Nuclear Magnetic Resonance T2 Research Articles

Cross-Scale Study on Lime Modified Phosphogypsum Cemented Backfill by Fractal Theory

Pore structure is a critical factor affecting the strength characteristics of mine backfill materials. In this paper, based on the mechanical tests on lime modified phosphogypsum cemented backfill (LMPGCB), the microstructure of the LMPGCB sample was characterized by nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) imaging. Based on the fractal theory (FT), the values expressing the fractal characteristics of the NMR T2 spectrum and SEM images were determined. The mathematical model between the NMR-FT fractal dimension and the SEM-FT box dimension was developed to infer the overall pore characteristics from various pore characteristics. The functional relationships between the strength and pore content and dimension were established, and the effects of pore structure on the strength were discussed. The results indicate that: (1) in NMR-FT fractal dimension, the fractal effect of various types of pores is better. The fractal dimension of the small pore is between 0.86–1.38, in which the overall trend first decreases and then increases. (2) In the SEM-FT box dimension, the fractal effect of the overall pore is better. The overall trend of box dimension decreased first and then increased at each kind of magnification. Furthermore, with the increase of magnification, the box dimension decreases. (3) There is a linear direct proportional relationship between the SEM-FT box dimension and the fractal dimensions of NMR-FT of small pores. The relationship between SEM-FT box dimension and NMR-FT fractal dimensions of various types of pores conforms to the plane relationship. There is a linearly increasing relationship between dimension and pore content and a decreasing relationship between dimension and strength.

Model Synthetic Samples for Validation of NMR Signal Simulations

Simulations of nuclear magnetic resonance (NMR) signal from fluids contained in porous media (such as rock cores) need to account for both enhanced surface relaxation and the presence of internal magnetic field gradients due to magnetic susceptibility contrast between the rock matrix and the contained fluid phase. Such simulations are typically focussed on the extraction of the NMR T2 relaxation distribution which can be related to pore size and indirectly to system permeability. Discrepancies between such NMR signal simulations on digital rock cores and associated experimental measurements are however frequently reported; these are generally attributed to spatial variations in rock matric composition resulting in heterogeneously distributed NMR surface relaxivities (ρ) and internal magnetic field gradients. To this end, a range of synthetic sediments composed of variable mixtures of quartz and garnet sands were studied. These two constituents were selected for the following reasons: they have different densities allowing for ready phase differentiation in 3D μCT images of samples to use as simulation lattices and they have distinctly different ρ and magnetic susceptibility values which allow for a rigorous test of NMR simulations. Here these 3D simulations were used to calculate the distribution of internal magnetic field gradients in the range of samples, these data were then compared against corresponding NMR experimental measurements. Agreement was reasonably good with the largest discrepancy being the simulation predicting weak internal gradients (in the vicinity of the quartz sand for mixed samples) which were not detected experimentally. The suite of 3D μCT images and associated experimental NMR measurements are all publicly available for the development and validation of NMR simulation efforts.

A modified nuclear magnetic model for characterizing the fluid signal response in hydrocarbon bearing rocks

The nuclear magnetic resonance (NMR) technique is a crucial means for formation evaluation. It can provide an accurate set of formation parameters in single-phase-flow reservoirs, such as porosity, permeability, and pore structure distribution; however, in most cases, there are hydrocarbon-water two-phase fluids in the reservoirs. The secondary change of the T2 distribution makes the accuracy of the evaluation a challenge. Hence, it is necessary to clarify the impact of changes in hydrocarbon content on the response of NMR signals before using the traditional evaluation methods. In this paper, we analyzed the response mechanism of NMR T2 distribution under different saturation conditions. Based on the theoretical derivation, we established a nuclear magnetic model that describes the relation among the T2 geometric mean value of water signal, hydrocarbon signal, and water saturation. The accuracy of each model part was verified by numerical simulation. In addition, the calculation results in core samples show that the proposed model is more effective and reliable than two existing models (the T2 saturation ratio model and the Xiao empirical model), especially in extreme conditions (irreducible water and hydrocarbon saturated samples). Finally, we applied the proposed model to field application in the Tertiary formation in the Bohai Bay Basin, China, which achieved good effects in the NMR fluid substitution and the NMR saturation evaluation.

