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Depleted gas fields are seen as promising options for geological storage of CO2. The advantage of hydrocarbon fields are that the characteristics, such as the storage capacity and the proven sealing capacity are known. This means that only limited uncertainty remains after a technical feasibility study. One important issue with depleted gas reservoirs is the low pressure, at the onset of injection. This may lead to adiabatic cooling (Joule-Thomson cooling or JTC). The temperature reduction associated with this JTC can lead to hydrate formation and freezing of the residual pore water, especially the injected fluid also has a low temperature. In a related paper, we report on the development of a simulator, which is capable of predicting the fate of transport of a CO2 in either the liquid gaseous and supercritical phase. The model only contains CO2 besides the aqueous phase. One of the remaining challenges is to predict the impact of the methane, which is still present in a depleted gas reservoir, on the phase behavior of CO2 and the thermal aspects. In this study simulations were performed on the P18-4 depleted gas field. The CO2 was injected at a minimum temperature of 12 oC into the reservoir formation with a temperature of 120 oC. The average injection rate during the demonstration phase was modelled with 1.1 Mton/yr over a period of 5 year. The injection of cold CO2 into P18-4 was modelled in TOUGH2 and a new module, which can handle not only the behaviour of CO2 but also of CH4. The original ECO2M module was initially designed for CO2 behaviour in gas reservoir and aquifers for brine-CO2 mixtures, including all possible phase transitions, in the absence of any other gas. The new TOUGH2 module, which we gave the name ECO2MG module, was extended in such a way that compositional flow simulations of two different gases can be modelled, namely CO2 and CH4. The ECOMG module can not only model the behaviour brine CO2 mixtures but also the behaviour of brine-CO2-CH4 mixtures including phase changes from gas to liquid and from super- and sub-critical conditions. Density, viscosity and enthalpy of the CO2-CH4 mixture is based on NIST data. |
