A New Apparatus for Coupled Low-Field NMR and Ultrasonic Measurements in Rocks at Reservoir Conditions

ABSTRACT

Models which describe the effect of pore fluids on elastic wave propagation in rocks are the basis for quantitative reservoir analysis. Laboratory ultrasonic measurements conducted on rock cores are used to develop and tune these models, which requires inputs such as fluid saturation and distribution, pore aspect ratio, wettability and fluid viscosity. Hydrogen (¹H) Nuclear magnetic resonance (NMR) is a technique that can be used to quantitatively describe some of these important parameters. Here we report on the design, construction and performance of a novel NMR-compatible rock core holder system allowing for the measurement of both ultrasonic P-wave velocities and NMR relaxation parameters in rock cores at reservoir pressure. Successful validation against a conventional benchtop ultrasonic measurement system was performed, whilst sequential NMR and ultrasonic measurements were demonstrated on a sandstone rock core at reservoir pressure as a function of variable brine/supercritical CO₂ saturation. This new apparatus represents the first coupled NMR and ultrasonic measurements of rocks at high temperature and pressure, and allows for a new approach to study pore scale saturation effects on elastic wave propagation in rocks.

INTRODUCTION

Low field Hydrogen (¹H) Nuclear Magnetic Resonance (NMR) is an important tool in petro-physics and is readily used to determine important reservoir parameters such as porosity, permeability, fluid type and saturation. Measurements are made both in situ in reservoirs using well logging tools, and ex situ on recovered reservoir plugs using benchtop spectrometers (Dunn et al. 2002). For quantitative assessment, low-field NMR employing magnetic field strengths less than 0.5 T have been shown to be more robust than higher fields as the impacts of internal magnetic field gradients are minimized (Straley et al. 1997; Mitchell et al. 2010a). Furthermore, due to the technological challenge of operating well logging tools on long wire lines in harsh reservoir conditions, only low magnetic fields (e.g. ∼<0.1T) are currently attainable (Mitchell & Fordham 2014).