Improving the knowledge of carbon storage and coal bed methane production by ‘in situ’ underground tests (Carbolab) : final report

Carbolab was an RFCS 4-year project aimed at gathering and analysing effects of CO2 injection in coal at a panel scale in an existing underground coal mine in the north of Spain. To achieve this goal, it was necessary to carry out many tasks to adapt and prepare the gallery. Additionally, we collected all the available information in relation to geology, geophysics, geomechanics and hydrogeology which allowed us to select the target coal seam. Anyway, there was a lack of information in relation to many aspects, which forced us to carry out additional studies in the zone. Almost 30 boreholes were sunk to characterise and monitor the area and to inject CO2. The injection itself lasted 2 months, being impossible to continue longer because of a break in the injection borehole casing. Concerning geophysical monitoring, we used seismic tomography, radar, microgravimetry, induced polarisation, spontaneous potential, resistivity and microseismicity. Only the last two methods showed a variation with CO2 injection. We also made a complete geochemistry monitoring, observing change of gas concentrations in coal and surrounding rocks, pH decrease, rise of conductivity and alkalinity, increase in Ca and Mg concentrations as well as a rise of CO2 dissolved values. In addition, we monitored a CO2 leak close to the injection borehole head that represented approximately 30 % of the CO2 injected. In parallel, we developed a model using TOUGH2 and another tool based on COMSOL Multiphysics software. Also, a detailed study was made to assess longterm safety.