Pore-fracture network alteration during forced and spontaneous imbibition processes in shale formation

Imbibition controls mass transfer in the complex pore-fracture network in shale, which may change the pore-fracture network and lead to a low efficient flowback of fracturing fluid. Thus, it is necessary to accurately characterize the pore-fracture network alteration during the imbibition process. In this study, forced and spontaneous imbibition tests were conducted under the confining pressure on selected shale core samples with induced fractures, collected from Longmaxi Formation, Sichuan Basin, China. The low-field nuclear magnetic resonance (NMR) spectrometer was employed to monitor the variation of pores and micro-fractures in the shale core samples during the dynamic imbibition process. In addition, optimal surface relaxivities, ranging from 0.019 to 0.033 μm/ms, were determined by comparing the NMR T2 distributions with the pore size distributions (PSDs) measured via high-pressure mercury intrusion (HPMI) tests. Then, the measured dynamic T2 spectra with three distinct peaks were converted into the corresponding PSDs to quantitatively analyze the number, size, and connectivity changes of small pores, large pores, and micro-fractures in shale. Results show that the total porosities of the four shale core samples are increased by 3.5%, 10.2%, 32.9%, and 36.3% after the imbibition tests. The forced imbibition leads to more remarkable improvements in the pore volume of large pores with radius between 0.2 and 3.6 μm. In contrast, the spontaneous imbibition results in more significant increases in the number and size of small pores with radius between 0.0004 and 0.36 μm. It is also found that the total porosity increment is primarily an outcome of small pore alteration during the imbibition. Moreover, the enlarged pores and micro-fractures are mainly categorized as the capillary bounded fluid pores and movable fluid pores, which significantly affect the efficiency of oil and gas transfer in shale. The findings of our study demonstrate the comprehensive effects of capillary force, clay hydration, osmotic potential, confining and pore pressures, and creep deformation and failure on the pore-fracture network alteration in shale and advance the understanding of the mechanisms behind the forced and spontaneous imbibition processes.

Quantitative Investigation of Water Sensitivity and Water Locking Damages on a Low-Permeability Reservoir Using the Core Flooding Experiment and NMR Test.

Production of oil and gas energy is often greatly hindered by reservoir formation damage, particularly the occurrences of water sensitivity and water locking damages on a low-permeability reservoir. For the purpose of this paper, a formation damage assessment methodology combining core flooding experiment and NMR (nuclear magnetic resonance) T2 relaxation tests is performed and applied to quantitatively determine water sensitivity/water locking damage on sandstone oil formation. XRD tests are used to analyze the mineral composition of cores. Core flooding experiments are designed to simulate the two damages and determine the permeability reduction. NMR tests are introduced to compare water saturation before and after flooding through rock cores, calculate the porosity damage and changes of the pore size, and analyze the mechanism of water sensitivity and water locking damages. Also, SEM experiments are used to determine the pore morphology before and after damage. Low-permeability sandstone rock cores cored from the Jilantai reservoir are assessed through this whole set of experiments. The results demonstrate that the permeability and porosity of core samples strongly decrease with the occurrence of water sensitivity/water locking damage, reflecting that the Jilantai reservoir has strong water sensitivity and is prone to be damaged by water locking. Compared with the previous formation damage assessment ideas, much attention is given to the microchanges of cores after damage, and using fluorinated oil instead of kerosene can help observe the distribution of water in rock core samples after each flooding by the NMR T2 spectra.

New insights into hydration-induced creep behavior of shale: A comparison study of brittle black shale and clayey oil shale at micro-scale

