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Title: A Sea Floor Survey of the Sleipner Field to Monitor CO2 Migration

Abstract

In the North Sea natural gas production field at Sleipner, CO{sub 2} is being separated from natural gas and injected into an underground saline aquifer, known as the Utsira formation, for environmental purposes. In this study, gravity measurements were made over the Sleipner CO{sub 2} injection site in 2002 and again in 2005 on top of 30 concrete benchmarks on the seafloor to study the behavior and physical properties of the injected CO{sub 2}. As the gas is injected, pore space water is replaced by gas, altering the bulk density of the formation. This results in a change in gravitational acceleration observed on the overlying sea floor. Our gravity measurements show a repeatability of 4.3 {micro}Gal for 2003 and 3.5 {micro}Gal for 2005. Forward models of the gravity change are calculated based on both 3-D seismic data and reservoir simulation models from other studies. These forward models indicate that the magnitude of maximum gravity change is primarily related to CO{sub 2} density rather than flow geometry. The time-lapse gravity observations best fit a high temperature forward model based on the seismically determined CO{sub 2} geometry, suggesting that the 3-D reflection seismics are imaging the geometry of the injected CO{sub 2},more » and that the in situ CO{sub 2} density is around 530 kg/m{sup 3}. Uncertainty in determining the average density using this technique is estimated to be {+-}65 kg/m{sup 3} (95% confidence), however, additional seismic surveys are needed before final conclusions can be drawn. Future gravity measurements will put better constraints on the CO{sub 2} density and continue to map out the CO{sub 2} flow.« less

Authors:
Publication Date:
Research Org.:
University Of California
Sponsoring Org.:
USDOE
OSTI Identifier:
902109
DOE Contract Number:
FC26-02NT41587
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
03 NATURAL GAS; ACCELERATION; BENCHMARKS; BULK DENSITY; CONCRETES; FLOORS; GEOMETRY; MONITORS; NATURAL GAS; NORTH SEA; PHYSICAL PROPERTIES; PRODUCTION; REFLECTION; SEAS; SEISMIC SURVEYS; SIMULATION; WATER

Citation Formats

Mark A. Zumberge. A Sea Floor Survey of the Sleipner Field to Monitor CO2 Migration. United States: N. p., 2005. Web. doi:10.2172/902109.
Mark A. Zumberge. A Sea Floor Survey of the Sleipner Field to Monitor CO2 Migration. United States. doi:10.2172/902109.
Mark A. Zumberge. Sat . "A Sea Floor Survey of the Sleipner Field to Monitor CO2 Migration". United States. doi:10.2172/902109. https://www.osti.gov/servlets/purl/902109.
@article{osti_902109,
title = {A Sea Floor Survey of the Sleipner Field to Monitor CO2 Migration},
author = {Mark A. Zumberge},
abstractNote = {In the North Sea natural gas production field at Sleipner, CO{sub 2} is being separated from natural gas and injected into an underground saline aquifer, known as the Utsira formation, for environmental purposes. In this study, gravity measurements were made over the Sleipner CO{sub 2} injection site in 2002 and again in 2005 on top of 30 concrete benchmarks on the seafloor to study the behavior and physical properties of the injected CO{sub 2}. As the gas is injected, pore space water is replaced by gas, altering the bulk density of the formation. This results in a change in gravitational acceleration observed on the overlying sea floor. Our gravity measurements show a repeatability of 4.3 {micro}Gal for 2003 and 3.5 {micro}Gal for 2005. Forward models of the gravity change are calculated based on both 3-D seismic data and reservoir simulation models from other studies. These forward models indicate that the magnitude of maximum gravity change is primarily related to CO{sub 2} density rather than flow geometry. The time-lapse gravity observations best fit a high temperature forward model based on the seismically determined CO{sub 2} geometry, suggesting that the 3-D reflection seismics are imaging the geometry of the injected CO{sub 2}, and that the in situ CO{sub 2} density is around 530 kg/m{sup 3}. Uncertainty in determining the average density using this technique is estimated to be {+-}65 kg/m{sup 3} (95% confidence), however, additional seismic surveys are needed before final conclusions can be drawn. Future gravity measurements will put better constraints on the CO{sub 2} density and continue to map out the CO{sub 2} flow.},
doi = {10.2172/902109},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Sat Dec 31 00:00:00 EST 2005},
month = {Sat Dec 31 00:00:00 EST 2005}
}

Technical Report:

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  • Since 1996, excess CO{sub 2} from the Sleipner natural gas field has been sequestered and injected underground into a porous saline aquifer 1000 m below the seafloor. In 2002, we carried out a high precision micro-gravity survey on the seafloor in order to monitor the injected CO{sub 2}. A repeatability of 5 {micro}Gal in the station averages was observed. This is considerably better than pre-survey expectations. These data will serve as the baseline for time-lapse gravity monitoring of the Sleipner CO{sub 2} injection site. A three-week trip to Statoil Research Centre in Trondheim, Norway, was made in the summer ofmore » 2003. This visit consisted of gathering data and collaborating with scientists working on the Sleipner project. The trip ended with a presentation of the seafloor gravity results to date at a SACS2 (Saline Aquifer CO{sub 2} Storage 2) meeting. This meeting provided the perfect opportunity to meet and gather information from the world's experts on the Sleipner project.« less
  • Since 1996, excess CO{sub 2} from the Sleipner natural gas field has been sequestered and injected underground into a porous saline aquifer 1000 m below the seafloor. In 2002, we carried out a high precision micro-gravity survey on the seafloor in order to monitor the injected CO{sub 2}. A repeatability of 5 {micro}Gal in the station averages was observed. This is considerably better than pre-survey expectations. These data will serve as the baseline for time-lapse gravity monitoring of the Sleipner CO{sub 2} injection site. A repeat survey has been scheduled for the summer of 2005. This report covers 9/19/03 tomore » 3/18/04. During this time, significant advancement in the 3-D gravity forward modeling code was made. Testing of the numerical accuracy of the code was undertaken using both a sheet of mass and a frustum of a cone for test cases. These were chosen because of our ability to do an analytic calculation of gravity for comparison. Tests were also done to determine the feasibility of using point mass approximations rather than cuboids for the forward modeling code. After determining that the point mass approximation is sufficient (and over six times faster computationally), several CO{sub 2} models were constructed and the time-lapse gravity signal was calculated from each. From these models, we expect to see a gravity change ranging from 3-16 {micro}Gal/year, depending on reservoir conditions and CO{sub 2} geometry. While more detailed modeling needs to be completed, these initial results show that we may be able to learn a great deal about the state of the CO{sub 2} from the time-lapse gravity results. Also, in December of 2003, we presented at the annual AGU meeting in San Francisco.« less
  • Since 1996, excess CO{sub 2} from the Sleipner natural gas field has been sequestered and injected underground into a porous saline aquifer 1000 m below the seafloor. In 2002, we carried out a high precision micro-gravity survey on the seafloor in order to monitor the injected CO{sub 2}. A repeatability of 5 {micro}Gal in the station averages was observed. This is considerably better than pre-survey expectations. These data will serve as the baseline for time-lapse gravity monitoring of the Sleipner CO{sub 2} injection site. A repeat survey has been scheduled for the summer of 2005. This report covers 3/18/04 tomore » 9/19/04. During this time, we participated in several CO{sub 2} sequestration-related meetings and conferences. On March 29, 2004, we participated in the 2004 Carbon Sequestration Project Review Meeting for the Department of Energy in Pittsburgh, PA. During the week of May 2, 2004, we attended and presented at the Third Annual Conference on Carbon Capture and Sequestration in Alexandria, VA. Finally, during the week of August 8, 2004, we took part in the U.S.-Norway, CO{sub 2} Summer School in Santa Fe, NM. Additional modeling was also completed, examining the seismic velocity pushdown estimates from the gravity models and the expected deformation of the seafloor due to the injected CO{sub 2}.« less
  • Since 1996, excess CO{sub 2} from the Sleipner natural gas field has been sequestered and injected underground into a porous saline aquifer 1000 m below the seafloor. In 2002, we carried out a high precision micro-gravity survey on the seafloor in order to monitor the injected CO{sub 2}. A repeatability of 5 {micro}Gal in the station averages was observed. This is considerably better than pre-survey expectations. These data will serve as the baseline for time-lapse gravity monitoring of the Sleipner CO{sub 2} injection site. A repeat survey has been scheduled for the summer of 2005. This report covers 9/19/04 tomore » 3/18/05. During this time, gravity and pressure modeling were completed and work graduate student Scott Nooner began writing his Ph.D. dissertation, of which this work is a the major part. Improvements to the gravimeters are also underway that will hopefully increase the measurement precision.« less
  • Since 1996, excess CO{sub 2} from the Sleipner natural gas field has been sequestered and injected underground into a porous saline aquifer 1000 m below the seafloor. In 2002, we carried out a high precision micro-gravity survey on the seafloor in order to monitor the injected CO{sub 2}. A repeatability of 4.3 {micro}Gal in the station averages was observed. This is considerably better than pre-survey expectations. These data will serve as the baseline for time-lapse gravity monitoring of the Sleipner CO{sub 2} injection site. This report covers 3/19/05 to 9/18/05. During this time, gravity and pressure modeling were completed andmore » graduate student Scott Nooner finished his Ph.D. dissertation, of which this work is a major part. Three new ROVDOG (Remotely Operated Vehicle deployable Deep Ocean Gravimeter) instruments were also completed with funding from Statoil. The primary changes are increased instrument precision and increased data sampling rate. A second gravity survey was carried out from August to September of 2005, allowing us to begin examining the time-lapse gravity changes caused by the injection of CO{sub 2} into the underground aquifer, known as the Utsira formation. Preliminary processing indicates a repeatability of 3.6 {micro}Gal, comparable to the baseline survey.« less