Pal. Jour. V.16, I.3, No.2 2017, 413-422 Copyright 2017 by Palma Journal, All Rights Reserved Available online at: http://palmajournal.org/ Simulating the Injection of Carbon Dioxide Gas and Impact of Asphaltene Precipitation on Permeability and Porosity the One Oil Reservoirs Yousef Amraei Astani, Department of Petroleum Engineering,Gachsaran Branch, Islamic Azad University, Gachsaran, Iran Roohollah Taghizadeh Corresponding Author, Department of Petroleum Engineering,Gachsaran Branch, Islamic Azad University, Gachsaran, Iran Abstract Porosity and permeability are two main parameters of reservoir conditions for enhanced oil recovery (EOR), which could be affected by majority of factors associated with reservoir like asphaltene. Asphaltene contains heterocyclic molecules formed of carbon, hydrogen and other elements such as sulfur, nitrogen and oxygen. In this study, based on the equation of state by entering the fluid composition, temperature, pressure and reservoir fluid tests, fluid model is provided for composite simulation. In the sector, the properties of asphaltene and the way of modeling it are determined by the eclipse-300 simulator. Then, carbon dioxide gas injection is simulated in it. Effective factor such as injection rate, precipitation rate and reduction of permeability and porosity are measured and the results have been analyzed and the appropriate scenario for EOR using carbon dioxide injection method is provided. With the increase in blockage parameter as a result of asphaltene, the production recovery factor is reduced to 2%; although the field of this blockage has more negative effect and needs improvement of eclipse asphaltene model. In the state of modeling permeability damage associated with changes in porosity, the recovery factor is equal to 32.2% and in second state that permeability damage is attributed to partial volume of asphaltene precipitation; the recovery factor is equal to 24.2%. Hence, according to simulation results, the permeability damage as a result of partial changes in volume of asphaltene precipitation has more negative effect compared to porosity damage. Introduction Precipitation of organic materials during oil production is a serious problem in Oil Industry. Asphaltene is introduced as one of the most problematic and the most unknown organic materials. Asphaltenes are organic and polyaromatic materials with high molecular weight and insoluble in normal pentane and normal heptane. It is believed that asphaltene is solved in oil or is existed in oil in for of colloid (stabled by means of resins adsorbed on their surface). Using chemical, mechanical or electric forces could distort thermodynamic balance of fluid and cause precipitation of asphaltene. Asphaltene precipitation may happen in any point of production from reservoir, in which the chemical stability balance of oil is distorted. Change in temperature, pressure, chemical compounds happened because of recovery of reservoir and carbon dioxide gas injection, using acid, mixing crude oil with diluters and other types of oil could be some instability forces. An oil fluid has 3 main components under atmospheric conditions: 1) oil component containing saturated hydrocarbons and aromatics 2) resin 3) asphaltene. The first component is nonpolar in terms of polarization or has average polarization; although asphaltenes and resins are polar components and this could cause their accumulation and formation of asphaltene. Presence of heavy elements in oil like resin and asphaltene prevent appropriate recognition of chemical structure of oil. Asphaltene is defined based on its solubility in different solvents. The term "asphaltene" was used for the first time by J.B Boussingault (1837) in France. Boussingault found about existence of components such as asphalt in east of France and in Peru and called them insoluble alcohols. Basically, Turpentine soluble solid particles are obtained from distillation and are called asphaltene, so that they are similar to asphalt in terms of Palma Journal
414 Y.A.Astani and R.Taghizadeh morphology. Asphaltene refers to components precipitated from crude oil. When bitumen or heavy oil is diluted by paraffin with low boiling point like normal pentane, normal heptane and other alkanes, asphaltene is precipitated. On the other hand, resins are soluble components in normal pentane and normal heptane; although they are insoluble in liquid propane. Moreover, it is true that resins are soluble in normal heptane; although they could not be extracted. However, about the first component, saturate hydrocarbons and aromatics, it could be mentioned that these components are soluble in normal pentane and normal heptane and could be also extracted from oil by same solvents. Literature review mingling displacement in oil production because of mechanisms such as oil and gas composition, reduced interfacial tension between oil and gas has high effectiveness. Mixture of oil and gas could reduce viscosity of oil and relative permeability of oil. Moreover, while contact of carbon dioxide with oil, the oil is swelled and its movement toward production well is facilitated (Kamalipout et al, 2014). In the process of carbon dioxide injection, in addition to enhance oil recovery, the greenhouse gas is trapped in underground structures. The two advantages have resulted in paying attention to EOR over the