Monitoring CO2 migration in an injection well: Evidence from MovECBM

Carbon dioxide (CO2) geological storage relies on safe, long-term injection of large quantities of CO2 in underground porous rocks. Wells, whether they are the conduit of the pumped fluid or are exposed to CO2 in the storage reservoir (observation and old wells) are man-made disturbances to the geological storage complex, and are thus viewed by some as a possible risk factor to the containment of the injected CO 2. Wells are composite structures, with an inner steel pipe separated from the borehole rock wall by a thin cement sheath (∼2 cm) that prevents vertical fluid migration. Both carbon steel and cement react in the presence of CO2, although evidence from production of CO2-rich fluids in the oil and gas industry and from lab experiments suggests that competent, defect-free cement offers an effective barrier to CO2 migration and leaks. However, reactivity of cement and steel may result in CO2 migration pathways degrading over time, thus in the leakage risk increasing during the life of the storage project. The issue then becomes how to best integrate preventive verification of zonal isolation/well integrity in the storage site monitoring plan. An analysis of the order of magnitude of possible CO2 leaks, and of their path to potable aquifers or the atmosphere, is also necessary to optimize the assurance (mitigation) monitoring of the storage site. Evidence gathered during the MovECBM project indicates that migration of small quantities of CO2 happened during injection in a coal seam in Southwest Poland. The evidence, gathered from casing and cement logging as well as soil gas monitoring over a 3-year period, was coupled with laboratory testing and extensive modeling of the chemo-mechanical behavior of cement and steel to determine if CO2 migration might have been responsible of the observed behavior. The three lines of evidence were: the detection of very small CO2 fluxes, coupled with less controversial helium concentration in soil; the occurrence of a thin pathway at the interface between cement and casing; and the change in mechanical properties of cement, suggestive of partial carbonation. Whereas the observations suggest that limited CO2 migration might have happened in the well, they are by no means proof that the migration did happen. Nonetheless, the integration of measurement and modeling yields important lessons for wellbore monitoring. First, it puts a probable ceiling on the order of magnitude of expected leaks from reasonably well-cemented wells at around 100 metric tons per year (less than 0.05% of the injected mass in a well like Sleipner or In Salah). It also suggests that cement may be a very effective leak detector: exposure to CO2 modifies its mechanical properties, which in turn can be detected using cement evaluation logs. Finally, coupling with dispersion modeling suggests the precision and accuracy required from soil gas and atmospheric monitoring, as well as the placement of sampling points; it also suggest that hysteresis, due to the accumulation in CO2 in surface aquifers and to the time required for it to be transported to the survey points, may delay initial detection; the same hysteresis may at the same time prolong the occurrence of CO2 shows long after the leak has stopped. © 2010 Elsevier Ltd. © 2011 Published by Elsevier Ltd

Short and Long Term Gas Hazard: The Release of Toxic Gases in the Alban Hills Volcanic Area (Central Italy).

In the Alban Hills area, strong areally diffuse and localised spot degassing processes occur. The gas comprises a large proportion of CO2, with minor CH4, H2S and Rn. These advective features are generated by fluid leakage from buried reservoirs hosted in the structural highs of the Mesozoic carbonate basement. Gas migration towards the surface is controlled by fault and fracture systems bordering the structural highs of the carbonate formations (e.g. Ciampino high). Both the sudden and catastrophic, and slow and continuous gas release at surface, of naturally occurring toxic species (CO2, H2S and Rn) poses a serious health risk to people living in this geologically active area. This paper presents data obtained from soil gas and gas flux surveys, as well as gas isotopes analyses, which suggest the presence of a deep origin gas flux enriched in carbon dioxide and minor species (CH4 and H2S), as well as a channelled migration of geogas mixtures having a Rn component which is not produced in situ. In regards to the health risk to local inhabitants, it was found that some anomalous areas had been zoned as parkland while others had been heavily developed for residential purposes. For example, many new houses were found to have been built on ground which has soil gas CO2 concentrations of over 70% and a CO2 flux of about 0.7 kg m_ 2 day_ 1, as well as radon values of more than 250 kBq/m3. In addition, an indoor radon survey has been conducted in selected houses in the town of Cava dei Selci to search for a possible correlation between the local geology and the radon concentration in indoor air. Preliminary results indicate high indoor values at ground floor levels (up to 1000 Bq/m3) and very high values in the cellars (up to 250.000 Bq/m3). It is recommended that land-use planners incorporate soil gas and/or gas flux measurements in the environmental assessment of areas of possible risk (i.e. volcanic or structurally active areas)

CO2Net: A marine monitoring system for CO2 leakage detection

Underwater oil and gas extraction and distribution, as well as the investigation of solutions for CO2 storage underwater, demand for new technologies to perform pervasive real life monitoring and control of underwater critical infrastructures. In this paper we present a system, named CO2Net, we have developed to perform accurate real-life monitoring of underwater CO2 storage infrastructures. The basic component of our system is the CO2Probe, a new underwater monitoring node which combines sensing, acoustic communications and networking capabilities. CO2Probes are connected via acoustic links in an underwater sensor network which provides robust, real-life communications of the monitored data both in single-hop and multi-hop deployments. The user has a real-time control on the monitoring system, being able to change alarm threshold values and sampling rates. The proposed CO2Net approach overcomes the major limitations of system currently available on the market, and provides a first easy to use, flexible and easy to extend, complete monitoring system for underwater infrastructures, based on the emerging underwater sensor networking paradigm. A first prototype of CO2Net has been tested during summer-fall 2011 at the NATO Undersea Research Centre (NURC) in La Spezia. Results of these experiments confirm system reliability, and its adaptability: all requested data where provided in real-time, the system was remotely accessible and end user could change monitoring parameters

