Petrophysical Measurements on Shales Using NMR

Abstract

Shales are important constituents of petroleum systems, and it is necessary to study their petrophysical properties as both reservoir components and as seals. Nuclear magnetic resonance has proven to be a good technique for measuring the reservoir engineering properties of rocks. This paper presents measurements of NMR relaxation in shales. It establishes that shale petrophysical information is accessible using standard NMR lab techniques employed in the oil industry. Even for good seals, shale porosity and pseudo-capillary pressure curves can be derived from NMR relaxation data. This opens the question as to whether NMR logging can be used to ascertain seal quality for oil and gas storage reservoirs and for CO2 disposal reservoirs. NMR measurements were performed in a low field spectrometer primarily on shales characterizing seals. Multiexponential T2 and T1 relaxation rates were determined from CPMG and inversion recovery experiments, respectively. Mean log T2 values of the undersaturated samples were about 0.4 ms. The T2 relaxation times increased after the shales were saturated but remained below 2 ms. Porosity for each sample was derived from the T2 magnetization and calibrated against a standard. Since NMR detects total porosity, the porosity was generally larger than that determined from laboratory gas flow-saturation techniques. The T2 relaxation rate distributions were normalized using the total magnetization calibration and integrated from the larger times to yield pseudo-capillary pressure curves. Introduction This study focuses on the applicability of the NMR to derive petrophysical properties to shales. Shales can be found in any of the constituents of the petroleum systems as seals, in reservoir rocks, or as source rocks. In any of these scenarios, it is fundamental to estimate porosity, permeability, fluid saturations, or capillary pressure. Common logging tools, such as resistivity, neutron and density logs, are sensitive to the presence of shales; therefore, NMR can be a valuable tool to study their properties1. A secondary objective of this investigation is to evaluate the capabilities of NMR in estimating seal quality of shales, which is fundamental to any exploratory or production study, as well as in environmental investigations, as for example the search of a well-sealed disposal aquifer for use in CO2 sequestration of residual gas from power plants. A more long-range objective might be to develop the ability to assess shale fabric while drilling by using a measurement-while-drilling NMR tool.

Experimental NMR Analysis of Oil and Water Imbibition during Fracturing in Longmaxi Shale, SE Sichuan Basin

“…Water Imbibition The relationships between NMR signals with T2 time smaller than 3 ms and water including clay-bound water, interlayer water and pore water remain controversial. The cutoff of T2 3 ms has been used to identify clay-bound water 54) 56) , but separate NMR signals of clay-bound water may be hard to identify because of coupling with the water within the clay size openings 57), 58) . As a result, interlayer water, clay-bound water and pore water will all exchange spins on the time scale of the measurement.…”

Micro-Structural Studies of Gas Shales

“…Surface relaxivities range from 0.00037 to .046 μm/ms for sands (Dunn et al 2002). NMR measurements on "as received" core have been performed by Howard (1991), Martinez and Davis (2000) and Dastidar (2007). Examples of T2 spectra are shown in Fig.…”

Pore Changes in Purple Mudstone Based on the Analysis of Dry-Wet Cycles Using Nuclear Magnetic Resonance

“…e T2 spectrum of pore water in a constant-temperature uniform magnetic field is mainly determined by ion surface relaxation [27,39], which mainly depends on the nature of the liquid and its affinity for the mineral surface or inner surface [40], so the quantification of pore water can be derived from the T2 spectrum. e T2 of bound water in porous materials is generally less than 3 ms, while the range for capillary water is 3-33 ms, while values >33 ms indicate a large amount of free water [41,42]. In this study, we used the above two critical values to divide the pore water component.…”