Nuclear Magnetic Resonance T2 Research Articles

A New Method for Determining Tight Sandstone Permeability Based on the Characteristic Parameters of the NMR T 2 Distribution

This paper proposes a new method to determine the permeability of tight sandstone using characteristic parameters of the nuclear magnetic resonance (NMR) transverse relaxation time (T 2) distribution. First, the Swanson parameters (T s ) and Capillary–Parachor parameters (T cp) are calculated as the percolation characteristic parameters (T c) of NMR T 2 distribution. The logarithmic mean (T lm), arithmetic mean (T am), and harmonic mean (T hm) are calculated as the pore structure characteristic parameters (T m) of NMR T 2 distribution. T x , the transverse relaxation time when the value of Y-axis is x% in the normalized accumulated T 2 distribution curve accumulated from long relaxation time part to short relaxation time part, is selected as a characteristic parameter of pore size distribution. Second, different T c, T m, T x , and NMR porosity (T por) values are selected to establish single-, double-, three-, and four- parameter models for estimating permeability. An analysis of the relationships between calculated permeabilities of different models and measured permeability in tight sandstone rocks indicated that the four-parameter model based on T cp, T 40, T am, and T por was the best model. Moreover, this model was superior to the calibrated Timur model and the calibrated SDR model for calculating permeability in tight sandstone reservoirs.

Dynamics of unloaded and green tea extract loaded lecithin based liposomal dispersions investigated by nuclear magnetic resonance T2 relaxation

Liposomes are lipid bilayer vesicles that can be used as encapsulation systems for bioactive agents to provide increased protection against environmental stresses (such as pH or temperature extremes). Time Domain Nuclear Magnetic Resonance (TD-NMR) that is based on differentiation of specimen contents with respect to magnetic relaxation rates provides detailed information on amount, state and distribution of water and oil and provide reproducible results on the samples. These make TD-NMR particularly suitable for time-dependent monitoring of emulsion system dynamics. In this study, spin-spin (T2) relaxation times and relaxation spectra were used for characterizing green tea extract loaded and unloaded liposomes prepared with soy (S75) and egg lecithins (E80) by different preparation methods (such as homogenization type, pressure and solvent type). Mean particle sizes of liposomes were found to be the most influential factor in shaping mono-exponential T2 relaxation times. The differences in particle sizes of E80 and S75 samples along with samples with different homogenization pressures could be monitored with T2 relaxation times. Additionally, T2 relaxation times were found to be correlated with particle shape irregularity, and chemical instability of samples due to lipid oxidation. With relaxation spectrum analysis, particular components in the sample could be distinguished (internal/external water and lipid bilayers), which gave more elaborate results on mechanisms of instability.

A new empirical method for constructing capillary pressure curves from conventional logs in low-permeability sandstones

Pore structure reflected from capillary pressure curves plays an important role in low-permeability formation evaluation. It is a common way to construct capillary pressure curves by Nuclear Magnetic Resonance (NMR) log. However, the method’s efficiency will be severely affected if there is no NMR log data or it cannot reflect pore structure well. Therefore, on the basis of J function and diagenetic facies classification, a new empirical model for constructing capillary pressure curves from conventional logs is proposed here as a solution to the problem. This model includes porosity and the relative value of natural gamma rays as independent variables and the saturation of mercury injection as a dependent variable. According to the 51 core experimental data sets of three diagenetic facies from the bottom of the Upper Triassic in the western Ordos Basin, China, the model’s parameters in each diagenetic facies are calibrated. Both self-checking and extrapolation tests show a positive effect, which demonstrates the high reliability of the proposed capillary pressure curve construction model. Based on the constructed capillary pressure curves, NMR T 2 spectra under fully brine-saturated conditions are mapped by a piecewise power function. A field study is then presented. Agreement can be seen between the mapped NMR T 2 spectra and the MRIL-P log data in the location of the major peak, right boundary, distribution characteristics and T 2 logarithmic mean value. In addition, the capillary pressure curve construction model proposed in this paper is not affected by special log data or formation condition. It is of great importance in evaluating pore structure, predicting oil production and identifying oil layers through NMR log data in low-permeability sandstones.