During hydraulic fracturing for shale gas development, water softening and creep deformation of shale rock are two common engineering problems that severely impair production performance. In order to understand the mineralogical and microstructural control on this mechanism from microscale, a brittle black shale from Lower Silurian Longmaxi Formation (named as LMX shale) and a clayey oil shale from Paleogene Youganwo Formation (YGW shale), which represent two members of shale family with different mineral constitutes and microstructures, were selected for hydration process under various durations (0.5 h–96 h) and temperatures (25 °C and 50 °C). The accompanied changes of microstructural and mechanical properties during hydration at microscale were investigated via nuclear magnetic resonance (NMR) and nanoindentation, respectively. Based on NMR T2 spectra, peak T2 time and water-infiltrated pore increment for both LMX and YGW shales rise sharply within the first 12 h, whereas remain invariable hereafter. And the values increase at higher temperature. For YGW shale, the maximum peak T2 time is lower but the incremental porosity is higher than LMX shale, respectively. Such difference indicates an occurrence of gradually enlarged and connected pores inside LMX shale after hydration, while a greater number of disconnected pores have been developed in YGW shale, comparatively. For both shales, great strength deterioration and creep enhancement are monitored within the first 12 h of hydration. The modulus reduction could be as high as 26.27% and 80.16% after 96 h soaking at 50 °C for LMX and YGW shales, respectively. Creep strain rate sensitivity increases from 0.026 to 0.234 for LMX shale, and 0.011 to 0.099 for YGW shale with intensified softening degree. The softening mechanism for LMX brittle shale is dominated by secondary pores and cavities generated as a result of dissolution of carbonate cementation and grain detachment after water infiltration, while microstructural integrity of YGW oil shale is degraded by clay swelling. Due to different mineral constitutes and microstructures, creep deformation for clayey YGW shale is mainly mediated by dislocation movement of fine clay grains, whereas diffusion creep plays the key role in coarse-grained LMX shale. This comparative study with two distinct shale samples provides the mineralogy- and microstructure-based framework for interpreting water softening process and creep mechanism of shale, which benefits hydraulic fracture design in different shale formations so as to enhance shale oil/gas production.

Advances in Characterizing Gas Hydrate Formation in Sediments with NMR Transverse Relaxation Time

The formation process, structure, and distribution of gas hydrate in sediments have become focal points in exploring and exploiting natural gas hydrate. To better understand the dynamic behavior of gas hydrate formation in sediments, transverse relaxation time (T2) of nuclear magnetic resonance (NMR) is widely used to quantitatively characterize the formation process of gas hydrate and the change in pore characteristics of sediments. NMR T2 has been considered as a rapid and non-destructive method to distinguish the phase states of water, gas, and gas hydrate, estimate the saturations of water and gas hydrate, and analyze the kinetics of gas hydrate formation in sediments. NMR T2 is also widely employed to specify the pore structure in sediments in terms of pore size distribution, porosity, and permeability. For the recognition of the advantages and shortage of NMR T2 method, comparisons with other methods as X-ray CT, cryo-SEM, etc., are made regarding the application characteristics including resolution, phase recognition, and scanning time. As a future perspective, combining NMR T2 with other techniques can more effectively characterize the dynamic behavior of gas hydrate formation and pore structure in sediments.

Pore structure evaluation in ultra-deep tight sandstones using NMR measurements and fractal analysis

The reservoir quality, pore spaces and pore size distributions of Bashijiqike sandstones in Kuqa depression were described by thin sections, SEM (scanning electron microscopy) analysis and nuclear magnetic resonance (NMR) tests. In order to quantitatively characterize the complexity of pore structure, NMR T2 (transverse relaxation time) spectrum was used for fractal analysis. This study unravels the relationships between petrophysical parameters, NMR parameters and fractal dimension of the ultra-deep tight sandstones. Primary intergranular pores are the main component of pore spaces, followed by secondary dissolution pores and micropores, and there contain microfractures. The NMR T2 spectra are either bi-modal or uni-modal distribution. The uni-modal T2 spectrum reflects uniform pore spaces. The coexistence of intragranular pores and intergranular pores leads to the bi-modal T2 spectrum. There is a positive correlation between fractal dimensions and T2gm (geometric mean of the T2 distribution). Consequently, four pore structure types are determined according to irreducible water content, T2gm, RQI (reservoir quality index), and the characteristics of individual pore structure are summarized. Because of the uniform pore space, Type I and Type IV have the lowest fractal dimensions. The structure of Type II and Type III are most heterogeneous due to their combination of intergranular and intragranular pores. The results help clarify the internal relationships between petrophysical parameters and microstructure, and have implications for pore structure evaluation of ultra-deep sandstones worldwide.

