An inverse method for estimating thickness and volume with time of a thin CO₂‐filled layer at the Sleipner Field, North Sea

Abstract

Migration of CO2 through storage reservoirs can be monitored using time lapse seismic reflection surveys. At the Sleipner Field, injected CO2 is distributed throughout nine layers within the reservoir. These layers are too thin to be seismically resolvable by direct measurement of the separation between reflections from the top and bottom of each layer. Here we develop and apply an inverse method for measuring thickness changes of the shallowest layer. Our approach combines differences in traveltime down to a specific reflection together with amplitude measurements to determine layer thicknesses from time lapse surveys. A series of synthetic forward models were used to test the robustness of our inverse approach and to quantify uncertainties. In the absence of ambient noise, this approach can unambiguously resolve layer thickness. If a realistic ambient noise distribution is included, layer thicknesses of 1–6 m are accurately retrieved with an uncertainty of ±0.5 m. We used this approach to generate a thickness map of the shallowest layer. The fidelity of this result was tested using measurements of layer thickness determined from the 2010 broadband seismic survey. The calculated volume of CO2 within the shallowest layer increases at a rate that is quadratic in time, despite an approximately constant injection rate into the base of the reservoir. This result is consistent with a diminished growth rate of the areal extent of underlying layers. Finally, the relationship between caprock topography and layer thickness is explored and potential migration pathways that charge this layer are identified.

Benchmarking of vertically-integrated CO2 flow simulations at the Sleipner Field, North Sea

“…Layer 9 sits at the top of the reservoir where the hydrostatic pressure is 8.2-8.9 MPa and temperature is 28.4-30.7 The existence of sub-vertical seismic chimneys described by Chadwick et al (2004) and by Cowton et al (2016) is consistent with vertical migration of CO 2 through the reservoir rocks. One major chimney correlates closely with the first observed accumulation of CO 2 in different layers.…”

“…that can be directly compared with the observed distribution obtained by analyzing seismic reflection surveys (Cowton et al, 2016). The geometry of the reservoir and its physical properties, for example the shape of the impermeable boundary along which CO 2 fluid is spreading, d(x, y), and the permeability, k(x, y), and porosity, φ(x, y), must be determined.…”

“…This restriction is a consequence of the limited vertical resolution since the thickness of a thin layer is difficult to seismically image. Recently, an inverse modeling technique has been developed for determining the thickness of thin CO 2 -filled layers by combining measurements of the amplitude of a reflection with small changes in two-way travel time between time-lapse surveys (Cowton et al, 2016). These authors applied this inverse method to each of the time-lapse seismic reflection surveys in order to accurately map the thickness of CO 2 -saturated rock within Layer 9 as a function of time.…”

The relaxation time for viscous and porous gravity currents following a change in flux

“…Previous work has shown that the seismic images of the spreading CO 2 plume may be successfully interpreted using the theory of axisymmetric gravity currents spreading in a porous medium (Bickle et al 2007). More recent processing of the seismic images of the spreading CO 2 plume also demonstrate that the flow is, to good approximation, unconfined and hence independent of depth of the aquifer (Cowton et al 2016). However, for projects in which injected CO 2 spans the depth of the aquifer the motion of ambient fluid (e.g.…”

A Low-Order Series Approximation of Thin-Bed PP-Wave Reflections

“…For some sedimentary basins, such as Williston Basin in America [5][6][7], Alberta Basin in Canada [8,9], SongLiao and Tarim Basins in China [10,11], lithologic reservoirs are composed of thin beds with the thicknesses below the vertical resolution limits [12]. Meanwhile, thin-bed problems are also faced in engineering exploration, such as monitoring CO 2 storage at Sleipner in the Norwegian North Sea [2,[13][14][15]. Therefore, seismic responses of a thin bed, which carry information of stratigraphic structure, lithology and pore fluid, are significantly important to crustal, exploration and engineering research.…”