High-Frequency (400 MHz) T2 Measurements Using a Custom-Built NMR Probe, Eagle Ford Shale, Gonzales and La Salle Counties, Texas

摘要

Nuclear magnetic resonance (NMR) has become an increasingly important tool for estimating porosity, permeability, and fluid characteristics in oil and gas reservoirs since its introduction in the 1950s. While NMR has become common practice in conventional reservoirs, its application is still relatively new to unconventional reservoirs such as the Eagle Ford Shale. Porosity and permeability estimates prove challenging in these exceptionally tight rocks where organic porosity and bound fluids are routinely below the detection limit and/or resolution of low-frequency (2 MHz or less) NMR. In response, a custom-built NMR probe has been constructed, capable of measuring 0.75-inch diameter, 0.45-inch length core plugs at a 400 MHz Larmor frequency. The NMR probe was validated by comparing high-frequency (400 MHz) data acquired in-house to low-frequency (2 MHz) data measured at a commercial laboratory. Twelve core plugs (0.75-inch diameter, 1-inch length) were cut from two Eagle Ford Shale subsurface cores located in Gonzales and La Salle counties, Texas. Low-frequency T2 distributions were measured twice: first after drying core plug samples in a vacuum oven and again after spontaneous imbibition with various brine solutions (deionized water, 8 wt.% KCl, or 17.9 wt.% KCl) for one week. For high-frequency NMR measurements, samples were trimmed to 0.45-inch lengths to fit inside the newly-built NMR probe, creating two sub-samples for each of the original core plugs. T2 distributions were first acquired “as-is” (e.g., without drying or saturation) then again after drying and spontaneous imbibition with the same brine solutions used in the low-frequency study. Qualitatively, high-frequency T2 distributions resemble low-frequency measurements; however, the absolute T2 values are routinely higher by one order of magnitude. The difference may be caused by data acquisition, data processing, enhanced fluid relaxation, magnetic field inhomogeneities, or some combination thereof. The project provides a proof-of-concept that T2 relaxation times can be measured in “conventional-sized” core plugs using 400 MHz NMR. Although limited in its outcomes, the study delivers promising results and elicits future research into utilizing high-frequency NMR spectroscopy as a petrophysical tool for unconventional reservoirs.