Bachman, Henry (Schlumberger) | Rampurawala, Mansoor Ali (Schlumberger Oilfield Eastern Limited) | Valori, Andrea (Schlumberger Middle East SA.) | Ali, Farhan (Schlumberger Middle East SA.) | Nicot, Benjamin (Schlumberger) | Sangani, Pratik (Saudi Arabian Chevron PZ) | Denny, Tim (Saudi Arabian Chevron PZ) | Newton, Steve
Unconventional resources merit strong attention in the oilfield today. Great value is placed on understanding the amount and viscosity of the oil which in turn guides the production strategy. We demonstrate a workflow employed in Middle East carbonate reservoirs containing intermediate to heavy oil. The workflow combines dielectric and nuclear magnetic resonance (NMR) log data, and it is complemented by laboratory measurements. The applicable range of viscosities is tens to thousands of centipoises.
Integrating dielectric and NMR log data provides a more accurate determination of saturation and viscosity. The workflow does not require any change to the processing of the raw data and is simple to implement. We demonstrate the workflow using log data along with correlations from lab measurements on core and bulk fluids. The workflow solves three key challenges for heavy oil analysis.
The first challenge is the overlap of heavy oil and bound water NMR signals, compounded by a radial variation of saturations near the wellbore. Conveniently, the physics of dielectric and NMR measurement results in depths of investigation of 1 to 4 inches. Dielectric-based analysis provides the total water-filled porosity, which improves the NMR-based saturation and fluid characterization. The second challenge comes from the effect of restricted diffusion of water molecules in pores. We show that a restricted diffusion model is required to obtain accurate saturations and viscosity. The third challenge is the reduced NMR sensitivity to viscosity for heavy oil. Lab measurements on core and bulk oil samples provide the parameters required to link NMR to viscosity.
We describe a workflow based on dielectric and NMR log data for a more accurate determination of fluid saturations and oil viscosity. The workflow takes into account a) the combined dielectric and NMR data; b) the carbonate restricted diffusion effect inherent in the NMR response for carbonates; and c) laboratory validation and interpretation support using core plugs in wells where the dielectric data was not available. We use log data covering a range of heavy oil viscosities to demonstrate the workflow in four wells.