Carl M. Edwards

10. NMR Imaging of Fluids and Flow in Porous Media

Publisher Summary This chapter discusses the application of the techniques of nuclear magnetic resonance (NMR) and nuclear magnetic resonance imaging (MRI) to probe fluids and flow in porous media, for which the imaging methods and interpretations of image data are often complicated because of the effects that the solid matrix has on the fluid. Although the fundamental imaging principles used in biomedical imaging are generally applicable to imaging fluid flow in porous media, obtaining quantitative images sensitive to only one of the desired NMR parameters in porous solids requires special considerations. The challenges and experimental techniques developed to probe fluids and flow in porous media are discussed in the chapter, along with some applications of MRI to processes in porous media. NMR provides many exciting new opportunities for probing fluid states and flow within porous media. It is a noninvasive method that is sensitive to molecular-level events within fluids, and with imaging, fluid states and properties can be resolved spatially. While other techniques, such as optical, ultrasonic, and x-ray methods, can be used to visualize fluid saturations and flow patterns, nuclear MRI provides unique and effective methods for obtaining quantitative information on the distribution and flow of fluids within porous media. MRI can image nuclear spin density, NMR relaxation processes, chemical compositions, and the fluid transport processes of diffusion and flow.

Lab NMR Study on Adsorption/Condensation of Hydrocarbon in Smectite Clay

Significant amounts of gas accumulations exist in unconventional gas plays. Current understanding held that in unconventional shale plays, natural gas was stored as “free” gas in pore spaces and as an “adsorbed” phase on clay minerals and surface of organic pores material. The adsorption of methane has been confirmed in lab experiments in high-pressured gas chambers. Our lab experiments indicated that hexane vapor could be adsorbed onto organic-rich shale core samples through capillary condensation and the signal could be detected by Nuclear Magnetic Resonance (NMR) instruments. This study further examines the capillary condensation of hexane vapor into clay minerals and the NMR response. Smectite samples from the Clay Minerals Society were used in the experiments. Two types of capillary condensation experiments were conducted: one with water vapor and the other with hexane vapor, both at room conditions. Weight gains indicated that some of the vapor condensed in the loose powder of smectite clay. NMR experiments were performed on vaporsaturated samples using a Maran 2 MHz spectrometer with an inter-echo time of 300 μsec. The T2 distributions of the water-vapor and hexane vapor-saturated smectite clay were both unimodal. The water vaporsaturated sample showed a T2 at 0.5 ms, while the hexane vapor-saturated sample showed a T2 between 1 and 6 ms. This was likely due to the fact that the smectite crystallites have a small charge that has a more pronounced effect on polarized molecules such as water, than on non-polarized molecules such as hexane.

Laboratory Investigation of NMR Crude Oils and Mud Filtrates Properties in Ambient and Reservoir Conditions

We present laboratory NMR measurements of 445 crude oil samples, four mud filtrates and four base oil samples, and 47 refined oil samples. In addition to measurements at ambient conditions, a subset of the samples were measured at high temperatures and high pressures for the purpose of investigating the temperature, pressure, chemical structure, and oxygen-saturation dependences of the NMR T2 relaxation time. We found that the relaxation time, T2, correlates with specific gravity better than with viscosity, especially for heavy oils. Molecular oxygen dissolved in the oils affects the light oils significantly and if uncorrected, the oxygen-saturated T2 departs significantly from known T2 viscosity and temperature correlations. We propose an approximate method to correct the effect of dissolved oxygen based on experimental investigation of oxygen-free and oxygen-saturated samples. We further studied the pressure dependence of the crude oils, OBMF, and refined oils and found that even for dead oils, T2 depends on pressure and the dependence fits a quadratic expression. Finally, we investigated the chemical structure dependence of the refined oil samples; we found that the viscosity-T2 correlation is different for ring-structured hydrocarbons from linear-chain structured hydrocarbons. On the other hand, alkanes, alkenes, and esters share the same correlation. † Now with Performance Plastic Products, Houston, Texas ‡ Summer intern. Permanent address: Chemical Engineering Department, Rice University, Houston, Texas Introduction Nuclear Magnetic Resonance (NMR) wireline logging, LWD, and laboratory core and fluid measurements are valuable for characterizing rock and reservoir fluid properties. Successful interpretation of NMR logs for hydrocarbon typing applications requires reliable correlations between NMR measurements (i.e., relaxation times and diffusion) and fluid properties (e.g., specific gravity, viscosity and Gas-Oil-Ratio (GOR). To date, several such correlations have been published and used in the industry that relate NMR relaxation times/diffusion with the fluid viscosity, temperature, or GOR. Many of these correlations are based on data acquired at ambient temperature and pressure. None of the correlations take into account the effect of molecular structure. We have investigated 445 crude oil and a handful of oil based mud filtrates (OBMF) and base oil samples. The crude oils were originally collected from oil fields located in five continents; many of their physical properties such API gravity and viscosity were measured before we acquired the samples. The viscosity of these samples range from 0.25 cP to 2860 cP at room temperature. We have measured NMR relaxation times at ambient conditions for all samples. In addition, a subset of samples was studied at temperatures up to 130C and pressures up to 4000 psi under oxygen-saturated and oxygenfree conditions. Further, a handful of OBMF samples were studied for pressure and base-oil dependence. In addition, 47 pure and mixtures of refined oil samples were studied to elucidate the chemical structure dependence of the T2 vs. viscosity and temperature correlations. The present study investigates some issues of common concern that are essential to NMR fluid typing interpretation, forward modeling, and log planning. Firstly, we have developed a method to measure both the oxygen-saturated and oxygen-free oil samples, as well as a method to correct for the effect of dissolved oxygen on T2 data. We demonstrate that the correlation between T2 and viscosity departs from the theoretical expectation for light oils without this correction. The correlation, however, is much improved after the correction. The data were also used to derive correlations between T2 and the specific gravity as well as temperature/viscosity. We find that T2 data correlates better SPE 90553 Laboratory Investigation of NMR Crude Oils and Mud Filtrates Properties in Ambient and Reservoir Conditions Chen, Songhua, SPE, Zhang, Gigi, Kwak, Hyung, Edwards, Carl M., Ren † , Jason, Chen ‡ , Jiansheng, Baker Atlas, Houston, Texas, U.S.A.