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Darabi, Hamed (The University of Texas at Austin) | Shirdel, Mahdy (The University of Texas at Austin) | Kalaei, M. Hosein (The University of Texas at Austin) | Sepehrnoori, Kamy (The University of Texas at Austin)
Abstract Asphaltene precipitation, flocculation, and deposition in the reservoir and producing wells cause serious damages to the production equipment and possible failure to develop the reservoirs. From the field production prospective, predicting asphaltene precipitation, flocculation, and deposition in the reservoir and wellbore essentially avoids high expenditures associated with the reservoir remediation, well intervention techniques, and field production interruption. Since asphaltene precipitation and deposition strongly depend on the pressure, temperature, and composition variations (e.g. phase instability due to CO2 injection), it is important to have a model that can track the fluid behavior during the entire production process from the injection well to the production well, which is absent in the literature. In this paper, a comprehensive thermal compositional coupled wellbore/reservoir simulator with a capability of modeling asphaltene phase behavior in the reservoir and the wellbores is presented to address the wellbore/reservoir interaction, the effect of asphaltene deposition on the flow prediction and long-term reservoir performance. Indeed, the simulator models multiphase fluid flow in the reservoir and the wellbore to enable comprehensive production system analysis. In addition, wettability alteration due to the asphaltene deposition on the rock surface is considered in our models. We present primary production and CO2 flood simulation cases to investigate the effect of asphaltene deposition on oil recovery. The results show that injection of the light components into the reservoir significantly increases the instability of asphaltene components in the reservoir where they can precipitate further around the wellbore and in the wellbore. The precipitated asphaltene in the reservoir can be carried into the wellbore and be combined with excess asphaltene formation and deposit in the wellbore. In addition, our simulation shows that well productivity decreases significantly in case of asphaltene precipitation and deposition during the production life of a reservoir.
Abouie, Ali (The University of Texas at Austin) | Rezaveisi, Mohsen (The University of Texas at Austin, BHP Billiton) | Mohebbinia, Saeedeh (The University of Texas at Austin, Halliburton) | Sepehrnoori, Kamy (The University of Texas at Austin)
Abstract Asphaltene deposition is known to be one of the major problems in oil fields. Asphaltene precipitation and deposition from the reservoir fluid can block pore throats or change the formation wettability in the reservoir. Furthermore, asphaltene precipitation and deposition result in partial to total plugging in the wellbore. Recent studies have shown that PC-SAFT EOS is a more appropriate and comprehensive thermodynamic model for simulation of asphaltene precipitation. The main objective of this paper is to implement PC-SAFT EOS into a compositional wellbore simulator to model asphaltene precipitation. Flocculation and deposition models are also integrated with the thermodynamic model to simulate the dynamics of asphaltene deposition along the wellbore. In addition, the capabilities of PC-SAFT and common-used Peng-Robinson equation of state are compared through fluid characterization to reproduce experimental precipitation data. The simulation results indicate asphaltene deposition profile and consequent decline in production rate. It is shown that the profile of asphaltene deposition is mostly governed by the precipitation condition and the deposition rate. Moreover, prediction capability of cubic equation of state is shown to give approximately similar results if additional precipitation data is available (e.g. lower onset pressure and maximum amount of precipitation). The prediction results of the developed tool are highly crucial to monitor the well performance, optimize the operating conditions of the field, and propose the remediation technique.
Abstract Particle deposition is a complex problem in oil fields which affects all aspects of petroleum production, processing, and transportation. Asphaltene deposition has been repeatedly reported in various oil fields with interest on how it impacts the development of a reservoir. In many of the reported cases, it has been indicated that asphaltene deposition damages the wellbore and production facilities more severely than the formation. A great deal of research has been conducted to study the phase behavior and dynamic aspect of asphaltene deposition. However, there is a lack of comprehensive integrated modeling of asphaltene deposition in the wellbore during the multiphase fluid flow. In this paper, we present an implementation of asphaltene precipitation and deposition models into a thermal, multiphase, multi-component wellbore simulator that can be coupled with a compositional reservoir simulator. A key contribution of this work is the development of a simulator for predicting the detrimental effects of asphaltene in a well. Simulation results can show where and when the presence of asphaltene particles severely damages the efficiency and the productivity of the wells. This prediction is highly crucial if it is aimed to control the well performance and to optimize productivity.
Peyman, Pourafshary (School of Mining and Geosciences, Nazarbayev University, Nur Sultan, Kazakhstan) | Majid, Zamani (Institute of Petroleum Engineering, University of Tehran, Tehran, Iran) | Muhammad, Hashmet Rehan (School of Mining and Geosciences, Nazarbayev University, Nur Sultan, Kazakhstan)
Prediction of asphaltene deposition in production system and design of production parameters adequately to control this issue is inevitable. We presented a transient model to predict asphaltene deposition along the tubing string in the production system. An accurate two-phase fluid flow model was coupled with a solid asphaltene precipitation model and a sub-layer particle deposition model in turbulent flow with the ability to predict the deposition of particles in vertical surfaces. Our procedure shows good agreement with the experimental work previously done to measure the rate of deposition of flocculated asphaltene particles via an accurate thermal apparatus at different temperatures and flow rates. The developed model was used to simulate the deposition of asphaltene in a real field. The results suggest that even with high flow rates, the deposited asphaltene caused a 2.5% reduction in wellhead pressure after 30 days of production. The developed model can predict the transient location of the asphaltene, onset pressure, and the profile of the deposited asphaltene in a wellbore versus time. In practice, the proposed model can be used for analysis of different production scenarios in a given well to minimize the possibility and extent of asphaltene deposition and enhance the production rate.
