Artificial Shale Research Articles

A comprehensive assessment of nanomaterials reinforced lightweight aggregate concrete containing high-volume artificial shale ceramsite

Poor mechanical behavior of lightweight aggregate concrete limits its application. To improve this situation, nano-silica (NS) and carbon nanotubes (CNTs) were used in this study. The impact of NS and CNTs on fresh and mechanical properties, general microstructure and interfacial transition zone (ITZ) of lightweight aggregate concrete containing high-volume artificial shale ceramsite (ASC-LAC) was evaluated. Furthermore, economic sustainability was conducted to provide a comprehensive evaluation on the overall performance of ASC-LAC. Results showed that the synergic effects of NS and CNTs were positive when the replacement of NS and CNTs was smaller than 2.5 wt% and 0.25 wt%, respectively. Incorporating NS caused a decrease in both slump and air content, while adding CNTs caused a decrease in slump but a slight increase in air content. ASC-LAC incorporating 1.0 wt% NS and 0.10 wt% CNTs was found to have the best mechanical performance given a rise of 47.7 % and 17.0 % in compressive and flexural strength, respectively. However, given a higher NS and CNTs replacement (i.e., 2.5 wt% and 0.25 wt%, respectively), a worsened ITZ with less C-S-H gel was identified, which was responsible for the decreased concrete performance. Multi-criteria optimization results showed that ASC-LAC at a mono doped of 1.0 wt% NS or 0.10 wt% CNTs or combined 1.0 wt% NS and 0.10 wt% CNTs had better overall performance considering five-dimensional indices, i.e., slump, air content, compressive strength, flexural strength and cost efficiency.

Quantitative characterizations of anisotropic dynamic properties in organic-rich shale with different kerogen content

Understanding the quantitative responses of anisotropic dynamic properties in organic-rich shale with different kerogen content (KC) is of great significance in hydrocarbon exploration and development. Conducting controlled experiments with a single variable is challenging for natural shales due to their high variations in components, diagenesis conditions, or pore fluid. We employed the hot-pressing technique to construct 11 well-controlled artificial shale with varying KC. These artificial shale samples were successive machined into prismatic shape for ultrasonic measurements along different directions. Observations revealed bedding perpendicular P-wave velocities are more sensitive to the increasing KC than bedding paralleling velocities due to the preferential alignments of kerogen. All elastic stiffnesses except C13 are generally decreasing with the increasing KC, the variation of C11 and C33 on kerogen content are more sensitive than those of C44 and C66. Apparent dynamic mechanical parameters (ν and E) were found to have linear correlation with the true ones from complete anisotropic equations independent of KC, which hold value towards the interpretation of well logs consistently across formations. Anisotropic mechanical parameters (ΔE and brittleness ΔB) tend to decrease with the reducing KC, with ΔB showing great sensitivity to KC variations. In the range of low KC (<10%), the VP/VS ratio demonstrated a linearly negative correlation with KC, and the VP/VS ratio magnitude of less than 1.75 may serve as a significant characterization for highly organic-rich (>10%) shale, compilation of data from natural organic rich-shales globally verified the similar systematic relationships that can be empirically used to predict the fraction of KC in shales.

Chemical structure and thermal maturation of the vitrinite-like maceral in the Lower Paleozoic shales: Insights from Raman spectroscopy of artificial pyrolysis residues

