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.
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.
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.
Based on the pseudo-analytical equation of electromagnetic log for layered formation, an optimal boundary match method is proposed to adaptively truncate the encountered formation structures. An ...efficient integral method is put forward to significantly accelerate the convergence of Sommerfeld integral. By asymptotically approximating and subtracting the first reflection/transmission waves from the scattered field, the new Sommerfeld integral method has addressed difficulties encountered by the traditional digital filtering method, such as low computational precision and limited operating range, and realized the acceleration of the computation speed of logging-while-drilling electromagnetic measurements (LWD EM). By making use of the priori information from the offset/pilot wells and interactively adjusting the formation model, the optimum initial guesses of the inversion model is determined in order to predict the nearby formation boundaries. The gradient optimization algorithm is developed and an interactive inversion system for the LWD EM data from the horizontal wells is established. The inverted results of field data demonstrated that the real-time interactive inversion method is capable of providing the accurate boundaries of layers around the wellbore from the LWD EM, and it will benefit the wellbore trajectory optimization and reservoir interpretation.
Characterization of pore throat size distribution (PTSD) in tight sandstones is of substantial significance for tight sandstone reservoirs evaluation. High-pressure mercury intrusion (HPMI) and ...nuclear magnetic resonance (NMR) are the effective methods for characterizing PTSD of reservoirs. NMR T2 spectra is usually converted to mercury intrusion capillary pressure for PTSD characterization. However, the conversion is challenging in tight sandstones due to tiny pore throat sizes. In this paper, the linear conversion method and the nonlinear conversion method are investigated, and the error minimization method and the least square method are proposed to calculate the conversion coefficients of the linear conversion method and the nonlinear conversion method, respectively. Finally, the advantages and disadvantages of these two different conversion methods are discussed and compared with field case study. The research results show that the average linear conversion coefficients of the 20 tight sandstone core plugs collected from Yanchang Formation, Ordos Basin of China is 0.0133 μm/ms; the average nonlinear conversion coefficient is 0.0093 μm/ms and the average nonlinear conversion exponent is 0.725. Although PTSD converted from NMR spectra by the nonlinear conversion method is wider than that obtained from linear conversion method, the nonlinear conversion method can retain the characteristic of bi-modal distribution in PTSD.
Azimuthal electromagnetic logging while drilling (LWD), which is capable of providing accurate position information approaching bed boundary, has been widely applied in real-time geosteering. For the ...inversion speed and precision of azimuthal electromagnetic LWD data, the key lies in the selection of proper inversion model and corresponding optimization algorithm. In this study, we first simplified the complex three-dimensional (3D) inversion of data into a series of one-dimensional (1D) inversion problems by using the dimensionality reduction scheme. Then, the feasibility and inversion performance of various 1D inversion models and different optimization methods were investigated, and the best combination between the inversion model and inversion algorithm was also obtained. Numerical simulation results show that the selection of 1D inversion model is dominated by the thickness of targeted beds, whereas the determination of inversion algorithm relies on the total layers amount of the inversion model. For the formation with thickness larger than 4.0 m, the single boundary inversion model and gradient optimization method are recommended. When the bed thickness is between 1.0 m and 4.0 m, the two-boundary inversion model instead of the single-boundary one is needed to estimate upper and lower boundaries around the borehole. For the inversion of azimuthal electromagnetic LWD data of thin layers, the multiple-boundary inversion model and the Bayesian algorithm should be employed.
The compressive strength is very important for petroleum and other engineering studies. However, the effect of pore size and fluid distribution on the rock’s strength is not fully understood. We ...developed comprehensive research to study the controlling factors of the compressive strength based on low field nuclear magnetic resonance (NMR) measurements and pseudo-triaxial compression test for tight sandstones. The relationship between the compressive strength and the NMR obtained parameters are investigated completely, aiming for a better estimation of the compressive strength using the NMR data. The result shows that the rock’s strength is strongly controlled by the pore size distribution and the fluid existing state. Generally, the compressive strength is negatively correlated with the average transversal relaxation time, the movable water saturation, and the porosity, but positively correlated with the irreducible water saturation. The result reveals that the rock with larger pore radius and higher percentage of movable fluid is easier to reach the failure state. Further, the precision of the empirical model by multiple regression of the geometric mean of the relaxation time and the porosity is greatly improved compared with the model established by the brittle minerals, which is potentially to be use for geophysical prospecting when the NMR logging data is available.
Highlights
The first time to use NMR to characterize the compressive strength.
Pore size control on the rock mechanical property is investigated.
Empirical equation is established to predict the compressive strength.
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