of the pH of brine and decreased with the increase of the salinity of brine. The chloride free cationic clay stabilizer can be used to inhibit the swelling of montmorillonite. (2) The environment of high temperature and high pH would promote the transformation of the clay minerals, and montmorillonite could be transformed into kaolinite and analcime; illite was relatively stable; kaolinite could be transformed into the water sensitivity of clay minerals I

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montmorillonite and analcime. The reservoir would be damaged by the swelling of montmorillonite, the migration of analcime, and so on. In order to reduce the transformation of clay minerals, the pH and temperature of steam liquid should be reduced as much as possible. (3) The dissolution of minerals at high temperature and high pH value was much higher than that at low temperature and low pH value. Moreover, when the pH was more than 9, the amount of mineral dissolution increased rapidly. The measures of controlling the pH of the steam within 9 and controlling the injection rate of steam could be taken to reduce the reservoir damage.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiangjun Liu: 0000-0002-0633-0989 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (NSFC; Grant No. 41772151). REFERENCES

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DOI: 10.1021/acs.energyfuels.7b03686 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.7b03686 Energy Fuels XXXX, XXX, XXX−XXX