Unconventional petroleum resources present in shales have recently seen increased development due to growing demand for energy worldwide, declining conventional petroleum discoveries, and improved technology for production. Shale reservoirs differ from more conventional reservoirs in both matrix composition and structure, tending to be low in porosity and ultralow in permeability, making traditional laboratory characterization methods difficult to apply. The noninvasiveness of nuclear magnetic resonance (NMR) makes it a favored technique for shale analysis. However, interpretation of NMR relaxometry results for shales is more complex than in traditional reservoirs. Surface relaxivity in conventional reservoirs is predominantly caused by the interaction of fluid molecules with paramagnetic impurities on pore surfaces. However, in shales, small pore sizes coupled with the presence of hydrogen-rich organic matter can result in more interactions. This article discusses different relaxation mechanisms that may be present in shales and situations where they are relevant. Surface relaxation in organic matter is likely caused by homonuclear dipolar coupling, not paramagnetic impurities, and as such, will have a significant temperature dependence. Due to the small pore sizes in shales, the effect of internal gradients on the signal appears negligible in almost all situations. Analysis of transverse relaxation resulting from organic solids by an inverse Laplace transform may lead to results that are anomalously short and overcall signal intensity. Diffusional coupling between the shale pores confounds interpretation of the NMR response as a pore size distribution. There also appears to be rapid exchange of magnetization between mobile and immobile phases in the samples. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part A 43A: 57–78, 2014.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/cmr.a.21302

Relaxation mechanisms and shales

Nuclear magnetic resonance (NMR) provides a powerful toolbox for petrophysical characterization of reservoir core plugs and fluids in the laboratory. Previously, there has been considerable focus on low field magnet technology for well log calibration. Now there is renewed interest in the study of reservoir samples using stronger magnets to complement these standard NMR measurements. Here, the capabilities of an imaging magnet with a field strength of 0.3 T (corresponding to 12.9 MHz for proton) are reviewed in the context of reservoir core analysis. Quantitative estimates of porosity (saturation) and pore size distributions are obtained under favorable conditions (e.g., in carbonates), with the added advantage of multidimensional imaging, detection of lower gyromagnetic ratio nuclei, and short probe recovery times that make the system suitable for shale studies. Intermediate field instruments provide quantitative porosity maps of rock plugs that cannot be obtained using high field medical scanners due to the field-dependent susceptibility contrast in the porous medium. Example data are presented that highlight the potential applications of an intermediate field imaging instrument as a complement to low field instruments in core analysis and for materials science studies in general.

Contributed Review: Nuclear magnetic resonance core analysis at 0.3 T

Fixed and pulsed gradient diffusion methods in low-field core analysis.

The inclusion of diffusion information with relaxation information has the potential to make contributions to the interpretation of NMR well logs. The results of NMR experiments can be displayed as a two-dimensional map or distribution of intrinsic diffusion coefficient (D) versus transverse relaxation time (T 2 ) called a D-T 2 map. These maps have already been proven useful as tools for fluid characterization, but there are other benefits to employing D-T 2 maps for interpretation. Divergence of the measured diffusion coefficient from bulk fluid values can give information about the system in which diffusion is occurring. For example, diffusion coefficient values above the bulk fluid diffusivity indicate the presence of internal field gradients, while values below bulk suggest the presence of restricted diffusion. The fluid characterization available with diffusion information also allows the observation of wettability alteration. When an oil phase relaxes and diffuses as bulk oil, the response will appear on the line corresponding to the correlation of diffusivity and relaxation time for hydrocarbons. Departure of the oil response from this line to shorter relaxation time is usually due to surface relaxation of the oil phase, indicating a change in wettability from water-wet conditions. This change can also be indicated by the water phase relaxation time shifting towards the bulk value, a phenomenon more easily monitored when oil saturations are low. The naturally oil-wet character of West Texas dolomites, as well as the wettability alteration due to oil-base drilling mud additives in Berea Sandstones, were both interpreted with Diffusion Editing NMR sequences. In these samples, the interpretation must be qualified with the condition that internal gradients will also result in departure of the oil response from the line for hydrocarbons. These examples will illustrate the process of generating and interpreting D-T 2 maps to observe internal gradients, multiple fluid saturations, and wettability changes.