years. Using the supercritical carbon dioxide in EOR projects could lead to more stability of displacement front, which could reduce negative effects of gravity separation and fingered phenomenon caused by differences in viscosity (Lee and Kam, 2013). Abedi and Torabi (2014) have conducted a study on oil recovery, asphaltene precipitation and permeability damage during carbon dioxide injection under miscible and immiscible terms in a light oil system. The study studied different parameters affecting this process and found that carbon dioxide injection as an increasing cycle has the oil recovery factor in light oil reservoirs. While injecting carbon dioxide in the reservoir, the gas is solved in oil and hydrocarbons lighter than oil are solved in carbon dioxide; although carbon dioxide is also solved in oil. The miscibility between carbon dioxide gas and oil happens just in rate higher than special pressure depending on density of fluids (oil and carbon dioxide gas) (NETL, 2010). Another study is conducted by Rajus and Farouk Ali on carbon dioxide injection in cores related to Leominster Reservoir to evaluate carbon dioxide injection in reservoirs with low thickness, similar to those done in Leometer field. The study observed that carbon dioxide injection and injection of mass of carbon dioxide displaced by water could be ineffective in oil efficiency. It was also proved that WAG processes are more effective when the injection is taken with higher water to carbon dioxide ratio. In this work, 4 mechanisms were presented for EOR including Expansion of oil, reducing its viscosity and reduced interfacial tension. An experimental study was conducted by Mangalsingg and Jagay on heavy oil, in which it was emphasized that solubility and permeability are the main processes in the effectiveness of carbon dioxide as EOR material. They took core injections on light to heavy oils with API degrees of 16-29. They found that compared to heavy oils, because of high content of methane in light oil, the higher amount of carbon dioxide is required for light oils. In light oils, carbon dioxide eliminated methane before being mixed by oil and changing its properties. Mathematical modeling hypotheses The multiphase flow simulation equation in the reservoir is as follows: The continuity equation (conservation of mass): iui Si i Qi x t Fluid flow equations (Darcy velocity): kkri ui Pi i z i Where; landa γ is equal to: Through combining the equations and using derivative method, a chain of equations of different phases is created as follows: 3-phase flow equation of water-oil-gas for different phases is:
Simulating the Injection of Carbon Dioxide Gas and Impact of 415 Oil phase equation: Gas phase equation: Moreover, passing the phases in reservoir is calculated as follows: k kri i i Bi Methodology Asphaltene contains heterocyclic molecules formed of carbon, hydrogen and other elements such as sulfur, nitrogen and oxygen. Resins have similar structure to asphaltene; although they are formed of smaller molecules and have more portions of paraffin chains. In normal state, asphaltene mostly contains large amount of nickel, iron and vanadium. Professor T.F Yen (1974) proposed an experimental formula C H NS O ( 74 87 2 ) for an average asphaltene structure. However, no exact molecular formula is existed for asphaltene and it differs from one reservoir to another. Pacheco-Sanchez et al (2004) proposed 4 different structures for asphaltene through imaging different types of asphaltene. Groenzin and Mullins (2000), Speight (1981), Zajac et al (1994) and Murgich et al (1996) are scholars who proposed different structures for asphaltene as it is presented in table 1. Table 1. Physical and chemical properties of asphaltene molecules (Pina A, 2006) Murgich et al. (1996) H159C138N3O3S2 1955.95 1574.6 84.74 8.19 2.15 1.64 3.28 24 Zajac et al. (1994) H63C57N1S1 794.2 637.3 86.2 8 1.76-4.04 9 Speight (1981) H79C80 N2 O1 S2 1149.76 892.7 83.58 7.01 2.44 1.39 5.58 14 Groenzin and Mullins (2000) H98C72S1 995.64 794.6 86.86 9.92 - - 3.32 7 Properties Formula Molecular mass (amu) Molecular size Å3 C H N O S Number of aromatic rings Small asphaltene components are soluble in oil fluids; although large components could be extracted from the solution in form of colloid particles. Asphaltene particles are existed in oil partially as solution and partially as colloid or micelle. The form of presence of these particles is highly depended on additional amounts of other particles in oil like paraffin, aromatics and resins. As asphaltene particles are highly tended to bond to each other, if paraffin is added to oil mixture, the small asphaltene particles in oil create a mass through bonding to each other and if the coagulating agents like resin are existed sufficiency in oil, the resins change the masses to spatial colloids through surface adsorption on them and remain as suspended particles in the oil. Stability of these spatial colloids is a function of density of coagulating agents and a part of surface of asphaltene mass occupied by these agents. Asphaltene masses are precipitated in oil under conditions that enough amount of resin is not existed in oil to cover asphaltene particles and help their suspension. Physical form of asphaltene and resin is illustrated in figure 1. Moreover, the interaction of asphaltene and resin is illustrated in figure 2.
