Abstract
In this work we calculate the gravity signature of small density changes in a real-case deep reservoir. Based on the 3D forward density modeling of constrained geometries and using parameters of the involved rocks and fluids, we compute the differential gravity signature before and after the production period of the Volve oil field in the North Sea. Causative sources of the retrieved residuals are spatially correlated with positions of the most productive wells, locating areas of maximum density change. Results show that the 4D gravity forward model is capable of resolving residual gravity signatures also for deep and small density changes. In particular, we locate ~-13 𝜇Gal gravity minima over the 2750 m deep reservoir, this minima was caused by the -53 kg m-3 density change related to production and injection activity.
Highlights
Using gravity data for sub-surface modeling of geological structures may represent a useful tool to address open questions about geological or geophysical processes at the crustal or local scale
Despite the results of the pre- and post-production, forward models are extremely similar in the way that it would not be possible to qualitatively locate any local undulation; the difference between these two gravity data highlights a ∼−13 μGal (−130 × 10−9 m s−2) minimum centered at 435256 E, 6478496 N (ED50, UTM 31N)
We successfully locate the causative source for −53 kg m−3 density changes due to fluid production from a reservoir by 3D forward calculation of gravity at pre- and post-production time steps
Summary
Using gravity data for sub-surface modeling of geological structures may represent a useful tool to address open questions about geological or geophysical processes at the crustal or local scale. It should be noted that in all these case histories, the top of the gravity source was always above 2500 m depth. The depth of the source is the most critical parameter affecting the resolvability of a gravity anomaly (e.g., Blakely, 1996). In the case of CO2 storage, the depth of 2500 m is considered a threshold between shallower ( resolvable) and deeper (very difficult to resolve) field applications (Cooper et al, 2009)
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