Vitrinite-like macerals (VLM) are ubiquitous in many pre-Devonian marine shales. The thermal maturity of these sediments, which are devoid of classical vitrinite, has been commonly evaluated using the reflectance of VLM as a substitute for vitrinite reflectance, but the molecular structure of VLM itself has seldom been examined. In this study, the immature VLM-rich Cambrian Alum shales from Scandinavia were selected and pyrolyzed at 300–550 ℃ to acquire a series of artificial pyrolysis samples with different ranks. The molecular structures and thermal evolution of VLM in natural and artificial shales were investigated by Raman spectroscopy. These Raman spectra and optical properties were compared with those of vitrinite, graptolite, and solid bitumen. The Raman spectra of VLM in natural samples of variable thermal maturities and immature samples altered by pyrolysis were generally comparable. The reduction of WD (D band position) and FWHM-G (full width at half maximum of the G band) and the increase of WG (G band position) and AD/AG (area ratio) in VLM spectra, which parallels the increasing thermal maturity of the samples, indicated an increased degree of aromatic conjugation and more structurally ordered carbon moieties. The increase in wavenumber difference between the D and G bands positions in Raman spectra (Raman band separation, RBS) demonstrated the gradual removal of oxygen-containing functional groups, the loss of aliphatic side chains, and the increased degree of aromatic ring condensation. In this way, with progressing maturity, the molecular structure of VLM gradually evolved from heterocyclic aromatic compounds and amorphous carbon structures at low maturity levels to structurally better-ordered, larger, aromatic fused-ring units characteristic of post-to-over mature samples. These molecular structures and their changes associated with thermal maturation were similar to those known from classical vitrinite. Aside from its reflectance, the thermal maturity of VLM-rich sediments could be assessed by Raman spectroscopy, notably through the RBS parameter of VLM Raman spectra, which closely correlated with VLM random reflectance throughout the reflectance interval between 0.5% and 5.5%.

The investigation of multiple factors on elastic anisotropy of artificial shale based on the orthogonal experiment

Elastic anisotropy of shales is critical to accurate constraints for rock physical models, quantitative interpretation and hydraulic fracturing. However, the causes of elastic anisotropy of shales are very complicated, and the understanding of how multiple influence factors affect the elastic anisotropy of shales is still not clear. Hence, the orthogonal experiment, as an effective multiple factors experimental method, is adopted in this study to analyze the effect of multiple factors for shale elastic anisotropy. Three factors, clay content, organic matter (OM) content and compaction stress are selected as independent variables, the orthogonal test table L16(43) with four levels for each factor is adopted. According to the designed orthogonal table, sixteen artificial shales are constructed based on the cold-pressing method, and all the dry artificial shales are measured by the ultrasonic measurements. The influence of each factor on the elastic anisotropy and the sensitivity orders of three factors are obtained using the range analysis. The orders of sensitivity for selected factors follow the sequence clay content > compaction stress > OM content for velocity anisotropy parameters. The compaction mechanism of artificial shales is also discussed by the compaction factor, which are positively correlated with the velocity anisotropy parameters. The clay platelets orientation distribution function (ODF) of samples is evaluated by a theoretical model, the ODF coefficients are significantly affected by the clay content and compaction stress, and W200 are much more sensitive to these factors than W400. The results can provide a critical rock physics basis for quantitative interpretation and reservoir prediction of the low-maturity or maturity shale reservoir.

Synergistic effects of nano-silica and fly ash on the mechanical properties and durability of internal-cured concrete incorporating artificial shale ceramsite

Artificial shale ceramsite, a lightweight aggregate, has been widely used in construction field. However, it can lessen concrete's mechanical properties due to its porous inner-structure. In this study, nano-silica (NS) and fly ash (FA) was adopted to improve the mechanical performance and durability of internal-cured concrete containing artificial shale ceramsite (ASC-ICC). The synergistic effects of NS and FA on ASC-ICC was assessed by performing hydration degree, mechanical properties, durability under combined sulfate and drying-wetting attack, SEM-EDS and XRD investigations. Results show that incorporating NS and FA has a positive synergistic effect when the dosage of NS and FA is less than 2.5 and 20 wt%, respectively. When the amount of FA increases to 40 wt%, both mechanical properties and durability of ASC-ICC suffer a significant decrease given that the loose microstructure due to insufficient early-age hydration provides more channels for sulfate ions to access the inner concrete. The enhancing effect of NS is improved by the internal curing of ASC since more calcium hydroxide can be produced to participate in the pozzolanic reaction with NS. Mixture C1F2 exhibits the best performance given an increase of 64.29% and 16.26% in the compressive and flexural strength (compared to mixture C0F1) and an increase of 6.46% in the flexural strength (in comparison to the initial value) even after 49 attack cycles (i.e., 294 d), which indicates that the combination of 1.0 wt% NS and 20 wt% FA is most conducive to improving ASC-ICC performance.

Investigations on the Performance of Shotcrete Using Artificial Lightweight Shale Ceramsite as Coarse Aggregate.