Low-Field NMR Relaxation Analysis of High-Pressure Ethane Adsorption in Mesoporous Silicas.

Understanding the behaviour of short-chain hydrocarbons confined to porous solids informs the targeted extraction of natural resources from geological features, and underpins rational developments in separation, storage and catalytic conversion processes. Herein, we report the application of low-field (12.7 MHz) 1 H nuclear magnetic resonance (NMR) relaxation measurements to characterise ethane dynamics within mesoporous silica materials exhibiting mean pore diameters between 6 and 50 nm. Our measurements provide NMR-based adsorption isotherms within the range 25-50 bar and at ambient temperature, incorporating the ethane condensation point (40.7 bar at our experimental temperature of 23.6 °C). The quantitative nature of the acquired data is validated via a direct comparison of NMR-derived excess adsorption capacities with ex situ gravimetric ethane adsorption measurements, which are demonstrated to agree to within 0.2 mmol g-1 of the observed ethane capacity. NMR relaxation time distributions are further demonstrated as a means to decouple interparticle and mesopore dominated adsorption phenomena, with unexpectedly rapid relaxation rates associated with interparticle ethane gas confirmed via a direct comparison with NMR self-diffusion analysis.

Effect of the Temperature and Tetrahydrofuran (THF) Concentration on THF Hydrate Formation in Aqueous Solution

The knowledge of the mechanism of clathrate hydrate formation is crucial for studying the formation of a natural gas hydrate and developing hydrate-based technologies. In this study, low-field nuclear magnetic resonance (NMR) is applied to observe the processes of tetrahydrofuran (THF) hydrate formation from the solutions with different THF concentrations, and from the analysis of the NMR transverse relaxation time (T2) of hydrates and solutions, the formation mechanism of THF hydrate is discussed. A quantitative relationship between the NMR T2 spectral area and the mass of THF hydrate is established, and it is applied to estimate the hydrate amount in the formation process of THF hydrate. The results obtained show that T2 of THF solution is controlled by the temperature and THF concentration: longer at a higher temperature and shorter at a higher THF concentration. Moreover, the initial THF concentration does not affect the final concentration of the remaining solution after hydrate crystallization, and the formation of THF hydrate from solution follows the thermodynamics of solution reaction, with the THF concentration of the solution ending at the phase line after hydrate crystallization. The kinetics of THF hydrate formation is affected by the temperature and THF concentration: a higher formation rate in the solution with stoichiometric composition (19.06 wt % THF) of THF hydrate than either THF-rich or water-rich solution and a lower formation rate at a higher temperature.

Influencing mechanics and correction method of nuclear magnetic resonance measurement in igneous rocks reservoir

Nuclear magnetic resonance (NMR) measurement provides lithology-independent porosity and pore structure for sedimentary reservoirs. Recently, oil and gas were discovered in igneous rock formations, however, NMR logging porosity is substantially lower than the true porosity, and the T2 distributions does not characterize the pore-size distributions. These observations limit the use of NMR logging in igneous rock formations and new research is needed to be developed. To address this gap in the literature, using numerical simulation of NMR behavior, we conduct a theoretical study to examine the impact of magnetic susceptibility, echo spacing, and fluid type on the T2 distributions in igneous rock. The simulation results and laboratory experiment show that, the paramagnetic minerals affect the magnetization of fluids in pores and rate of the echo decay, and NMR porosity is thus significantly reduced. To effectively correct the NMR measurement, a new approach is proposed to improve the underestimation of NMR porosity and T2 distribution shifting to shorter times. For a pore model, the corrected porosity and T2 distribution are both in good agreement with the model. In a case study from Chepaizi area of Junggar Basin, China, the corrected NMR porosity matches well with core measurement, and the corrected T2 distribution shifts to longer times more than before. Moreover, bound fluid volume and movable fluid volume are both corrected, and the NMR permeability also agrees well with core laboratory measurement, which proves that the correction method effective. Therefore, this technology enables NMR log for evaluating the igneous rock reservoir quality.

