Formatted Title
In Situ NMR Measurements to Quantify Crude Oil at Bemidji Crude Oil Spill Site
Background/Objectives
Nuclear magnetic resonance (NMR) is a powerful and non-destructive technology that directly measures hydrogen nuclei in the formation fluids. In geophysics, NMR logging tools have been widely used for several decades for petroleum exploration in the oil and gas industry. More recently, smaller diameter, lower cost NMR logging systems have been adopted for groundwater investigations to directly measure hydrogeologic properties of native formation. Leaks and spills of crude oil and petroleum products are a major source of soil and groundwater contamination worldwide. NMR represents a useful tool for direct detection and monitoring of hydrocarbons (e.g., crude oil) and its mobilization and transport in the shallow subsurface. One of the methods to distinguish between hydrocarbons and water in a formation is diffusion-based NMR. This measurement is conducted in the presence of a strong magnetic field gradient and can differentiate and quantify formation fluids based on their diffusivity. Diffusion NMR measurements are nondestructive and can provide quantification and monitoring of hydrocarbon degradation. The primary objective of our project is to use a high-resolution Direct Push NMR (DP NMR) probe to define the presence and distribution of crude oil in shallow soils and related unconsolidated materials of the sough plume of the Bemidji crude oil spill site and correlate these findings with the Geoprobe optical image profiler (OIP) probe that detects NAPL based on fluorescence.
Approach/Activities
We used high-resolution DP NMR to detect and quantify the crude oil amendments at Bemidji crude oil spill site. We compared the DP NMR results with results obtained from OIP that uses ultraviolet (UV) or green (G) light source. To define the locations for DP NMR investigations we first conducted a series of OIP-UV and OIP-G logs within the south plume area. The OIP logs were used to evaluate the ability of the NMR probe to detect crude oil. These measurements are significantly quicker as compared to DP NMR and provided accurate co-localization of crude oil. After the locations were identified, we conducted a high-resolution DP NMR log (vertical resolution of 6 inch) to obtain parameters such as water content, relative porosity, and hydraulic conductivity. We collected a series of diffusion-NMR experiments along the log to quantify the crude oil at specific levels. We will present field results obtained using OIP, DP NMR, and diffusion-based NMR that were used to evaluate and quantify the NAPL crude oil at Bemidji site.
Results/Lessons Learned
We will present for the first-time diffusion-based NMR results obtained using small diameter and high-resolution DP NMR technology. Comparisons of NMR and HPT logs have shown a good correlation between high and low permeability zones. The OIP probe could detect NAPL above and below the water table that was associated with high fluorescence images. Diffusion NMR was able to directly detect and quantify the volumetric fraction of crude oil in-situ within the formation above and below water table.