Anssi Myrttinen

Field-based stable isotope analysis of carbon dioxide by mid-infrared laser spectroscopy for carbon capture and storage monitoring

A newly developed isotope ratio laser spectrometer for CO2 analyses has been tested during a tracer experiment at the Ketzin pilot site (northern Germany) for CO2 storage. For the experiment, 500 tons of CO2 from a natural CO2 reservoir was injected in supercritical state into the reservoir. The carbon stable isotope value (δ(13)C) of injected CO2 was significantly different from background values. In order to observe the breakthrough of the isotope tracer continuously, the new instruments were connected to a stainless steel riser tube that was installed in an observation well. The laser instrument is based on tunable laser direct absorption in the mid-infrared. The instrument recorded a continuous 10 day carbon stable isotope data set with 30 min resolution directly on-site in a field-based laboratory container during a tracer experiment. To test the instruments performance and accuracy the monitoring campaign was accompanied by daily CO2 sampling for laboratory analyses with isotope ratio mass spectrometry (IRMS). The carbon stable isotope ratios measured by conventional IRMS technique and by the new mid-infrared laser spectrometer agree remarkably well within analytical precision. This proves the capability of the new mid-infrared direct absorption technique to measure high precision and accurate real-time stable isotope data directly in the field. The laser spectroscopy data revealed for the first time a prior to this experiment unknown, intensive dynamic with fast changing δ(13)C values. The arrival pattern of the tracer suggest that the observed fluctuations were probably caused by migration along separate and distinct preferential flow paths between injection well and observation well. The short-term variances as observed in this study might have been missed during previous works that applied laboratory-based IRMS analysis. The new technique could contribute to a better tracing of the migration of the underground CO2 plume and help to ensure the long-term integrity of the reservoir.

A Brief Overview of Isotope Measurements Carried Out at Various CCS Pilot Sites Worldwide

About 1800 geochemical measurements including more than 1000 isotope analyses have been published as a result of geochemical monitoring programs established at several CO2 storage and enhanced oil and gas recovery projects worldwide. These projects are briefly discussed here in order to compare sampling techniques to obtain fluid and gas samples for chemical and isotopic analyses. In all the projects, changes of stable isotope ratios of CO2 and dissolved inorganic carbon in samples obtained pre- and post-injection were used to quantify solubility and ionic trapping of CO2 via stable isotope mass balances. Further applications include, monitoring of underground CO2 migration and early detection of potential CO2 leaks into overlying formations. Other benefits of these stable isotope tracers include a better understanding of water-rock-gas interactions with CO2 under supercritical conditions and often highly saline reservoir fluids that are present in the storage formations. While the results of these projects need further laboratory and experimental confirmation, further increase in field applications of stable isotope tracer techniques are anticipated with the introduction of new portable laser stable isotope mass spectrometers.

Stable carbon isotope fractionation data between H2CO3* and CO2(g) extended to 120 °C

RATIONALE Literature data on experimentally derived equilibrium stable carbon isotope fractionation (10(3) lnα(13) C) between H2 CO3 (*) (H2 CO3  + CO2(aq) ) and gaseous CO2 (CO2(g) ) are so far only available up to 60 °C and were typically determined at or near atmospheric pressures. Here we experimentally expand this dataset to temperature and pressure conditions close to the supercritical state for CO2 . The objective is to improve the applicability of stable carbon isotopes as a tracer in environments where such conditions prevail. METHODS Eighteen stable carbon isotope laboratory experiments were conducted in a steel vessel. Deionised water that was acidified with hydrochloric acid (HCl, 1 N) to a pH of 2.4 was equilibrated with CO2(g) at pressures (pCO2 ) of 55 bar for durations between 2 and 188 h. The experiments were conducted at 20, 60, 80, 100 and 120 °C. H2 CO3 (*) and CO2(g) were sampled separately and their carbon isotope ratios were determined by isotope ratio mass spectrometry. RESULTS At 20 °C, average 10(3) lnα(13) CH2CO3 * -CO2(g) values of -1.0 ± 0.1 ‰ were observed with a preference for (12) C in H2 CO3 (*) consistent with previous research. At elevated temperatures of 120 °C, 10(3) lnα(13) CH2CO3 * -CO2(g) values decreased to an average value of -0.7 ± 0.1 ‰. The resulting temperature dependence for carbon isotope fractionation between H2 CO3 (*) and CO2(g) was 10(3) lnα(13) CH2CO3 * -CO2(g)  = (0.0025 ± 0.0004) T(°C) - (1.0 ± 0.03) ‰. Carbon isotope equilibrium between H2 CO3 (*) and CO2(g) was reached within reaction times of 18 h and mostly within 5 h or less. CONCLUSIONS 10(3) lnα(13) CH2CO3 * -CO2(g) data are now available for temperatures up to 120 °C and for pressures of up to 55 bar. The results suggest that higher pCO2 levels possibly shorten carbon isotope equilibration times. These data are critically important for using δ(13) C values as tracers, for instance at geological CO2 sequestration sites and corresponding natural analogues.

Advances in Stable Isotope Monitoring of CO2 Under Elevated Pressures, Temperatures and Salinities: Selected Results from the Project CO2ISO-LABEL

The BMBF project CO2ISO-LABEL (Carbon and Oxygen ISOtopes under extreme conditions LABoratory EvaLuations for CO2-storage monitoring) investigated stable isotope methods in laboratory studies for transferral to carbon capture and storage (CCS) field sites including enhanced gas and oil recovery (EGR and EOR). The isotope composition of injected CO2 and water are useful tracers for migration and water-rock-gas interactions during such operations. However, quantification of carbon and oxygen equilibrium isotope effects at elevated pressures and temperatures are so far scarce. They thus need more investigations under p/T conditions that are characteristic for reservoirs and overlying aquifers. With this, the main objective of the project was to improve stable carbon and oxygen isotope methods for monitoring CO2 storage sites and their impact of injected CO2 on reservoir geochemistry under controlled laboratory settings. An important finding was that isotope fractionations of carbon between CO2 and dissolved inorganic carbon (DIC) were not significantly different from each other in experiments with pure CO2 and pressures between 59 and 190 bar. Furthermore, influences of rock types (limestone, dolomite and sandstone) and fluid salinities were found to be negligible for carbon isotope fractionation between CO2 and DIC. Another finding was that water oxygen isotope ratios changed systematically in response to different CO2/H2O molar ratios in closed system equilibration experiments. This helps to reconstruct the amounts of CO2 that equilibrated with formation waters. Results of the project will enable better assessment of geochemical conditions in underground carbon storage sites or other subsurface systems where large amounts of CO2 interact with water and rocks.

Reservoir and Cap Rock Monitoring

One aim of the CLEAN project was to develop and test monitoring methods for the reservoir cap rock and the reservoir itself. It is shown here that advanced injection and production profile evaluation can be achieved using a combination of pressure, temperature and spinner flow meter data. Using distributed temperature sensing, temperature profiles in gas-filled wells can be acquired, as the sensor cable can be stationary during the measurement allowing for simultaneous thermal equilibration along the entire logged profile. A first field test of the developed hybrid wireline logging system was successfully performed under static conditions and the feasibility of warm-back monitoring was shown based on the results of numerical simulations for a possible CO2 injection scenario. A combined approach using the developed hybrid system for enhanced production logging during injection followed by warm-back monitoring within a subsequent shut-in period would allow for accurate determination of the spatial extent and injectivity of individual CO2 injection intervals.