Changes in the properties of conventional crude oil before and after CO2 flooding

Abstract

In order to enhance oil recovery of a conventional oil reservoir by CO₂ flooding, the changes in the properties of the crude oil before and after CO₂ flooding are systematically investigated by on-site sampling and experimental testing. The results show that, after CO₂ flooding, the light hydrocarbons of the produced crude oil is increased and the heavy hydrocarbons of the produced crude oil is decreased due to the deposition of resins and asphaltenes in the pores of the formation. In addition, the produced fluid (a mixture of oil and water) has a high water separation rate, the oil- water interface has a high tension value, and the crude oil has a high acid value and a low viscosity. The conclusions can provide a certain guidance for high-efficiency development of a conventional oil reservoir by CO₂ flooding.     

烃组分对CO2驱最小混相压力的影响

针对CO_2-EOR原油组分对混相能力影响的问题,应用界面张力消失法设计了不同碳数烃组分、不同族烃组分、不同含量烃组分混合模拟油与CO_2的最小混相压力实验,分析不同族烃组分与CO_2最小混相压力的变化规律,探寻原油中影响CO_2驱最小混相压力的关键组分。研究表明:原油中不同组分与CO_2的最小混相压力不同,相同碳数烃组分最小混相压力依次为:烷烃、环烷烃、芳香烃;同族烃的碳数越小,最小混相压力越小;相同碳数烃类的混合组分模拟油的最小混相压力小于单一烃组分的最小混相压力;原油中低碳数烷烃含量增加,最小混相压力降低,高碳数芳香烃含量增加,最小混相压力升高。该研究结果为多种类型油藏实施CO_2驱提高采收率提供了数据材料及理论支撑。     

Influence of injection pressure and injection volume of CO2 on asphaltene deposition

To further improve the oil displacement effect by CO₂ flooding, the trends and conditions of asphaltene deposition under different injection pressures and injection volumes of CO₂ were studied by SDS solid phase deposition testing system, high temperature and high pressure microscope, and P-X phase diagram. When the mole fraction of CO₂ in crude oil increases to a certain value, asphaltene deposition appears. The lower the pressure, the lower the mole fraction of CO₂ in crude oil causing the asphaltene deposition there is. After the onset of asphaltene deposition, the degree of deposition increases with an increase in pressure. The amount of the deposited asphaltene under miscible displacement is the highest, under near-miscible displacement is the second highest, and under immiscible displacement is the lowest. When the dissolution of CO₂ in crude oil reaches the saturation point, the asphaltene deposition becomes slow. Besides, it is feasible to prevent or reduce the asphaltene deposition by adjusting the thermodynamic parameters according to the phase behaviors of the CO₂-crude oil system. The experimental results can provide theoretical basis for optimization design of the parameters of CO₂ flooding.     

Influence of pressure and CO2 content on the asphaltene precipitation and oil recovery during CO2 flooding

During CO₂ flooding, the crude oil is treated with CO₂, and meanwhile it is displaced by CO₂. Based on the two processes, the influence of pressure and CO₂ content on the asphaltene precipitation and oil recovery efficiency are systematically investigated by indoor simulation experiment. With the increase of the pressure or CO₂ content during CO₂ treatment, the amount of asphaltene precipitation can be increased to a certain value. Correspondingly, the degrees of the changes of oil-water interface, the compositions of crude oil, and reservoir permeability are positively correlated with the amount of asphaltene precipitation. However, during the process, the oil recovery has an optimal value due to the combined action of asphaltene precipitation and the improvement of flow performance of the crude oil. These conclusions can provide a basis for high efficiency development of low permeability oil reservoirs by CO₂ flooding.     

