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| Titel |
Sensitivity study of pulsed neutron gamma monitoring to evaluate saturation changes for depleted gas field conditions in the context of CO2 storage |
| VerfasserIn |
Gunther Baumann, Jan Henninges, Marco De Lucia |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250048368
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| Zusammenfassung |
| Saline aquifers and depleted gas fields have a high potential for CO2 storage operations in
order to reduce emissions into the atmosphere. One important question in the context of CO2
storage is the monitoring of saturation changes caused by displacement of the in-situ fluids in
the pore space. Pulsed neutron gamma (PNG) logs are widely used for formation
evaluation through casing in oil and gas fields. Based on these experiences the task is to
study the potential of PNG monitoring to evaluate saturation changes due to CO2
injection under the low contrast conditions in depleted gas fields at the borehole
scale.
A neutron generator emits bursts of high energy neutrons periodically, which interact with
the surrounding borehole and formation. Due to the collision with these neutrons, atoms
from the surrounding environment emit gamma rays with characteristic energies
depending on their atomic number. After elastic and inelastic scattering with the
atoms, going along with short time gamma ray emission, the neutrons reach the low
energy thermal level. At this thermal energy level the neutrons are finally captured
mainly by hydrogen and chlorine also with a corresponding emission of gamma
rays. This long period gamma ray emission is the basis for pulsed neutron gamma
logging from capture processes. The value of the probability to capture thermal
neutrons is the capture cross section (Σ) which is derived from the decline of gamma
rays with time. Monitoring of saturation changes is performed in time lapse mode
comparing baseline measurements before injection with repeat measurements during
injection.
The high Σ contrast between brine and CO2 results in a high sensitivity to evaluate
saturation changes. Therefore the PNG method is successfully applied in saline aquifers to
evaluate the saturation changes from CO2 injection. Moreover, dry-out effects together with
resulting salt precipitation can affect the Σ reading, and has therefore to be carefully taken
into account. Ignoring the salt precipitations e.g. the chlorine would result in an
underestimation of the CO2 saturation.
This contribution shows a sensitivity analysis for PNG monitoring to evaluate
saturation changes for depleted gas fields under conditions similar to the Altmark site,
Germany. The Σ contrast between the formation gases, mostly N2, CH4, and the
injected CO2 is in the range of typical pulsed-neutron gamma tools accuracy and
therefore difficult to detect. Taking into account possible drying effects especially in
injection wells the Σ contrast is quite higher and is the subject of the sensitivity study. |
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