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14Journal of Siberian Federal University. Chemistry 2022 15(2): 186-196
DOI: 10.17516/1998-2836-0283 УДК 622.276.1; 622.276.4; 622.276.6
Laboratory Testing of Acidic EOR Oil-Displacing Compositions Based on Surfactants, Inorganic Acid Adduct and Polyols
Mechrob R. Sholidodov, Vladimir V. Kozlov*, Liubov K. Altunina, Vladimir A. Kuvshinov and Liubov A. Stas'eva
Institute of Petroleum Chemistry, SB RAS Tomsk, Russian Federation
Received 17.02.2022, received in revised form 23.03.2022, accepted 12.04.2022
Abstract. The paper presents the results of laboratory testing of new acid compositions based on surfactants, an adduct of inorganic acid, and polyols (glycerol and sorbitol) in the process of displacement of a heavy high-viscosity oil. The experiments were carried out using an equipment for physical modeling of the oil displacement process. The effectiveness of the new acidic compositions was evaluated in respect to the conditions of the fields at the early and late stages of development. As a result of the study, it has been found out that the use of acid compositions based on glycerol and sorbitol led to an equalization of filtration flows with an increase in reservoir sweep, return to the initial permeability and, as a consequence, to a significant increase in the oil displacement efficiency both at low and high temperatures.
Keywords: heavy high-viscosity oil, acid composition, polyols, enhanced oil recovery, physical modeling, oil displacement coefficient.
Citation: Sholidodov, M.R., Kozlov, V.V., Altunina, L.K., Kuvshinov, V.A. and Stas'eva, L. A. Laboratory testing of acidic EOR oil-displacing compositions based on surfactants, inorganic acid adduct and polyols. J. Sib. Fed. Univ. Chem., 2022, 15(2), 186-196. DOI: 10.17516/1998-2836-0283
© Siberian Federal University. All rights reserved
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0). Corresponding author E-mail address: kozlov_vv2004@mail.ru
Лабораторные испытания кислотных нефтевытесняющих композиций на основе ПАВ, аддукта неорганической кислоты и полиолов для увеличения нефтеотдачи пластов
М. Р. Шолидодов, В. В. Козлов, Л. К. Алтунина, В. А. Кувшинов, Л. А. Стасьева
Институт химии нефти СО РАН Российская Федерация, Томск
Аннотация. В работе представлены результаты лабораторных модельных испытаний новых кислотных композиций на основе ПАВ, аддукта неорганической кислоты и полиолов (глицерина и сорбита) в процессе вытеснения тяжелой высоковязкой нефти. Эксперименты проводили с использованием оборудования для физического моделирования процесса нефтевытеснения. Эффективность новых кислотных композиций оценивали применительно к условиям месторождений, находящихся на ранней и поздней стадиях разработки. В результате исследований установлено, что использование кислотных композиций на основе глицерина и сорбита приводит к выравниванию фильтрационных потоков с увеличением охвата пласта, восстановлению первоначальной проницаемости и, как следствие, к существенному приросту коэффициента нефтевытеснения как при низких, так и при высоких температурах.
Ключевые слова: тяжелая высоковязкая нефть, кислотная композиция, полиолы, увеличение нефтеотдачи, физическое моделирование, коэффициент нефтевытеснения.
Цитирование: Шолидодов, М. Р. Лабораторные испытания кислотных нефтевытесняющих композиций на основе ПАВ, аддукта неорганической кислоты и полиолов для увеличения нефтеотдачи пластов / М. Р. Шолидодов, В. В. Козлов, Л. К. Алтунина, В. А. Кувшинов, Л. А. Стасьева // Журн. Сиб. федер. ун-та. Химия, 2022, 15(2). С. 186-196. DOI: 10.17516/19982836-0283
Introduction
The world resources of heavy and bituminous oil being estimated at 750 billion tons significantly exceed the reserves of light oil. Canada and Venezuela are endowed with the largest oil reserves. Hence, the oil reserves of Canada are 386 billion tons, where 25 billion tons are recoverable oils. Venezuela's reserves are estimated at 335 billion tons, of which 70 billion tons are recoverable oil. Mexico, the United States, Russia, Kuwait and China also have significant reserves [1, 2]. Heavy oils and natural bitumens are characterized by a high content of aromatic hydrocarbons, resin-asphaltene substances, a high concentration of metals and sulfur compounds, high values of density and viscosity, increased coking properties, which leads to high production and processing costs of such an oil.
