Monitoring a 120-kg CO2 injection in a coal seam with continuous gas and microseismic measurements (European RFCS CARBOLAB research project)

The main goal of the CARBOLAB research project was to fill the gap concerning the lack of data and comprehension of the physicochemical mechanisms that exist between laboratory experiments on gas adsorption properties of coal and full-scale CO2 injection tests from the surface. Gathering Spanish (HUNOSA, AITEMIN), French (BRGM, INERIS) and Polish (GIG) partners, the project contained three main actions : laboratory tests, numerical modelling and in-situ injection at a seam scale. The injection site was within an existing underground infrastructure, a crosscut of a coal mine located in Asturias (North of Spain). HUNOSA built an experimental site by drilling 21 boreholes from a crosscut that intersects an unexploited 2-m width vertical coal. Geochemical and geophysical monitoring boreholes were drilled in coal close to the injection point and in the surrounding rock strata. INERIS performed passive seismic monitoring, continuous gas monitoring in boreholes, and also gas flux measurements at the floor and on the wall of the crosscut. We will here present our monitoring plan, describe our equipments and discuss data from baseline, injection and post-injection phases. Data from passive seismic monitoring show a correlation between microseismic activity recorded in the monitored zone and the variations in CO2 injection pressure. Data from gas monitoring show CH4 (initial gas in place) and CO2 concentration variations in time both in coal and surrounding rocks. Flux measurements helped us to determine CH4 and CO2 quantities leaking from the rock mass to the crosscut. Monitoring equipments have highlighted two main leakage pathways. Due to leakage, gas migration was not monitored in coal because the quantity of CO2 really injected in coal ( 120 kg) created a plume whose radius was smaller than expected by modelling and therefore gas didn’t reach the monitoring boreholes. Here, passive seismic monitoring and continuous gas monitoring proved to be valuable tools to observe gas migration and the behaviour of the rock during the injection test. They also helped us to understand the discrepancies between observations and predictions. Therefore the results will help us to draw recommendations and to design future in-situ injection operations in coal, as well as CO2-enhanced coal bed methane recovery (CO2-ECBM) operations