Interfacial Interactions Between Crude Oil and CO2 Under Reservoir Conditions

Abstract

In this paper, an experimental technique was developed to study the interfacial interactions between crude oil and CO₂ under reservoir conditions. By using the axisymmetric drop shape analysis (ADSA) for the pendant drop case, this new technique makes it possible to measure the interfacial tensions (IFTs) between crude oil and solvents, such as CO₂, at high pressures and elevated temperatures. The major component of this experimental setup is a see-through windowed high-pressure cell. In this study, the IFT of the crude-oil–CO₂ system was measured as a function of pressure at two fixed temperatures. It was found that, due to mutual interfacial interactions between crude oil and CO₂, their dynamic IFT gradually reduces to a constant value, i.e., the equilibrium IFT. The major interfacial interactions observed in this study include light-ends extraction and initial turbulent mixing. At T = 58°C, the equilibrium IFT reaches 1–2 dyne/cm when P ≥ 13.362 MPa, and only partial miscibility is achieved even up to P = 28.310 MPa. Thus, this experimental study shows that only partial miscibility can be obtained in most CO₂ flooding reservoirs. In addition, it is expected that the observed light-ends extraction and initial turbulent mixing phenomena may have significant effects on ultimate oil recovery and long-term CO₂ sequestration.     

CO2 miscible flooding in low permeability sandstone reservoirs and its influence on crude oil properties

Miscible CO₂ injection process has become widely used technique for the enhanced oil recovery in low permeability reservoirs. Core flooding experiments and field test of CO₂ miscible flooding in low permeability sandstone reservoirs and its influence on crude oil properties was studied. The results showed that CO₂ miscible flooding in low permeability sandstone reservoirs can enhance oil recovery both in laboratory study and field test. The permeability of sandstone reservoirs decreased during CO₂ miscible flooding due to the precipitation of asphaltene of crude oil. The precipitation of asphaltene lead to a reduction of asphaltene content and the apparent viscosity of crude oil. A further study on inhibitors and removers for asphaltene deposits from crude oil should be investigated to prevent and remove asphaltene deposits in low permeability sandstone reservoirs.     

延长油田致密砂岩油藏CO2驱油机理研究

CO_2驱是提高低渗透油田产量、缓解温室效应的有效途径。针对鄂尔多斯盆地油藏压力系数低、原油轻质组分含量高的特点,通过PVT和最小混相压力等测试分析方法,揭示了低压、低孔、低渗油藏CO_2驱提高采收率主要机理。开展了CO_2注入储层与无机、有机物作用后的沉淀研究,表明CO_2在无机盐溶液中不会形成沉淀堵塞孔隙,CO_2与有机质作用后沉积点高于油藏压力,且注入压力越高,CO_2在地层原油中的溶解能力越强,目标区块CO_2注入后不易形成沥青质沉淀。物模驱替实验结果表明,均质岩心的采出程度明显高于非均质岩心,且随着岩心非均质性的增加,水驱采出程度、气驱采出程度及最终采出程度均明显下降。     

模拟延长油田CO_2驱油过程原油结蜡特性研究

通过对某CO2驱油区块的不同CO2分压条件下原油结蜡倾向和原油结蜡特性评价,计算出原油的结蜡速率、结蜡率及蜡含量,分析了原油的析蜡曲线特性。结果表明,CO2分压增大时原油的结蜡速率和结蜡率提高,蜡含量降低,蜡析出更明显;经CO2处理作用后原油析蜡点向低温方向移动,其蜡含量也相应地比未处理油样的蜡含量降低。     

Study on the blockage in pores due to asphaltene precipitation during different CO2 flooding schemes with NMR technique

CO₂ flooding is an effective way in the tertiary oil recovery. While asphaltene often precipitates from the crude oil during the CO₂ flooding, and the mechanisms of blockage resulting from asphaltene precipitation is still unclear in different CO₂ flooding schemes. In this work, pure-CO₂ flooding, water-alternating-CO₂ flooding (WAG), and CO₂-foam flooding were applied to conduct the core-flooding experiments. Then, as for each flooding scheme, we quantitatively investigated the blockage degree in different pores due to asphaltene precipitation with nuclear magnetic resonance (NMR) technique. Tests results show that CO₂-foam flooding has a relatively higher blockage degree both in the smaller pores and the larger pores than WAG and pure-CO₂ flooding. Although pure-CO₂ flooding has the least asphaltene precipitation and blockage degree among three flooding schemes, its oil recovery degree is far less than the other two flooding schemes. Compared with pure-CO₂ flooding and CO₂-foam flooding, WAG flooding has the highest oil recovery and an acceptable asphaltene precipitation.     

