Nuclear Magnetic Resonance T2 Research Articles

Research on Adaptability Evaluation Method of Polymer by Nuclear Magnetic Resonance Technology.

In order to study the matching relationship between polymer(HPAM) molecular weight and reservoir permeability, in this paper, the injection performance of polymers with different molecular weights in rock cores with different permeability is studied. Using nuclear magnetic resonance technology combined with conventional core displacement equipment, the change law of the displacement process was analyzed from three aspects of nuclear magnetic resonance T2 spectrum, core layering, and imaging. Finally, the fluidity of the polymer solution in the core was analyzed by injection pressure control features. The experimental results show that the polymer solution with a molecular weight of 25 million has the best retention effect in the core flooding experiment and can stay in the dominant channel of the core for a long time to control the water flooding mobility. In rocks with a permeability of 500, 1000, and 2000 mD, subsequent water flooding can expand the swept volume by about 25% compared with polymer flooding. This method can effectively establish the adaptability matching relationship between the polymer molecular weight and the reservoir permeability.

Evaluation of Fluid Saturation in Shale Using 2D Nuclear Magnetic Resonance

Two-dimensional (2D) nuclear magnetic resonance (NMR) has become an indispensable tool for fluid saturation evaluation in shale reservoir. NMR spectrum derived from echo data with a low signal-to-noise ratio, however, cannot be effectively used for fluid feature extraction and saturation calculation. In this study, we proposed a fluid saturation evaluation method that combines morphology, nonnegative matrix decomposition, and fully constrained least squares. Moreover, an Akaike information criterion (AIC)-based method was proposed to determine the number of fluid types. A shale formation data set with varying fluid saturation was processed to validate the effectiveness of the proposed method. The results show that the proposed method can efficiently and accurately obtain the NMR T1–T2 feature and saturation of different fluid types and has advantages over the comparable method in both computational efficiency and accuracy. Furthermore, the impact of the structure size was investigated, demonstrating that the proposed method is very fault tolerant.

A novel approach for estimating pore size distribution and capillary pressure in hydrocarbon zone through hydraulic flow unit framework using NMR log

Nuclear magnetic resonance (NMR) logging imparts a wealth of information regarding the petrophysical characterization of rocks including pore size distribution and capillary pressure. However, the NMR data can only be comprehensively utilized when formation pores are fully water-saturated. As with hydrocarbon saturation increase in rock pores the NMR's transverse relaxation curve (T2 distribution) gets distorted and this in turn causes flawed measurement of petrophysical and fluid properties. Therefore, the estimation of representative pore structure and capillary pressure from hydrocarbon zone utilizing T2 distribution has traditionally been quite arduous. The previous correction methods in this regard mostly require expensive core data for their implementation, which makes them less desirable. In this study, a novel workflow has been introduced which does not necessitate expensive core data for its application. The new method employs the concept of hydraulic flow units (HFUs), which are zones within a reservoir having consistent petrophysical and geological properties to subdivide the reservoir rock while investigating the T2 distribution character variation through a statistical approach to determine the most optimally informed reconstruction of effected T2 distributions in hydrocarbon zone for accurate petrophysical characterization. After implementing the novel workflow on a carbonate reservoir having gas, the before and after comparative analysis of T2 distribution, pore size distribution (PSD) and capillary pressure curves show promising results. The novel workflow establishes a robust and cost-effective methodology for NMR T2 distribution correction in hydrocarbon zone for un-cored and partially cored wells.

Mixed Wettability Modeling and Nuclear Magnetic Resonance Characterization in Tight Sandstone