Effects of maternal vitamin D3 status on quality characteristics of pork batters in offspring pigs during cold storage

Seventy-two offspring pigs (150 d) from 27 sow (9 sows per group) fed different vitamin D3 levels were used to evaluate the effects of maternal vitamin D3 status and postmortem cold storage on cooking loss, color, texture profile analysis (TPA), low-field nuclear magnetic resonance (LF-NMR) T2 relaxation times and dynamic rheological of gel properties of chilled pork batters in offspring pigs. Sows were allotted to low, normal, and high dietary vitamin D3 groups which contained 200 (LD), 800 (ND), and 3200 (HD) IU of vitamin D3/kg basal diet, respectively. Results showed that HD group had higher a* value, hardness, cohesiveness, springiness, chewiness, and storage modulus (G') values, while had lower cooking loss, L* value, T21 and T22 relaxation time compared with LD group during postmortem cold storage period (P < 0.05). In addition, cold storage time increased cooking loss and b* values of LD and ND groups, TPA parameters, T22 relaxation time and G' values of all experimental groups, while decreased a* and L* values of LD and ND groups (P < 0.05). The results revealed that maternal high-dose vitamin D3 levels influenced these indicators of chilled pork batters in offspring pigs, which improved the quality attributes during postmortem cold storage.

Study on the pore structure, fluid mobility, and oiliness of the lacustrine organic-rich shale affected by volcanic ash from the Permian Lucaogou Formation in the Santanghu Basin, Northwest China

Lacustrine organic-rich shale with extremely heterogeneous pore structure is widespread in the second member of the Permian Lucaogou Formation (P2l2) of the Santanghu Basin. However, information on the pore structure, fluid mobility, and oiliness characteristics is still lacking, as well as the interaction between them in the organic-rich shales remains controversial. To address these problems, this study analyzed the influencing factors of pore structure, oiliness, and fluid mobility of the P2l2 shale and constrained the lower limit of movable fluid pore throat radius based on thin sections, routine core analysis, whole rock X-ray diffraction (XRD), scanning electron microscopy (SEM), high-pressure mercury injection (HPMI), nuclear magnetic resonance (NMR), constant-rate mercury injection (CMI), and nano-CT scanning data. The P2l2 shale was characterized by fine-grained minerals of felsic (tuffaceous material) and carbonate (dolomite), with poor reservoir physical properties and strong oil-bearing heterogeneity. Micro- and nano-scale pores are the main type of storage space. Among them, most of the microfractures emit green to yellow hydrocarbon fluorescence regardless of whether they are filled or not, as do the edges of dolomite grains. While the edges of the felsic minerals are characterized by stronger orange yellow fluorescence of organic matter. According to the morphology and parameters of HPMI and NMR curves, the pore structure can be divided into four types. From type 1 to type 4 pore structure, the maximum mercury saturation decreases from 95 to 47 % (avg. 70 %), indicating poor pore connectivity, and the pore throat radius that mainly contributes to permeability is > 0.04 μm. Meantime, the NMR T2 spectrum gradually transitions from right unimodal to left unimodal behavior, with a gradual decrease in the saturation of movable fluid and a transition of the mineral type from carbonate to felsic. Fluid mobility is not only influenced by macropores, but also related to the content of small pores (T2 < 1 ms) and the mineral type. On this basis, the lower limit of the movable fluid pore throat radius in the P2l2 organic-rich shale is finally determined to be 40 nm.

Characterization of Nanoscale Pores in Tight Gas Sandstones Using Complex Techniques: A Case Study of a Linxing Tight Gas Sandstone Reservoir

