In general speaking, the porosity obtained by nuclear magnetic resonance (NMR) measurement is independent of lithology. However, when the matrix has paramagnetic impurity, such as hematite which is one of the paramagnetic mineralogy of the matrix in volcanic rock, the NMR measurement results will be influenced by the internal magnetic field induced by the matrix paramagnetic mineralogy. In the paper NMR transverse relaxation time T2 of the volcanic breiccia samples have been measured with different echo spacing. The experimental results show that T2 measurement is strongly dependent of echo spacing because of internal magnetic field induced by paramagnetic impurity of the matrix, and it also has an obvious impact on NMR porosity.
Volcanic breccia is one kind of the volcanoclastic rock, which is mainly composed of the volcanic schiefer with the diameter exceeding 4mm, and widespread in the earth. We pay much attention to study the physical characteristics of the volcanic breccia because oil and gas has been founded in igneous reservoir in several basins in China in the recent years, such as Junggar Basin, Songliao Basin and so on. The main characteristics of the volcanic breccia are much dense, low porosity and low permeability, and complicated double pore configuration mainly composed of the vug and fracture. It is hard to identify and evaluate pore fluid of the volcanic rock because of complex well logging response.
The porosity from neutron well logging, density well logging and acoustic well logging often influenced by the matrix properties, furthermore the pore fluid must be known for porosity calculation. And it is still difficult to get a real porosity with the above well logging data. NMR well logging response mainly comes from the hydrogen proton of the pore fluid, and it can obtain porosity independent of matrix (Kleinberg et al, 1994). At present NMR well logging with T2 relaxation time is widely used in fluid typing, reservoir parameters calculation such as porosity, permeability, saturation and so on. And it takes an important role in formation evaluation.
The magnetic susceptibility contrast between the matrix and pore fluid will induce the internal magnetic field if the matrix contains the paramagnetic impurity, and give rise to the obviously effect on NMR measurement (Foley, 1996; Bergman et al, 1996; Hurlimann, 1998; Zhang G Q, et al, 1998 and 2001; Anand, et al, 2005 and 2007). Scott (1995) studied the internal field of sandstone with iron and glauconitic using NMR core and log measurement, and the results imply that the internal field will reduce the T2 measurement. Lattorraca and Dunn (1995) investigated the magnetic susceptibility contrasts effects on T2 logging, and they show that local field fluctuations will cause increase in the T2 relaxation rate. Zhang G Q and Hirasaki (1998 and 2001) have theoretically and experimentally investigated the internal gradient on T2 measurement, and the results show that the internal magnetic field of the rock is very complicated. Anand and Hirasaki (2005 and 2007) have studied the internal field on NMR measurement, and brought forward a new relaxation mechanism called "secular" relaxation.