Rock wettability significantly impacts fluid distributions in reservoirs and enhanced oil recovery. As the large variety of mixed-wet tight formations becomes increasingly evident, research evaluating wettability has become increasingly important. The nuclear magnetic resonance (NMR) technique is suitable for wettability characterization, but the simplified rock models currently used in NMR studies, that is, entirely water-wet or oil-wet or classified as water- or oil-wet based on pore geometry, may result in inaccurate pore-fluid distributions and wettability estimates. In this paper, a new pore-scale mixed-wet rock model was constructed that considers the contact angle and wetting area. A tight sandstone rock saturated with oil–water is presented as an example to investigate T2 responses and NMR T2-based wettability indices for different wettability features. In the model, a mixed-wet digital rock saturated with oil and water is constructed using the sedimentological process method and the Lattice Boltzmann method. Then, the random walk method is applied to simulate T2 responses in cases with varied wettability properties, such as contact angle, wetting area ratio, and fluid saturation. Finally, the apparent contact angle cosine is used as the wettability indicator for comparison with published NMR T2-based wettability indices. The results showed that the combination of the wetting area ratio (determined by the pore size cutoff), actual contact angle of wetting surfaces, and fluid saturation significantly impacted T2 distributions of fluid-saturated mixed-wet rocks. Simulations and experiments showed that three NMR T2-based wettability indices were inappropriate for wettability assessment in weakly wet or intermediate-wet rocks. This was because those NMR indices are based on wetting areas and do not account for the effect of local contact angles of wetting surfaces. These investigations have provided a theoretical basis for interpreting NMR T2 characterizations in rocks with mixed wettability.

NMR-data-driven prediction of matrix permeability in sandstone aquifers

Predicting the matrix permeability of subsurface sandstone aquifers is a formidable challenge. One test showing great promise is nuclear magnetic resonance (NMR), as it is the only advanced hydrogeophysical technique non-invasively measuring subsurface pore structure information and being applied in downhole well-log analysis with core calibration. Unfortunately, the classical NMR-based permeability models suffer from more than two empirical constants that limit prediction accuracy greatly. In this study, a NMR-data-driven method was reported with nine machine learning models. Results indicate that the type of input data strongly influences the machine learning modeling of matrix permeability. Using the cumulative T2 relaxation data instead of the original T2 data, a stronger agreement was found between experimental results and NMR estimates of matrix permeability from the gradient boosting decision tree model automatically tuned by GridsearchCV. The correlation coefficient reaches 0.92 with the lowest MSE of 0.12. Due to the nature of “black box” of machine learning model, the visualization and interpretation of the optimal model were performed through the Shapley Additive exPlanations method. The 38th variable of 501 ms in the cumulative T2 spectrum positively contributes the most to the matrix permeability. However, the 26th variable of 31.6 ms provides the largest negative contribution to the final predicted matrix permeability. This work aims to provide a new idea to the “from curve-to-value” prediction process for a rapid and accurate matrix/bedrock permeability estimation via NMR T2 data in deep sandstone aquifers and extensive porous rocks.

Application of Multifractal Analysis Theory to Interpret T2 Cutoffs of NMR Logging Data: A Case Study of Coarse Clastic Rock Reservoirs in Southwestern Bozhong Sag, China

The Paleocene Kongdian Formation coarse clastic rock reservoir in Bozhong Sag is rich in oil and gas resources and has huge exploration potential. However, the coarse clastic rock reservoir has the characteristics of a complex pore structure and strong heterogeneity, which restrict the accuracy of evaluating the reservoir’s physical properties, such as porosity and permeability, for field evaluation. Nuclear magnetic resonance (NMR) technology has become a popular methods for unconventional reservoir evaluation because it can obtain abundant reservoir physical property information and because of its ability to identify fluid characteristics information. The transverse relaxation time (T2) cutoff (T2C) value is an important input parameter in the application of NMR technology. The accuracy of the T2C value affects the accuracy of the reservoir evaluation. The standard method for determining the T2C value requires a series of complicated centrifugation experiments in addition to the NMR experiments, and its application scope is limited by obtaining enough core samples. In this study, 14 core samples from the coarse clastic rock reservoir in the southwestern Bozhong sag of the Bohai Bay Basin were selected, and NMR measurements were carried out under the conditions of fully saturated water and irreducible water to determine the T2C value. Based on the multifractal theory, the NMR T2 spectrum of the saturated sample was analyzed, and the results show that the NMR T2 distribution of the saturated sample has multifractal characteristics, and the multifractal parameter Dq and the singular intensity range Δα have a strong correlation with the T2C value. Thus, based on multiple regression analyses of the multifractal parameters with the experimental T2C value of 10 core samples, we propose a method to predict the T2C value. After applying this method to 4 samples that were not used in the modeling, we confirmed that this method can be used to predict the T2C value of core samples. Furthermore, we expanded this method to the field application of a production well in Bozhong sag by adding an empirical index in the model. The new model can be used to directly calculate the T2C value of NMR logging data, and it does not require any other extra data, such as those from core analysis. This method is applicable in fast reservoir evaluations by only using NMR logging data in the field. The research results improve the accuracy of field NMR logging reservoir evaluations.

