A region-partitioned nuclear magnetic resonance <i>T</i>2 spectra correction method in volcaniclastic reservoirs

Nuclear magnetic resonance (NMR) logging, one of geophysical exploration methods, is very sensitive to pore fluid and is promising in unconventional reservoir exploration. However, the volcaniclastic reservoir is characterized by tightness, low porosity, and the micro- and nano-pore development. A number of short relaxation signals acquired by NMR logging instruments may be lost, since the echo spacing is not sufficiently short for downhole NMR measurements. Additionally, paramagnetic minerals in volcaniclastic rocks, such as iron and manganese, increase the internal magnetic field gradient, enhancing diffusion relaxation of pore fluids, and thus leading to the complex relationship between T2 spectra and pore size. Based on the above factors, a region-partitioned T2 spectra correction method in volcaniclastic reservoirs was proposed in this paper. The T2 spectra were divided into short (T2 &amp;lt; 10 ms) and long (T2 ≥ 10 ms) relaxation parts. For short relaxation correction, the relationship model between the normalized interval porosity at TE = 0.6 ms and the normalized T2 spectra signal intensity at TE = 0.1 ms, as well as the relationship model between the signal intensity of the shortest relaxation peak at TE = 0.1 ms and TE = 0.6 ms, was constructed and employed. For long relaxation correction, the internal gradient Gint determined from T2–G spectra was utilized to calculate and remove diffusion relaxation from T2 spectra at TE = 0.6 ms. The proposed method's effectiveness is validated through simulations and field NMR data processing.