Nuclear magnetic resonance T2 spectrum: multifractal characteristics and pore structure evaluation

Pore structure characteristics are important to oil and gas exploration in complex low-permeability reservoirs. Using multifractal theory and nuclear magnetic resonance (NMR), we studied the pore structure of low-permeability sandstone rocks from the 4th Member (ES4) of the Shahejie Formation in the south slope of the Dongying Sag. We used the existing pore structure data from petrophysics, core slices, and mercury injection tests to classify the pore structure into three categories and five subcategories. Then, the T 2 spectra of samples with different pore structures were interpolated, and the one- and three-dimensional fractal dimensions and the multifractal spectrum were obtained. Parameters α (intensity of singularity) and f (α) (density of distribution) were extracted from the multifractal spectra. The differences in the three fractal dimensions suggest that the pore structure types correlate with α and f (α). The results calculated based on the multifractal spectrum is consistent with that of the core slices and mercury injection. Finally, the proposed method was applied to an actual logging profile to evaluate the pore structure of low-permeability sandstone reservoirs.

Combined interpretation of NMR and TGA measurements to quantify the impact of relative humidity on hydration of clay minerals

Reliability of core measurement and analysis can be highly dependent on the humidity of measurement environment. However, impact of humidity on core measurements is often assumed to be negligible. This paper quantified the influence of relative humidity on the hydration of clay minerals (i.e., montmorillonite, illite, chlorite, and kaolinite) using nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA). The influence of clay hydration on the assessment of fluid volume in organic-rich mudrocks was also investigated. The NMR T2 (spin-spin relaxation) magnetic decay of the clay samples showed existence of water even after the samples were dried in an oven at 95°C. The TGA measurements in clay minerals confirmed the weight loss between 95°C and dehydroxylation temperature, which is approximately the same volume as that recorded by NMR measurement in dried samples. This recorded water volume corresponded to desorption of water molecules bound to interlayer cations (i.e., interlayer water). In the case of the hydration water, the NMR-based water vapor adsorption experiments showed that the volume of water accumulated on the surface of clay minerals increased significantly as the relative humidity increased. The cation exchange capacity (CEC) measurements of clay minerals demonstrated that the volume of hydration water was strongly related to the CEC. The volume of interlayer water was, however, not related to the CEC. In the case of organic-rich mudrocks, water vapor adsorption experiments showed >250% change in water volume when relative humidity varies in the range of 0% to 95%, which further verified the significant impact of humidity on core measurements and the necessity of applying corrections for such an impact.

Nuclear magnetic resonance features of low-permeability reservoirs with complex wettability

The nuclear magnetic resonance T2 spectra of low-permeability reservoirs with complex wettability were studied using the samples from the Chang 8 Member, Upper Triassic Yanchang Formation, Ordos Basin, China. Abnormally high resistivity and normal resistivity core samples were selected. NMR T2 spectra under different wettability and water saturation conditions, contact angles and Amott wettability indexes were designed and tested. The test results show that under fully brine-saturated condition, the NMR T2 spectra of normal resistivity core samples reflect surface relaxation of water, while the samples with abnormally high resistivity exhibit wide unimodal T2 spectrum, consisting of both surface and volume relaxation of water, which indicates that these cores are not fully water-wet after washing oil. In the process of oil displacing water, the NMR T2 spectra of normal resistivity core samples present bimodal feature, and those of abnormal high resistivity core samples (both non-ageing and ageing) mainly show the same unimodal feature as those measured under fully brine-saturated condition. Based on these results, it can be inferred that the wettability change of abnormally high resistivity core samples to oil-wet has basically completed during oil displacing water process, and the ageing process has little effect on the wettability of abnormally high resistivity core samples. In the process of water displacing oil to residual oil, the NMR T2 spectra of abnormally high resistivity core samples generally show trimodal feature, among which, the shortest relaxation time spectrum peaks coincide with that under irreducible water saturation condition, the moderate ones reflect surface and volume relaxation of residual oil, and the longest ones reflect surface and volume relaxation of water in large pores.

