N2-CO2 co-injection field test at the Ketzin pilot CO2 storage site

Abstract

In summer 2013, a four week N2-CO2 co-injection field test was conducted at the Ketzin pilot site. Major objectives were (i) demonstrating the technical feasibility of a continuous N2-CO2 co-injection scenario, (ii) monitoring wellhead and reservoir pressure, (iii) monitoring spreading and behavior of the CO2-N2 gas mixture in the reservoir, and (iv) analyzing potential chromatographic effects within the reservoir. 10,000 L (10 Nm3) of krypton (Kr) were injected as an additional conservative chemical tracer prior to injection of the N2-CO2 gas mixture. For the field test CO2 from a natural CO2 source with a much heavier carbon isotope composition (δ13C=-3.4±0.2‰) was injected instead of the previously used industrial CO2 (δ13C=-30.6±0.4‰) from a refinery process to allow for examination of isotopic effects. Vital parameters during monitoring include N2 and CO2 injection rates, pressure and temperature at injection and observation wells, and reservoir pressure, respectively. A capillary riser tube was used to collect reservoir fluid and gas samples. These were analyzed for gas and carbon isotope compositions. Preliminary results show successful realization of the N2-CO2 co-injection field test next to effective and permanent monitoring of the injected gases (N2, CO2 and Kr) and vital storage parameters (wellhead pressures, reservoir pressure and reservoir temperature). During the field test, 32 t of N2 and 613 t of CO2 were co-injected in total to ensure a CO2:N2 volume ratio of approximately 95:5. Despite some variation of both N2 and CO2 injection rates, wellhead pressure and reservoir pressure were well controlled during the entire field test, and thereafter. Based on δ13C data, the gas mixture arrived after about 17 days at the first observation well (Ktzi 203) located at about 25 m distance to the injection well (Ktzi 201). Increasing Kr concentrations at Ktzi 203 positively correlate with both increasing N2 concentrations and δ13C values. Additionally, the δ13C data also indicate mixing between natural and industrial CO2 within the reservoir.