416 Y.A.Astani and R.Taghizadeh Asphaltene Resin Figure 1: The image taken of asphaltene and resin Figure 2: The forces applied by micelles of asphaltene on each other [Al-Kafeef, 2001] Simulation method For this research, eclipse-300 simulation is used. A reservoir model sector is also used to simulate carbon dioxide gas injection. Firstly, the data of a reservoir in Iran are made with average properties and random properties. Then, in a 5-point injection model, carbon dioxide injection is simulated. Modeling fluid model containing asphaltene is taken using reservoir fluid composition using PVTi software. In this software, according to the equation of state, through putting composition of fluid, temperature, pressure and reservoir fluid experiments, the fluid model is prepared for compositional simulation. In the simulated model sector, the asphaltene model was activated and the properties of asphaltene and the modeling procedure was determined using eclipse-300 simulator. Then, carbon dioxide injection is simulated in the software. Effective factors such as injection rate, precipitation level and reduced permeability are measured and the results are analyzed and appropriate scenario is provided for EOR using carbon dioxide injection method. Figure 3 has illustrated diagram of liquid density with adjusted experimental data.
Simulating the Injection of Carbon Dioxide Gas and Impact of 417 Figure 3: Diagram of liquid density of sample well The diagram of gas to oil ratio is also illustrated in figure 4 and it has been observed that in higher pressures of saturate pressure (1964feet), the diagram has been fixed. Figure 4: Diagram of gas to oil ratio in well liquid sample Figure 5: Diagram of oil viscosity of well liquid sample For purpose of simulating, a 5-point model sector is selected. Because of complexity and time consuming nature of simulation of processes in the asphaltene reservoir liquid, some structural simplifications are taken like lack of inputting the fault. The schematic of applied model is illustrated in figure 6. The depth from ground level to the first layer of reservoir is equal to 9000ft.
418 Y.A.Astani and R.Taghizadeh Figure 6: Applied model, depth from ground level to the first layer of reservoir if equal to 9000ft Distribution of permeability in model is presented in the schematic in figure 7. The permeability values vary in range of 275 to 525 MDARCY. Figure 7: Permeability distribution in reservoir model Distribution of porosity values of model in range of 15-30% is illustrated in the schematic in figure 8. Figure 8: Porosity distribution in reservoir model The oil saturate distribution is considered same (84%) in whole model and is illustrated in the schematic in figure 9.
Simulating the Injection of Carbon Dioxide Gas and Impact of 419 Figure 9: Oil saturate of reservoir model The initial reservoir pressure at the depth of 9052 feet is equal to 4757 feet. The contact surface of water and oil is in depth of 11813feet. In this model, an injection well and 4 production wells are used as 5-point injection model and carbon dioxide injection is done to enhance oil recovery in asphaltene-containing reservoir. The composition of injected gas contains just carbon dioxide gas. Simulation scenarios for injection of carbon dioxide with regard to effect of asphaltene are as follows: Carbon dioxide gas injection Change in carbon dioxide injection rate Change in asphaltene precipitation parameters Simulation results Carbon dioxide gas injection simulation is done with the injection rate of 5000ft 3 per day in two states. It has been taken in the first state regardless of asphaltene (blue diagram) and in second state, with regard to asphaltene (green diagram). The oil recovery factor diagram is both states in illustrated in the figure. The diagram in figure 10 shows reservoir permeability in asphaltene-contained state of oil reservoir. With the precipitation of asphaltene, the permeability is damages and is reduced. Changes in permeability are considered based on partial volume of asphaltene precipitation. Figure 10: Permeability damage as a result of asphaltene precipitation In the grids near the injection wells, because of high changes in reservoir liquid, asphaltene precipitation is more than other sections and as a result, the damage of formation and permeability is also more than other sections. Asphaltene precipitation and deposition in reservoir could block the impasse of pores and this could affect permeability and porosity significantly. Increased blockage of pores could reduce permeability and porosity of reservoir rock. In figure 11, the effect of blockage parameter of pores in asphaltene modeling during carbon dioxide gas injection is illustrated.