T 1–T 2 Correlation and Biopolymer Diffusion Within Human Osteoarthritic Cartilage Measured with Nuclear Magnetic Resonance

Cartilage is a load-bearing tissue that provides smooth articulation during motion of human joints like the knee and hip. Cartilage deterioration in the form of osteoarthritis (OA) causes painful joint motion in more than 100 million patients worldwide, and thus there is great interest in improving our understanding of cartilage to further clinical treatment. Previous studies have examined many aspects of cartilage mechanics, including the flow of interstitial water and repulsion of neighboring glycosaminoglycan chains. However, the contributions of specific molecules to overall tissue properties remain unclear. In this study, we use nuclear magnetic resonance (NMR) diffusometry and relaxometry to examine the molecular dynamics of water and cartilage polymers in OA human articular cartilage. To our knowledge, this is the first identification of two macromolecular populations corresponding to collagen and proteoglycan in human cartilage through their diffusive properties. Further, we performed NMR T1–T2 correlation studies on human cartilage and observed two populations of water distinguished by differing NMR relaxation corresponding to a solid-like component and a liquid-like component. These results provide fundamental insight on the water behavior and polymeric interactions that drive the functional mechanics of cartilage. This study provides a basis to both expand our understanding of basic cartilage mechanics and provide molecular dynamics data for design of novel biomaterials to improve joint health.

AN EXPERIMENTAL STUDY ON RESISTIVITY AND CONDUCTIVE MECHANISM IN LOW‐PERMEABILITY RESERVOIRS WITH COMPLEX WETTABILITY

AbstractAs the clay film developed in low‐permeability lithologic reservoirs absorbs oil, reservoirs become oil‐wet, which results in abnormally high resistivity oil‐water layers and water layers. Such layers bring great challenges to the identification of oil and water layers. In order to make clear the resistivity response characteristics and conductive mechanism in reservoirs with different wettability, cores were selected in Chang 8 formation, Yanchang group, Upper Triassic, the western Ordos basin of China. Experiments were conducted with these samples to simulate the process of oil displacing water, ageing and water displacing oil. In addition, the contact angles of thin slices after washing oil were also tested. The experimental results show that abnormally high resistivity cores are not completely water‐wet after washing oil. However, the formation factors of abnormally high resistivity cores and normal resistivity cores have little difference. In the process of oil displacing water, the relationship between normal resistivity core resistivity index and water saturation is linear in a log‐log plot, while that of abnormally high resistivity cores is convex. The resistivity of abnormally high resistivity cores remains unchanged during ageing process. Combining the analysis of nuclear magnetic resonance (NMR) T2 spectra under different conditions, it can be inferred that wettability of abnormally high resistivity cores has become less water‐wet after oil displacing water, whereas the ageing process has little effect on the wettability. In the process of water displacing oil, the relation between abnormally high resistivity core resistivity index and water saturation is almost linear, which shows that the conductive structure of rock is not changed. Based on the analyses and discussions of these experimental results, a hydrocarbon accumulation mode and the corresponding conductive mechanism are proposed for low‐permeability reservoirs with complex wettability. It shows that abnormally high resistivity water layers are caused by the destruction of continuous conductive paths under oil‐wet condition. The researches and experiments are of great importance for understanding the conductive mechanism, identifying oil and water layers and evaluating water flooded formations in low‐permeability reservoirs with complex wettability.

