Magnetic Resonance Transverse Relaxation Time Research Articles

Fine quantitative characterization of high-H2S gas reservoirs under the influence of liquid sulfur deposition and adsorption

In order to clarify the influence of liquid sulfur deposition and adsorption to high-H2S gas reservoirs, three types of natural cores with typical carbonate pore structures were selected for high-temperature and high-pressure core displacement experiments. Fine quantitative characterization of the cores in three steady states (original, after sulfur injection, and after gas flooding) was carried out using the nuclear magnetic resonance (NMR) transverse relaxation time spectrum and imaging, X-ray computer tomography (CT) of full-diameter cores, basic physical property testing, and field emission scanning electron microscopy imaging. The loss of pore volume caused by sulfur deposition and adsorption mainly comes from the medium and large pores with sizes bigger than 1 000 μm. Liquid sulfur has a stronger adsorption and deposition ability in smaller pore spaces, and causes greater damage to reservoirs with poor original pore structures. The pore structure of the three types of carbonate reservoirs shows multiple fractal characteristics. The worse the pore structure, the greater the change of internal pore distribution caused by liquid sulfur deposition and adsorption, and the stronger the heterogeneity. Liquid sulfur deposition and adsorption change the pore size distribution, pore connectivity, and heterogeneity of the rock, which further changes the physical properties of the reservoir. After sulfur injection and gas flooding, the permeability of Type I reservoirs with good physical properties decreased by 16%, and that of Types II and III reservoirs with poor physical properties decreased by 90% or more, suggesting an extremely high damage. This indicates that the worse the initial physical properties, the greater the damage of liquid sulfur deposition and adsorption. Liquid sulfur is adsorbed and deposited in different types of pore space in the forms of flocculence, cobweb, or retinitis, causing different changes in the pore structure and physical property of the reservoir.

Evaluating Pore Structure of Shale Reservoirs through Pore Space Multifractal Characterization

Characteristics of pore structure are crucial to oil exploration and development in shale reservoirs, but practical evaluation of shale pore structure remains a longstanding challenge. In this study, based on nuclear magnetic resonance transverse relaxation time (T2) spectra under water-saturated and irreducible conditions, total pore space and moveable fluid space were skillfully determined. Meanwhile, multifractal spectra were used to characterize T2 distributions of total pore space and movable fluid space. The results show characteristic parameters of high probability measure areas of multifractal spectra (multifractal parameters) have strong negative correlations with T2 geometric mean value, and average radii of pore and throat increase with a decrease in the multifractal parameters, indicating the shale reservoirs with low values of the multifractal parameters may have good pore structure. Moreover, clay mineral content has strong positive effects on the multifractal parameters. In contrast, (quartz+feldspar) mineral content negatively influences on the multifractal parameters. Finally, new accurate models for permeability estimation in core scale and well-logging scale of shales are both proposed based on the multifractal parameters, which are also applicable for the other reservoirs. Overall, this study is important and timely for evaluating pore structure and predicting permeability in shale reservoirs.

Impact of storage time and temperature of salad heads on the quality of fresh-cut Cichorium endivia

The impact of temperature and storage time of escarole salad heads on the quality of processed, fresh-cut salad was investigated. Based on temperatures recorded in a fresh-cut processing plant, salad heads were stored for four different storage times (0, 5, 9, and 12 d) at four temperatures (4, 7, 10, and 12 °C) before being processed. A range of quality criteria (technical yield, global visual aspect, pink cut surfaces, mechanical texture, aerobic microflora, respiration rate, atmosphere composition in fresh-cut salad pouches, nuclear magnetic resonance transverse relaxation time) were measured on processing day, after 7 d storage at 4 °C of the fresh-cut salad packaged in pouches, and 1 d after pouch opening. The results are presented for salads grown in southeast France and early-season harvest, validated by experimental repetitions with a late-season harvest and salads from another geographical origin. Storage of salad heads caused deterioration of all quality attributes except for total aerobic bacteria. The global visual aspect was the most sensitive to changes in storage conditions of salad heads (significant reduction in quality for 5 d over 7 °C). In contrast, mechanical texture (maximum load for the shear test) was only significantly different for the fresh-cut salad prepared from salad heads stored for 9 d at 12 °C. For all quality criteria, fresh-cut salads processed from salad heads stored for 5 d at 4 and 7 °C were not significantly different from those of non-stored salad heads. For storage time of 5 d or less, a temperature of 7 °C is likely as good as 4 °C for escarole salad-head storage intended for fresh-cut processing.

