Anisotropic velocity changes in seismic time‐lapse data

Abstract

Reservoir production causes subsurface deformations and changes in seismic velocity. These deformations and velocity changes can be monitored using time-lapse seismic data. A fundamental challenge in the interpretation of time shifts observed in time-lapse data is the decomposition of the time delay into a spatial compaction component and a velocity change component. Several authors (Hatchell and Borne, 2005; Janssen et al., 2006) have published the application of pragmatic linear relationships between overburden stretching and velocity changes which, have proved applicable in a wide range of geological settings. In this study, we predict velocity changes through coupled reservoir and geomechanical modeling of the subsurface stress state and subsequent application of a rock physics model that relates changes in subsurface stress and strain to velocity changes. We expand on previously published work by investigating the anisotropy of the velocity changes and relate these to field observations of offset-dependent timelapse time-shifts from the South Arne Field. Our modeling shows that in the overburden, vertical seismic velocities decrease over time, causing traveltimes in a monitor survey to increase compared to traveltimes in a base survey. We furthermore predict that horizontal seismic velocities in the deep overburden increase and for seismic waves propagating at intermediate angles (approx. 20°-30°), the velocity changes are minimal. This suggests, that time-shifts between base-and monitor surveys are largest for zero-offset data and will gradually decrease as a function of offset. We test this prediction on a 4D field data set at South Arne, North Sea. For zero-offset data, we find a maximum traveltime increase in the overburden of 6ms. Time-shifts show a dependence on observation angle, with large offset time-shifts in the overburden being up to 50% smaller than near-offset time-shifts.

Review Paper: A semi‐empirical model of strain sensitivity for 4D seismic interpretation

“…In practice, lateral variations must imprint upon the trends we predict from anisotropy through equation (33). Our result is consistent with previous work by Herwanger et al (2007) who established through numerical modelling that a decrease with offset could be achieved using an anisotropic velocity variation calibrated by the third-order coefficients on Prioul et al (2004). Here, it is observed that the time-shift gradient is determined predominantly by the first term containing g = (v S /v P ) 2 , and to a lesser degree by the second term containing g and B T /B N .…”

“…Our result is consistent with previous work by Herwanger et al . () who established through numerical modelling that a decrease with offset could be achieved using an anisotropic velocity variation calibrated by the third‐order coefficients on Prioul et al . ().…”

Modeling and analysis of compaction-induced traveltime shifts for multicomponent seismic data

“…The magnitudes of the two-way traveltime shifts shown in Figures 8, 9, 10, and 11 are higher than those typically observed in field data (Guilbot and Smith, 2002;Hatchell and Bourne, 2005;Herwanger et al, 2007;Hodgson et al, 2007;Rickett et al, 2007). Apart from differences due to the use of shot records rather than poststack migrated data, these disagreements may be caused by integration of rock physics and empirical data in the geomechanical modeling.…”

“…Previous research based on geomechanical and seismic methods includes two main directions. One of them focuses on zero-offset time-lapse data governed by vertical stress/strain (Landrø and Stammeijer, 2004;Hatchell and Bourne, 2005;Roste, 2007), while the other considers a full triaxial stress field (Herwanger et al, 2007;Sayers and Schutjens, 2007;Scott, 2007;Herwanger and Horne, 2009;. Whereas the approach based on vertical strain helps evaluate compaction limits, horizontal and shear stress/strain are also induced during production, generating a heterogeneous, anisotropic velocity field inside and around the reservoir.…”

Sensitivity of compaction-induced multicomponent seismic time shifts to variations in reservoir properties

“…For this study, reflectors A, B, and C ( Figure 2) are used to measure time shifts of P-, S-and PS-waves for a range of pressure drops and to evaluate the linearity of the pressure dependence of time shifts. P-wave time shifts in Figure 4 are somewhat larger than typical values measured in the field (Guilbot and Smith, 2002;Hatchell and Bourne, 2005;Herwanger et al, 2007;Hodgson et al, 2007;Rickett et al, 2007;Staples et al, 2007;De Gennaro et al, 2008). Smaller values of the third-order stiffnesses C αβγ , or a decrease in the effective stress coefficient (α) at larger pressure drops will reduce the magnitude of modeled time shifts.…”