Seismic detection of CO2 leakage along monitoring wellbores

Abstract

A pilot carbon dioxide (CO 2 ) sequestration experiment was carried out in the Michigan Basin in which ~10,000 tonnes of supercritical CO 2 was injected into the Bass Island Dolomite (BILD) at 1050 m depth. A Passive Seismic Monitoring (PSM) network was operated before, during and after the ~17 day injection period. The seismic monitoring network consisted of two arrays of eight, three-component sensors, deployed in two monitoring wells at only a few hundred meters from the injection point. 225 microseismic events were detected by the arrays. Of these, only one event was clearly an injectioninduced microearthquake. It occurred during injection, approximately 100 m above the BILD formation. No events, down to the magnitude -3 detection limit, occurred within the BILD formation during the injection. The observed seismic waveforms associated with the other 224 events were quite unusual in that they appear to contain dominantly compressional (P) but no (or extremely weak) shear (S) waves, indicating that they are not associated with shear slip on faults. The microseismic events were unusual in two other ways. First, almost all of the events occurred prior to the start of injection into the BILD formation. Second, hypocenters of the 94 locatable events cluster around the wells where the sensor arrays were deployed, not the injection well. While the temporal evolution of these events show no 2 correlation with the BILD injection, they do correlate with CO 2 injection for enhanced oil recovery (EOR) into the 1670 m deep Coral Reef formation that had been going on for ~2.5 years prior to the pilot injection experiment into the BILD formation. We conclude that the unusual microseismic events reflect degassing processes associated with leakage up and around the monitoring wells from the EOR-related CO 2 injection into the Coral Reef formation, ~700 m below the depth of the monitoring arrays. This conclusion is also supported by the observation that as soon as injection into the Coral Reef formation resumed at the conclusion of the BILD demonstration experiment, seismic events (essentially identical to the events associated with the Coral Reef injection prior to the BILD experiment) again started to occur close to a monitoring arrays. Taken together, these observations point to vertical migration around the casings of the monitoring wellbores. Detection of these unusual microseismic events was somewhat fortuitous in that the arrays were deployed at the depth where the CO 2 undergoes a strong volume increase during transition from a supercritical state to a gas. Given the large number of pre-existing wellbores that exist in depleted oil and gas reservoirs that might be considered for CO 2 sequestration projects, passive seismic monitoring systems could be deployed at appropriate depths to systematically detect and monitor leakage along them.

CO2 geological storage — Geotechnical implications

“…Remote sensing of air composition (7) CO 2 atmospheric concentration by hyperspectral remote sensing of vegetative stress and long open path infrared absorption Applicable at large scales (1) (Emberley et al, 2004;Gunter et al, 2000;Newell et al, 2008); (2) laboratory studies (Lei and Xue, 2009;Shi et al, 2007;Xue et al, 2005) and pilot tests (Bohnhoff et al, 2010;Daley et al, 2008;Daley et al, 2007;Onishi et al, 2009); (3) (Gasperikova and Hoversten, 2006;Nakatsuka et al, 2010); (4) (Bielinski et al, 2008); (5) (Charpentier et al, 2009); (6) (Oldenburg et al, 2003); (7) (Leuning et al, 2008;Oldenburg et al, 2003); (8) (Strazisar et al, 2009); (9) (Arts et al, 2004); (10) (Alnes et al, 2008); (11) (Alnes et al, 2008;Kempka et al, 2008); (12) (Wielopolski and Mitra, 2010) − 716 − KSCE Journal of Civil Engineering (15) where S CO2 ≤ S perc , the maximum saturation of CO 2 at percolation. Figure 10 shows the variation of P-wave velocity V P [m/s] and electrical conductivity σ form as a function of the CO 2 saturation S CO2 .…”

CO2‐Speicherung. Chancen und Risiken

“…in den Bohrungen, gibt es auch seismische Verfahren, die eingesetzt werden können, um Leckagen früh-zeitig festzustellen und Gegenmaßnahmen zu ergreifen [22]. Da nachgewiesenermaßen Undichtigkeiten auftreten kön-nen und auch beobachtet werden, müssen CO 2 -Speicher umfangreich überwacht werden.…”

Oscillation of fluid‐filled cracks triggered by degassing of CO2 due to leakage along wellbores

“…All these events have consistent first arrivals with a phase velocity of 4.8 ± 0.2 km/s across the arrays (Figure 4a), but different dominant frequencies as described above. As this is the compressional wave velocity of the surrounding rock formation, these first arrivals are interpreted to be P waves and provide evidence that the events are located very close to the wellbores [ Bohnhoff et al , 2010b]. Because the phase velocity rapidly increases for hypocenters laterally offset from the array, an offset of 10, 20 or 50 m from the seismometer at the end of an array would result in a phase velocity of 5.3, 5.8 or 7.6 km/s, respectively, clearly much higher than the formation velocity.…”

“…As a result, we cannot use the 3D particle motion of the P onset as additional parameter to further refine the hypocenter precision. However, we note that recordings of all seven calibration shots conducted after the deployment of the geophones and prior to the onset of the CO 2 injection into the BILD Formation resulted in linear particle motions at the sensors allowing to determine the orientation of the individual geophones at depth (see Bohnhoff et al [2010b] for details). To quantify whether our near‐source recordings were in fact obtained in the seismic near field we systematically determine the amplitude decay across the respective sensor arrays.…”