Experimental Study of the Flow Mechanism of Deep Lacustrine Carbonate Reservoirs in the Yingxi Area of the Qaidam Basin

Many of the exploration and development theories derived for conventional clastic rock reservoirs are not applicable to pore-fracture lacustrine carbonate reservoirs. The fluid flow mechanisms under reservoir conditions are still unclear. Therefore, in this study, the rock samples were characterized using X-ray diffraction (XRD), porosity-permeability analysis, scanning electron microscopy (SEM), plain thin sections, and casting thin sections. The core samples were classified into two types (fractures and matrix pores) based on their reservoir spaces. The core flow experiments were performed under reservoir conditions using reservoir core plugs. The experimental results demonstrate that the cores, especially the fractured cores, have a strong stress sensitivity. The oil phase flow in the core has the characteristics of non-Darcy flow, and the threshold pressure gradient is 0.01–0.003 MPa/m. Additionally, for the oil-water two-phase flow in the fractured core, the water phase relative permeability of the residual oil is high. In contrast, the water phase relative permeability of the matrix core is less than 0.2. The nuclear magnetic resonance (NMR) transverse relaxation time ( T 2 ) spectra were used to analyze the differences between the water flooding characteristics of the two pore structures. The experimental results show that the peaks of the T 2 spectra after water flooding are lower than those before water flooding, and the matrix cores have a better oil displacement effect. The relaxation time of 0.1–10 ms makes the greatest contribution to the water flooding efficiency. The micropores smaller than 10 μm in diameter play an important role in the water flooding of the matrix core. These results will provide theoretical basis for solving the difficult problems of developing deep lacustrine carbonate reservoirs.

Revisiting movable fluid space in tight fine-grained reservoirs: A case study from Shahejie shale in the Bohai Bay Basin, NE China

Assessment of movable fluid is a key topic in production of tight formations. Evaluation of the movable fluid was usually conducted on tight sandstone formations while rarely on black organic-rich shales. To analyze the movable fluid properties in tight black shales, we document the basic geochemical and petrological features of Paleogene Shahejie shale and carry out quantitative analyses of the movable fluid properties. A modified method is proposed to estimate the pore size distribution. Results indicate that Shahejie shale is characterized by a low porosity and under-saturated of movable fluid. Further, porosity and saturation of movable fluid in shales are mainly determined by the heterogeneity of pore structure. Besides, micropores and mesopores with pore radius smaller than 6.69–18.25 nm (the lower limits are calculated for the collected samples based on the low-field Nuclear Magnetic Resonance (LF-NMR) transverse relaxation time ( T 2 ) spectra, i.e., the lower limit is different for each sample) provide substantial movable space for the fluid. Previously, relaxation rate that related to conversion from T 2 spectra of porous media to pore size distributions is often regarded as constant as well as thickness of film fluid that produces pore surface relaxation. However, our work indicates that both the relaxation rate and thickness of film fluid are variables as a function of the pore radius. This study enriches the application of movable fluid properties in tight reservoirs formations, especially shales, and lays a foundation for tight porous media research using LF-NMR in future. • Movable fluid saturation indicates the exploitable rate of reservoir. • Movable fluid porosity is used to calculate the exploitable degree of reservoir. • The newly proposed method first employs nonconstant ρ 2 and h. • ρ 2 and h increase predictably as pore radius increases.