Lithotype characterizations by Nuclear Magnetic Resonance (NMR): A case study on limestone and associated rocks from the eastern Dahomey Basin, Nigeria

Three representative rock types (limestone, sandstone, and shale) and glauconite samples collected from Ewekoro Quarry, eastern Dahomey Basin in Nigeria were characterized using low field 2 MHz and 20 MHz Nuclear Magnetic Resonance (NMR) techniques. NMR T2 relaxation time decay measurement was conducted on disc samples under partial water-saturation and full water-saturation conditions using CPMG spin-echo routine. The T2 relaxation decay was converted into T2 distribution in the time domain to assess and evaluate the pore size distribution of the samples. Good agreement exists between water content from T2 NMR distributions and water imbibition porosity (WIP) technique. Results show that the most useful characteristics to discriminate the different facies come from full saturation NMR 2 MHz pore size distribution (PSD). Shale facies depict a quasi-unimodal distribution with greater than 90% contribution from clay bound water component (T2s) coupled to capillary bound water component (T2i) centred on 2 ms. The other facies with well connected pore structure show either bimodal or trimodal T2 distribution composed of the similar clay bound water component centred on 0.3 ms and quasi-capillary bound water component centred on 10 ms. But their difference depends on the movable water T2 component (T2l) that does not exist in the glauconite facies (bimodal distribution) while it exists in both the sandstone and limestone facies. The basic difference between the limestone and sandstone facies is related to the longer T2 coupling: T2i and T2l populations are coupled in sandstone generating a single population which convolves both populations (bimodal distribution). Limestone with a trimodal distribution attests to the fact that carbonate rocks have more complex pore system than siliclastic rocks. The degree of pore connectivity is highest in sandstone, followed by limestone and least in glauconite. Therefore a basic/quick NMR log run on samples along a geological formation can provide precise lithofacies characterization with quantitative information on pore size, structure and distributions.

Experimental study on the wettability of Longmaxi gas shale from Jiaoshiba gas field, Sichuan Basin, China

Wettability plays a significant role in exploration and exploitation of shale gas. However, the characterization and evaluation of the wettability of gas shale is a challenge due to the complexity and heterogeneity of components and pore structure. The objective of this paper is to characterize the wettability of gas shale and analyze its relationships with mineral composition, total organic carbon (TOC), and nuclear magnetic resonance (NMR) T2 spectrum by a series of parallel experiments of oil (water) contact angles, NMR, and rock composition analysis on core samples of Longmaxi Formation drilled from shale gas wells in Jiaoshiba gas field, Sichuan Basin, China. The study shows that: ① the gas shale is both water-wet and oil-wet and prone to be oil-wet with water contact angle (θw) ranging from 32° to 41.5°; ② the mineral composition together with TOC jointly affects θw; the quartz mineral and TOC have a negative effect on θw, whereas clay minerals have a positive influence on θw of gas shale; ③ the θw shows distinctive correlations to the characteristic of T2 spectrums; Recognized as a crucial parameter describing the characteristic of the T2 spectrum, the geometric mean of T2 spectrum (T2g) is considered as a preferable model parameter to relate the θw and the T2 spectrum. Thus, novel equations for predicting contact angle by NMR are proposed.

Detecting Microbially Induced Calcite Precipitation in a Model Well-Bore Using Downhole Low-Field NMR.

Microbially induced calcite precipitation (MICP) has been widely researched recently due to its relevance for subsurface engineering applications including sealing leakage pathways and permeability modification. These applications of MICP are inherently difficult to monitor nondestructively in time and space. Nuclear magnetic resonance (NMR) can characterize the pore size distributions, porosity, and permeability of subsurface formations. This investigation used a low-field NMR well-logging probe to monitor MICP in a sand-filled bioreactor, measuring NMR signal amplitude and T2 relaxation over an 8 day experimental period. Following inoculation with the ureolytic bacteria, Sporosarcina pasteurii, and pulsed injections of urea and calcium substrate, the NMR measured water content in the reactor decreased to 76% of its initial value. T2 relaxation distributions bifurcated from a single mode centered about approximately 650 ms into a fast decaying population (T2 less than 10 ms) and a larger population with T2 greater than 1000 ms. The combination of changes in pore volume and surface minerology accounts for the changes in the T2 distributions. Destructive sampling confirmed final porosity was approximately 88% of the original value. These results indicate the low-field NMR well-logging probe is sensitive to the physical and chemical changes caused by MICP in a laboratory bioreactor.

