Distinguishing Shale from Water Utilizing Extra Deep Azimuthal Resistivity and Inversion Technology for the Grane Field

Extra Deep Azimuthal Resistivity logging (EDAR), while drilling (LWD) enables detection, measurement and visualization of the reservoir architecture and multiple bed boundaries up to 30m (100ft) distance from the wellbore. Whilst this technology successfully allows the user to understand reservoir architecture, heterogeneity and fluid contacts, it is challenging to distinguish shale from water due to very similar resistivities.

On the Grane field, the Heimdal reservoir sand is injected into the Lista shale resulting in irregular reservoir base and roof topographies. The reservoir sections are planned with respect to minimizing the risk of drilling into shales and faults as well as to optimize drainage of initial and slumped oil in the area. The Lower Lista has a resistivity of 0.8–1.1 Ohmm, and the water in the ‘slumped Oil’ or produced oil zone has a similar resistivity range. Identifying and thus avoiding the Lower Lista shale is challenging and critical for the success of the well. Because shale resistivity is anisotropic, meaning the resistivity value is not equal in all directions, we can use this to distinguish shale from water, which is isotropic.

This case study illustrates that by using EDAR supported by multi-component inversions, how it was possible to ascertain the anisotropy value for each resistive layer thus identify and shale from water. This approach can differentiate between water and shale up to 5m TVD above the conductive boundary, and then used to optimally place the well and accurately predict the shale entry and exit.