A method to predict the resistivity index for tight sandstone reservoirs from nuclear magnetic resonance data

The relationship between water saturation and resistivity index in tight sandstone reservoirs cannot be simply expressed using the Archie equation. This makes the saturation exponent difficult to determine and the water saturation estimation significantly challenging. Based on fractal theory and the Archie equation, a theoretical power function relationship is used to predict the resistivity index using the nuclear magnetic resonance (NMR) transverse relaxation time. In this study, 36 core samples, which were recovered from tight gas sands of the Upper Triassic Xujiahe Formation in the central Sichuan Basin, southwestern China, were studied using laboratory NMR and resistivity experiments to verify the reliability of the proposed relationship. The results of this study show that this theoretical relationship is only effective for core samples that contain similar pore structures and physical properties. To precisely predict the resistivity index from NMR data in formations with complicated pore structures, these 36 core samples were classified into three types based on the pore structure and physical properties. For each type of core sample, the parameters used in this relationship were calibrated, along with the relationships between the water saturation and resistivity index and the saturation exponents. Finally, the predicted saturation exponents and the experimental results were compared and validated using two tight sandstone reservoirs located elsewhere in China. Using this proposed method, tight sandstone reservoir saturation exponents were predicted from NMR data. Combining the existing cementation exponent prediction technique, the indispensable input parameters in the Archie equation were acquired, and water saturations were accurately estimated in tight sandstone reservoirs.

Effect of CO2-brine-rock reactions on pore architecture and permeability in dolostone: Implications for CO2 storage and EOR

Geologic carbon sequestration (GCS) is considered a feasible technology for storing substantive volumes of greenhouse gases in subsurface geological formations. In the reservoir, far from carbon dioxide (CO2) injection wells or in post-injection scenarios, diffusion dominates over advection. This condition conjoins with spatially distributed geochemical reactions to induce heterogeneous changes in pore architecture, i.e. pore body and throat sizes or surface roughness. These changes can affect CO2 transport properties and storage capacity. In this work, we investigated mineral dissolution and precipitation in dolomite samples saturated with a CO2-saturated brine at 93 °C and 34.5 MPa, aged without flow. Two rock types samples, i.e. intergranular- and vuggy-dominant, were selected to investigate changes in pore size, porosity and permeability under reactive conditions. Mineral dissolution and precipitation were characterized using scanning electron microscopy. Changes in pore size were quantified via time-domain nuclear magnetic resonance (TD-NMR) transverse relaxation time (T2) and diffusion coefficient (D) distributions. We show that mineral dissolution likely occurs in highly permeable pathways. These observations are confirmed through analysis of (T2) and diffusion coefficient (D) distributions. In contrast to results during CO2-enriched brine continuous injection, mineral precipitation was observed in micropores. The leftward shift of the T2 peaks, corresponding to micropores, also evidenced mineral precipitation in low-permeability zones. However, microscale alterations resulted only in a subtle increase in porosity and permeability. Results in this study shed light on effects of geochemical reactions on alteration of rock properties in diffusion-dominated regions during CO2 storage.

A new method for predicting capillary pressure curves based on NMR echo data: Sandstone as an example

The mercury injection capillary pressure (MICP) curves have been widely used for the evaluation of the pore structure. In practical application, the MICP curves are usually obtained by the mercury injection experiment, but the experimental measurement can not obtain the MICP curves of the reservoir continuously. Since the pore size distribution obtained from the nuclear magnetic resonance (NMR) transverse relaxation time (T2) distribution is related to the pore-throat size distribution obtained from the MICP curve, so in previous studies, the MICP curves were predicted based on the NMR T2 distribution. However, the NMR T2 distribution obtained by the inversion of the NMR echo data has uncertainty, which affects the prediction accuracy of the MICP curves. In this paper, a new method for predicting the MICP curves of sandstone based on NMR echo data is proposed for the first time. Multiple characteristic parameters of the NMR echo data are calculated. The relationship between the parameters and the mercury saturation (Shg) is established at each capillary pressure point to predict the MICP curves. And the parameters of the MICP curves and the physical parameters are selected to create a reservoir classification index. The results show that the method for predicting the MICP curves of sandstone based on NMR echo data has high prediction accuracy and strong stability, and the reservoir classification index provides an accurate classification of sandstone reservoirs.