Dissolution behaviour in carbonate reservoirs during WAG injection: A preliminary experimental study

In this study, a core flooding experiment using a water-alternating-gas (WAG) injection was conducted to evaluate its impact on the petrophysical properties of an initially oil-saturated heterogeneous carbonate core sample. Carbon dioxide (CO2) and synthetic formation brine were injected (0.5 pore volume CO2 alternating with 0.5 pore volume brine) alternately following establishment of waterflooding residual oil saturation under reservoir conditions. Gas porosity, gas permeability, NMR (nuclear magnetic resonance) T2 measurements, and X-ray CT scanning were conducted preand post-core flooding. The results show that CO2-WAG injection resulted in substantial additional oil recovery (~30 %) under the applied experimental conditions. The results also show an increase in the permeability of the tested sample from 1.5 to 16 mD, which could be attributed to mineral dissolution. X-ray CT imaging shows signs of excessive mineral dissolution and formation of wormhole structures. It is believed that dissolution within the tested core plug caused the WAG fluids to follow the newly wormhole (causing them to enlarge further), and consequently bypassing many parts of the sample. Therefore, despite a significant increase in oil recovery, a large amount of oil is still left behind.

A Novel Method to Enhance the Inversion Speed and Precision of the NMR T2 Spectrum by the TSVD Based Linearized Bregman Iteration

The low-field nuclear magnetic resonance (NMR) technique has been used to probe the pore size distribution and the fluid composition in geophysical prospecting and related fields. However, the speed and accuracy of the existing numerical inversion methods are still challenging due to the ill-posed nature of the first kind Fredholm integral equation and the contamination of the noises. This paper proposes a novel inversion algorithm to accelerate the convergence and enhance the precision using empirical truncated singular value decompositions (TSVD) and the linearized Bregman iteration. The L1 penalty term is applied to construct the objective function, and then the linearized Bregman iteration is utilized to obtain fast convergence. To reduce the complexity of the computation, empirical TSVD is proposed to compress the kernel matrix and determine the appropriate truncated position. This novel inversion method is validated using numerical simulations. The results indicate that the proposed novel method is significantly efficient and can achieve quick and effective data solutions with low signal-to-noise ratios.

Experimental investigation of nanofluid enhanced oil recovery by spontaneous imbibition.

Nanofluids have been recently proposed as new chemical agents for enhanced oil recovery. In this study, in order to reflect the effect of nanofluids on imbibition, the imbibition performance of manganese chloride (MnCl2) solution, sodium dodecylbenzene sulfonate (SDBS) solution, and silica (SiO2) nanofluids were studied by a spontaneous imbibition experiment at 25 °C and 0 MPa. The oil production from pores with different sizes and the imbibition efficiency were tested by nuclear magnetic resonance T2 spectroscopy and metering in spontaneous imbibition. In addition, the interfacial tensions between the imbibition liquids and oil were tested. The changes in the contact angle of the core slice before and after immersing in imbibition liquids were measured. The silica nanofluid is used as the imbibition liquid, and the shift of the T2 spectral peak to the left is not obvious and shifted by only 23.95-25.72 ms, the change in the contact angle is 6.63°-12°, the interfacial tension between the nanofluid and the simulated oil is 0.25-0.41 mN m-1, and the imbibition efficiency was slightly improved with increasing nanoparticle concentration, up to 57.40%, which improved by 16.14% and 32.95%, respectively, compared to the surfactant solution and the manganese chloride solution. This shows that the silica nanofluid can effectively improve oil production in small pores, reduce oil-water interfacial tension, and change rock wettability.