Pore structures with rich nanopores and permeability in tight gas reservoirs are poorly understood up to date. Advanced techniques are needed to be employed to accurately characterize pore structures, especially tiny pores which include micron and nanopores. In this study, various experimental techniques such as scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) T 2 , nitrogen adsorption method, and NMR cryoporometry (NMRC) are combined to interrogate the complex pore systems of the tight gas reservoir in the Linxing formation, Ordos Basin, China. Results show that tight gas sandstones are primarily comprised of residual interparticle and clay-dominated pores. Clay and quartz are two dominate minerals while pyrite occupies a nontrivial amount as well. The permeability of tight gas sandstones is very low, exhibiting an extremely poor positive correlation with porosity. While pore types and relative pore contents are more influential factors on the permeability, accurate characterization of pore size distribution is critical for the permeability of tight gas sandstones. Therefore, complementary characterization methods are carried out, indicating that neither small pores with radii &lt; 100 nm (around peak 1 in NMR T 2 distribution) nor large pores with radii &gt; 5 μ m (around peak 3 in NMR T 2 distribution) control the permeability by analyzing the connectivity of the pores in various size ranges, but rather pores averaging approximately 350 ± X nm (around peak 2 in NMR T 2 distribution) have sufficient connectivity to host and transmit hydrocarbons. The pore size of tight gas sandstones is dominated by the clay-rich mineral assemblage. The study shows that the NMRC technique can be a very promising method, especially when referred to as a promising “roadmap” on how to interrogate tight formations such as the tight gas sands or even shale especially for the nanopore characterization.

Insights into the pore structure and oil mobility in fine-grained sedimentary rocks: The Lucaogou Formation in Jimusar Sag, Junggar Basin, China

Unconventional shale/tight oil resources in fine-grained sedimentary rocks play essential roles in global energy structure. Fine-grained sedimentary rocks are characterized by complex pore assemblage and connectivity. Insights into the links between pore structure and movable oil are important for exploration and production of tight/shale oil or gas in fine-grained sedimentary rocks. A series of analyses including thin sections, mercury injection capillary pressure (MICP), and nuclear magnetic resonance (NMR) were carried out to unravel the linkage between pore structure and oil movability. Various centrifugation speeds were used in the NMR tests to remove the movable oil to obtain the movable oil saturation corresponding to various centrifugation speed. Fractal dimension is calculated from NMR T2 spectrum. Then four types of pore structures were divided according to the parameters derived from the MICP and NMR results, and the characteristics of various pore structure are summarized. From type Ⅰ to type Ⅳ, the threshold pressure, irreducible water saturation (Swi), and the fractal dimension increased while the SHgmax, rapex, T2gm, porosity, permeability, and movable oil saturation decreased. Then the key factors controlling the movable oil saturation were investigated, including permeability, porosity, T2gm, rapex, and mineral composition of the rocks. The movable oil saturation is dominated by porosity, T2gm, fractal dimension, and rapex. Then a quantitative evaluation model of the movable oil saturation was established. In addition, the NMR logs and the derived parameters were utilized to evaluate the reservoir quality and pore structures in a single well. At last the T2 value of 25 ms (the ratio of the T2 components >25 ms to the total T2 components) was used to calculate the movable oil saturation in the upper member of the Lucaogou Formation, and the T2 value of 28 ms was used for the lower member of the Lucaogou Formation in the Jimusar Sag. The results are testified by the high-speed (10000 r/min) centrifugation NMR measurements. The results above help improve understanding of tight/shale oil enrichment and provide a new model for calculating the movable oil saturation in fine-grained sedimentary rocks.

Method to Generate Random Element Equivalent Core Models Based on the NMR T 2 Spectrum for Waterflooding in Tight Sandstone.

Numerical simulation based on the widely used homogeneous equivalent core model can solve the problem of high cost and long duration of coreflooding experiments. However, using the homogeneous equivalent core model, it is difficult to reflect the characteristics of the core interior during waterflooding. In this paper, we provide a method to generate random element equivalent core models based on the nuclear magnetic resonance (NMR) T2 spectrum, and it can divide permeability regions by granularity. The permeability calculation formula derived from the complementary correlation principle ensures that most areas of the core satisfy the correlation between permeability and T2 relaxation time. Moreover, the generation method can guarantee that the random element equivalent core model is consistent with the homogeneous equivalent core model in terms of the geometric mean of permeability. The simulation results show that the high-resolution random element equivalent core model can better simulate microcosmic fingering inside the core during waterflooding. Nevertheless, the proposed method has some limitations emanating from the demarcation criteria and the porosity assumption. Furthermore, the generation method is expected to be extended to simulate enhanced oil recovery (EOR) mechanisms on the core scale after waterflooding.