Research progress of <sup>10</sup>B concentration and distribution measurement in boron neutron capture therapy

Boron neutron capture therapy (BNCT) is a dual tumor radiotherapy method combining boron-10 containing drugs and heavy ion therapy, which can kill cancer cells with almost no damage to normal cells, and achieve precise cell-scale treatment. However, the BNCT does not really enter into clinical application stage though it has been developed for nearly 70 years. The methods of measuring the concentration distribution of boron-10 containing drugs in vivo cannot meet the clinical needs, which is one of the core problems affecting the effect and safety of clinical application of BNCT. This article reviews the current methods of measuring the concentration distribution of boron-10 drugs, including the invasive estimation methods that have been used in clinical practice, the researching single photon emission tomography (SPECT) method, positron emission tomography (PET) method, nuclear magnetic resonance (NMR) method, etc., and analyzes the advantages and limitations of current methods. Based on the characteristics of the low gyromagnetic ratio of the boron-10 element and the extremely short magnetic resonance transverse relaxation time, the research ideas of ultra-short echo time magnetic resonance boron-10 quantitative imaging are proposed. The feasibility of quantitative measurement of boron-10 distribution in vivo based on ultrashort echo time magnetic resonance imaging is analyzed theoretically.

Characterization of pore structure of tight sandstone reservoirs based on fractal analysis of NMR echo data

The evaluation of the pore structure of tight oil and gas reservoirs has great significance for the characterization of the reservoir properties and the prediction of reservoir productivity. Fractal analysis is an effective method commonly used to evaluate reservoir pore structure. In previous studies, the fractal dimension of nuclear magnetic resonance (NMR) transverse relaxation time (T2) distribution was calculated to evaluate reservoir pore structure. However, due to the low signal-to-noise ratio (SNR) of the NMR echo data in tight sandstone reservoirs, the T2 distribution obtained by inversion of the echo data with low SNR has great uncertainty, which affects the evaluation accuracy of reservoir pore structure. In this paper, the fractal dimension of the NMR echo data (D_ECHO) is calculated to evaluate the pore structure of tight sandstone reservoirs for the first time. Fast Fourier transform is performed with the NMR echo data and then the D_ECHO is calculated. We analyze the correlation between the fractal dimension and the physical parameters and pore structure parameters, the results showed that 1) compared with the calculation of the fractal dimension of the T2 distribution, the calculation of the D_ECHO has higher stability, which can avoid the influence of the inversion uncertainty of echo data with a low SNR on the evaluation of the reservoir pore structure; 2) the fractal dimension is closely related to pore structure parameters, D_ECHO are well suited for characterizing the pore structure of tight sandstone reservoirs. As the heterogeneity of reservoirs increases, the D_ECHO increases. Numerical simulations, core experiments and NMR logging data processing results verify the effectiveness of the proposed method.

Integration of NMR and Conventional Logs for Vuggy Facies Classification in the Arbuckle Formation: A Machine Learning Approach