Investigation of Oil Saturation Development behind Spontaneous Imbibition Front Using Nuclear Magnetic Resonance T2

Spontaneous imbibition is a critical mechanism for the development of water-wet fractured reservoirs. In order to improve the ultimate oil recovery, it is important to understand the change of in situ oil saturation during the spontaneous imbibition process. In this study, spontaneous imbibition experiments of two ends open (TEO) are conducted using unconsolidated sand packs. The sand packs are filled with quartz sands of three different particle sizes respectively and are fully oil-saturated. Nuclear magnetic resonance (NMR) T2 is used to monitor the saturation development behind spontaneous imbibition front. For porous media of the same lithology, the imbibition speed and final oil recovery decline with the reduction of average pore size. As the imbibition front constantly moves forward, the change of oil saturation behind the imbibition front does exist, and the major decrease of oil saturation happens in the large pore space. In terms of a particular region behind the spontaneous imbibition front, with the progression of the front, the oil saturation gradient in the area declines. Specifically, the dramatic gradient descent occurs when the spontaneous imbibition front just passes by. The smaller the average pore size is (the larger the mesh of sand is), the more rapid the saturation changes behind imbibition front. For porous media of small pore size, even when the imbibition front has moved far away, oil saturation still changes a lot.

Numerical simulation and parameter analysis of NMR T2–D distributions of tight sandstone saturated with a gas–water two-phase fluid

Two-dimensional nuclear magnetic resonance (NMR) transverse relaxation time–diffusion coefficient (T2–D) distributions can simultaneously provide with pore-fluid parameters of the relaxation and the diffusion, making it possible to distinguish fluid types based on the differences in the two parameters. Hence, T2–D distributions are advantageous over one-dimensional T2 distributions for the identification of fluids because the T2 distributions of different fluids sometimes overlap. In this paper, we simulated NMR measurements with diffusion-editing pulse sequences in tight sandstone saturated with a gas–water two-phase fluid using the random-walk method, and the simulated echo trains were inverted to obtain the T2–D distributions. The effect of fluid saturation on the T2–D distributions of tight sandstone was analyzed, and we studied the effects of the observation parameters–echo spacing TE, magnetic field gradient G, and the group of echo spacing TE1–in diffusion-editing pulse sequences and data signal-to-noise ratios (SNRs) on T2–D distributions of tight sandstone with different gas saturations. The simulated results showed that the water signal in T2–D distributions of tight sandstone gradually moved to a lower diffusion coefficient and a shorter transverse relaxation time with decreasing water saturation, but the gas signal remained fixed in position. With increasing echo spacing TE or magnetic field gradient G, the gas signal in the T2–D distributions moved to shorter transverse relaxation times. The effects of echo spacing TE or magnetic field gradient G on the position of the water signal in the T2–D distributions can be ignored. T2–D distributions for the hybrid echo spacing TE1 group is much better at reflecting pore-fluid information than ones with the short echo spacing TE1 group or the long echo spacing TE1 group. When data SNRs are low, identifying pore fluids based on T2–D distributions is not reliable due to the partial overlap of the water and gas signals.

Insight into the Pore Structure of Tight Sandstones Using NMR and HPMI Measurements

Laboratory measurements including porosity, permeability, high-pressure mercury intrusion (HPMI), nuclear magnetic resonance (NMR) measurements, and microscopic analysis of thin sections and scanning electron microscopy (SEM) were performed to provide insights into the microscopic pore structure of the Xujiahe Formation tight sandstones in Sichuan Basin. The relationships between microscopic pore structure parameters, such as pore geometry, pore size distribution, pore network, and macroscopic consequences, such as permeability, reservoir quality index, Swanson parameter, fractal dimension as well as NMR parameters, have been investigated. The results show that the pore systems are dominated by secondary dissolution porosity with minor amounts of primary porosity and microfracture. NMR T2 pore size distributions are either uni- or multimodal. Long T2 components are not frequently present due to the lack of the macropores, whereas as shorter T2 components dominate the T2 spectrum. T2gm (the geometric mean ...