Experimental investigation of interfacial tension and oil swelling for asphaltenic crude oil/carbonated water system

The phenomenon of oil swelling at the oil-carbonated water (CW) system could be an important mechanism during the water alternating gas (WAG) injection process. Nevertheless, the study of the main mechanisms during water flooding (WF) is a complex topic that has not been well revealed so far, especially for asphaltenic crude oil (ACO) systems. Hence, the main goal of this experimental work is to determine the influence of carbon dioxide (CO₂) within the water phase in the interfacial tension (IFT) between water and crude oil for an extensive range of pressures between 400 psi and 2000 psi (i.e. 2.76–13.79 MPa), under two temperatures of 313.15 and 323.15 K (i.e. 40 and 50 °C) by axisymmetric drop shape analysis (ADSA) method. The experimental results demonstrate that the water/ CW and crude oil IFTs decline with time. The value of dynamic IFT (DIFT) between CW and crude oil decreased about 6 mN/m in comparison with the oil–water DIFT. As a result of the CO₂ solubility, the crude oil droplet swells with increasing pressure. When the temperature rises, the effects of increasing entropy phenomena and decline of liquids density is dominant compared to the solubility of CO₂. Thus, the volume of oil droplet increases with temperature, unexpectedly. In addition, as thetemperature increases the water/CW-Oil IFT is slightly reduced over a wide range of pressure evaluated. Nevertheless, there is a slight increase as the pressure increases for the water–oil system. According to the predicted results, interfacial tension of the CW-oil system declines with increasing pressure until the solubility of CO₂ is reached to a maximum value and then approximately remains changeless.     

X油藏注气混相驱可行性实验研究

针对X底水油藏油井注水后综合含水上升过快的问题,利用HB70/300型高压物性分析仪开展了该区块原油相态特征实验、注气相态特征实验,并运用细管法开展了注CO₂最小混相压力实验。对比分析了CO₂和N₂两种性质气体注入前后原油的相态特征变化,确定了该区块原油注CO₂最小混相压力,为X油藏注气提高采收率可行性提出依据。实验结果表明,X油藏原始地层压力为46.01 MPa,原油饱和压力为11.06 MPa,注N₂后饱和压力上升迅速,在原始地层条件下难以实现混相,表现出典型的非混相特征;注CO₂后饱和压力上升较平缓,细管法测得的最小混相压力为28.03 MPa,说明利用CO₂可实现CO₂的混相驱替,而且最终的驱替效果比较理想。说明该油藏可开展注CO₂混相驱,为进一步的开发方案调整提供了依据和合理的建议。      

On the prediction of solubility of alkane in carbon dioxide using the LSSVM algorithm

The increasing global energy demand and declination of oil reservoir in recent years cause the researchers attention focus on the enhancement of oil recovery approaches. One of the extensive applicable methods for enhancement of oil recovery, which has great efficiency and environmental benefits, is carbon dioxide injection. The CO₂ injection has various effects on the reservoir fluid, which causes enhancement of recovery. One of these effects is extraction of lighter components of crude oil, which straightly depends on solubility of hydrocarbons in carbon dioxide. In order to better understand of this parameter, in this study, Least squares support vector machine (LSSVM) algorithm was developed as a novel predictive tool to estimate solubility of alkane in CO₂ as function of carbon number of alkane, carbon dioxide density, pressure, and temperature. The predicting model outputs were compared with the extracted experimental solubility from literature statistically and graphically. The comparison showed the great ability and high accuracy of developed model in prediction of solubility.     

