There is a growing interest in characterizing the pore structure of reservoir rocks using the low field nuclear magnetic resonance technique. The transverse surface relaxivity is widely appreciated ...as the most significant parameter to connect the transverse relaxation (T2) time and the pore size by scientists. In this study, we reviewed long-established and recently developed methods to predict the surface relaxivity over the last few decades. Moreover, the advantages, shortcomings, as well as the applications of the methods were discussed. What's more, the potential causes of affecting the surface relaxivity such as mineralogical compositions, reservoir conditions, fluid properties, and magnetic field strengths were investigated to analyze the controlling factors of the surface relaxivity. Based on the review and analysis, we recommended a composited experiment to measure the surface relaxivity and proposed a possible workflow to obtain the variable surface relaxivity.
The review section shows that the surface relaxivity is generally calculated based on the specific surface area. There is no universally accepted method since each of the experiments only probes a part of the pore system or resolves pore size at a different length scale, and it is better to characterize the full-scale pore space using multi-scale experiments. In addition, the constant surface relaxivity may not hold true for heterogeneous reservoir rocks with wide pore size distributions and the presence of paramagnetic minerals. It is a trend to use more than one surface relaxivity according to petrophysical facies or pore types. Multiple regressions may also serve as an effective way to get variable surface relaxivities. Further work needs to be done to develop a comprehensive understanding of the surface relaxivity, especially the influence of the spatial distribution of paramagnetic minerals which are extensively distributed in unconventional reservoirs such as shale and coal.
•Conventional methods to quantify the surface relaxivity of reservoir rocks are reviewed.•Influential factors of the surface relaxivity are discussed and analyzed thoroughly.•Recommended methods of transforming relaxation time to pore radius are addressed.
Display omitted
•More excellent capability to reveal narrow peaks than traditional algorithms.•First introduced L1 penalty term to constrain the sparsity of the spectra.•The effectiveness, noise ...vulnerability are verified by simulations and experiments.•FISTA used to solve L1 regularization problems is effective and robust.
NMR relaxometry has been used as a powerful tool to study molecular dynamics. Many algorithms have been developed for the inversion of 2D NMR relaxometry data. Unlike traditional algorithms implementing L2 regularization, high order Tikhonov regularization or iterative regularization, L1 penalty term is involved to constrain the sparsity of resultant spectra in this paper. Then fast iterative shrinkage-thresholding algorithm (FISTA) is proposed to solve the L1 regularization problem. The effectiveness, noise vulnerability and practical utility of the proposed algorithm are analyzed by simulations and experiments. The results demonstrate that the proposed algorithm has a more excellent capability to reveal narrow peaks than traditional inversion algorithms. The L1 regularization implemented by our algorithm can be a useful complementary to the existing algorithms.
SUMMARY
Permeability is a critical factor in evaluating the fluid flow capacity and production performance of natural gas hydrate reservoirs. The similarity of electrical conduction and hydraulic ...flow makes it possible to predict reservoir permeability using electrical data. Clarifying the relationship between the permeability and resistivity of sediments with different hydrate growth habits contributes to the efficient exploration and development of natural gas hydrate resources. In this work, normalized permeability and the resistivity index models for grain-coating (GC) and pore-filling (PF) hydrates are developed based on the fractal geometry theory, forming a new relationship between normalized permeability and resistivity index. The empirical exponent is determined by fractal dimension. Meanwhile, we selected five sets of 3-D computed tomography (CT) images of quartz sand with different particle sizes, GC and PF hydrate digital rocks are constructed using random simulation methods. The numerical simulation of permeability and resistivity index is carried out, based on the pore microstructure images, the box counting method was used to calculate the fractal dimension and analyse the relationship between pore space and transport paths. Furthermore, the pore radius, throat radius and pore connection number are extracted through the pore network method to study the evolution of pore space. The results show that the tortuosity fractal dimension is a critical parameter in the relationship between normalized permeability and resistivity index. The proposed analytical expressions are validated by laboratory and well log data, and the exponent ranges cover existing hydrate permeability–resistivity index data. The models provide the possibility to predict the normalized permeability of hydrate reservoirs based on electrical data alone.
The static rock elastic parameters are important in the petrophysical evaluation of unconventional reservoirs since many of them need the fracturing technology for development. However, it often ...fails to establish models of the static rock elastic parameters through geophysical well logging data. We developed a preliminary research to explore the relationship between the static elastic parameters and the low field Nuclear Magnetic Resonance (NMR) parameters based on joint measurements of the NMR responses and rock mechanical properties. The geometric mean, the arithmetic mean of the transversal relaxation time, the cutoff value, as well as the bin porosities are considered to investigate their relationships with the static rock elastic parameters. The result revealed that the static Young's modulus is strongly correlated with the NMR parameters, whereas the static Poisson's ratio is slightly influenced by the pore size properties. This study provides a new perspective in the application of the low field NMR data.