In order to significantly increase the oil recovery of developed reservoirs, where from it is no longer possible to extract residual oil reserves by traditional methods, it is necessary to apply new oil production
technologies. Currently, the most used methods of enhanced oil recovery are waterflooding, methods of thermal treatment of formation, such as cyclic steam soaking and injection of hot water or steam into the formation, and physicochemical methods. Each of these methods has its own field of application [3, 4].
Physicochemical methods of enhanced oil recovery are mainly based on the displacement of oil by aqueous solutions of various chemical reagents, which improve or change as necessary the displacing properties of water. The most common technologies are based on the introduction of water-soluble surfactants, polymers, acids and alkalis into the oil reservoir [5-9]. In particular, oil well acidizing includes the injection of acid (mud acid, HCl, etc.) into the well in order to increase the productivity or injectivity of the well, as well as to treat the bottomhole formation zone [10]. Surfactant-based chemical compositions for oil recovery enhancement affect the following interrelated factors: interfacial tension at the oil-water, water-rock, and oil-rock boundaries, which is due to the adsorption of surfactants at the interface. In addition, the effect of surfactants is manifested in a change in the selective wetting of the rock surface with water and oil, in the rupture and washing off of an oil film from the surface of rocks, in the stabilization of oil dispersion in water, in an increase in the coefficients of oil displacement by the water phase during forced displacement and capillary imbibition, as well as in an increase in the relative phase permeabilities. [11, 12].
This paper presents the results of laboratory testing of new acidic chemical oil-displacing compositions developed at the Institute of Petroleum Chemistry of the Siberian Branch of the Russian Academy of Sciences (IPC SB RAS).
Acidic Chemical Oil-Displacing Compositions
In recent years, in order to increase the oil recovery efficiency by increasing the permeability of reservoir rocks and the productivity of producing wells, as well as by increasing the oil displacement efficiency, acidic oil-displacing compositions have been developed at the Institute of Chemistry of the Siberian Branch of the Russian Academy of Sciences (Tomsk, Russia) (Table 1). These compositions are based on an aqueous solution of a mixture of water-soluble surfactants, an adduct of an inorganic acid, urea, and polyhydric alcohols (glycerol or sorbitol) [13-16].
Inorganic acid contained in the developed acid compositions, in particular, a weak boric acid (pKa = 9.2) forms a boric acid glycerol complex when interacting with glycerol. It is three to four orders of magnitude stronger (pKa = 5.7-6.5), so it is capable of dissociating into ions as a strong monobasic acid characterized by low pH values. In an aqueous solution, when boric acid interacts with a polyol, a dynamic equilibrium of two successive stoichiometric reactions is established [17, 18]. Figure 1 shows a diagram of the formation of a complex of boric acid and glycerol.
At low temperatures of 20-40 °C, the interaction of the acidic composition with carbonate rocks results in the return of the original reservoir permeability without the formation of insoluble products
Table 1. Physicochemical characteristics of acidic oil-displacing compositions
Acidic oil-displacing compositions Viscosity, mPas Density, g/cm3 PH
Glycerol-based oil-displacing compositions 12.8 1.187 2.37
Sorbitol-based oil-displacing compositions 1.83 1.073 3.27
H»C-QH
+ 3H,0
II»C-OH
Fig. 1. Formation of a boric acid glycerol complex
and in a decrease in the swelling of clays, while the released CO2 dissolves in the oil reducing its viscosity by 2-6 times. This results in the additional displacement of residual oil from both high-permeability and low-permeability zones of the formation.
In addition, urea, contained in the composition, dissolves in water. It is quite stable up to 70 °C, but at high temperatures (under heat exposure) it hydrolyzes with the formation of carbon dioxide and ammonia [19]. As a result, an alkaline ammonia-borate buffer system is formed having a high capacity in the range of of 9.0-10.0 pH units, which provides optimal conditions for surfactant operation and effective oil displacement. Figure 2 shows the scheme of urea hydrolysis with the formation of carbon dioxide and ammonia [20].
Carbon dioxide dissolves in oil, which leads to a decrease in its viscosity. In addition, carbon dioxide and ammonia in the vapor phase contribute to the preservation of the vapor-gas mixture at a temperature below the steam-condensate temperature, which increases the efficiency of the transfer of oil components by the distillation mechanism [21].
Being compatible with saline formation waters, the developed compositions have a low freezing point (-20 ^ -60 °C) and a low interfacial tension at the boundary with oil. They do not form any precipitated hardness upon dilution but have a beneficial effect on the reservoir rock, since they preserve its original permeability, reduce the swelling of clay minerals in the reservoir rock and poorly react with carbonate rocks [19].
