CO2 degassing of geothermal fluids during coreflood experiments

Abstract

In the energy transition from fossil fuels to less polluting renewable energy sources geothermal energy are considered as a promising technology. Gasses such as CO2 are often dissolved in geothermal waters. With the extraction of these fluids from the reservoir, a change in pressure will occur towards the extraction well which may cause dissolved gas to exsolve. The exsolved gas may clog the pores of the reservoir rock near the extraction well and therefore reduce the effective permeability, which can result in reduced production of geothermal waters. This project is aimed at experimentally investigating the conditions of the onset of the degassing process and what the influence of the degassing process is on the permeability. Therefore Bentheimer and Berea sandstone core flood experiments were performed, using either tap water or brine with different CO2 concentrations (0.2 1.3 mol/L) and at temperatures between 30 and 90°C. Flow rates were varied/kept constant, between 90 2 bar and between 5 and 22 bar respectively.

At 30 °C and up to 50 bar the onset of the degassing process is controlled by Henry’s law, i.e. it is governed by the solubility of gaseous CO2. The onset of the degassing process is not influenced by the pore size and initial permeability. At these conditions the effective permeability decreases by a factor 2 to 5 in the Bentheimer sandstone core and by about a factor 10 in the Berea sandstone core. This change in effective permeability is gradual in the Bentheimer sandstone while in the Berea sandstone the change is nearinstant. For rocks with small pore sizes and low initial permeability, the reduction in effective permeability is larger and the rate of permeability decrease is faster.

Experiments at temperatures between 30 and 90°C show that with increasing temperature Henry’s law becomes increasingly inaccurate to find the onset of the degassing process. The onset degassing pressure increases with temperature but are significantly lower than Henry’s law. This inaccuracy can partly be explained by the fluid in the core not reaching the right temperature or by the values used for extrapolating Henry’s law for different temperatures. In experiments performed with a 1M NaCl brine the pressure at which the degassing process starts is higher than for experiments performed with tap water. The increased salinity does not influence the change in effective permeability due to degassing. The reason for this is that the change in interfacial tension between CO2 does not change sufficiently between tap water and Brine to cause differences in the degassing process.

The results obtained from this research can be used for successful management of geothermal projects. It shows at what conditions the degassing process starts and what the effect of this process is. Once the conditions are known they can be avoided. The research also shows the degassing process and its effects can be reversed.