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

Abstract

The CO 2 storage operation at Sleipner in the Norwegian North Sea provides an excellent demonstration of the application of time-lapse surface seismic methods to CO 2 plume monitoring under favourable conditions. Injection commenced at Sleipner in 1996 with CO 2 separated from natural gas being injected into the Utsira Sand, a major saline aquifer of late Cenozoic age. CO 2 injection is via a near-horizontal well, at a depth of about 1012 m 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.A comprehensive time-lapse surface seismic programme has been carried out, with 3D surveys in 1994, 1999, 2001, 2002, 2004, 2006 and 2008. Key aims of the seismic monitoring are to track plume migration, demonstrate containment within the storage reservoir and provide quantitative information as a means to better understand detailed flow processes controlling development of the plume in the reservoir.The CO 2 plume is imaged as a number of bright sub-horizontal reflections within the reservoir, growing with time ( Figure 1). The reflections mostly comprise tuned wavelets arising from thin (mostly < 8 m thick) layers of CO 2 trapped beneath very thin intra-reservoir mudstones and the reservoir caprock. The plume is roughly 200 m high and elliptical in plan, with a major axis increasing to over 3000 m by 2008. As well as its prominent reflectivity, the plume also produces a large velocity pushdown caused by the seismic waves travelling more slowly through CO 2 -saturated rock than through the virgin aquifer. This paper summarises some of the quantitative methods that have been applied to the Sleipner seismic datasets.

Quantitative evaluation of thin-layer thickness and CO2mass utilizing seismic complex decomposition at the Ketzin CO2storage site, Germany

“…However, analysis of AVO may be invalid, if thin-layer interference occurs (Swan 1991;Juhlin & Young 1993;Chadwick et al 2010). At the Ketzin site, Ivanova et al (2013a) investigated these two effects using the modelled AVO response and concluded that it is difficult to discriminate between these two effects in the field data due to the limited signal-to-noise ratio and reservoir heterogeneity.…”

“…The thickness can be determined based on the relationship between the tuning frequency and temporal thickness using spectral decomposition. Several spectral decomposition methods, such as the continuous wavelet transform (CWT) and the Wigner-Ville distribution, have been applied to CO 2 storage sites to qualitatively and quantitatively assess the injected CO 2 (Chadwick et al 2010;Ravazzoli & Gómez 2014). White et al (2013) used the smoothed pseudo Wigner-Ville distribution (SPWVD) to analyse the topmost supercritical CO 2 layer at the Sleipner gas field.…”

Quantitative seismic characterization of CO2 at the Sleipner storage site, North Sea

“…Time-lapse seismic analysis has been the main focus to qualitatively map the CO 2 migration process Chadwick et al, 2010). Eight repeated surface seismic surveys have been acquired by 2014 at Sleipner field.…”

Determining changes in CO2 saturation from time-lapse measurements using ghost reflections retrieved by seismic interferometry

“…Time-lapse seismic measurements have proven their merit in monitoring changes in the reservoirs layer during CO 2 sequestration (Arts et al, 2002;Chadwick et al, 2010;Lumley, 2010;Ivanova et al, 2012). Results at the Sleipner underground storage site in the North Sea show a very strong time-lapse seismic response, both in terms of high seismic amplitudes as well as strong timeshifts (Arts et al, 2004b).…”