In this study, a type of artificial lightweight shale ceramsite (ALSC) was used as the coarse lightweight aggregate for shotcrete (LAS), of which the mechanical properties, chloride penetration ion resistance, and rebound behavior were investigated. Based on the experimental results on compressive, tensile, and bond strength, LAS meet the strength requirements, and the replacement rate of fly ash (FA) and silica fume (SF) are suggested to be kept at ~15% and 10%, respectively, to result in the best mechanical properties of LAS. Adding FA and SF to the mixture significantly improved the chloride ion penetration resistance (CPR) of LAS because of morphology effects and secondary hydration of FA and SF that lead to a denser microstructure of the mixture. The electric flux and chloride ion migration coefficient (DRCM) of LAS decreased by 56% and 67%, respectively, with FA increasing from 0 to 10%. The electric flux and DRCM further decreased by 71% (153C) and 66% (3.24 m2/s), respectively, with FA increasing from 10 to 20%. As 5–10% SF was further added, the electric flux and DRCM of LAS decreased to extremely low levels; for instance, with FA = 10% and SF = 10%, DRCM = 1.61 m2/s, and the electric flux was too small and could be ignored. The contact stresses between aggregate and shotcrete mixtures were measured to investigate the rebound trend of ALSC in shotcrete. According to the analyses of the theoretical model of the rebound behavior of aggregate in shotcrete proposed by Armelin and Banthia, because of the reduced contact stresses between ALSC and mortar and the smaller density of LAS compared with normal-weight aggregate, the rebound rate of ALSC was about half of that of normal-weight aggregate in the shooting process of the shotcrete.

Modeling of Compaction Trends of Anisotropic Elastic Properties of Shales

AbstractModeling of anisotropic elastic moduli of shales is a long‐standing problem in rock physics. Every year, industry and academia conduct hundreds of measurements of elastic and geomechanical properties of shales complemented with detailed mineralogical analysis, electron microscopy, and microtomographic images. However, a predictive theoretical model, which would be able to predict elastic properties of shale based on its detailed characterization, is still lacking. This is due to the complex multicomponent nature of shales which leads to multiparametric character of the modeling. It is extremely challenging to separate and investigate effects of these multiple parameters individually on natural shale samples. In this work, we analyze a unique data set of elastic properties measured on two artificial shales compacted with carefully controlled mineralogy, microstructure, and porosity. We developed a new modeling approach that consists of four steps that take into account effects of (1) the orientation of clay particles, (2) porosity, (3) silt fraction, and (4) the stiffness of contacts between clay particles. Three of these four steps require only measurable parameters and, only at the final step, the effect of unknown parameters (namely, contact properties of clay platelets) are accounted for. Further, the obtained contact properties are shown to be in a good agreement with the contact properties estimated for natural shale samples. Finally, we demonstrate the effect of each of these identified parameters on elastic coefficients and Thomsen's anisotropic parameters.

Laboratory observation of velocity anisotropy affected by clays and microcracks in artificial clay-rich shale samples

Clays play important roles in velocity anisotropy. Unlike brittle minerals, e.g., quartz, ductile clays are commonly more easily affected by mechanical compaction. Hence, clays tend to have a platy structure that might introduce significant velocity anisotropy. Cracks developed in clay-rich shales also have substantial influences on the shale elastic anisotropy and hydraulic stimulation. However, crack developments are sometimes unknown, and the effects of clays and cracks are ambiguous due to the complexity of shale properties. In this study, we construct artificial clay-rich shale samples which contain 40% clays by weight (kaolinite, smectite, and illite, respectively). The artificial shale samples are cored into shale-90 samples (along the bedding-parallel direction) and shale-0 samples (along the bedding-normal direction), and the P- wave and S- wave velocities are measured as the axis pressure (Pa) increases from 15 MPa to 50 MPa, while the confining pressure (Pc) remains at 15 MPa. We evaluate the slopes of the fitting line of velocity versus pressure, and estimate the crack density. We analyze the velocity anisotropy effect from clays and cracks based on laboratory experiments. Clays with an apparent platy grain structure (e.g., illite) introduce higher anisotropy, indicating that velocity anisotropy is dependent on the clay lamination structure. The comparison suggests that the mineralogy effects are the dominant factors in the anisotropy of clay-rich artificial shale samples, and the microcrack effects further enhance the velocity anisotropy.

