Physico-thermodynamic fundamentals of phase transformation in multicomponent hydrocarbon systems under deep gas-condensate reservoir
Fataliyev Vugar Maharram oglu
It is known that, complex phase transformation can be observed during exploitation of gas-condensate reservoirs which significantly impacts on the effectiveness of a reservoir development project. The complicity of these processes and lack of base knowledge about the nature of the occurrences cause some difficulty in modeling and forecasting the natural ongoing processes in the reservoir and building the reservoir development projects. From this point of view, the purpose of targeted subjects and the importance of the obtained results in presenting the work make practical and theoretical sense and also have fundamental values.
The aerosol state of the gas-condensate mixture above the retrograde condensation pressure has been investigated based on fundamental principles of colloidal systems and assessed the importance of this state on gascondensate reservoir development effectiveness. Also, the impact on the reservoir exploitation parameters of the reservoir fluid components which have high solubility capability in hydrocarbon liquids was investigated.
Apart from the three fundamental forces of colloidal systems which operate on fine particles in a solution which are: a gravitational force; a viscous drag force, and the "natural" kinetic energy of particles and molecules in porous media or in reservoir conditions, surface energy and force between the rock surface and dispersed phase played a significant part. To support this conclusion, the reason why at a given constant temperature the retrograde dew point pressure of the gas-condensate system in the porous media is 20-25% higher than in the free cell was also explained. Also included, the Differential Condensation and Constant Composition Expansion test procedures have been developed in order to improve the retrograde dew point pressure accuracy.
As a result of thermodynamic test analysis of gas-condensate fluids it has been discovered that there is very sensitive phase behavior takes place between gas and liquid components in thermobaric condition above the retrograde condensation. Based on these obtained results and the reasons for the existence of the dispersed or free liquid hydrocarbons (DLH) in the pressure range above the retrograde dew point have been described.
In accordance with experimental data, it was found that in the critical state of gas-condensate mixture the amount of gas components in the gas phase necessary for the appearance of retrograde processes has a minimum value. Also, the research indicated that when changing pressure isothermally or changing temperature isobaric in the region above the retrograde condensation gas-condensate system could change from “gas in the liquid” to the “liquid in gas” or visa verse colloidal state. These observations have a proven base on thermodynamic tests carried out on “Bulla-deniz” (Azerbaijan) gas-condensate field samples.
Fundamental aspects of the impact connate water on the hydrocarbon mixture phase change have also been studied. A more in-depth analysis carried out here on the basis of experimental and theoretical studies has identified three main areas of influence of residual water in the process. Depending on the conditions of the water in the reservoir: vapor, free liquid and irreducible water affects the phase transformation of the system and reservoir development in different ways.
The reasons and importance of instability of the thermodynamic equilibrium state of gas-condensate systems during exploitation of reservoirs has been investigated. For this purpose, the energy balance equations have been used which includes and reflects the change of the internal kinetic and potential energies of fluids during depleting reservoir pressure. The obtained results give a wide understanding of the source and type of reservoir energy, and also presents the requirements for selection of effecttive agents which can be used for maintaining reservoir energy.