Research on the Performance of Titanium Gypsum Concrete Based on Calcium-Silicon-Sulfur Ratio

Based on the high sulfur content in titanium gypsum, the concept of the calcium-silicon-sulfur (Ca/Si/S) ratio was proposed. The Ca/Si/S ratio of concrete was adjusted by changing the titanium gypsum, fly ash, and cement content. The effects of different Ca/Si/S ratios on the mechanical properties, hydration products, and concrete microstructure were investigated by nuclear magnetic resonance, uniaxial compression, and scanning electron microscopy. The result shows: (1) The compressive strength of concrete mixed with titanium gypsum increases first and then decreases with the Ca/Si/S ratio decrease. When the Ca/Si/S ratio is 1:0.85:0.10, the strength reaches the peak and is lower than the blank group. (2) The microstructure indicates the addition of titanium gypsum can effectively stimulate the activity of fly ash. Still, too much or too little titanium gypsum will hamper concrete strength development. (3) Titanium gypsum concrete’s nuclear magnetic resonance T2 spectrum has two characteristic peaks. With the Ca/Si/S ratio decreasing, the micropores in the concrete expand towards the macropores. The compressive strength is negatively correlated with the proportion of macropores and is positively correlated with the proportion of no-capillary pores.

A nuclear magnetic resonance based quantification of pore water distribution in unsaturated soils

This study presents a novel approach to quantify the distribution characteristics of pore water in unsaturated soil by using the nuclear magnetic resonance (NMR) technology. The SWCC and NMR T2 curves under different matric suction were measured during the drying process of soil specimen, and a conversion relationship between T2 and pore radius r of was determined. The method using capillary drainage model to simulate soil drying process was feasible, and pores of unsaturated soil were divided into saturated pore, unsaturated pore, and dry pore when the suction exceeded the air entry value. Pore water distribution curve based on T2 well described the distribution characteristics of pore water in unsaturated soil; the quantification of pore water distribution was realized through a defined index named pore water damage potential (Mcp), and a good mathematical relationship was observed between the relative damage degree and the matric suction. By introducing the Mcp, a modified VG model that reflected the characteristics of pore water distribution was in a good quantitative agreement with the experimental observations. The results achieved provided a valuable reference for revealing the internal mechanism of SWCC and establishing mechanical model of unsaturated soil based on pore water distribution.

Damage constitutive model and mechanical properties of jointed rock mass under hydro-mechanical coupling

The damage evolution law and mechanical property weakening mechanism of jointed rock mass under hydro-mechanical coupling are important scientific problems in deep rock mass excavation. In this paper, the mineral composition of rock specimens was analyzed, and the distribution law of nuclear magnetic resonance T2 spectrum of granite under different axial stress was studied. Then the mechanical properties of jointed rock samples under triaxial stress and seepage pressure were studied. Based on the analysis of the stress field at the joint tip, the damage model of the jointed rock mass was obtained. Furthermore, the tensor of damage variables was analyzed, and the stress intensity factor under seepage pressure was calculated. The seepage pressure promotes the damage evolution process of jointed rock mass specimens and accelerates the strength failure of jointed rock mass specimens. The damage constitutive model of jointed rock mass under seepage pressure was established and verified. Based on the weakening law of seepage pressure, the developed hydro-mechanical coupling constitutive model was customized and imported into FLAC3D with the calculation function of the Mohr-Coulomb constitutive model as the compilation template. A three-dimensional numerical simulation model of underground rock mass engineering was established to analyze the stability of rock mass with different joint dip angles. For the surrounding rock with the dominant angle of 85° in the mine, its stability is poor, and the support needs to be strengthened.

Finite Element Simulation of NMR spectra and spin-spin relaxation of confined fluids under intense magnetic field gradients

Magnetic field gradients, generated as the magnetic response of solids and liquids to an applied field, directly affect the lineshapes and spin-spin relaxation time (T2) in Nuclear Magnetic Resonance (NMR) experiments and, therefore, the interpretation of fluid content in porous rocks. This paper presents a numerical study of pulse NMR spectra and T2 relaxation of fluids under realistic magnetic field gradients using the software COMSOL. Two physics sections are coupled: first, the internal magnetic field gradient is calculated in the whole system, comprising the solid matrix and a fluid mixture of brine and oil inside. Then, in the second, the NMR signal of the liquids, the Free Induction Decay (FID), is obtained by solving the Bloch–Torrey–MacConnell equations over the pore volume. The calculation of the NMR spectrum follows from the FID Fourier Transform (FFT). For the T2 calculations, a CPMG sequence is built by concatenating different studies. The obtained T2 reproduces the bulk value of the relaxation time, the dependence on the pore radii, and the relaxivity value correctly. Different magnetic properties, coupling parameters, pore sizes, and geometries are considered.