Determination of NMR T2 cut-off for clay bound water in shales: A case study of Carynginia Formation, Perth Basin, Western Australia

Low-field Nuclear Magnetic Resonance (NMR) has proved to be a valuable tool for the petrophysical characterization of conventional reservoirs, but its effective application to unconventional reservoirs is still under research. Pore structure characterization of shales is particularly challenging due to the complexity of the pore network and the small size of pores.Using low-field NMR, we performed transverse relaxation (T2) experiments on samples from the Perth Basin, Western Australia. The samples were initially saturated with KCl brine to obtain the total NMR porosity and T2 distribution, then centrifuged and finally oven-dried at increasing temperatures. T2 spectra were also acquired after centrifuging and heating the samples. Our results indicate that most of the transverse relaxation occurs below 3ms in saturated samples and that a conventional centrifuge cannot remove water from the smaller pores, making the commonly accepted clay bound water cut-off unsuitable for shales. Furthermore, the results from NMR experiments performed on the oven-dried shale samples suggest that the water content remains relatively constant after heating them above 65°C. The calculated T2 cut-off for clay bound water is between 0.22 and 0.26ms for the samples studied.The methodology presented in this paper can be replicated in other formations to find a suitable T2 value for clay bound water, which can be a good indication of potentially producible porosity and can also be used for permeability estimation.

NMR research on deterioration characteristics of microscopic structure of sandstones in freeze–thaw cycles

In order to study the deterioration characteristics of the microscopic structure of sandstones in freeze–thaw cycles, tests of 180 freeze–thaw cycles were performed on sandstone specimens. The nuclear magnetic resonance (NMR) technique was applied to the measurement of sandstone specimens and analysis of the magnetic resonance imaging. Then, the fractal theory was employed to compute the fractal dimension values of pore development of rocks after different freeze–thaw cycles. The results show that the mass and porosity of rocks grow with the increase of freeze–thaw cycles. According to the NMR T2 distribution of sandstones, the pore sizes of rock specimens increase after 180 freeze–thaw cycles, especially that of the medium-sized and small-sized pores. The spatial distribution of sandstone pores after freeze–thaw cycles has fractal features within certain range, and the fractal dimension of sandstones tends to increase gradually.

New characterization on the petrophysical characteristics of tight sandstone reservoirs in Yanchang Formation Ordos Basin China

New characterization on petrophysical characteristics of tight sandstone reservoirs is of an important value to exploration and exploitation of oil and gas. In this paper, low-field nuclear magnetic resonance, combined with casting thin sections, laser scanning confocal microscopy, scanning electron microscopy and pressure-controlled porosimetry were applied to investigate the pore structure characteristics of nine tight sandstone samples of Yanchang Formation in the Upper Triassic Ordos Basin China. And then, based on the nuclear magnetic resonance T2 spectrum distribution and previous theoretical models, a new model is proposed to predict permeability of tight sandstone reservoirs. Petrophysical characteristics are qualitatively–quantificationally analyzed by a combination of various experiments, which is considered to be a new method that will be widely used in future research. Results indicate that nuclear magnetic resonance T2 spectrum distribution has a close relationship with characteristics of pore types and pore radius distribution. Three T2 spectrum peaks are identified by the relaxation time at 0.1–10 ms, 10–100 ms and >100 ms correspond to micropores (<0.2 µm), mesopores (0.2–4.9 µm) and macropores and fractures (>4.9 µm), respectively, which is well consistent with results from casting thin sections, laser scanning confocal microscopy, scanning electron microscopy and pressure-controlled porosimetry. Compared with the existing several classical models, the new permeability model can better estimate the permeability of tight sandstone samples. The new characterization method and new model can accurately evaluate petrophysical characteristics of typical tight sandstone samples, which have a great value for nuclear magnetic well logging in the exploration and exploitation of tight sandstone reservoirs in the Ordos basin.