Summary Diagenetic effects in carbonate rocks can enhance or occlude depositional pore space. Reliable identification of porosity-enhancing diagenetic features (e.g., vugs and fractures) is essential for petrophysical characterization of reservoir properties (e.g., porosity and permeability), construction of geological and reservoir models, reserve estimation, and production forecasting. Challenges remain in characterizing these diagenetic features from well logs as they are often mixed with variations in mineral and fluid concentrations. Herein, we explore a data-driven approach that is based on a comprehensive well log data set from the Arbuckle Formation in Kansas to classify vuggy facies in carbonate rocks. The available well log data include conventional logs (gamma ray (GRTC), resistivity (RT), neutron/density porosity (NPHI/RHOB), photoelectric factor (PE), and acoustic slowness) and nuclear magnetic resonance (NMR) transverse relaxation time (T2) logs. We parameterized the measured T2 distribution using a multimodal lognormal Gaussian density function and combined the resulting Gaussian parameters with conventional logs as inputs into three supervised machine learning (ML) algorithms; namely, support vector machine (SVM), random forest (RF), and artificial neural network (ANN). The facies labeling data used in this study were based on visual examination of vug sizes from core samples, which include five classes; namely, nonvuggy, pinpoint-size, centimeter-size, fist-size, and super-vuggy. In total, 80% of the data set was used as the training set, and a fivefold cross validation was used for hyperparameter tuning. We conducted a detailed comparison of the above three ML algorithms on the basis of different combinations of features. The highest classification accuracy achieved on the holdout testing set is 84% using SVM on a combination of conventional logs and selected NMR Gaussian parameters as inputs. In general, inclusion of conventional log data improves the prediction accuracy compared with using NMR data alone. Feature selection improves the performance for SVM and ANN but is not recommended for RF.

MRI T2 and T2* relaxometry to visualize neuromelanin in the dorsal substantia nigra pars compacta

Visualizing gradual changes in neuromelanin distribution within the substantia nigra is an important metric used to monitor the progression of Parkinsonism. This study aimed to identify the origin of the mismatch region between magnetic resonance transverse relaxation times (T2 and T2*) in the substantia nigra and investigate its feasibility and implications for in vivo detection of neuromelanin as a clinical biomarker. The relationships between neuromelanin distribution assessed by histological staining and the area of T2 and T2* mismatch determined by high- and low-resolution magnetic resonance relaxometry at 7T were directly compared in two normal and one depigmented substantia nigra collected at postmortem. In vivo feasibility of assessing T2 and T2* mismatch, clinically, was investigated using 3T magnetic resonance imaging. In the normal postmortem substantia nigra tissue, the T2 and T2* mismatch region exhibiting a linear pattern was strongly colocalized with neuromelanin distribution along the dorsal substantia nigra pars compacta, but a negligible amount of dorsal mismatch was observed in the depigmented brain. The regions of T2 and T2* mismatch from MRI, neuromelanin pigments from histology, and elevated iron signals from mass spectrometry were spatially overlapped for a normal postmortem brain. In preliminary in vivo studies, a similar, linear T2 and T2* mismatch region was observed in the dorsal area of the substantia nigra in eight normal subjects; this mismatch was significantly obscured in eight Parkinson’s disease patients. The length of the dorsal linear mismatch line based on the T2*-T2 mask was significantly shorter in the Parkinson’s disease patients compared to normal controls; this result was corroborated by reduced striatal uptake of [18F] FP-CIT dopamine transporters assessed by positron emission tomography scans. In conclusion, the measurement of T2 and T2* mismatch could serve as a complementary imaging biomarker to visualize the dorsal region of the substantia nigra pars compacta, which contains large amounts of neuromelanin.

Classification of tight sandstone reservoirs based on NMR logging

The traditional reservoir classification methods based on conventional well logging are inefficient for determining the properties, such as the porosity, shale volume, J function, and flow zone index, of the tight sandstone reservoirs because of their complex pore structure and large heterogeneity. Specifically, the method that is commonly used to characterize the reservoir pore structure is dependent on the nuclear magnetic resonance (NMR) transverse relaxation time (T2) distribution, which is closely related to the pore size distribution. Further, the pore structure parameters (displacement pressure, maximum pore-throat radius, and median pore-throat radius) can be determined and applied to reservoir classification based on the empirical linear or power function obtained from the NMR T2 distributions and the mercury intrusion capillary pressure curves. However, the effective generalization of these empirical functions is difficult because they differ according to the region and are limited by the representative samples of different regions. A lognormal distribution is commonly used to describe the pore size and particle size distributions of the rock and quantitatively characterize the reservoir pore structure based on the volume, mean radius, and standard deviation of the small and large pores. In this study, we obtain six parameters (the volume, mean radius, and standard deviation of the small and large pores) that represent the characteristics of pore distribution and rock heterogeneity, calculate the total porosity via NMR logging, and classify the reservoirs via cluster analysis by adopting a bimodal lognormal distribution to fit the NMR T2 spectrum. Finally, based on the data obtained from the core tests and the NMR logs, the proposed method, which is readily applicable, can effectively classify the tight sandstone reservoirs.

