Cross‐matching with interpreted warping of 3D streamer and 3D ocean‐bottom‐cable data at Valhall for time‐lapse assessment

Abstract

A B S T R A C TLegacy streamer data and newer 3D ocean-bottom-cable data are cross-matched and analysed for time-lapse analysis of geomechanical changes due to production in the Valhall Field. The issues relating to time-lapse analysis using two such distinctly different data sets are addressed to provide an optimal cross-matching workflow that includes 3D warping. Additionally an assessment of the differences between the imaging using single-azimuth streamer and multi-azimuth ocean-bottom-cable data is provided. The 3D warping utilized in the cross-matching procedure is sensitive to acquisition and processing differences but is also found to provide valuable insight into the geometrical changes that occur in the subsurface due to production. As such, this work also provides a demonstration of the use of high-resolution 3D interpreted warping to resolve the 3D heterogeneity of the compaction and subsidence. This is an important tool for Valhall, and possibly other fields, where compaction and subsidence (and monitoring thereof) are key factors in the reservoir management since the predominant observed production-induced changes are compaction of the soft, high-porosity chalk reservoir, due to pore-pressure reduction, and the resultant overburden subsidence. Such reservoir compaction could have significant implications for production by changing permeabilities and production rates. Furthermore the subsidence effects could impact upon subsea installations and well-bore stability. Geomechanical studies that have previously been used to model such subsidence and compaction are only constrained by observed surface displacements and measured reservoir pressure changes, with the geological overburden being largely neglected. The approaches suggested herein provide the potential for monitoring and assessment in three dimensions, including the probable heterogeneity and shearing, that is needed for full understanding of reservoir compaction and the resultant effects on the overburden to, for example, mitigate well-bore failures.

Time-lapse full-waveform inversion with ocean-bottom-cable data: Application on Valhall field

“…Pressure depletion of the highly porous rocks leads to significant reservoir compaction, which drives the production and induces the subsidence of the overburden structures (Barkved and Kristiansen, 2005). Significant 4D seismic time shifts due to reservoir compaction have been observed in a previous study by crossmatching of 3D streamer data collected in 1992 and 3D OBC data collected in 1998 (Hall et al, 2005). Acoustic impedance changes that reflect the depletion of the reservoir have been derived from amplitude differences by comparing marine streamer surveys in 2002 and 1992 (Barkved and Kristiansen, 2005).…”

“…Since 1998, OBC data have been collected in the Valhall field in the North Sea (Hall et al, 2005). A permanent OBC system was installed in 2003 to enable frequently repeated time-lapse surveys to help manage the field.…”

When geophysics met geomechanics: Imaging of geomechanical properties and processes using elastic waves

“…Although some studies have been relatively successful at matching the observations, in a general sense, and have allowed predictions about future displacements, e.g., [57,74], they cannot capture the full heterogeneity of the system. [16] observed differences in the travel-times from the surface to the reservoir level between two seismic images from different times during the production of hydrocarbons from a subsurface reservoir; such analysis has also been carried out more recently for other fields, e..g., [24,2,18], and increasingly since. Such an approach sheds new light on subsurface processes in terms of the combined effects of vertical displacements and strain-induced velocity changes, but the resolution of true 3D effects (shear and lateral displacements/strains, e.g., [79,74]) cannot be assessed properly as the analysis is just carried out in the vertical direction.…”

“…The proposed method is essentially a form of 3D digital image correlation, a method that is increasingly used in experimental mechanics (e.g., [85,84]). Figure 11 shows an example (from [24,19]) of the results from such an analysis applied to a compacting North sea chalk reservoir, in which the downward displacement of the top of the reservoir unit around different producing wells can be seen in the vertical component of the displacement vectors. The horizontal components, shown in the zoom, also indicate some structure associated with the compaction/subsidence bowls, although these are less well defined due to the lower level of variation along geologic horizons in the seismic images compared to in the vertical direction (roughly perpendicular to the geologic layering).…”

“…The analysis of [24,19] discussed above was for the same North Sea reservoir analysed for seismic anisotropy presented in the preceding section and the regions shown in Figs. 10 and 11 are approximately the same.…”