Summary Asphaltene precipitation from reservoir crudes has been modeled using liquid-liquid equilibrium; the precipitated heavy phase is assumed to be in the liquid state and to consist of only asphaltene and resin. The light liquid phase is assumed to consist of the monomers of all the components and the micelles. A thermodynamic micellization model and the Peng-Robinson equation of state (EOS) description of all monomers fully describe the thermodynamic equilibrium state. From the direct minimization of the Gibbs free energy of the liquid-liquid system, the composition and the amount of each of the phases are calculated. The above model is used to calculate precipitation from three different reservoir crudes. Once vapor-liquid equilibrium parameters are obtained, there is no further adjustment of the parameters of the EOS for precipitation calculations. The predictions from the thermodynamic micellization model are in good agreement with the data. The effect of pressure, temperature, and composition on precipitation is properly predicted by the model. Introduction The modeling of asphaltene precipitation from crudes has been a challenge. Part of the challenge is due to the unique features of asphaltene precipitation in comparison to other types of precipitation such as wax precipitation. For example, an increase in pressure increases the wax appearance temperature in crudes (i.e., enhances wax precipitation), while pressure increase may inhibit asphaltene precipitation. Wax precipitation is strongly dependent on temperature, asphaltene precipitation may not be affected by temperature, and temperature may weakly enhance asphaltene precipitation or it may inhibit precipitation. The composition effect is also very different for wax and for asphaltene precipitation. At high temperatures, an increase in concentration of light hydrocarbons such as C3 and C4 and nonhydrocarbons such as CO2 decreases the wax appearance temperature; on the other hand, an increase in the amount of these components can significantly enhance asphaltene precipitation. The precipitated wax phase does not contain asphaltenes and resins; the amount of paraffins may be very small in the precipitated asphaltene phase. Another distinct feature of wax and asphaltene precipitation is the state of the precipitated phase. The precipitated wax phase is in the solid state while at high reservoir temperatures, as we will discuss later in this paper, the precipitated asphaltene phase may be in the liquid state. The objective of this paper is to propose a thermodynamic model that can predict all the features of asphaltene precipitation including pressure, temperature, and composition effects. Prior to the description of the thermodynamic model, we will first discuss the reversibility of asphaltene precipitation in light of new measurements on reversibility, and then review the literature on the state of the precipitated phase. Reversibility. Reversibility of asphaltene precipitation is often an issue. Hirschberg et al. observed reversibility of asphaltene precipitation with pressure at 367.15 K. The pressure reversibility addressed by Hirschberg et al. at high temperatures seems to be accepted by others. Reversibility with respect to composition at low temperatures is still unresolved. Rassamdana et al. performed experiments at room temperature to study the reversibility of asphaltene precipitation with respect to composition. Normal hexane was mixed and then stripped from the crude. They observed that part of the precipitated asphaltene redissolved into solution and concluded that the asphaltene precipitation process is partially reversible. Chung et al. studied reversibility using n -pentane. They found only 23% of the precipitate redissolved back in the crude. Recently, Ramos et al. experimentally verified that the asphaltene precipitation-dissolution process in liquid titration is reversible. Using a method similar to Ref. 4, they precipitated asphaltenes by n-heptane and n-decane, then reduced the precipitant/crude ratio by either evaporating the diluent or adding fresh oil. Full redissolution could not be observed at room temperature after continuous stirring for 24 hours. However, when ultrasound was used for the mixing, complete redissolution was observed immediately, demonstrating that the process is reversible. Ramos et al. also observed the redissolution of the precipitated asphaltene in a crude/n-heptane mixture by adding toluene. The dissolution of the precipitated asphaltene at room temperature is a kinetically slow process and, therefore, the reversibility may require a very long time. Cimino et al. and Hirschberg et al. also comment that the titration experiments are reversible. At high temperatures (reservoir temperatures), several experimental observations have shown the reversibility of asphaltene precipitation from the change of pressure or composition. Using the reversible thermodynamics of the asphaltene precipitation, Hirschberg and Hermans introduced an aggregation model for asphaltenes and resins to describe the asphaltene precipitation in crude. The model can describe asphaltene precipitation at high dilution ratios when a crude is diluted with normal alkanes (nC5,nC7, and nC10) at 298.15 K. Based on the evidences above, we postulate that the asphaltene precipitation process can be thermodynamically reversible, especially at reservoir temperatures; the application of an equilibrium thermodynamic model, therefore, is justified. Next, we discuss the issue of the state of the precipitated phase. State of the Precipitated Phase. Most of the models developed for asphaltene precipitation are based on the assumption that the precipitated phase is in the solid state. This assumption may be true at room temperature but may not be valid at reservoir temperatures, which are often higher than 330 K. Kokal et al. observed that the precipitated phase is in the form of dark solid particles when a crude was mixed with propane at room temperature. When the same crude was diluted with n-pentane or a heavier normal alkane at room temperature, the precipitated phase had a crystalline state. Chung et al. found the precipitate to be in a solid state when a crude was mixed with n-pentane.