Pore size distribution and reservoir characterization: evaluation for the Eocene beach-bar sequence, Dongying Depression, China

Pore size distribution and associated heterogeneities of thinly bedded beach-bar sandstone reservoir from upper fourth member of Eocene Shahejie Formation (Es4s) are characterized with the help of mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR) transversal relaxation time (T2) data. Permeability and porosity are two represented important characteristics of rocks that control the movement and storage of fluids. The aim of this paper is to establish relationships between pore throat sizes and reservoir quality. The results derived from thin-section petrography, scanning electron microscopy (SEM), MICP, NMR T2 relaxation time, and 3D micro-CT (μ-CT) are compared to characterize pore space dimensions and types comprehensively. Average pore throat size from MICP ranges between 0.47 μm and 2.83 μm while maximum pore throat size ranges between 2.48 μm and 7.36 μm. The combination of pore size distribution obtained from MICP and NMR seems appropriate to cover the range of pore size from beach-bar sand and overcome the individual method limits. Afterwards, digital 3D μ-CT tomographic images are used to characterize and visualize pore space and pore network model to compare with the experimental data. MICP and NMR experiment showed generally bimodal (meso and micro) pore size distribution. Usually mesopore corresponding to intergranular pores is dominant, while heavily cemented sandstones show large amounts of intercrystalline micropores. Complex and heterogeneous beach-bar sandstone reservoir requires comprehensive study program to evaluate the reservoir.

Petrophysical Parameter Calculation Based on NMR Echo Data in Tight Sandstone

Bound water saturation (Swb) and permeability are the important petrophysical parameters. However, the traditional methods for the calculation of Swb and permeability based on the nuclear magnetic resonance (NMR) transverse relaxation time (T2) distributions are not applicable for tight sandstone because the NMR data have a low signal-to-noise ratio and the calculation models have certain disadvantages. In this paper, we present a new method for the calculation of Swb and permeability using integral transforms (ITs) of the NMR echo data in tight sandstone. First, the tapered cutoff function was established using the Laplace transform function of the exponential hyperbolic sine function based on the film model; Swb was directly calculated from the echo data using the exponential hyperbolic sine transform without an inversion of the T2 distribution. The position and slope of the median point, both of which control the shape of the tapered cutoff function, were determined by core experiments. Subsequently, the tapered cutoff function representing the proportion coefficients of the free fluids in the pore space was added to the arithmetic mean of the T2 (T2am) as a weight to construct the weighted T2am (T2wam). T2wam was highly suitable for reflecting the seepage ability of the pores and was directly calculated from the NMR echo data using an IT without an inversion. A new permeability calculation model was developed based on T2wam. The numerical simulation and core experimental results verified the effectiveness of the new method.