420 Y.A.Astani and R.Taghizadeh Figure 11: The effect of blockage of pores on oil recovery With the increase in blockage parameter as a result of asphaltene, the oil recovery is reduced to 2%. However, in field reality, this blockage has significant negative effects and the eclipse asphaltene model should be improved. Asphaltene precipitation could damage the reservoir and affect fluid flow in reservoir. Asphaltene precipitation in reservoir could reduce liquid volume. In this scenario, the changes in permeability damage of reservoir are measured in two different states of modeling in simulation. In the first state, permeability damage is associated with porosity changes and in second state, the permeability damage is attributed to partial volume of asphaltene precipitation. In both states, using Einstein 1-parameter model, changes in viscosity are modeled. In figure 12, the oil recovery factor is illustrated for both states of modeling permeability damage. Figure 12: The diagram of oil recovery factor in both states of formation damage In the state of modeling permeability damage associated with porosity changes, the recovery factor is equal to 32.2% and in second state that permeability damage is attributed to partial volume of asphaltene precipitation; the recovery factor is equal to 24.2%. Hence, according to simulation results, permeability damage as a result of partial volume changes of asphaltene precipitation has more negative effects. The oil production rate for both states of permeability damage is illustrated in figure 13. Figure 13: The diagram of oil production rate in both states of modeling permeability damage
Simulating the Injection of Carbon Dioxide Gas and Impact of 421 In the state of permeability damage associated with partial volume of asphaltene precipitation, oil production rate is declined rapidly. However, in long-term, the production rate in this state is fixed in higher rate because of reduction of asphaltene precipitation over the time. In this reservoir, carbon dioxide injection has led to enhanced oil recovery and asphaltene precipitation has not been regarded as a critical problem. Hence, the carbon dioxide injection method could be used to enhance oil recovery in this reservoir. Moreover, according to the weaknesses of eclipse simulator in simulation of asphaltene precipitation while carbon dioxide injection, experimental tests should be performed for more careful analysis in the studied reservoir. Conclusion In the state of modeling permeability damage attributed to porosity changes, the recovery factor is equal to 32.2% and in second state that permeability damage is attributed to partial volume of asphaltene precipitation; recovery factor is equal to 24.2%. Hence, according to simulation results of permeability damage, as a result of partial volume changes, asphaltene precipitation has more negative effects compared to porosity damage. Aphaltene precipitation in reservoir could lead to blockage of pore impasse and this could have significant effect on permeability and porosity. Increased blockage of pores could reduce permeability and porosity of reservoir rock. As the aim in EOR method sis to enhance the recovery factor to several percentages, it is cost-effective to prevent reduced recovery as a result of precipitation of asphaltene. Moreover, in case of injecting carbon dioxide and increased precipitation rate of asphaltene, porosity and permeability is reduced and this method may face challenge. On the other hand, carbon dioxide injection has led to enhancement of oil recovery. The lower the level of asphaltene precipitation is, the better results of carbon dioxide injection would be. With the increased value of blockage parameter as a result of asphaltene precipitation, the oil production is reduced to 2%. However, in field reality, this blockage has significant negative effects and the eclipse asphaltene model should be improved. Aspahltene precipitation could damage the reservoir and affect fluid flow in reservoir. Asphaltene precipitation in reservoir could reduce liquid volume. Suggestions For further studies, the suggestions are as follows: To investigate the effect of reservoir parameters on asphaltene precipitation like reservoir rock type, pore size distribution, and wettability in empirical studies. To measure the effect of injecting different types of gas like nitrogen and hydrocarbon gases on asphaltene precipitation and to use CT Scan device to observe reservoir's permeability damage. To take empirical investigations about the effect of temperature and pressure while injection of carbon dioxide gas on asphaltene precipitation and to measure the effect of asphaltene precipitation inhibitors while injecting gas. The suggestion is to conduct further studies and empirical investigations on properties of injection liquid and the injection conditions and their impact on asphaltene precipitation. In order to obtain more exact results, it would be better to conduct the study in different reservoirs such as sand, rock, carbonated and cracked reservoirs. Also, comprehensive plans should be considered to prevent precipitation of asphaltene in the reservoir. It would be better to simulate alternative injection method of carbon dioxide and water for EOR of a reservoir with asphaltene-containing oil.
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