A Wellbore Model for Geothermal Wells Containing Considerable Amount of CO2

Abstract

Geothermal reservoirs can contain considerable amounts of carbon dioxide (CO2) dissolved in geothermal waters. CO2 content of geothermal waters could reach 4-5 percent by mass in some geothermal wells. Using the well-known Henry’s law for the solubility of CO2 in water is a common practice in geothermal reservoir engineering; however, applicability of Henry’s law is limited with low concentration of CO2; around 2-3 percent by mass. In this study, we propose a numerical wellbore model for the prediction of temperature and pressure profiles when geothermal waters contain significant amounts of CO2. An equation of state for pure CO2 and water with or without salinity is integrated within the wellbore model. The equation of state implemented predicts both phase equilibrium and volumetric properties accurately for temperature range of 273.15-1273.15 K and pressure range of 0-800 MPa for CO2. A solubility model based on the equation of state is introduced and compared with Henry’s law based models as well as experimental data in the literature. Then a wellbore model is developed for the prediction of temperature and pressure profiles along the well. Energy and momentum equations are coupled through the solubility model based on the equation of state. The model is also capable of estimating CO2 content by history matching pressure and temperature data and also estimating the depth where CO2 comes out of the solution in the wellbore. Henry’s law based model and equation of state based model is compared in their outcomes of estimated CO2 content. When compared with the real field data from a well in a western geothermal field in Turkey, the matches obtained for pressure and temperature are quite good.