NMR Diffusion Editing for D?T2 Maps: Application to Recognition of Wettability Change

Relationship between susceptibility induced field inhomogeneities, restricted diffusion, and relaxation in sedimentary rocks

We present a nuclear magnetic resonance (NMR) based study on a series of sandstone cores from a major reservoir in the Norwegian Sea. The cores have varying amounts of chlorite and were prepared in different saturation states. NMR measurements were performed using the standard Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence and the new diffusion editing method that is designed to separate diffusion and relaxation effects. This procedure generally results in more reliable Sw values and we demonstrate here that it can also be used to derive an indicator of chlorite content. Since the measurement of diffusion editing can be performed with logging tools, this technique can be used directly in a reservoir for the improved determination of saturation and to estimate the chlorite content, with important implications for the assessment of reservoir quality. When the samples were saturated with a mixture of refined oil and brine it was generally difficult to separate the contributions of the two phases in the CPMG relaxation measurements. The relaxation time of the oil often overlapped significantly with the T2 distribution of the brine signal. To overcome this problem, we used the technique of diffusing editing to obtain simultaneously diffusion and relaxation information and its correlation. This was achieved by preceding the standard short-echo-spacing CPMG sequence by an editing sequence that attenuates the amplitude of the signal according to diffusion in the applied gradient. In the current work, we implemented the diffusion editing by increasing the first two echo spacings systematically. Relaxation information is obtained from the signal decay after the diffusion encoding. This effectively orthogonalizes the diffusion and relaxation information and allows the extraction of diffusion – relaxation distribution functions. These two-dimensional D – T2 maps can be used to extract information about important reservoir parameters such as water saturation, oil viscosity, wettability state and hydrocarbon-corrected bound-fluid volume. For the samples with low chlorite concentration, the diffusion – rela xation distribution functions clearly separated the signal into oil and water contributions. For samples with higher chlorite concentrations, the D – T2 maps showed an additional significant contribution at apparent diffusion coefficients in excess of bulk oil or water. In these

/pdf/sca2003-26-application-of-nmr-diffusion-editing-as-chlorite-4k1eo4sqqs.pdf

Sca2003-26: application of nmr diffusion editing as chlorite indicator

We present a nuclear magnetic resonance (NMR) based study on a series of sandstone cores from a major reservoir in the Norwegian Sea. The cores have varying amounts of chlorite and were prepared in different saturation states. NMR measurements were performed using the standard Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence and the new diffusion editing method that is designed to separate diffusion and relaxation effects. This procedure generally results in more reliable S w values and we demonstrate here that it can also be used to derive an indicator of chlorite content. Since the measurement of diffusion editing can be performed with logging tools, this technique can be used directly in a reservoir for the improved determination of saturation and to estimate the chlorite content, with important implications for the assessment of reservoir quality. When the samples were saturated with a mixture of refined oil and brine, it was generally difficult to separate the contributions of the two phases in the CPMG relaxation measurements. The relaxation time of the oil often overlapped significantly with the T 2 distribution of the brine signal. To overcome this problem, we used the technique of diffusion editing to obtain simultaneously diffusion and relaxation information and its correlation. This was achieved by preceding the standard short-echo-spacing CPMG sequence by an editing sequence that attenuates the amplitude of the signal according to diffusion in the applied gradient. In the current work, we implemented the diffusion editing by increasing the first two echo spacings systematically. Relaxation information is obtained from the signal decay after the diffusion encoding. This effectively orthogonalizes the diffusion and relaxation information and allows the extraction of diffusion - relaxation distribution functions. These two-dimensional D - T 2 maps can be used to extract information about important reservoir parameters such as water saturation, oil viscosity, wettability state and hydrocarbon-corrected bound-fluid volume. For the samples with low chlorite concentration, the diffusion - relaxation distribution functions clearly separated the signal into oil and water contributions. For samples with higher chlorite concentrations, the D - T 2 maps showed an additional significant contribution at apparent diffusion coefficients in excess of bulk oil or water. In these samples, the presence of chlorite gives rise to internal gradients in the adjacent pore space that exceeds the externally applied gradient. This leads to an increased diffusive decay that can be characterized by a large apparent diffusion coefficient. We found that the chlorite concentration in the sample is correlated with the fraction of signal that exhibits such large apparent diffusion coefficients.

Application of NMR Diffusion Editing as Chlorite Indicator

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Sca2003-26: application of nmr diffusion editing as chlorite indicator

Application of NMR Diffusion Editing as Chlorite Indicator