EMinar 3.10: Nigel Clegg - Ultra-deep 3D EM anisotropy inversion: Imaging complex fluid boundaries

Описание: Fluid contacts in mature oil fields are often complex, having undergone changes in terms of vertical depth shift and lateral variation because of years of production and injection. This can be exaggerated by depositional, structural, and lithological elements, which can influence the movement of fluids through the reservoir. Ultra-deep EM tools are deployed to identify the positions of formations and fluids relative to the well bore. Where low resistivity water filled zones may be present, low resistivity shales can be indistinguishable from these sands at distance.
One defining factor is anisotropy; inverting for resistivity anisotropy provides a potential tool to differentiate shales from water filled sand, without necessarily penetrating them. Clean water filled sand will be relatively isotropic compared to anisotropic laminated shale.
This presentation demonstrates how inverting for anisotropy as well as resistivity in 3D allows these zones to be identified and the impact that even very thin shale zones can have on the reservoir. In a Tri-lateral well placed in a complex turbidite reservoir, initial 1D inversions for resistivity clearly show a variable low resistivity contact both above and below the target reservoir. Above the well is the overlying shale caprock. Below the well, correlation with offset data indicates that this is the oil/water contact. However, this lower boundary is not confined to a single horizontal plane and has a more complex morphology than expected. 3D inversion for anisotropy shows a thin layer with high anisotropy bounding the oil/water contact, potentially explaining its morphology.

Material first presented at the SPWLA 63rd Annual Logging Symposium, Stavanger, Norway, June 2022.