Synthesis, Property and Mechanism Analysis of a Novel Polyhydroxy Organic Amine Shale Hydration Inhibitor

Based on the adsorption mechanism analysis of polyhydroxy organic compound on a shale surface, a novel polyhydroxy organic amine shale hydration inhibitor N, N, N′, N′-tetrakis (2-hydroxyethyl) ethylenediamine (THEED) was synthesized via a nucleophilic reaction by using diethanolamine and dibromoethane as raw materials. Its structure was characterized by Fourier transform infrared spectrometry (FTIR), Hydrogen Nuclear Magnetic Resonance (1H NMR) and Liquid Chromatography Mass Spectrometry (LCMS). The inhibition performance of THEED was studied by a shale rolling recovery experiment, a linear expansion experiment and the particle size distribution experiment. Results showed the shale rolling recovery rate in 2.0 wt % THEED solutions was up to 89.6% at the rolling condition of 100 °C × 16 h, and the linear expansion height of artificial shale core in 2.0 wt % THEED solutions was just 4.74 mm after 16 h. The average particle size of Na-bentonite (Na-MMT) in fresh water was 8.05 μm, and it was observed that the average particle size has been increased to 124 μm in 2.0 wt % THEED solutions. The shale hydration inhibition mechanism of the novel inhibitor THEED was analyzed by FTIR, Scanning Electron Microscopy (SEM) and X-ray diffractometry (XRD), we concluded that the nice shale hydration inhibition performance of THEED was achieved by means of intercalation and adsorption onto the surface of shale. The superior shale hydration inhibition property makes THEED promising in maintaining wellbore stability in drilling engineering.

Experimental study of waste tire rubber, wood-plastic particles and shale ceramsite on the performance of self-compacting concrete

In recent decades, the utilization of waste tires, plastic and artificial shale ceramsite as alternative fine aggregate to make self-compacting concrete (SCC) has been recognized as an eco-friendly and sustainable method to... | Find, read and cite all the research you need on Tech Science Press

Casing Shear Deformation Analysis of Shale Gas well Based on Shale Formation Strength Reduction Method

The problem of casing deformation caused by fault slippage due to the hydraulic fracturing process in Changning-Weiyuan area, Sichuan Province, has seriously affected the economic benefits of shale gas wells. In order to clarify the relationship between the strength of fractured rock mass, the size of fractured rock mass, and the parameters of cement sheath, based on the characteristics of artificial shale formation during hydraulic fracturing, the shale strength is reduced. The finite element model of the casing, cement sheath and formation is analyzed, and the deformation law of the casing during the shale strength reduction process is analyzed. The results show that the greater the reduction of the strength of the shale, the greater the risk of shear deformation of the casing. The fracturing height has a great influence on the casing deformation, which increases substantially linearly, and the crack width has little effect on the casing deformation. 2 The Young’s modulus of the cement sheath has little effect on the deformation of the casing, and reducing the Young’s modulus of the cement sheath cannot effectively alleviate the deformation of the casing. Increasing the wall thickness of the cement sheath increases the contact area between the cement sheath and the formation, and the degree of deformation increases. 3 increasing the wall thickness of the casing can alleviate the deformation degree of the casing, and the wall thickness of the casing should be optimized during the fracturing process. The research results have a certain guiding role for the prevention and treatment of casing deformation.

Coupled measurements of hydraulic permeability and full stiffness tensor compaction trends in artificial shales

Understanding of compaction trends of elastic and hydraulic properties in anisotropic shales is crucial for exploration of energy resources, ecological disposal of nuclear waste, and hydrogeological applications. However, complexity of the natural shale mineralogy and lack of quality data available for analysis results in poor knowledge of these compaction trends. Careful control over the shale mineralogy, pore fluid composition and applied stresses allows us to simulate the natural environments and acquire quality data on the properties of artificial shales. Here for the first time we present methodology and describe a setup that allows simultaneous acquisition of all five independent elastic constants and extremely low hydraulic permeability values of transversely isotropic artificial shale samples during mechanical compaction experiments (porosity decrease from 40% to 10%). Hydraulic permeabilities of artificially compacted samples are comparable to the ones of natural shales. Permeability drops exponentially with compaction. Silt fraction and clay mineralogy are the two key parameters that are responsible for broad variations of permeability in shales with the same porosity. We provide analytical equations that allow calculating permeability if porosity and silt fraction are known. Elastic constants of clay matrix exhibit positive linear trends with the porosity decrease. Small variations in clay mineralogy have a minor effect on absolute values of elastic coefficients or anisotropy but lead to noticeable increase of the compressional (VP) to shear (VS) velocities ratio at the same porosity. Finally, strong correlations (R2 above 0.95) of the hydraulic permeability with acoustic impedance and VP/VS ratio are observed for all the prepared samples.

