Volumetric bounds on subsurface fluid substitution using 4D seismic time shifts with an application at Sleipner, North Sea

Abstract

A method is presented for the volumetric estimation of subsurface fluid substitution based 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 fluid mixing to patchy fluid mixing. 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, fluid mixing and layer thickness has a severe impact on the convergence of these mass bounds. That is, fluid substitution scenarios with patchy fluid mixing and roughly linear velocity-saturation relationships allow for more accurate fluid quantification than scenarios associated with uniform mixing. Application of the method to two 4D seismic monitor data sets from Sleipner results in CO 2 mass bounds that are consistent with the true injected masses of CO 2 . Moreover, a linear relationship between progressively developing 4D time-shifts and known injected CO 2 mass is observed, suggesting that the evolving patterns of fluid saturation and fluid mixing in the CO 2 plume have remained roughly constant with time.

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

“…On the 2006 vintage, they get saturations ranging from 20% (deeper layers) to 90% (870 m deep layer), but P-wave velocities are very low, approximately 1200 m∕s. Boait et al (2012) and Bergmann and Chadwick (2015), using volumetric estimations, derive similar conclusions about the fluid distribution; i.e., the saturation is most likely uniform in high saturation zones and following patchy mixing laws elsewhere. Golding and Huppert (2011) agree with Queißer and Singh (2013) that saturation is probably greater than 30%.…”

“…Mohapatra et al (2012) conduct laboratory experiments on partially saturated sandstone (gas-brine, oil-brine, and CO 2 -brine) and show that the P-wave velocities and impedances are in good agreement with laboratory measurements except for liquid CO 2 flooding. Usually, Gassmann fluid substitution (Gassmann, 1951) combined with effective fluid bulk modulus (calculated by averages) are used for estimation of saturation from seismic velocities or time shifts (Bergmann and Chadwick, 2015). Otherwise, the Gassmann equation is a zero-frequency approximation and the computation of the effective fluid bulk modulus strongly depends on saturation distribution.…”

The first post‐injection seismic monitor survey at the Ketzin pilot CO2 storage site: results from time‐lapse analysis

“…() are affected by considerable uncertainties related to the choices of input parameters (Ivanova et al . ; Bergmann and Chadwick ; Ivandic et al . ).…”

“…In order to compute the range of possible CO 2 masses in the layer, Bergmann and Chadwick () proposed the generalised velocity–saturation relationship that is parameterised by the patchiness parameter p and the velocity change ∆v . The velocity change is given by the difference of the velocities for full CO 2 saturation, v 2 = v , and full brine saturation, i.e., ∆v = v 2 – v 1 .…”

“…Set of velocity–saturation relationships computed from equation using a variable patchiness parameter (marked graphs). Figure modified after Bergmann and Chadwick ().…”