Mutual Interactions of CO2/Oil and Natural Gas/Oil Systems and Their Effects on the EOR Process

Mutual interactions between oil and gas are critical factors affecting the gas enhancing oil recovery (EOR) process. Focusing on CO₂/oil and natural gas/oil systems, their interactions are researched and compared by extraction capacity and solubility measurement experiments. Core flood tests are also implemented to determine the effects of interactions on oil recovery. Results show that CO₂ can extract more light oil from the original and its extraction efficiency can reach 59.3% at 46 MPa, whereas that of natural gas is only 7.3%. However, heavy components content and viscosity of the residual oil processed by CO₂ increases significantly because of extraction, while natural gas does not affect the composition of the residual so remarkably. With increased pressure, solubility of CO₂ and natural gas in a light oil present a linear growth trend with similar rate, but the former is greater than the latter by about 130m³/m³. Core flood tests show that, for the continuous gas injection in the secondary oil recovery process, recovery of CO₂ flood is about 20% higher than that of natural gas due to the late breakthrough of CO₂, as most of the crude oil is produced before breakthrough.     

Effect of pressure,temperature, and mass fraction of CO2 on the stability of the asphaltene constituents in crude oil

This paper focus on the main influence factors (temperature, pressure, and mass fraction of CO₂) on the state of asphaltene in the crude oil during CO₂ flooding by using high temperature and high pressure microanalysis system of solid precipitation. For the simulated oil sample – CO₂ system, the state of asphaltene is not affected by temperature within this range of 50°C to 100°C, the particle size of the asphaltene has an increase with the increase of the pressure from 8MPa to 40 MPa. When the mass fraction of CO₂ is less than 35%, the state of the asphaltene has not changed and the asphaltene particles are in a suspension state. When the mass fraction of CO₂ increases to 40%, the aggregation of the asphaltenes occurs and then form precipitation. With the further increase of the mass fraction of CO₂, the particle of the asphaltene aggregates has a significant increase. For the field development project design of CO₂ flooding, the influence of the temperature can be ignored, the appropriate mass fraction of CO₂ is below 35% and the gas injection pressure should maintain a relatively low value. The results can provide a theoretical basis to avoid the asphaltene precipitation during CO₂ flooding.     

Evaluation of Interfacial Tension Between Zahra Oil and ASP Solution

Alkaline-surfactant-polymer (ASP) flooding has been proved to be an effective enhanced oil recovery (EOR) method. Reduction of interfacial tension (IFT) between crude oil and ASP solution is the main mechanism in ASP flooding. Evaluating IFT between crude oil and ASP solution is a key parameter for ASP flooding in laboratory experiments or field projects. In order to obtain good result of ASP flooding in the reservoir in Zahra field, the influence of the concentration of Na₂CO₃ on IFT between Zahra crude oil and ASP solution with three different surfactants, BHJC, SS-231, and SS-233, was researched. IFT was measured with surface tension meter SVT20N, Dataphysics Co. Germany, at 72°C. For the view of IFT result anionic surfactant BHJC is more suitable for the Zahra oil field. This research is helpful for practical application of ASP flooding in Zahra oil field.     

Modeling CO2 miscible flooding for enhanced oil recovery

The injection of fuel-generated CO2 into oil reservoirs will lead to two benefits in both enhanced oil recovery (EOR) and the reduction in atmospheric emission of CO2. To get an insight into CO2 miscible flooding performance in oil reservoirs, a multi-compositional non-isothermal CO2 miscible flooding mathematical model is developed. The convection and diffusion of CO2-hydrocarbon mixtures in multiphase fluids in reservoirs, mass transfer between CO2 and crude, and formation damages caused by asphaltene precipitation are fully considered in the model. The governing equations are discretized in space using the integral finite difference method. The Newton-Raphson iterative technique was used to solve the nonlinear equation systems of mass and energy conservation. A numerical simulator, in which regular grids and irregular grids are optional, was developed for predicting CO2 miscible flooding processes. Two examples of one-dimensional (1D) regular and three-dimensional (3D) rectangle and polygonal grids are designed to demonstrate the functions of the simulator. Experimental data validate the developed simulator by comparison with 1D simulation results. The applications of the simulator indicate that it is feasible for predicting CO2 flooding in oil reservoirs for EOR.     