Experimental Investigation of Evolution of Pore Structure in Longmaxi Marine Shale Using an Anhydrous Pyrolysis Technique

To better understanding the evolutionary characteristics of pore structure in marine shale with high thermal maturity, a natural Longmaxi marine shale sample from south China with a high equivalent vitrinite reflectance value (Ro = 2.03%) was selected to conduct an anhydrous pyrolysis experiment (500–750 °C), and six artificial shale samples (pyrolysis products) spanning a maturity range from Ro = 2.47% to 4.87% were obtained. Experimental procedures included mercury intrusion, nitrogen adsorption, and carbon dioxide adsorption, and were used to characterize the pore structure. In addition, fractal theory was applied to analyze the heterogeneous pore structure. The results showed that this sample suite had large differences in macropore, mesopore, and micropore volume (PV), as well as specific surface area (SSA) and pore size distributions (PSD), at different temperatures. Micropore, mesopore, and macropore content increased, from being unheated to 600 °C, which caused the pore structure to become more complex. The content of small diameter pores (micropores and fine mesopores, &lt;10 nm) decreased and pores with large diameters (large mesopores and macropores, &gt;10 nm) slightly increased from 600 to 750 °C. Fractal analysis showed that larger pore sizes had more complicated pore structure in this stage. The variance in pore structure for samples during pyrolysis was related to the further transformation of organic matter and PSD rearrangement. According to the data in this study, two stages were proposed for the pore evolution for marine shale with high thermal maturity.

Theoretical Modeling of Dielectric Properties of Artificial Shales

Accurately modeling the anisotropic dielectric properties of shales is important for the interpretation of dielectric data acquired from shales as source rocks and unconventional reservoirs. We have developed a multiphase incremental model for the frequency dependent anisotropic dielectric properties of sedimentary rocks and presented an approach based on the developed model to simulate the measured anisotropic dielectric behaviors of artificial shales. The new model was built based on the theoretical basis of differential effective medium models for any number of mineral grain components aligned in any direction and was shown to be independent of the mixing order. The model incorporates the measured orientation distribution function of the clay particles to determine the shale dielectric anisotropy, and the frequency dependent dielectric behaviors of the wet clay minerals are obtained by inverting the dielectric properties of the artificial sample composed of clay and the same brine as in other artificial shales. The modeling technique combined important polarization mechanisms in the intermediate frequency range and was shown to give satisfactory fit to the measured frequency dependent anisotropic relative permittivity and conductivity of the artificial shales with varying silt contents by using a reasonable aspect ratio and constant dielectric parameters for the silt grains.

Compaction trends of full stiffness tensor and fluid permeability in artificial shales

Summary We present a methodology and describe a set-up that allows simultaneous acquisition of all five elastic coefficients of a transversely isotropic (TI) medium and its permeability in the direction parallel to the symmetry axis during mechanical compaction experiments. We apply the approach to synthetic shale samples and investigate the role of composition and applied stress on their elastic and transport properties. Compaction trends for the five elastic coefficients that fully characterize TI anisotropy of artificial shales are obtained for a porosity range from 40 per cent to 15 per cent. A linear increase of elastic coefficients with decreasing porosity is observed. The permeability acquired with the pressure-oscillation technique exhibits exponential decrease with decreasing porosity. Strong correlations are observed between an axial fluid permeability and seismic attributes, namely, VP/VS ratio and acoustic impedance, measured in the same direction. These correlations might be used to derive permeability of shales from seismic data given that their mineralogical composition is known.