Quantitative Investigation on Natural Gas Flooding Characteristics in Tight Oil Cores after Fracturing Based on Nuclear Magnetic Resonance Technique

SummaryTight reservoirs are mainly developed by injecting various gases after fracturing. However, the formed fractures are complex, and different fracture conditions have an important impact on the gas injection effect. In addition, natural gas is considered to be suitable for the development of tight reservoirs in China because of the abundant gas source and no corrosion. For this paper, the natural gas injection experiments were studied by combining mercury intrusion porosimetry (MIP) measurements and nuclear magnetic resonance (NMR) measurements. The method can be used to study the distribution characteristics of core pore structure and the recovery characteristics of oil in different pore spaces.In this work, the tight cores of the Changqing Oil Field were selected for fracturing for the natural gas flooding experiments. At first, the distribution characteristics of the core pore structure were studied based on the MIP and NMR measurements. The conversion relationship between the core pore throat radius and the relaxation time (T2) was decided. The NMR T2 distribution was transformed into the distribution of oil in pore space with different throat radii. Then, the gasflooding experiments were conducted to study the oil recovery law of tight cores with different fracture conditions. Finally, the recovery characteristics of oil in different pore spaces were analyzed based on the NMR results of cores.The results show that the pore throat radius of the core is mainly distributed in the range of 0.001 to 10 μm. The oil is mainly stored in the pore space whose pore throat radius ranges from 0.01 to 1 μm. The natural gas also mainly drives the saturated oil in the pore space with a pore throat radius of 0.01 to 1 μm. The increase in fracture area improves the distribution of oil in the larger pore space. In the process of natural gasflooding, with the increase of gas injection, the oil began to be recovered, and then gas was observed at the end of the core. With the continuous injection of natural gas, the rate of recovering oil gradually slowed down, and finally gas breakthrough occurred. The displacement oil process of the nonfractured core was uniform and slow. However, the oil and gas rapidly flowed along the fracture when the natural gas displaced the oil in the fractured core. The oil in the matrix was poorly recovered. Gasflooding mainly recovered the saturated oil in the matrix of nonfractured cores and the saturated oil in the fracture of fractured cores. As the fracture length increased, the oil recovery became lower and the gas breakthrough occurred earlier. The higher fracture density increased the fracture area, which also increased the oil recovery and caused a more intense gas breakthrough.In this paper, the displacement law of tight oil cores by injecting natural gas and the recovery characteristics of oil in different space pores were illuminated. The results can provide theoretical guidance for the formulation of the natural gas injection development plan in tight reservoirs.

Influence of Sand Production Damage in Unconsolidated Sandstone Reservoirs on Pore Structure Characteristics and Oil Recovery at the Microscopic Scale.

Unconsolidated sandstone reservoirs have low rock strength and are easily damaged by sand production. To evaluate the effect of the microscopic pore system on the degree of recovery and sand production damage, five typical unconsolidated sandstone core samples were selected. The nuclear magnetic resonance T2 spectrum of the water-flooding experiments was used to analyze the oil displacement mechanism and sand production damage of the pore throat structure, and the control mechanism of the injection parameters was used to evaluate the recovery and sand production damage degree. The results showed that the recovery of the unconsolidated sandstone core samples was the highest when the injection volume was 6 PV, and the overall pore throat recovery increased from 14.37 to 48.72%. The recovery and sand production damage increased with increasing injection pressure. The overall pore throat recovery was the highest when the pressure was 20 MPa (48.42%), and the sand production damage index was the maximum when the pressure was 25 MPa (19.98%). Under a lower injection pressure, sand production damage mainly originates from loose sand. The main target of loose sand production damage is a smaller pore throat, and the sand production index increases slowly. The recovery increased with the pressure and increased relatively quickly. Under a higher injection pressure, sand production damage mainly comes from loose sand and skeleton sand, causing damage to both smaller and larger pore throats. The sand production damage is positively correlated with injection pressure. The recovery slows down with an increase in pressure or even decreases.

