(404g) CO2 Storage through ECBM Recovery: Experiments and Models
Deep unmineable coal seams have been proposed as a possible underground geological location for permanent storage of carbon dioxide (CO2) (White et al., 2005). The recovery of the coal bed methane can be enhanced by injecting CO2 in the coal seam at supercritical conditions. Through an in situ adsorption/desorption process the displaced methane is produced and the adsorbed CO2 is permanently stored. This process is called enhanced coal bed methane recovery (ECBM) and it is a technique under investigation as a possible approach to the geological storage of CO2 in a carbon dioxide capture and storage (CCS) system (Mazzotti et al., 2009). ECBM recovery is not yet a mature technology, in spite of the growing number of pilot and field tests worldwide that have shown its potential and highlighted its difficulties.
The present study investigates both experimentally and theoretically, two fundamental aspects related to ECBM. First, high-pressure adsorption studies are needed to estimate the storage capacity of coal and to describe the adsorption/desorption dynamics. Nine different coal samples obtained from different locations around the world were investigated in terms of adsorption capacity of CO2, CH4 and N2. For the experiments a Magnetic Suspension Balance (Rubotherm, Germany) is used (Ottiger et al., 2006, Ottiger et al., 2008a, Ottiger et al., 2008b) . Moreover, a technique is presented to perform high pressure adsorption measurements on wet samples. Adsorption isotherms have been obtained at different temperatures, i.e. between 33°C and 60°C and up to 200 bar, using both pure gases and mixtures. To assess the reliability of the obtained adsorption data, the reproducibility between the isotherms measured by two different laboratories making use of two different gravimetric methods was tested. Parameters affecting coal adsorption capacity, such as temperature, depth and rank are also discussed.
The second aspect deals with the volume increase (swelling) of the coal when exposed to an adsorbing gas. Being at a depth of several hundred meters, the coal seam is subjected to a lithostatic pressure, which doesn't allow it to expand; the coal porosity has therefore to accommodate for volumetric changes caused by CO2 adsorption, with a consequent dramatic reduction of the permeability (Reeves, 2004). This phenomenon has to be quantified, since, besides affecting the CO2 injectivity and CH4 recovery, it could impede an optimal exploitation of the coal seam.
A setup has been built to better reproduce the conditions in a coal seam. It allows the injection of different gases through an intact coal core confined under an external pressure (Pini et al., 2009). Helium is used to study the effects of the confining pressure on the permeability of the sample, whereas CO2 and N2 to quantify those of adsorption and swelling. The transient step method is used to carry out the flow experiments; the sample is connected to two reservoirs: a pressure change is imposed at one end of the sample and the system is allowed to equilibrate to a new pressure level. The interpretation of the experiments is done by describing the dynamics of the process with a suitable model, which consists mainly of mass balances for the gas flow, adsorption isotherms and a stress-strain relationship for the change in porosity and permeability caused by swelling and confinement. This technique has been extended to a multicomponent system, where CO2 or N2 is injected in a coal sample pre-saturated with methane, allowing therefore studying the ECBM process in the laboratory.
The model used to predict the flow behavior observed in the laboratory represents an important tool to investigate several parameters affecting the ECBM process. Typical ECBM scenarios can be simulated and the effect of changing composition of the injected gas, i.e. pure CO2, N2 or mixture of them, on the displacement dynamics can be studied. It can be shown that the injection of pure CO2 displaces the CH4 through a sharp front, due to the higher adsorptivity of the former compared to the latter. On the contrary, when pure N2 is injected the front is much smoother, resulting in a produced stream of CH4 polluted with N2. As expected, injection of a mixture of CO2 and CH4 results in the appearance of both the abovementioned phenomena. Moreover, injection of mixtures rich in N2 improves significantly CH4 production compared to the case where pure CO2 is injected. This effect can be attributed to the higher permeability following N2 injection in the coal bed compared to CO2. Due to the early breakthrough of N2 at the production well, however, there is a trade-off between the enhanced CH4 recovery and the purity of the produced gas. Useful design criteria can be therefore derived from this type of studies.
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