Effects Of Downhill Running On Muscle MRI T2 In Young mdx Mice

Previous studies have demonstrated that downhill treadmill running causes skeletal muscle damage that can be detected with magnetic resonance transverse relaxation time (T2) in adult dystrophic mice (mdx). However, young mdx mice (under 12 weeks of age) are characterized by a peak inflammatory phase with greater heterogeneity among muscles, potentially making it more difficult to detect T2 changes. PURPOSE: To determine whether muscle damage following downhill running can be detected in young mdx mice using proton magnetic resonance imaging (MRI) and spectroscopy (MRS). METHODS: C57BL/10ScSn-DMDmdx (mdx, n=5) and wild-type C57BL/10ScSn (controls, n=5) male mice of 6-9 weeks of age performed downhill running on a treadmill (14% grade at 8-12m/min for 45-60 min). MRI/MRS was conducted prior to and 24 hours following running in the mice hindlimbs. T2-weighted, multiple-slice, single spin-echo MR axial images were acquired (TR 2s, TE 14/40 ms, 12 slices) from the hindlimbs. MRI T2 values were calculated on a pixel-by-pixel basis for the anterior compartment (AC), posterior compartment (PC), and the deep medial region between the tibia and fibula (MC). In addition, single voxel 1H-MRS data were acquired from the soleus and gastrocnemius using stimulated echo acquisition mode (STEAM; TR 9 s, 32 TE’s exponentially spaced: 5-288 ms, 4 phase cycles) with a 4.7 T Varian/Agilent MR system. RESULTS: At baseline, T2 was elevated (p<0.05) in mdx mice (26.8±1.2ms) compared to controls (24.8±0.9 ms). Following downhill running, the mdx mice had elevated (p<0.05) T2 values compared to baseline when a composite of the compartments were compared (Pre: 26.8±1.2ms; Post: 28.8±1.4 ms). The MC was typically (80%) the most affected hindlimb region in the mdx mice. Similarly, 1H-MRS derived T2 values were increased (p=0.05) in a composite measure of the soleus and gastrocnemius after downhill running (29.8±4.2ms) in mdx compared to before downhill running (26.4±2.8ms). There were no significant changes in T2 in control mice after performing the downhill running protocol. CONCLUSIONS: Overall, our findings support the use of downhill running combined with MR T2 measures as a valuable approach for testing potential therapeutic interventions in young dystrophic mice. Funding Source: NIH (NIAMS) R01 AR070101.

Nuclear magnetic resonance surface relaxivity of coals

Nuclear magnetic resonance (NMR) transverse relaxation time (T2) measurements of fully water-saturated rock samples can provide a reference for pore size distribution (PSD). Commonly, surface relaxivity of the rock must be known before obtaining an absolute PSD sample using NMR data. There are many reports concerning the surface relaxivity of sandstones, shales, and carbonate rocks, however, little research has been performed on the surface relaxivity of coals, which limit the application of the NMR in PSD evaluation of coals. To obtain a standard surface relaxivity for PSD measurements of coals, we performed NMR, low-temperature N2 adsorption (LTNA), and mercury intrusion porosimetry (MIP) measurements for 15 bituminous and anthracite coals, whose Ro range from 0.52% to 3.07%. The results show that the values of surface relaxivity from LTNA (ρ2-SVR) are inconsistent with those from MIP (ρ2-MIP) for all samples. Moreover, the applications of ρ2-SVR and ρ2-MIP for PSD conversions are valid only for the smaller pores and larger pores, respectively. Evidently, both these two surface relaxivities cannot be used individually to calculate full-scale PSD. To obtain the actual surface relaxivity (ρ2) for full-scale PSD, we rebuilt a full-scale PSD by combining the pores smaller than 25 nm from LTNA and the pores larger than 25 nm from MIP. The results indicate that the calculated ρ2 provides excellent function for PSD transformation for all coal samples. Finally, the references of ρ2 for different coals are provided, i.e., values of 2.1 μm/s for sub-bituminous coal (low-rank coal), 3.0 μm/s for bituminous coal (medium-rank coal) and 1.6 μm/s for anthracite coal (high-rank coal). Using these referential surface relaxivities, a T2 distribution from NMR measurement can be converted to a PSD, which is applicable not only in the laboratory but also in field applications such as well logging.