Characterization of Full Pore and Stress Compression Response of Reservoirs With Different Coal Ranks

The accurate characterization of coal pore structure is significant for coalbed methane (CBM) development. The splicing of practical pore ranges of multiple test methods can reflect pore structure characteristics. The pore\fracture compressibility is the main parameter affecting the porosity and permeability of coal reservoirs. The difference in compressibility of different coal rank reservoirs and pore\fracture structures with changing stress have not been systematically found. The pore structure characteristics of different rank coal samples were characterized using the optimal pore ranges of high-pressure mercury intrusion (HPMI), low-temperature liquid nitrogen adsorption (LT-N2A), low-pressure carbon dioxide adsorption (LP-CDA), and nuclear magnetic resonance (NMR) based on six groups of different rank coal samples. The compressibility of coal matrix and pore\fracture were studied using HPMI data and NMR T2 spectrum under effective stress. The results show that the more accurate full pore characterization results can be obtained by selecting the optimal pore range measured by HPMI, LT-N2A, and LP-CDA and comparing it with the NMR pore results. The matrix compressibility of different rank coal samples shows that low-rank coal &amp;gt; high-rank coal &amp;gt; medium-rank coal. When the effective stress is less than 6 MPa, the microfractures are compressed rapidly, and the compressibility decreases slowly when the effective stress is more than 6 MPa. Thus, the compressibility of the adsorption pore is weak. Nevertheless, the adsorption pore has the most significant compression space because of the largest proportion in different pore structures. The variation trend of matrix compressibility and pore\fracture compressibility is consistent with the increase of coal rank. The compressibility decreases with the rise of reservoir heterogeneity and mechanical strength. The development of pore volume promotes compressibility. The research results have guiding significance for the exploration and development of CBM in different coal rank reservoirs.

Machine learning in the classification of lithology using downhole NMR data of the NGHP-02 expedition in the Krishna-Godavari offshore Basin, India

We apply machine learning techniques to classify the lithology using downhole nuclear magnetic resonance (NMR) data of gas hydrate reservoirs in the Krishna-Godavari (KG) Basin, eastern Indian offshore. We choose two sites of the second expedition of the Indian National Gas Hydrate Program (NGHP-02) for our study, one with clay dominated site NGHP-02-01 from Area E and another with silt/sand dominated site NGHP-02-05 from Area C. We obtain five distinct classes in NMR signals at two sites using unsupervised techniques such as Davies-Bouldin index, silhouette, self-organizing map, k-means clustering and emergent self-organizing map. Obtained classes are interpreted using the NMR T2 distribution pattern with time in terms of clay, silt, sand and pebble/gravels with depth and reconciled with gamma-ray, density, porosity, resistivity, velocity and photoelectric factor. Our results demonstrate the lithology as silty-clay with minor sand at Hole NGHP-02-01A, and silt, sand with less clay and minor pebble/gravel at Hole NGHP-02-05A. The presence of gas hydrate is identified by observing an overall low amplitude T2 distribution curve, which is due to the reduction of pore space by solid gas hydrate. The maximum concentration of gas hydrate of about 43% of pore space estimated using NMR- and density-porosity are distributed mainly in silty-clay at Hole 01A and of about 75% in clayey-silt and silty-sand at Hole 05A. Low amplitude NMR T2 signals observed in clay-dominated sediments at higher relaxation time may be due to the fracture-filled gas hydrates at both holes. Interestingly, high porosity (90–95%), high permeability (4 mD), low bulk-density (1.2 gm/cm3) and low photoelectric factor (0.8 b/electron) are found in sandy layers within the gas hydrate-stability zone at Hole 05A, which possibly comprising buried channels. Comparison of the lithology obtained from other log data with that from NMR data and core samples illustrates that the NMR data are most effective for characterization of reservoirs, except for a high concentration of gas hydrate in a sandy/coarse-grained reservoir, which needs further studies.

