Integrating Diffusion-Based NMR and Multi-Frequency Dielectric Measurements to Identify Moveable Heavy Oil Zones in Lower Fars Formation: A Case Study From Kuwait

Abstract

The Lower Fars Formation in Kuwait houses one of the largest accumulations of heavy oil in the Middle East. It is a very shallow, unconsolidated clastic reservoir of Miocene Age. Four discrete sand bodies are developed within this formation, having vertical and lateral variation in fluid properties. Identifying movable heavy oil zones is one of the key challenges in assessing the production potential of these reservoirs. Formation evaluation using conventional measurements fail to provide an accurate assessment of oil mobility. Downhole fluid sampling, which can provide detailed fluid properties, is quite challenging owing to difficulties in getting the heavy oil to flow and inherent operational complications. Further, fluid sampling provides point data which does not account for continuous variation in fluid properties. In order to identify movable heavy oil zones and optimize formation sampling, a workflow has been designed integrating diffusion-based nuclear magnetic resonance (NMR) and multi-frequency dielectric measurements. Conventional log analysis is performed to identify potential hydrocarbon bearing zones. Diffusion based NMR is then analyzed to identify tar zones. The NMR signal in tar decays too quickly to be detected by the logging tools resulting in a deficit in NMR porosity when compared to total porosity derived from conventional logs. Zones with significant deficit porosity are classified as tar bearing. Multi-frequency, multiple-spacing dielectric measurement allows determination of accurate flushed zone water saturation at two depths of investigation. Thus, in zones with shallow invasion, as is the case in viscous oil reservoirs, moveable hydrocarbon can be directly detected. Oil viscosity and permeability are derived by combining NMR and dielectric measurements. While the NMR log is deficient in distinguishing bound water and heavy oil signals, the accurate water-filled-porosity (PHIW) from dielectric log fosters accurate NMR fluid characterization. Zones to be sampled were then selected taking into account the borehole condition, presence of moveable hydrocarbon, permeability and viscosity. The workflow was tested on several exploration wells in Kuwait. The results allowed accurate zone selection to optimize sampling. Oil samples were collected from zones identified as bearing moveable heavy oil. Tar zones flowed only water in spite of pumping out for an extended period, validating the workflow.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015