Sodium diffusion in heterogeneous porous media: Connecting laboratory experiments and simulations

Sodium has been suggested as a tracer for brine in reservoir formations where a significant amount of sodium ion is found mainly in the aqueous phase. Fortunately, the most abundant sodium isotope, 23Na, is Nuclear Magnetic Resonance (NMR) active, making it possible to study the structure and dynamical properties of both bulk and pore confined fluid systems. We investigated the diffusion of key dissolved sodium species in bulk solution and porous media as a function of composition, ionic strength, and permeability of the carbonate samples using NMR relaxometry and pulsed-field gradient (PFG) NMR techniques. We use Indiana limestone as an example of natural porous media and water as a freely diffusible tracer and carrier. We demonstrate apparent diffusion measurement of sodium based on changes in spin–spin relaxation time (T2) signal in pore confinement. The diffusion rate of sodium decreases with increasing counter-ion size. This effect is greater at higher ionic strengths and lower chemical potentials in porous media. The reactive transport code, CrunchFlow, was used to complement the NMR experiments to simulate diffusion behavior in porous media. Combining these two methods provides a powerful approach to estimating effective diffusion coefficients in heterogeneous matrices. The modeling considers the influence of physical properties (porosity and tortuosity) and chemical properties (geochemical composition and chemical gradients). The results presented in this work highlight the advantage of measuring apparent diffusivity using NMR T2 relaxometry in conjunction with numerical simulation to derive effective diffusivity and the corresponding matrix properties (i.e., tortuosity) of the system.

EFFECTS OF MATERNAL VITAMIN D3 LEVELS DURING PREGNANCY ON PORK QUALITY CHARACTERISTICS OF OFFSPRING PIGS

Introduction. Maternal vitamin D3 levels have long lasting consequences on meat quality and growth performance. The present study was conducted to investigate maternal vitamin D3 status in sows during pregnancy on meat quality attributes, chemical composition, and low field nuclear magnetic resonance (LF-NMR) T2 relaxation times of longissimus dorsi muscle in offspring pigs.
 Materials and methods. A total of 27 sows (41st day of pregnancy) were randomly allotted to low (LD), normal (ND), and high (HD) dietary vitamin D3 groups (containing 200, 800 and 3200 IU of vitamin D3/kg experimental diet, respectively). In each group were 3 replicates with 3 sows per replicate. From parturition to weaning, all lactating sows fed the ND diet. At 150 days of age, six offspring pigs in each group (2 offspring pigs with similar body weight per replicate, gender balanced) were selected and slaughtered to evaluate pork quality characteristics.
 Results and discussion. The results showed that maternal vitamin D3 supplementation decreased L*, b* value, shear force, cooking loss, T21 and T22 relaxation times, while increased a* value, pH value, water holding capacity (WHC), crude protein (CP), crude fat (CF), calcium (Ca), and phosphorus (P) content in longissimus dorsi muscle of offspring pigs. These results demonstrated that high-dose maternal vitamin D3 level could improve meat quality characteristics of offspring pigs, and it will provide a scienfific basis for the early nutritional regulation of offspring prok quality.
 Conclusions. The findings indicated that maternal vitamin D3 supplementation has positive effects on quality characteristics of longissimus dorsi muscle, and improved eating quality of meat in offspring pigs.

The influence of freeze–thaw cycles on the mechanical properties of paleosols: based on a multiscale research

To investigate the multiscale effects of freeze–thaw cycles on the mechanical properties and structural damage of paleosols, remodeled paleosol specimens at natural moisture content were subjected to multiple freeze–thaw cycles, followed by scanning electron microscopy, nuclear magnetic resonance (NMR) pore testing, and triaxial shear testing, and then the shear strength deterioration mechanism of paleosols was elaborated on from three aspects: fine, mesoscopic, and macroscopic. The main experimental results were as follows: (1) at the fine level, the NMR T2 spectrum distribution curve showed one primary and two secondary peaks, in which the main spectrum occupied the majority, and the spectrum area showed an exponential function distribution relationship with the number of freeze–thaw cycles. With the accumulation of freeze–thaw cycles, the medium and large pores increased significantly. (2) At the mesoscopic level, when the specimens underwent freeze–thaw cycles, the interparticle contact pattern and particle morphology changed and the particle roundness increased. As the freeze–thaw cycle continued, fissures gradually developed and increased the most after the first freeze–thaw cycle, but the probability entropy of soil particles showed a decreasing trend with the increase of the number of freez–thaw cycles. (3) At the macro level, the number of freeze–thaw cycles gradually accumulated, the specimen stress–strain curve softened significantly, the shear strength deterioration effect was obvious, the deterioration value was the largest after one freeze–thaw cycle and gradually stabilized after 10 cycles, and the deterioration effect of cohesion was greater than that of the internal friction angle.

