Movable oil identification and viscosity estimation in Lower Fars heavy-oil reservoir: A case study. 9th Middle East Geosciences Conference, GEO 2010.

Reservoir fluid typing is one of the key parameters in well completion and field development planning. While the resistivity and nuclear logs provide basic information about fluid type, detailed but non-continuous fluid profiling is obtained from down-hole pressure-volume-temperature (PVT) sampling. The recent advancement in nuclear magnetic resonance (NMR) logging helps immensely for the continuous fluid identification. The Lower Fars Formation in Kuwait is a shallow unconsolidated sandstone reservoir containing heavy oil. The oil viscosity in the field varies from tens to thousands of centipoises both vertically and laterally. In-place PVT-quality fluid sampling with wireline formation testers in this low-pressure reservoir is quite challenging and time consuming. The deployment of advanced NMR logging technique was successful in identifying movable oil and providing a continuous oil viscosity profile. The presence of clay within heavy-oil sand affects fluid identification as the clay-bound water and heavy-oil NMR signals overlay and occur at fast relaxation domain. The standard diffusion method has poor resolution at early T2 domain and interpretation suffers from the effect of restricted diffusion. The advanced NMR logging tool provides measurement at multiple radial depths and the diffusion measurement is found useful in identifying movable oil in such environments. An integrated approach combining advanced NMR log with the nuclear and resistivity logs is used to identify mov-able oil and fluid-type variation and to estimate a continuous oil viscosity profile. NMR station measurements helped to enhance signal to noise ratio to increase confidence in log interpretation. The viscosity profile estimated using this approach correlates quite well with the PVT sample analysis available in the field. The next logical step is the optimization of workflow to produce consistent and more quantitative viscosity results, which may require lab NMR measurement of Lower Fars oil samples and core calibration.