Evolution of nanopore structure in lacustrine organic-rich shales during thermal maturation from hydrous pyrolysis, Minhe Basin, Northwest China

The nanometer-scaled pore systems of gas shale reservoirs have a prominent contribution for gas storage. To obtain information about the characteristics of the nanopore structure within lacustrine organic-rich shales during their thermal evolution, artificial shale samples with different thermal maturities were obtained from a hydrous pyrolysis experiment. Nitrogen adsorption, field emission scanning electron microscopy, and porosity tests were used to investigate the characteristic pore structures of lacustrine shales with different thermal maturities from the Minhe Basin. The results show that the total organic carbon content decreased from 41.89% (unheated) to 27.7% (370℃) and that organic pores, intragranular pores of pyrite, and intergranular pores of clay minerals began to form with an increase in the simulated temperature and pressure. The porosity increased from 3.57% (unheated) to 26.09% (350℃) and then decreased to 20% (370℃) on the whole. The pore sizes were distributed from 1.7 to 500 nm, and the average pore diameter first showed a decreasing trend and then an increasing trend. The cumulative pore volume and cumulative specific surface area both presented a slowly increasing trend from an unheated status to 325℃, exhibited a rapid increase at 350℃, and then showed a slow increase at 370℃. This study could provide a reference for the exploration of shale gas in lacustrine shales with different thermal maturities.

Experimental Characterization of Dielectric Properties in Fluid Saturated Artificial Shales

High dielectric contrast between water and hydrocarbons provides a useful method for distinguishing between producible layers of reservoir rocks and surrounding media. Dielectric response at high frequencies is related to the moisture content of rocks. Correlations between the dielectric permittivity and specific surface area can be used for the estimation of elastic and geomechanical properties of rocks. Knowledge of dielectric loss-factor and relaxation frequency in shales is critical for the design of techniques for effective hydrocarbon extraction and production from unconventional reservoirs. Although applicability of dielectric measurements is intriguing, the data interpretation is very challenging due to many factors influencing the dielectric response. For instance, dielectric permittivity is determined by mineralogical composition of solid fraction, volumetric content and composition of saturating fluid, rock microstructure and geometrical features of its solid components and pore space, temperature, and pressure. In this experimental study, we investigate the frequency dependent dielectric properties of artificial shale rocks prepared from silt-clay mixtures via mechanical compaction. Samples are prepared with various clay contents and pore fluids of different salinity and cation compositions. Measurements of dielectric properties are conducted in two orientations to investigate the dielectric anisotropy as the samples acquire strongly oriented microstructures during the compaction process.

Compaction of quartz-kaolinite mixtures: The influence of the pore fluid composition on the development of their microstructure and elastic anisotropy

Shales play important roles in sedimentary basins, acting as both seals and reservoir rocks, and knowledge of their anisotropic velocity trends is of practical importance for correct seismic image processing and inversion, and for seismic to well tie. Whereas velocity-depth trends are extensively studied for conventional reservoirs and critical factors that control their compaction are well understood, little work has been done on shales. Compaction trends of shaly formations are controlled by a number of parameters such as clay mineralogy, silt fraction and depositional environment. This large number of parameters, which are difficult if not impossible to control in naturally deposited shales, make the study of shale compaction a statistically complex, multivariate and non-unique problem. In such a situation, laboratory experiments that allow precise planning of mixture mineralogy and chemical compositions of pore fluids as well as an accurate control of microstructural changes seems to be an attractive alternative.In this work, we have conducted two sets of compaction experiments on quartz-kaolinite mixtures with 100%, 75% and 60% of kaolinite powders (dry weight). In the first set, the use of a KCl brine solution to saturate the mixtures has led to the aggregation of the existing clay particles with each other and with silt particles. In the other set, the mixtures have been treated with a dispersant to separate existing clay aggregates into individual platelets. The mixtures have been further compacted in an oedometric cell at uniaxial stresses up to 30 MPa. Compressional (in vertical and horizontal directions) and shear (in vertical direction) ultrasonic wave velocities have been measured during these compaction experiments and the P-wave anisotropy has been estimated. The effect of the chemical composition of the pore fluid on shale microstructure has been studied using the micro-CT and SEM image analysis. Neutron diffraction experiments have been conducted to understand the orientation of clay platelets at the final stage of the compaction.Our research shows that the chemical composition of pore fluid significantly affects the microstructure of the clay component and via this the elastic properties of the artificial shales. The samples with the dispersed clay microstructure (DCM) have shown larger elastic anisotropies at the same porosity and significantly larger anisotropies at the same stress. The dispersed clay platelets have orientated preferably parallel to bedding plane while the clay platelets in the samples with the aggregated clay microstructure (ACM) show a wider angle distribution in the absence of quartz particles (100% kaolinite samples). In the case of 40% of quartz fraction though, the clay particles are more aligned in the ACM sample than in the DCM one.