Table 1 shows the physicochemical characteristics of acidic oil-displacing compositions based on surfactants and polyols (glycerol and sorbitol).
(NH2)2CO ^ CO2 + H2O
■ C UN BIO IH
Fig. 2. Hydrolysis of urea with the formation of carbon dioxide and ammonia
High oil-displacing capacity, compatibility with saline formation waters, and a decrease in clay swelling promote an additional displacement of residual oil from both high-permeability and low-permeability zones of the formation. At high temperatures, the composition chemically evolves to become an alkaline oil-displacing composition with a high buffer capacity, providing effective oil displacement and prolonged impact on the reservoir.
It should be noted that acidic oil-displacing compositions prepared on the basis of glycerol have a higher viscosity than compositions based on sorbitol; therefore, they are better in displacement of more viscous oils. For the displacement of light oils (piston displacement), it is preferable to use acidic compositions based on sorbitol.
Experimental
For laboratory studies of acidic oil-displacing compositions, four models of a heterogeneous reservoir were prepared (as applied under the conditions of the Permocarbon reservoir of the Usinskoye oilfield), each model consisted of two columns with different gas permeability parameters.
The Permian-Carboniferous reservoir of the Usinskoye oilfield is located in the depth interval 1100-1500 m. Under initial conditions, the oil of the Permian-Carboniferous reservoir is characterized by high values of dynamic viscosity about 710 mPas, which is due to the high content of asphaltene-resin components. Permian-Carboniferous deposits have an extremely heterogeneous geological structure. Hence, their reservoirs are of a complex type: cavernous-porous, fractured-porous, and fractured-cavernous-porous. The current state of reservoir development is characterized by a high degree of water cut in the produced products with low development of geological oil reserves [22]. This creates the prerequisites for the use of various methods of enhanced oil recovery, in particular, for the use of chemical compositions. Since the average reservoir temperature is 23 °C, thermal oil recovery methods are widely used in the oilfield.
The columns were filled with disintegrated carbonate core material using a shaker (GE0L-3915100000). Next, the gas permeability values were measured according to the method GOST 23409.6-78. Then the columns were sequentially saturated with models of formation water (salinity 62.1 g/l) and formation oil (heat-stabilized oil with the addition of 30 % wt of kerosene) from the Usinskoye field (Table 2).
Table 2. Characteristics of models of a heterogeneous reservoir
Model Column Gas permeability, ^m2 Ratio of gas permeabilities of models Oil viscosity, mPa/s Pore volume, cm3 Initial oil saturation,%
1 1 2.30 1.92:1 64 43.0 93.2
2 1.20 42.2 87.7
2 1 0.44 1.76:1 60 46.0 73.8
2 0.25 39.7 70.5
4 1 1.10 1.23:1 55.3 43.0 64.6
2 0.89 43.7 62.6
5 1 1.86 3.64:1 52.6 37.4 63.0
2 0.51 35.7 64.4
The gas permeability value of the columns of the heterogeneous reservoir model was in the range of 0.51-2.30 ^m2. The permeability ratio of the columns of each model varied within the range 1.23: 1-4.07: 1. The viscosity of the oil was in the range of 47.2-64.0 mPa/s. The initial oil saturation of the prepared models averaged 67.6 %.
A laboratory testing of the acidic compositions was carried out using an installation for studying filtration characteristics (KATAKON LLC, Russia) as applied to the conditions of the Permian-Carboniferous deposit of the Usinskoye field. The installation, consisted of two columns with a volume of 125 cm3 (core holders), filled with disintegrated core material and having different values of gas permeability made it possible to simulate reservoir heterogeneity.
The study of the process of oil displacement with the use of acidic compositions was carried out under conditions simulating the natural mode of development at a temperature of 20-23 °C, as well as under steam-thermal and steam cyclic exposure at a temperature of 150 °C.
The study of the effect of acid compositions on the oil displacement process was carried out as follows. First, oil was displaced with water until the products were completely water-flooded from both columns at a given temperature. Temperatures, pressure values at the inlet and outlet of the columns, volumes of displaced oil and water from each column were measured every 5-15 minutes. The obtained data were used to calculate the pressure gradient grad P (MPa/m), filtration rate V (m/ day), fluid mobility k/^ (^m2/mPa^s), and coefficient of oil displacement by water Cd (%). After the oil was displaced by water, a slug of the oil-displacing composition was simultaneously injected into both columns, pushed with water for a predetermined distance, and thermostated for a certain time. Then the water injection was continued. The measurement of the above parameters - temperature, inlet and outlet pressure, volumes of displaced oil and water from each column - was continuously carried out every 5-15 minutes. In addition, the pH of the fluid at the outlet from the columns and the concentration of urea included in the compositions were determined. The obtained data were used to calculate the pressure gradient, filtration rate, fluid mobility, and the absolute oil displacement coefficient by composition and water.
