Modelling reinjection of two-phase non-condensable gases and water in geothermal wells

Abstract

Steam production from high enthalpy geothermal systems is frequently accompanied by the emission of non-condensable gases (NCGs), initially dissolved in the liquid phase or mixed in the vapour phase at depth in the reservoir. Capturing and reinjecting geothermal gases (CO2, CH4, NH3, H2S, H2, …) together with condensed steam leads to a significant improvement of the environmental profile of geothermal power systems and helps with reservoir recharge and pressure support. Nowadays, there are several ongoing projects targeting the minimalization of the environmental impact of geothermal exploitation through reinjection of the NCGs. For low NCG content, the gases can be fully dissolved into the condensed water and reinjected. However, for a high concentration of NCGs, the dissolution is only partial, and a two-phase mixture needs to be reinjected to achieve this goal. We present here the numerical results for the reinjection process in two different geothermal sites and for two different injection scenarios, focusing on the case of two-phase (liquid–gas) flows. Three commercial software (UniSim®, PIPESIM, and OLGA) and one in-house developed code (GWellFM) were used, and their results were compared. The injection should always take place inside a liquid column in the well to avoid degassing and accumulation of the NCGs. When targeting high pressure reservoirs or when injecting high ratios of water to NCGs flow rates, mixing of the two phases should take place on the surface. Whereas for low pressure reservoirs or for high gas contents mixing must occur deep in the well through one or several mixing points to ensure either complete dissolution of the gas or the downward flow of a two-phase mixture with minimum compression on the surface. The uncertainties of two-phase downward flows and water/NCGs mixtures characterisations introduce additional complexities in the modelling of the reinjection process.