Andy Chadwick

Quantitative analysis of time-lapse seismic monitoring data at the Sleipner CO2 storage operation

The CO2 storage operation at Sleipner in the Norwegian North Sea provides an excellent demonstration of the application of time-lapse surface seismic methods to CO2 plume monitoring under favorable conditions. Injection commenced at Sleipner in 1996 with CO2 separated from natural gas being injected into the Utsira Sand, a major saline aquifer of late Cenozoic age. CO2 injection is via a near-horizontal well at a depth of about 1012 m below sea level (bsl) some 200 m below the reservoir top, at a rate approaching 1 million tonnes (Mt) per year, with more than 11 Mt currently stored.

Axisymmetric gravity currents in a porous medium

The release from a point source of relatively heavy fluid into a saturated porous medium above an impermeable boundary is considered. A theoretical relationship is compared with experimental data for the rate of propagation of the front of the resulting gravity current and its shape. A motivation of the study, the problem of carbon dioxide sequestration, is briefly discussed.

Seismic monitoring at the Sleipner underground CO2 storage site (North Sea)

Abstract The growing emissions of greenhouse gases, especially CO2, are seen worldwide as one of the major causes of climate change. International treaties like the Kyoto Protocol are supposed to contribute to reducing the emission of greenhouse gases. Underground sequestration has the potential to play an important role in keeping large volumes of CO2 from escaping into the atmosphere in the short term. The first case of industrial scale CO2 storage in the world (close to one million tonnes per year since 1996) is taking place at the Sleipner underground CO2 storage site in the North Sea offshore Norway. Careful monitoring of the behaviour of the storage facility is required to establish its safety. To this end, two time-lapse seismic surveys have been acquired; the first repeat survey was completed in October 1999 and the second in October 2001. The presence of CO2 beneath thin intra-shale layers within the reservoir has caused significant changes both in reflection amplitudes (up to a factor 10) and in travel time (more than 40ms) through the CO2 plume (the velocity push-down effect). Some aspects of the interpretation of these time-lapse seismic surveys will be presented here.

Recent time-lapse seismic data show no indication of leakage at the sleipner CO2-injection site

This chapter highlights that the injection of CO2 into the Utsira Sand0 has been in progress for more than seven years, with an annual rate of approximately one million tones. This project is considered as the first industrial-scale, environmentally driven CO2 injection project in the world. In consequence, a European research project has been organized around it, with special focus on monitoring and simulation. To that end, four seismic surveys have so far been acquired, one prior to injection, and three afterwards in 1999, 2001 and 2002. In this chapter results from the 2002 survey are presented for the first time and compared to findings from the previous surveys. The major conclusion is that there are still no indications of CO2 leakage to levels shallower than the Utsira Sand.

Regional Study of the Neogene Deposits in the Southern Viking Graben Area - a Site for Potential CO2 Storage

Since 1996 Statoil has injected CO2 separated from gas of the Sleipner Vest field into saline reservoir sands of the Utsira Forrnation in the central North Sea at a depth of approximately 900 m. This is the first case of industrial scale CO2 storage in the world (1 million tons per year). The Saline Aquifer CO2 Storage (SACS) project is conducted by a consortium of oil companies and research institutions. This paper presents regional geological interpretation in the southem Viking Graben area. Sands of the Mio-Pliocene Utsira Formation and the Nordland Shale have been identified and their stratigraphical architecture outlined from seismic sections and well-logs. Detailed geophysical and geological aspects at the injection site are treated in Arts et al. (this volume) and Brevik et al. (this volume).

Volumetric Bounds on Subsurface Fluid Substitution Using 4D Seismic Time-shifts with an Application at Sleipner

A method for volumetric estimation of subsurface fluid substitution is presented that relies on the analysis of 4D seismic time-shifts. Since time-shifts cannot resolve for fluid saturation and layer thickness simultaneously without additional constraints, mass estimates are derived from the complete set of possible fluid saturations and layer thicknesses. The method considers velocity-saturation relationships that range from uniform saturation to patchy saturation. Based on a generalized velocity-saturation relationship that is parameterized by the degree of patchiness, explicit upper and lower fluid mass bounds are provided. We show that the inherent ambiguity between fluid saturation and layer thickness has a severe impact on the convergence of these mass bounds. That is, roughly linear velocity-saturation relationships with patchy saturation tend to provide significantly better accuracy in a mass interpretation than the strongly non-linear velocity-saturation relationships associated with homogeneous saturation. The method is validated at the Sleipner storage site, where injected fluid masses are known. Moreover, a linear relationship between 4D time-shifts and injected mass is observed, suggesting that the evolving patterns of fluid saturation and fluid mixing in the CO2 plume at Sleipner have remained roughly constant with time.

CO2 Injection Induced Dispersion And Attenuation

Summary Injection of CO2 in a geological subsurface will cause changes in seismic velocities, dispersion and attenuation, which result in changes in seismic wave propagation and seismic imaging. Estimating seismic attenuation changes from the seismic section with CO2 plume can provide some valuable information about CO2 storage, migration and possible leakage. We apply a frequency-dependant AVO inversion (FDAI) and Q value estimation method to a seismic dataset acquired in a CO2 injection area. The computed and inverted dispersion and attenuation results reveal the distribution of the CO2 injected into the reservoir sand. There are significant dispersion and attenuation in the CO2 plume, but the attenuation change is complex around and within the plume as the pressure changes after CO2 injection.