Results and discussion
Figures 3-6 and Table 3 show the results of determination of the filtration characteristics and the oil displacement coefficient under conditions simulating those of reservoir, at low temperatures in natural recovery drive after preliminary treatment with acidic surfactant compositions based on glycerol (models 1 and 2, Figures 3 and 5) and sorbitol (models 3 and 4, Figures 4 and 6).
The model of formation water of the Usinskoye field was filtered through the oil-saturated models of the heterogeneous reservoir of the Usinskoye field until the full water cut of the products at the outlet from the models. The oil displacement coefficient ranged from 43.5 to 51.6 % for all models (Table 3).
Then, a slug of an acidic oil-displacing composition based on glycerol (models of a heterogeneous reservoir No. 1 and 2) in a volume of 0.5 of the model pore volume and a slug of an acidic oil-displacing composition based on sorbitol (models of a heterogeneous reservoir No. 3 and 4) were injected at 23 °C in the direction 'well - reservoir'. The maximum pressure gradient when injecting acidic compositions was 10.2 MPa/m. The models of the heterogeneous reservoir were kept for 24 hours and then the water filtration was resumed. The treatment of heterogeneous reservoir models with acidic oil-displacing compositions resulted in the additional oil displacement. Due to the treatment with the compositions
Fig. 3. Filtration characteristics of models of a heterogeneous formation made of carbonate core material: gas permeability of the first and second columns 2.30 and 1.20 ¡¡m2, respectively (a); gas permeability of the first and second columns 0.44 and 0.25 ¡m2, respectively (b) after treatment with an acidic oil-displacing composition based on glycerol
Fig. 4. Filtration characteristics of models of a heterogeneous formation made of carbonate core material: gas permeability of the first and second columns 1.10 and 0.89 ^m2, respectively (a); gas permeability of the first and second columns 1.86 and 0.51 ^m2, respectively (b) after treatment with an acidic oil-displacing composition based on sorbitol
the oil displacement efficiency increased from 1.1 to 12.6 % for high-permeability columns and from 10.2 to 21.3 % for low-permeability columns, respectively (Table 3). During filtration, the fluid mobility in the columns significantly decreased, while the pressure gradient increased from 2.075 -6.125 to 7.45-7.875 MPa/m for all models of a heterogeneous reservoir.
The next step was to simulate the oil displacement process under the combined treatment with acidic oil-displacing compositions and heat treatment. For this purpose, the temperature of the heterogeneous reservoir models was raised to 150 °C and held for 24 hours. After thermostating, the filtration of the formation water of the Usinskoye field was continued. As the temperature in the models has risen to 150 °C and the fluid in the columns has become more mobile, while the gradient of pressure created when injecting formation water into the reservoir model decreased. Due to the filtration of formation water at 150 °C, the oil displacement coefficient has increased from 1.1 to 11.1 % in high-permeability columns and from 0 to 0.5 % in low-permeability columns.
After that, acidic oil-displacing compositions were re-injected into the model of a heterogeneous reservoir at 150 °C (two slugs of a sorbitol-based composition were sequentially injected into the model of a heterogeneous reservoir № . 3). The heterogeneous reservoir models were kept for 24 hours and
then the injection of formation water was resumed. At the same time, additional oil displacement was observed. Due to the treatment with compositions at 150 °C the increment in the oil displacement coefficient reached 9.5 % for high-permeability columns and from 2.8 to 11.1 % for low-permeability columns.
In all experiments performed, the equalization of filtration flows (changes in fluid mobility in the columns) was observed (Figures 5-6).
An increase in the oil displacement efficiency due to the use of acidic compositions occurred both at low and high temperatures. Hence, the increment of the oil displacement coefficient due to the treatment with compositions at 23 °C ranged from 1.1 to 12.6 % for high-permeability columns and from 10.2 to 21.3 for low-permeability columns, respectively. In the case of the treatment with compositions at 150 °C it ranged from 1.1 to 4.8 % for high-permeability columns and from 8.1 to 14.5 % for low-permeability columns (Table 3). The maximum gradient of pressure created during filtration was in the range of 0.12-8.3 MPa/m for heterogeneous reservoir models.