Classification of Tight Sandstone Reservoirs Based on the Nuclear Magnetic Resonance T2 Distribution: A Case Study on the Shaximiao Formation in Central Sichuan, China

The classification of tight sandstone reservoirs is of significance for hydrocarbon exploration and production. Different from conventional reservoirs, tight sandstone reservoirs are characterized by a complex pore structure and strong heterogeneity. Tight sandstone reservoir classification may not be reliable with conventional reservoir classification methods. In this paper, a classification method of tight sandstone reservoirs was proposed on the basis of the nuclear magnetic resonance (NMR) transverse relaxation (T2) distribution, and it was employed for reservoir classification on the Shaximiao Formation in central Sichuan, China. As a result of the complex pore structure of a tight sandstone reservoir, the NMR T2 distribution is characterized by the presence of multiple peaks. Therefore, the three-peak Gaussian function was used to fit the T2 distribution and obtain the characteristic parameters. On the basis of high-pressure mercury intrusion (HPMI) experiments, the correlation between the characteristic parameters of the NMR T2 distribution and pore structure parameters and petrophysical properties was analyzed, and then the optimal characteristic parameters of the NMR T2 distribution were selected according to the results of correlation analysis and used to establish the pore structure index. In combination of petrophysical properties with the pore structure index, the model for classification of tight sandstone reservoirs was established by the naive Bayesian method based on hierarchical clustering, and the accuracy of the model was verified by k-fold cross-validation. Finally, the effectiveness of the proposed method was verified using the actual NMR logging data in conjunction with the oil test data. The results showed that the method for classification of tight sandstone reservoirs based on the NMR T2 distribution can provide effective guidance for production capacity evaluation.

Determination of in situ hydrocarbon contents in shale oil plays. Part 2: Two-dimensional nuclear magnetic resonance (2D NMR) as a potential approach to characterize preserved cores

Determination of the in situ hydrocarbon content and occurrence characteristics of shale oil reservoirs is crucial for reserve prediction and efficient development. Although many scholars have studied the fluid saturation of conventional coring shale using solvent and thermal extraction methods, with time consuming and only volumes or mass of oil and water were given, which could not reveal the occurrence characteristics of oil and water in samples. This study proposes a two-dimensional (2D) nuclear magnetic resonance (NMR) T1–T2 mapping method to systematically assess the in situ water and oil content, distribution, and their evaporative loss laws of preserved shales. Sixty preserved shales with different maturities were selected from the first member of the Qingshankou Formation in the northern Songliao Basin, China. Each preserved sample was divided into two parts and analyzed using a high-frequency (22 MHz) NMR and Dean–Stark extraction respectively. Furthermore, a time-elapsed NMR T1–T2 mapping was performed on a preserved shale core sample after varying the exposure times in an open environment. Results show that T1–T2 mapping can be used to determine the porosity, water and oil contents, and saturations of preserved shales. Oil content measured using T1–T2 mapping was consistent with that obtained by Dean–Stark extraction, with a correlation coefficient (R2) as high as 90% when crude oils of varying maturities were used for NMR calibration.The T1/T2 of oil inpreserved shale showed a negative trend with maturity, and the T2 of oil was often higher than that of water, indicating oil is enriched in the relative macropores. After exposing the preserved shale to open air for 150–200 h, 1) the total loss ratio of oil and water was approximately 80%; 2)the fluid evaporation loss rate follows the orders: oil > water and fluids in larger pores > smaller pores; 3)the free oil content decreased by approximately 92%, whereas the absorbed oil content varied slightly. As a nondestructive, rapid, and minimal sample preparation method, 2D NMR can efficiently assess both fluids content and distribution of a core sample in only 5 min, thus displaying its great advantages and applications for preserved shales with fast fluid evaporation to determine the in situ fluid content.