Experimental Investigation on the Effects of CO2 Displacement Methods on Petrophysical Property Changes of Ultra-Low Permeability Sandstone Reservoirs Near Injection Wells

The petrophysical properties of ultra-low permeability sandstone reservoirs near the injection wells change significantly after CO2 injection for enhanced oil recovery (EOR) and CO2 storage, and different CO2 displacement methods have different effects on these changes. In order to provide the basis for selecting a reasonable displacement method to reduce the damage to these high water cut reservoirs near the injection wells during CO2 injection, CO2-formation water alternate (CO2-WAG) flooding and CO2 flooding experiments were carried out on the fully saturated formation water cores of reservoirs with similar physical properties at in-situ reservoir conditions (78 °, 18 MPa), the similarities and differences of the changes in physical properties of the cores before and after flooding were compared and analyzed. The measurement results of the permeability, porosity, nuclear magnetic resonance (NMR) transversal relaxation time (T2) spectrum and scanning electron microscopy (SEM) of the cores show that the decrease of core permeability after CO2 flooding is smaller than that after CO2-WAG flooding, with almost unchanged porosity in both cores. The proportion of large pores decreases while the proportion of medium pores increases, the proportion of small pores remains almost unchanged, the distribution of pore size of the cores concentrates in the middle. The changes in range and amplitude of the pore size distribution in the core after CO2 flooding are less than those after CO2-WAG flooding. After flooding experiments, clay mineral, clastic fines and salt crystals adhere to some large pores or accumulate at throats, blocking the pores. The changes in core physical properties are the results of mineral dissolution and fines migration, and the differences in these changes under the two displacement methods are caused by the differences in three aspects: the degree of CO2-brine-rock interaction, the radius range of pores where fine migration occurs, the power of fine migration.

Characterization of Pore Structures of Tight Sandstone Reservoirs by Multifractal Analysis of the NMR T2 Distribution

Characterization of the pore structure of tight sandstone reservoirs carries great significance for the evaluation of the reservoir storage and transport properties, “sweet spots” prediction, and reservoir development. In this paper, the pore structures of tight sandstone reservoirs are characterized on the basis of the multifractal analysis of nuclear magnetic resonance (NMR) transverse relaxation time (T2) distributions. For the T2 distribution models established using the mixed Gaussian distribution function, the characteristic parameters of the high probability measure areas (the right branch of the generalized fractal dimension spectrum and the left branch of the singularity spectrum) are closely related to the model parameters (weight coefficients and standard deviations of the short relaxation component). For the T2 distributions of the tight sandstone samples, the results of the relationships between the multifractal characteristic parameters of the T2 distributions and the petrophysical parameter...

Performance of Relative Permeability and Two-Phase Flow Parameters Under Net Effective Stress in Water Wet Porous Media: A Comparative Study of Water–Oil Versus Silica Nanofluid–Oil

While studies on the stress sensitivity of relative permeability using conventional fluids have varying and inconclusive results, the effect of stress on relative permeability using nanofluids has not been reported. In this study, the effect of net effective stress on relative permeability and two-phase flow parameters using conventional and nanofluid was investigated. Nanoparticle retention in pores was examined by changes in nuclear magnetic resonance transverse relaxation time distribution. Nanoparticle deposition affected big pores more than small pores. As net effective stress was increased during water flooding, end-point oil and water saturations increased and end-point oil relative permeability decreased. Nanofluid–oil relative permeability showed contrasting results depending on whether the sample had adsorbed nanoparticles prior to relative permeability measurements. For the sample that had adsorbed nanoparticles prior to subsequent stress increments, the effect of stress was dominated by the effect of nanoparticle adsorption. However, the effect of stress took precedence over nanoparticle adsorption in samples that had no nanoparticle deposition prior to relative permeability experiments. A comparison of water–oil and nanofluid–oil relative permeabilities showed similar initial water saturations and end-point oil relative permeability changes but differing residual oil saturations and end-point water relative permeability. Mobility ratio increased in water flooding as stress increased but decreased after an initial increase in nanofluid flooding. Displacement efficiency decreased with increased stress for both water and nanofluid flooding. The results provide insights into the effect of stress on relative permeability using conventional fluids and silica nanofluid.