Despite the fact that the viscosities of the applied glycerol or sorbitol-based compositions were different, the results of filtration tests showed in all experiments the equalization of filtration flows. In addition, all compositions revealed a high efficiency in the process of heavy oil displacement.
1 column 2 column 1 column 2 column
Fig. 5. Changes in fluid mobility in models of a heterogeneous formation made of carbonate core material: gas permeability of the first and second columns is 2.30 and 1.20 ^m2, respectively (a); gas permeability of the first and second columns is 0.44 and 0.25 ^m2, respectively (b)
Fig. 6. Changes in fluid mobility in models of a heterogeneous formation made of carbonate core material: gas permeability of the first and second columns is 1.10 and 0.89 ^m2, respectively (a); gas permeability of the first and second columns is 1.86 and 0.51 ^m2, respectively (b)
Table 3. Summary of the conducted experiments
Mobility ratio Oil-displacement coefficient
No. of model (experiment) No. of column Gas permeability of column, ^m2 Before injection of composition Displacement with water / water and composition, % After injection of Increment of the oil the oil-displacement coefficient due to the application of the composition, %
composition 23 °C 150 °C S
1 1 2.30 23.5:1 2.75:1 29.6 / 45.8 12.6 3.6 16.2
2 1.20 3.60 / 38.5 20.4 14.5 34.9
2 1 0.44 6.69:1 1.32:1 43.0 / 53.6 7.8 2.8 10.6
2 0.25 44.2 / 66.5 12.8 9.5 22.3
3 1 1.11 10.9: 1 5.4:1 38.5 / 49.4 6.1 4.8 10.9
2 0.89 27.6 / 50.9 10.2 13.1 23.3
4 1 1.86 3.3:1 1.0:0.7 49.5 / 51.8 1.1 1.1 2.20
2 0.51 4.10 / 33.5 21.3 8.1 29.4
Potentiometric analysis of water samples taken at the outlet from the heterogeneous reservoir model showed an increase in the pH values to a maximum of 8.9 pH units, while the pH values are determined from the hydrolysis of urea and depend on the temperature of the experiment. Hence, at low temperatures, the pH values ranged from 5.6 to 7.0 pH units, while at high temperatures they lay in the range of 8.5-8.9 pH units, which is optimal to ensure the most effective detergent ability of surfactants.
The concentration of urea at the outlet from the models of a heterogeneous reservoir determined by the photocolorimetric method depends on the temperature of model. So, at temperatures below 90 °C, partial hydrolysis of urea and a significant concentration of urea at the outlet were observed, while at a temperature of 150 °C and a sufficient time of holding the composition in the model of a heterogeneous formation almost complete hydrolysis took place, and the concentration of urea decreased to trace values. Following on from the results of the experiments, the amount of urea in the sampled water was 21.7-95.4 % of the initial concentration on a whole for the models. This suggests different degrees of urea hydrolysis. The low degree of urea hydrolysis is primarily due to the low temperature. At an experimental temperature of 150 °C, the degree of urea hydrolysis tends to 100 %.
Conclusions
As a result of the studies, it was found out that the injection of acidic compositions based on surfactants, glycerol or sorbitol leads to the equalization of filtration fluid flows in reservoir models. This makes it possible to increase the oil displacement and the sweep efficiencies through water and steam flooding. The acidic components of the oil-displacing composition interact with the rock, which allows returning or preserving its permeability. As a result, the injectivity of the wells increases or persists. In addition, a significant increase in the oil displacement efficiency is observed under the natural recovery drive and in the case of simulation of thermal steam stimulation. The increment of the oil displacement coefficient due to the treatment with compositions at 23 °C ranged from 1.1 to 12.6 % for high-permeability columns and from 10.2 to
21.3 for low-permeability columns, respectively. In the case of the treatment with compositions at 150 °C it ranged from 1.1 to 4.8 % for high-permeability columns and from 8.1 to 14.5 % for low-permeability columns. The total increase in the oil displacement coefficient over the entire experiment was from 2.20 to 34.9 %. Thus, it was found out that acid compositions based on a surfactant, an inorganic acid adduct, polyol, and urea are effective for enhancing oil recovery from high-viscosity oil deposits due to an increase in the oil displacement and sweep efficiencies through water and steam flooding.
The work was carried out within the framework of the state assignment of the Institute of Petroleum Chemistry SB RAS, funded by the Ministry of Science and Higher Education of the Russian Federation.
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