Allaying public concern regarding CO2 geological sequestration through the development of automated stations for the continuous geochemical monitoring of gases in the near surface environment

The principle concern for both regulators and the public at large regarding the large-scale application of on-shore geological carbon dioxide (CO2) sequestration is related to possible health risks due to the leakage of CO2 at surface. Development and "proof of concept" of the automated geochemical monitoring station during research is seen as an important step forward in providing tools that help in assuring the public at large about the safety of geological CO2 sequestration. The advances in microchip technology, portable analytical instrumentation, and decreasing prices means that, once fully developed, a significant number of these relatively low cost stations might be deployed above an injection site to monitor CO2 leaks. These might be particularly useful if placed around any deep wells that penetrate the reservoir or deep aquifer in which CO2 is being injected, as gas injection tests have shown how breakthrough of monitored gases is often first observed in the vicinity of the injection borehole, implying that fracturing during drilling provided preferential flow pathways. Preliminary data provided by the development and installation of a prototype automated geochemical-geophysical monitoring station in the San Vittorino area has shown that this technology exists and that it can provide, together with other techniques, a useful tool for the safety assessment and monitoring of geological sequestration sites. © 2005 Elsevier Ltd. All rights reserved

The study of CO2 natural reservoirs to develop criteria for risk assessment and safety strategy.

Due to the large volumes produced, carbon dioxide (CO2) is the main anthropogenic compound identified as affecting the stability of the Earth’s climate. The injection and storage of anthropogenic CO2 in deep geologic formations (i.e. saline aquifers, hydrocarbon reservoirs, and unmineable coal seams) is a feasible strategy for rapidly reducing CO2 emissions to the atmosphere. However, a lack of public support, due to concerns over risks, could potentially block the widespread implementation of this promising technology. Only by facing these legitimate concerns will we be able to assure the public that (i) the scientific knowledge exists to select the best and safest sites; (ii) the techniques and approaches exist to monitor the safety of these sites during operation and post-injection; and (iii) the technologies exist if it is necessary to remedy a leak. There is the potential to examine some of the many naturally occurring CO2 reservoirs that occur throughout the world, some of which have trapped gas over geologic time periods while others leak due to gas migration along faults and fracture networks. These natural reservoirs occur in volcanic, geothermal, and sedimentary basin settings, and the unique geological and structural characteristics of each site can give important information regarding CO2 migration mechanisms at the required timescales and within complex, heterogeneous geological settings. While well-sealed, natural CO2 reservoirs can help us to understand the processes that isolate CO2, and other gases in the subsurface, leaking sites can be used to study gas migration processes along unsealed fault and fracture zones. In this work we present results from gas migration studies conducted at sites which represent the two extremes of gas leakage found in nature. These two sites are used to highlight how faulting style and near surface geology/hydrogeology can influence the migration of deep origin gases towards the ground surface

A comparison of leaking and non-leaking CO2 reservoirs as natural analogues for geological CO2 sequestration (Sesta and Latera geothermal fields, central Italy),

The Latera and Sesta areas are two geothermal fields located in central Italy which both exhibit significant quantities of associated CO2 gas. The primary difference between these two sites, however, is that there is large scale leakage of CO2 to the atmosphere in the Latera area while the CO2 at Sesta was only discovered at depth during exploratory geothermal drilling. These sites were thus selected as natural analogues for the geological sequestration of CO2 and were studied with both geophysical and shallow geochemical techniques in order to better understand the differences between a leaking and a sealed CO2 reservoir. The Latera geothermal field is located within the extinct Latera caldera, a large, elliptical, NNE-SSW trending structure which has long been know for carbonate rich springs and CO2-rich gas vents at surface. Sesta, on the other hand, is located within a NNW-SSE trending graben having associated boundary faults and a thick Pliocene clay cover. The CO2 at both sites is believed to be the result of decarbonisation of carbonate minerals due to the local high heat flow. Work conducted on these sites included soil gas surveys to delineate zones of elevated CO2 leakage and gas flux measurements to quantify the amount of CO2 leaking to the atmosphere. In addition electrical tomography surveys were performed at the Latera site to delineate migration pathways (faults and fracture networks). Soil gas results from Latera show CO2 concentrations which range from normal values of 1% up to 95% in gas vents cores. Associated with the elevated CO2 values are anomalous concentrations of trace gases which are transported from depth within the carrier stream, including H2S, He, H2 and CH4. Gas flux measurements range from 3 x 10-7 to 7.7 x 10-5 m3 m-2 sec- 1, with measurements on one gas vent (70 m2) indicating a total mass flux of about 108 kg day-1. In sharp contrast Sesta data showed low CO2 soil gas concentrations, on the order of shallow biological production, and gas flux rates which were 3 orders of magnitude lower than those observed at Latera. Gas migration to surface at Latera thus appears to occur along sub-vertical collapse structures associated with caldera formation, whereas the thick clay sequences at Sesta appear to have prevented large-scale escape of deep gases along the local graben boundary faults. The authors gratefully acknowledge the support of the EC in partly funding this work (EC Nascent project)