A novel pulse sequence for shale characterization by low-field NMR

In recent years, unconventional reservoir, such as gas shale and oil shale, is gaining increasing exploration interests. The NMR response of shale differs significantly from conventional reservoir as it contains kerogen in the matrix and viscous bitumen in the pore. In 2014, Washburn applied binomial-edited CPMG(BE-CPMG) pulse sequence to shale characterization for the first time. ^[1] Magnetization transfer among solids, semi-solids and liquids was capitalized to provide an indirect method of detecting solid organic materials in shales.
Due to the presence of short relaxation components in shale, large echo time in CPMG results in the signal loss of short relaxation components. ^[2] In addition, the limit of electronic circuit, especially for NMR well logging tools, results in relative long damping time which poses another challenge to the detection of short relaxation signal. Free induction decay, which is acquired almost immediately after the application of 90 degree pulse, have the inherent advantage of accurately measuring short T₂ components. Therefore, Binomial-edited FID (BE-FID) pulse sequence is proposed here to better detect solid organic materials in shales both in laboratory and downhole. Combined with FID, BE-FID pulse sequence has the potential application for characterization of organic matter in shale.

Fig1.(a)CPMG;(b) BE-CPMG;(c) Contrast of the signals acquired with FID and CPMG; (d) Experimental data of shale sample acquired by FID and CPMG; (e) Schematic of BE-FID pulse sequence; (f) Experimental data of shale sample acquired by FID and BE-FID