NMR Considerations in Shales at Elevated Temperature

Abstract

The process of hydrocarbon production from unconventional resource shales begins with identifying zones in the well which have the most favorable features – in terms of fluid content, fluid flow and rock mechanical properties. Both in the oilfield and in the laboratory, Nuclear Magnetic Resonance (NMR) based techniques are used to characterize fluids in rock samples. Rock properties like fluid-filled porosity, fluid type, wettability, pore size distribution can be estimated using NMR. By comparing the laboratory results with the field data, the relationship between rock properties and production behavior can be better understood. Routine laboratory NMR measurements are made at room temperature while oilfield NMR tools capture measure data at reservoir temperature (usually over 100°C). The difference in temperature makes the direct comparison of lab and log data risky. While considerable literature exists on probing porous media with NMR at elevated temperature, no reference exists on how temperature impacts NMR measurements in shales. The present thesis summarizes 2 MHz NMR experiments made on shales and conventional reservoir rock samples at reservoir temperature (110°C) and one atmospheric pressure. Based on our results we outline key considerations in interpretation of 2 MHz NMR measurements in porous media at elevated temperature which include: 1) temperature correction of NMR porosity used for conventional rocks does not apply to shales. 2) mobilization of heavy hydrocarbon and adsorbed water at elevated temperature leads to increase in measured NMR porosity at elevated temperature in shales. 3) A revised equation is proposed to predict the NMR porosity of shales at elevated temperature