Determination of gas-water seepage characteristics with consideration of dynamic pore-throat structure in a tight sandstone gas formation

In this paper, a pragmatic and integrated technique has been developed to experimentally determine the gas-water seepage characteristics with consideration of dynamic pore-throat structure in a tight sandstone gas formation due to a decrease in pore pressure. Conventional experiments were conducted to obtain porosity and permeability of core samples collected from a tight sandstone gas reservoir in the Ordos basin. Then, gas and water flow experiments integrated with the nuclear magnetic resonance (NMR) measurement were employed to obtain the dynamic characteristics of relative permeability curves due to changes in pore-throat structure, while gas permeabilities under different conditions are corrected with consideration of the slippage effect and dynamic pore-throat characteristics. Subsequently, irreducible water saturation and water blocking coefficient were obtained by combining the measured NMR T2 spectrum and gas-water volume, which were used to examine the effects of dynamic evolution of a pore-throat structure on the gas-water micro-distribution and percolation capacity. In the case that the decreased pore pressure inevitably leads to the contraction of pores and throats, although the absolute value of permeability decrease is larger for samples with higher permeability, the relative proportion of permeability decrease is larger for samples with lower permeability, indicating a stronger stress sensitivity. With such a decrease of pore pressure, gas and water relative permeability curves are shifted downwards to the right-hand side, during which the flowing ability of the gas-water system is weakened. The increase in irreducible water saturation is found to be greater in the larger throats, while the relative permeability characteristics and irreducible water saturation of a core sample with a higher permeability have larger changes due to the influence of the dynamic pore-throat structure under the same stress variation. The change in the pore-throat structure due to a decrease in pore pressure imposes the greatest impact on the seepage characteristics at the initial stage.

Adaptation of Crushed Rock Analysis to Intact Rock Analysis To Improve Assessment of Water Saturation and Fast Pressure Decay Permeability

Summary Legacy crushed rock analysis, as applied to unconventional formations, has shown great success in evaluating total porosity and water saturation over the previous three decades. The procedure of crushing rock into small particles improves the efficiency of fluid recovery and grain volume measurements in a laboratory environment. However, a caveat to crushed rock analysis is that water and volatile hydrocarbons evaporate from the rock during the preparatory crushing process, causing significant uncertainty in water saturation assessment. A modified crushed rock analysis incorporates nuclear magnetic resonance (NMR) measurements before and after the crushing process to quantify the volume of fluid loss. The advancements improve the overall total saturation quantification. However, challenges remain in the quantification of partitioned water and hydrocarbon loss currently derived from the NMR spectrum along with its uncertainty. Furthermore, pressure decay permeability from crushed rock analysis has been reported to have two to three orders of magnitude difference between different laboratories. The calculated pressure decay permeability of the same rock could even vary by several orders of magnitude with different crushed sizes, which questions the quality of the crushed pressure decay permeability. In this paper, we introduce an intact rock analysis workflow on unconventional cores for improved assessment of water saturation and enhanced quantification of fast pressure decay matrix permeability from intact rock. The workflow starts with acquisition of NMR T2 and bulk density measurements on the as-received state intact rock. Instead of crushing the rock, the intact rock is directly transferred to a retort chamber and heated to 300°C for thermal extraction. The volumes of thermally recovered fluids are quantified through an image-based process. The grain volume measurement and a second NMR T2 measurement are performed on post-retort intact rock. The pressure decay curve during the grain volume measurement is then used for calculating the pressure decay matrix permeability. Total porosity is calculated using the bulk volume and grain volume of the rock. Water saturation is quantified using the total volume of recovered water. In addition, the twin as-received-state rocks are processed through the crushed rock analysis workflow for an apple-to-apple comparison. Meanwhile, the pressure decay permeability of the post-retort intact sample is cross-validated against the steady-state gas permeability of the same post-retort sample. The introduced workflow has been tested successfully on different formations, including Bakken, Bone Spring, Eagle Ford, Cotton Valley, and Niobrara. The results show that total porosities calculated from intact rock analysis are consistent with total porosities from crushed rock analysis, while water saturations from the new workflow are an average 8% saturation unit (SU) [0.2 to 0.7% porosity unit (PU) of bulk volume water (BVW)] higher than those from the prior crushed rock workflows. The study also indicates that for some formations (e.g., Bone Spring), the fluid loss during the crushing process is dominated by water; however, for some other formations (e.g., Bakken), the hydrocarbon loss is significant. Pressure decay permeability quantified using intact rock analysis is also confirmed within an order of magnitude of steady-state matrix permeability.