The porous and low-permeability characteristics of a shale gas reservoir determine its high gas storage efficiency, which is manifested in the extremely high breakthrough pressure of shale. ...Therefore, the accurate calculation of breakthrough pressure is of great significance to the study of shale gas preservation conditions. Based on a systematic analysis of a low-field NMR experiment on marine shales of the Longmaxi Formation in the Sichuan Basin, a shale gas breakthrough pressure determination technique different from conventional methods is proposed. The conventional methods have low calculation accuracy and are a tedious and time-consuming process, while low-field NMR technique is less time-consuming and of high accuracy. Firstly, the NMR T2 spectrum of shale core sample in different states is measured through low-field NMR experiment. The NMR T2 spectra of sample in water-saturated state and dry state are combined to model the mathematical relationship between shale gas breakthrough pressure and NMR T2 spectrum. It is found that the gas breakthrough pressure is power-exponentially related to the geometric mean of NMR T2 spectrum and positively related to the proportion of micropores. Accordingly, the shale gas breakthrough pressure is quickly and accurately calculated using continuous NMR logging data and then the sealing capacity of the shale caprocks is evaluated, providing basic parameters for analyzing unconventional hydrocarbon accumulation, preservation and migration. This technique has been successfully applied with actual data to evaluate the sealing capacity of shale caprock in a shale gas well in the Sichuan Basin. It can provide a good basis for the evaluation and characterization of shale oil and gas reservoirs.
We conducted comprehensive numerical simulations to probe the low field nuclear magnetic resonance (NMR) relaxation of the clay-rich shale in inhomogeneous magnetic field. Under the guidance of the ...Bloch equation, the relaxation property of a unimodal distributed clay-rich shale is simulated using the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. Both the magnetization and the transverse relaxation time (T2) spectrum are obtained to investigate influential factors such as the excitation pulse angle, the refocusing pulse angle, the pulse duration, the echo spacing, as well as the off-resonance frequency. The simulation result showed that the field inhomogeneity contributes great influence on the NMR relaxation, particular in high magnetic field strength and high off-resonance frequency. We observed that in standard CMPG pulse sequence, the amplitude of the T2 distribution is negatively correlated with the off-resonance frequency and can be compensated by the empirical regression. Moreover, the excitation pulse angle poses great impact on the amplitude of the T2 distribution, but has less influence on the T2 distribution and its peak value. On the contrary, both the amplitude and the peak value of the T2 spectrums are also impacted by the refocusing pulse angle, especially in higher magnetic field inhomogeneity. The refocusing pulse angle is preferable higher than 150° for the clay-rich shale since it helps the spins to rephrase quickly. In additional, the echo spacing is ideally reduced to as low as its minimal value for the measurement of samples with short relaxation component. The result provides comprehensive knowledge on influential factors of the NMR relaxation of the clay-rich shale, which is useful for the optimization of the pulse sequence, the acquisition conditions, as well as the manipulation of the magnetization data.
•The magnetization evolution is developed by the matrix rotation method.•NMR responses of the tight sand at representative magnetic fields are simulated.•Joint effect of the field inhomogeneity and the pulse sequence are investigated.
We monitored the gas hydrate through low‐field nuclear magnetic resonance measurement. An observed decrease of the relaxation time (T2) intensity corresponds to the formation process, whereas an ...increase of the intensity corresponds to the dissociation process. The right domain of the spectrum with T2 larger than 10 ms disappears gradually with the formation time, whereas the left domain with T2 smaller than 1 ms remains invariant, indicating the gas hydrate forms preferentially in larger pores. In addition, the right domain increases rapidly with the dissociation time, revealing that the gas hydrate preferentially decomposes in large pores. The spectrum distributions move toward the fast relaxation domain with the growth of gas hydrate, because the generated gas hydrate occupies the large pore and accelerate the relaxation rate. There is no obvious relationship between the gas hydrate saturation and the porosity, whereas the volume and preliminary dissociation ratio are strongly correlated with the porosity.
Key Points
An equipment to form the methane gas hydrate in porous rock is developed
Low‐field NMR responses of the gas hydrate bearing samples are measured and analyzed during the formation and dissociation processes
The formation and dissociate behaviors and habits for gas hydrate bearing samples are investigated
We developed a numerical simulation algorithm to explore the nuclear magnetic resonance (NMR) response of the porous media based on the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence and the Bloch ...equation. The evolution of the magnetization vector of two representative pores at different pulse properties, including the excitation angle, the refocusing angle, the phase angle, as well as the pulse duration are simulate to understand the NMR relaxation signals. The result showed that the normalized magnetization is symmetrical with the excitation angle and positive with the T2 spectrum's amplitude when the excitation angle is less than 90°. In additional, the refocusing angle has no clear influence on the NMR response. The phase angle of the excitation pulse is inversely correlated with the echo amplitude and can be neglected when the value is lower than 15°. The phase angle of the refocusing pulse causes the zig-zag phenomenon, but the response of the even echoes is not disturbed. Moreover, the influence of the pulse duration should not be neglected at higher values, particularly for the mesopore. The simulation results are helpful for the design and optimization of the pulse sequence, and the data manipulation of the measured signals.
•The magnetization evolution under the CPMG pulse sequence is established.•The NMR responses of reservoir rock is simulated and analyzed.•The influences of the pulse sequence properties on the NMR relaxations are investigated.
