HR: 08:00h
AN: GC31A-01    [PDF]
TI: Seafloor micro-gravity survey of the Sleipner CO2 sequestration site
AU: * Nooner, S L
EM: snooner@ucsd.edu
AF: Scripps Institution of Oceanography, Univ California San Diego 9500 Gilman Dr Dept 0225, La Jolla, CA 92093-0225 United States
AU: Zumberge, M A
EM: zumberge@ucsd.edu
AF: Scripps Institution of Oceanography, Univ California San Diego 9500 Gilman Dr Dept 0225, La Jolla, CA 92093-0225 United States
AU: Eiken, O
EM: oei@statoil.com
AF: Statoil Research Centre, Rotvoll, N-7005, Trondheim, 7005 Norway
AU: Stenvold, T
EM: torste@statoil.com
AF: Statoil Research Centre, Rotvoll, N-7005, Trondheim, 7005 Norway
AU: Sasagawa, G S
EM: gsasagawa@ucsd.edu
AF: Scripps Institution of Oceanography, Univ California San Diego 9500 Gilman Dr Dept 0225, La Jolla, CA 92093-0225 United States
AB: The Sleipner fields make up a natural gas production area in the North Sea operated by Statoil, the major Norwegian oil company. The gas recovered there has an excess CO2 content of about 7 percent, which is separated out and collected. CO2 collected from most production areas is released into the atmosphere, increasing the atmospheric content of this greenhouse gas. Sleipner is the world's first large scale concentrated CO2 sequestration project. Each year 1 MT of CO2 is injected into a large, deep saline reservoir called the Utsira formation. This is a high porosity sandstone aquifer capped by low porosity shale 720 m below the seafloor. The multi-institutional SACS (Saline Aquifer CO2 Storage) group began geologic analysis, reservoir simulations, seismic modeling, and geophysical monitoring of Sleipner in 1998. The primary monitoring technique is 4-D or time-lapse seismics. The results show a clearly defined CO2 bubble. However, uncertainties exist such that the density, thus mass, of CO2 within the Utsira sand is not precisely known. Preliminary gravity modeling indicates that the density changes due to a year of CO2 injection should cause a change in the local gravity ranging from 5-15 microGal (the range is due to an uncertainty in the reservoir temperature). In August of 2002, we carried out the first phase of a seafloor gravity survey, establishing a baseline for time-lapse gravity monitoring of the CO2 bubble as well as obtaining a gravity data set for initial reservoir modeling. Gravity was measured on the seafloor above the Sleipner CO2 injection site from the 15th to the 21st of August, 2002, on top of 30 concrete benchmarks, which were permanently deployed on the seafloor spaced from 300 to 500 m apart. The area spans about 7 km E-W and 3 km N-S and has a water depth of 80 m. Gravity measurements were gathered using ROVDOG (ROV deployed Deep Ocean Gravimeter), a package consisting of three relative gravimeters. In relative gravity surveys, the uncertainty is given by the repeatability of the measurements, thus each benchmark was visited at least three times. Based on repeatability, the uncertainty in the station averages is estimated to be 2.5 microGal. For time-lapse measurements, there is additional uncertainty associated with the reference null level, determined from stations outside the CO2 area, of about 1-2 microGal. Therefore, the final detection threshold for time-lapse changes is about 5 microGal. Modelling of the data from our initial gravity survey, in conjunction with seismic and borehole data, is a first attempt at estimating the CO2 density and mass within the reservoir. Subsequent surveys that provide time-lapse data will be an independent and more reliable means to estimate these quantities. The time scale for a repeat survey is 2 to 5 years.
DE: 1219 Local gravity anomalies and crustal structure
DE: 1294 Instruments and techniques
DE: 1610 Atmosphere (0315, 0325)
DE: 1694 Instruments and techniques
DE: 3010 Gravity
SC: Global Climate Change [GC]
MN: 2003 Fall Meeting