Note Of Co <sub> 2 </ Sub> Containing Asphaltenes And Heavy Crude Oil Deposition Mechanism

Gas injection especially carbon dioxide injection is regarded as one of the most efficient methods of oil development because carbon dioxide can enhance oil recovery obviously by decreasing viscosity of crude oil, reducing interfacial tension, and swelling oil. However, gas injection for enhancing oil recovery, such as steam injection, carbon dioxide injection and nitrogen gas injection etc., may cause asphaltene deposit, which seriously influences production. Oil production practice indicates that gas injection can easily trigger heavy organic matters deposit, such as asphaltene, resin and wax, in asphaltenic oil. Many domestic and oversea oil fields have reported this phenomenon. Thus, based on the investigation of comprehensive literatures about the asphaltene deposition caused by carbon dioxide injection in reservoirs, this paper tries to explain deposition mechanism and its influence on development dynamic during the process of carbon dioxide injection for recovering heavy oil by analyzing asphaltenic oil phase change characteristic and establishing percolation model.First of all, asphaltene deposition experiments are introduced, and then, based on the theory of fluid thermodynamics, multiphase equilibrium thermodynamic model for asphaltenic oil gas system is established and the variation trends of thermodynamic parameters of alphaltene with quantity of injected carbon dioxide are analyzed. Last, compositional model considering asphaltene deposition, adsorption, plugging and non-Newton characteristic under low pressure gradient during gas injection is constructed. The influences of oil rheological property and asphaltene deposition and adsorption on oil recovery are discussed through the established compositional model. Main achievements in the paper are as follows,1. Based on fluid thermodynamics theory, multiphase equilibrium thermodynamic models of asphaltenic oil gas system, vapor-fluid-fluid, vapor-fluid-solid and vapor-fluid-fluid-solid, are constructed. Second liquid fugacity is derived with strict fugacity formula. The fugacity of asphaltene solid is developed from fugacity formula of pure component liquid made by Firoozabadi.2. Heat capacity at constant pressure and fusion enthalpy of asphaltene deposition during carbon dioxide injection are qualitative analyzed for the first time; the variation trends of thermodynamic parameters of alphaltene with quantity of injected carbon dioxide are explained.3. Noticing that pressure-temperature phase diagram can characterize second liquid phase and the relationship between second liquid phase and asphaltene precipitation,...