This paper presents the first utilization of Logging While-Drilling Nuclear Magnetic Resonance (LWD NMR) and azimuthal resistivity inversion to characterize the Zubair reservoir in the North Kuwait. The Zubair formation is a complex sandstone reservoir. In general, in highly deviated wells, formation complexity causes polarization horns in the resistivity measurements. This effect is leading to both inaccurate resistivity values and related saturation calculations. Therefore, LWD NMR which is insensitive to polarization horns was used to accurately calculate saturation values. The accurate saturation calculation values in highly deviated well has been demonstrated in well-A. Subsequently, the NMR data were used to analyze the grain size distribution in the Zubair reservoir to be modelled and correlated to the lateral extension of the sand and shale bodies generated by azimuthal resistivity inversion.
The LWD NMR tool was deployed in the Dual Wait Time (DWT) mode enabling to differentiate between hydrocarbon and water. The reliability of the saturation profile was later on confirmed by the production testing run in Well-A. The LWD NMR saturation was compared with production testing from one layer of the Zubair formation. Grain size analysis from the LWD NMR was evaluated using the following parameters: T2 distribution, total and effective porosity, and saturation.
The LWD NMR saturation for Well-A was confirmed by the production test result. The initial production test from 1B_LCH layer in Zubair formation showed a 12% water cut, which after six months increased to 34%. The water increase was clearly observed from the LWD NMR saturation, which also showed two different saturation profiles in this 12-feet thick layer sand-body. However, this saturation differential was not clearly observed from the LWD propagation resistivity.
The production test confirmation from Well-A showed that the LWD NMR saturation profile was reliable and could be used in Well-A to validate potential low-resistivity pay zones.
Azimuthal resistivity inversion was performed by means of a newly developed algorithm which used the omni-directional and extra-deep LWD resistivity measurements of the Multi-Component-while-Drilling (MCWD) data. The algorithm is based on a 1D anisotropic layered model.
Based on LWD NMR saturation, Well-B was found to have three potential hydrocarbon-bearing intervals with low resistivity pay zones. These intervals had a gross thickness in the range from 15ft to 25ft. The inversion result in Well-B successfully showed the lateral continuity of the sand and shale layers. Some thin layers were seen clearly from the MCWD inversion but were not shown by the standard distance-to-bed boundary inversion algorithm.
Grain size analysis provides supporting geological evidence to help assess reservoir quality. LWD NMR saturation profile and grain size analysis, together with sand and shale lateral extension from azimuthal resistivity inversion, provide an integrated solution that can characterize a complex sandstone reservoir and improve the geological model.