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Abstract
An integrated model was developed that includes economic and technical aspects of CO2-EOR and sequestration projects. Based on This model, a numerical simulator is developed to predict the performance of project and determine optimum rate of injection rate in different conditions. Also, an analytical model is developed and compared with numerical method. The results show that numerical simulator is a reliable tool for optimization of injection rate, whereas analytical method is not, because of its assumptions and approximations. Sensitivity analyses are done with both numerical and analytical method. The results
show that in larger and more homogeneous reservoirs, optimum injection rate is higher. Lower oil price and higher oil viscosity, require lower injection rate to make the project more economical.

Introduction
Underground storage of carbon dioxide (CO2) is attracting considerable interest worldwide as a means of avoiding continued release of CO2 from anthropogenic sources. Global warming is a term used to describe the observed increases in the average temperature of the Earth's atmosphere and oceans. The average global temperature rose 0.6 ± 0.2 °C over 150 years, and the scientific opinion on climate change is that it is likely that "most of the warming observed over the 20th century is attributable to human activities" (Houghton et al 1996; Jones and Briffa 1992). Deep ocean and geologic sequestration are the only choices to dispose large amount of CO2 by safely and economically for long term periods. Geologic sequestration, a prospective technology to reduce large amount of CO2 released into the atmosphere, involves the capture of CO2 from hydrocarbon emissions, transportation of compressed CO2 from the source to the field, and injection and storage of CO2 into the subsurface. CO2 sequestration into depleted oil reservoirs, which are very close to their economic lifetime, has advantages
when compared with other familiar projects. First of all, a structural trap has already been available in the reservoir to hold the injected CO2. Secondly, reservoir is well characterized in terms of porosity, permeability, faults and rock integrity. Usually the presence of core sample and seismic data with many others make easy to decide the capability of CO2 sequestration in a storage site. Finally and most importantly, there is an invaluable experience since the CO2 injection into oil reservoirs has been in practice for enhanced oil recovery (EOR) for more than 35 years (GCEP Technical Report 2003). Also due to interfacial effects, CO2 injection has benefits over N2 injection or hydrocarbon gas injection (Ghoodjani and Bolouri 2011). However, the dissolution of CO2 in oil, either in miscible or immiscible flooding, causes the asphaltenes to precipitate and as a consequence to deposit which results in formation damage and wettability alteration (Srivastava and Huang 1997; Sim et al. 2005; Hayashi and Okabe 2010; Bolouri and Ghoodjani 2011). Here, it is important to realize that in CO2-EOR the main purpose is to maximize oil recovery with the minimum quantity of CO2 while a maximum amount of CO2 is aimed to store in a sequestration. Thus, enhancing oil recovery in a sequestration is an optimization process that requires careful analysis. 

In this paper, the main purpose of this study is to optimize the CO2 injection rate to reach maximum profit. An integrated technical-economic model is developed and used in numerical simulator for estimating best CO2 injection rate. Also, a semianalytical solution for finding optimized CO2 injection rate is introduced; however, the analytical method is not recommended due to considered simplifications and approximations in the model development.