Shale gas in China: Characteristics, challenges and prospects (I)

Abstract This paper mainly discusses the industrialization progress, “sweet spot” evaluation criterion, E&P technologies, success experiences, challenges and prospects of China's shale gas. Based on the geologic and engineering parameters of the Fuling, Changning and Weiyuan shale gas fields in the Sichuan Basin, this paper points out that China's shale gas has its particularity. The discoveries of super-giant marine shale gas fields with high evolution degree (R o =2.0%−3.5%) and ultrahigh pressure (pressure coefficient=1.3−2.1) in southern China is of important scientific significance and practical value to ancient marine shale gas exploration and development to China and even the world. It's proposed that shale gas “sweet spots” must be characterized by high gas content, excellent frackability and good economy etc. The key indicators to determine the shale gas enrichment interval and trajectory of horizontal wells include “four highs”, that is high TOC (>3.0%), high porosity (>3.0%), high gas content (>3.0 m 3 /t) and high formation pressure (pressure coefficient>1.3), and “two well-developed” (well-developed beddings and well-developed micro-fractures). It's suggested that horizontal well laneway be designed in the middle of high pressure compartment between the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation. The mode of forming “artificial shale gas reservoir” by “fracturing micro-reservoir group” is proposed and the mechanisms of “closing-in after fracturing, limiting production through pressure control” are revealed. Several key technologies (such as three-dimensional seismic survey and micro-seismic monitoring of fracturing, horizontal wells, “factory-like” or industrialized production mode, etc.) were formed. Some successful experiences (such as “sweet spot” selection, horizontal well laneway control, horizontal length optimization and “factory-like” production mode, etc.) were obtained. The four main challenges to realize large-scale production of shale gas in China include uncertainty of shale gas resources, breakthroughs in key technologies and equipment of shale gas exploration and development below 3 500 m, lower cost of production, as well as water resources and environment protection. It is predicted that the recoverable resources of the Lower Paleozoic marine shale gas in southern China are approximately 8.8×10 12 m 3 , among which the recoverable resources in the Sichuan Basin are 4.5×10 12 m 3 in the favorable area of 4.0×10 4 km 2 . The productivity of (200−300)×10 8 m 3 /a is predicted to be realized by 2020 when the integrated revolution of “theory, technology, production and cost” is realized in Chinese shale gas exploration and development. It is expected in the future to be built “Southwest Daqing Oilfield (Gas Daqing)” in Sichuan Basin with conventional and unconventional natural gas production.

Changes in elastic properties of artificial shales due to compaction

SUMMARY The effects of compaction on elastic properties of shales and their anisotropy are important for seismic imaging, seismic to well tie and borehole stability issues. Compaction trends in shales remain poorly studied, but it is well known that porosity of shales rapidly decreases with the increase of burial depth due to mechanical compaction and chemical transformation of clays in particular. These processes affect all the physical properties of shales including their elastic moduli, electrical conductivity and permeability. In this experimental work we study changes in the anisotropic elastic properties of artificial shales caused by mechanical compaction. Investigation of anisotropy is performed on two specimens made of kaolinite and quartz powder mixtures: (1) 75% / 25 % and (2) 60% / 40%, respectively. An uniaxial stress is applied progressively to achieve distinct levels of porosity. Ultrasonic P- and S-wave velocities in the specimens are measured at every stage of the compaction. Thomsen’s anisotropy parameters are calculated from these velocities. Qualitative characteristic of microfabric anisotropy is performed using micro-CT image analysis. The results allow to conclude that at a given level of porosity of the specimen 2, with 40 % of quartz, has higher compressional and shear velocities than the specimen 1 with only 25% of quartz. However, the specimen 1 shows higher degree of elastic anisotropy than the specimen 2 due to higher fraction of anisotropic clay.