Oil recovery and CO2 storage in CO2 flooding

In this study, the interfacial tension (IFT) of crude oil-carbon dioxide mixtures was measured to determine the minimum miscibility pressure. CO₂ flooding with sand packs, long cores, and heterogeneous cores was conducted to investigate the oil recovery and storage efficiency. The experiment results show that the interfacial tension decreases linearly with increasing pressure at two different pressure ranges. Under immiscible condition, the oil recovery and storage efficiency are increased by 30.1% and 52.4% when the injection pressure is increased from 13 to 22 MPa, and improved by 16.3% and 22.04% when the permeability is decreased from 270 to 10 mD, respectively. Under miscible condition, increase of injection pressure can only lead to much slower increase of oil recovery and storage efficiency, and permeability almost has no influence on oil recovery and storage efficiency. The oil recovery and storage efficiency can be remarkably reduced by heterogeneity. Water alternating CO₂ injection can improve the oil recovery and storage efficiency by 35.5% and 13.55%, respectively, compared with continuous injection.     

国外CO2驱流度控制进一步扩大波及体积方法

CO₂驱油是一种有效的提高采收率方法,矿场成功应用已有60多年。理论上微观驱油效率接近100%,但相对原油,CO₂的低黏度低密度与储层的非均质性导致的黏性指进和重力分离两大典型问题大大降低了气体波及系数。本文综述了国外各种流度控制方法,包括气水交替、聚合物直接稠化CO₂、CO₂泡沫驱、气-化学剂联合方法及气体辅助重力泄油,简要介绍了CO₂驱扩大波及体积的其他方法,分析了每种方法的机理及优缺点,力图为我国CO₂驱油进一步扩大波及体积这一难题积累先进做法和经验。     

Accelerated oxidation during air flooding in a low temperature reservoir

To ensure the safety of application of air flooding in a low temperature reservoir, the accelerated oxidization of oxygen and the crude oil is studied at low temperature. The stabilized chlorine dioxide solution can be used as a type of catalyst, which can accelerate the reaction rate of oxygen and the crude oil to decrease the concentration of remaining oxygen when the gas reaches the production well. The experimental results of oxidization show that O₂ concentration is greatly reduced and CO₂ concentration is greatly increased on condition with catalyst. In addition, the amount of heavy components can be further reduced and the amount of light components can be further increased. These can ensure the safety of application of air flooding in low temperature reservoir. The experimental results of adaptability of the accelerated oxidation under different water saturation show air flooding can be better used in the low-temperature reservoir with low water saturation on condition with catalyst but cannot be used in the low-temperature reservoir with high water saturation. The experimental results can provide theoretical basis for the application of air flooding in a low-temperature reservoir.     

Hybrid connectionist model determines CO2–oil swelling factor

In-depth understanding of interactions between crude oil and CO₂ provides insight into the CO₂-based enhanced oil recovery (EOR) process design and simulation. When CO₂ contacts crude oil, the dissolution process takes place. This phenomenon results in the oil swelling, which depends on the temperature, pressure, and composition of the oil. The residual oil saturation in a CO₂-based EOR process is inversely proportional to the oil swelling factor. Hence, it is important to estimate this influential parameter with high precision. The current study suggests the predictive model based on the least-squares support vector machine (LS-SVM) to calculate the CO₂–oil swelling factor. A genetic algorithm is used to optimize hyperparameters (γ and σr2) of the LS-SVM model. This model showed a high coefficient of determination (R² = 0.9953) and a low value for the mean-squared error (MSE = 0.0003) based on the available experimental data while estimating the CO₂–oil swelling factor. It was found that LS-SVM is a straightforward and accurate method to determine the CO₂–oil swelling factor with negligible uncertainty. This method can be incorporated in commercial reservoir simulators to include the effect of the CO₂–oil swelling factor when adequate experimental data are not available.