Fractal characterization of pores in shales using NMR: A case study from the Lower Cambrian Niutitang Formation in the Middle Yangtze Platform, Southwest China

The fractal dimensions of the Lower Cambrian Niutitang Formation shales in the Middle Yangtze Platform, Southwest China were investigated for pore structure and fractal characteristics using low-field nuclear magnetic resonance (NMR) and N2 adsorption. The field emission scanning electron microscopy (FE-SEM), N2 and CO2 adsorption, NMR, porosity and permeability analysis were performed on six shale samples collected from outcrops. The NMR T2 distributions are consistent with the combination of pore size distributions from N2 and CO2 adsorption. The discrepancy of fractal dimensions between NMR and N2 adsorption resulting from the pore size distribution indicates that NMR fractal dimensions provide more accurate insight into the fractal characteristics of pore structure in shales than N2 adsorption fractal dimensions. The results show that the min(DNMR), with respect to the shorter relaxation times, reflects the roughness of the micropore surface area and the max(DNMR), with respect to the longer relaxation times, reflects pore volume for larger pores. The surface fractal dimension min(DNMR) has a positive correlation with TOC contents and thermal maturity, attributable to the conversion of kerogen and occurrence of organic matter pores. Min(DNMR) increases with increasing quartz content, while max(DNMR) increases with increasing clay content. The porosity from helium gas injection ranges from 2.8% to 6.7%, and permeability is between 3.55 and 6.62 × 10−2 mD. The min(DNMR) has a positive correlation with permeability, and the max(DNMR) exhibits a positive correlation with porosity, indicating that the volume fractal dimension max(DNMR) and surface fractal dimension min(DNMR) provide new indicators to measure porosity and permeability, respectively.

Developing a new NMR-based permeability model for fractured carbonate gas reservoirs

Fractured gas-bearing carbonates are dual porosity media characterized by their complex pore structure and heterogeneity. In reservoir evaluation, the pore fluids could accurately be characterized by nuclear magnetic resonance (NMR) well logging. However, the NMR-based permeability models of fractured carbonates are limited due to lack of understanding of how fractures affect NMR T2 spectrum. Based on the inevitable influence of fractures on reservoir permeability conditions, the current research investigates the effects of different fracture parameters on the resultant T2 spectrums and permeability values of the fractured gas-bearing carbonates. Firstly, a carbonate core sample having no fractures was scanned by CT imaging and its digital model was reconstructed by digital core algorithms. Secondly, fracture having various parameters dip angle, length, aperture and density were inserted into that digital core model to represent different fractured carbonate rock. In the next step, random walk simulation and lattice Boltzmann method were respectively used to simulate corresponding T2 spectrums and permeabilities of these models. We introduced a new kind of spectrum called ‘T2 spectrum of fractures’ or ‘T2f spectrum’ and used it to study effects of different fracture parameters on the NMR T2 spectrum. Finally, based on the quantitative analysis, a novel permeability model was presented for the fractured carbonates. Combined with nuclear magnetic resonance well logging data, this novel approach was used in the interpretation process of well logs. Compared to the existing models of permeability, this new model better matched the real-field permeabilities and greatly improved the accuracy of permeability interpretation in the studied fractured carbonate gas reservoir.

Experimental investigation on the relevance of mechanical properties and porosity of sandstone after hydrochemical erosion

Under the effect of chemical etching, the macroscopic mechanical properties, mesoscopic structure, mineral content, and porosity of rocks undergo significant changes, which can lead to the geological disasters; thus, an understanding of changes in the microscopic and macroscopic structure of rocks after chemical etching is crucial. In this study, uniaxial mechanical tests and nuclear magnetic resonance (NMR) spectroscopy were carried out on sandstone samples that had been previously subjected to chemical erosion under different pH values. The aim was to study changes in properties and mechanical characteristics, including deformation and strength characteristics, of the rock, and microscopic pore variation characteristics, and to perform preliminary studies of the chemical corrosion mechanism. Results show that different chemical solutions have a significant influence on the uniaxial compressive strength, the axial strain corresponding to the peak axial stress, elastic modulus, etc. With the passage of time, porosity increases gradually with exposure to different chemical solutions, and exposure to chemical solutions results in large changes in the NMR T2 curve and T2 spectrum area. Sandstone exposed to different chemical solutions exhibits different corrosion mechanisms; the root cause is the change of mineral.