ReactVessel: An ActiveX Component in Visual Basic for Numerical Simulation of H2O-CO2 Interaction in a Geothermal Reservoir

Abstract

An ActiveX component, ReactVessel, is written in Visual Basic 6.0 in order to study the H2O-CO2 interaction in a geothermal reservoir. The reservoir is considered as a reaction vessel of constant volume, which contains a varying amount of H2O, CO2 and alkalinity, caused by the addition of acid HCl and base NaOH. There may be three phases in the Reaction Vessel: liquid, vapor and solid. The minerals NaHCO3, Na2CO3 and NaOH may precipitate only in the extreme case while all the water is converting into the vapor phase or when there is an initial high concentration of the minerals in the liquid phase. However, the solid phase is not considered here since the thermodynamic data of the solid species are not available at high temperature (T) and pressure (P).

Although there is only one independent variable from T and P, both T and P increase along the saturation curve. Therefore, the equilibrium constants are fitted in a quadratic polynomial of P and inverse of T. Using ReactVessel a demonstration program is developed in order to simulate the reservoir characteristics during the heating and exploitation stage. The pressure in the reaction vessel is, in general, first controlled by CO2, then by water vapor and in the last stage again by CO2, while all the water is converting to the liquid phase.

On extracting vapor from the reaction vessel, the total amount of CO2 decreases, while it remains constant on extracting liquid phase. In other words, CO2 remains mostly in the vapor phase except for highly alkaline fluid. The variation of gaseous species concentration with time may provide useful information on the geothermal reservoir fluid characteristics.