Unconventional Natural Gas Resources Research Articles

Production Prediction and Influencing Factors Analysis of Horizontal Well Plunger Gas Lift Based on Interpretable Machine Learning

With the development of unconventional natural gas resources, plunger gas lift technology has gained widespread application. Accurately predicting gas production from unconventional gas reservoirs is a crucial step in evaluating the effectiveness of plunger gas lift technology and optimizing its design. However, most existing prediction methods are mechanism-driven, incorporating numerous assumptions and simplifications that make it challenging to fully capture the complex physical processes involved in plunger gas lift technology, ultimately leading to significant errors in capacity prediction. Furthermore, engineering design factors and production system factors associated with plunger gas lift technology can contribute to substantial deviations in gas production forecasts. This study employs three powerful regression algorithms, XGBoost, Random Forest, and SVR, to predict gas production in plunger gas lift wells. This method comprehensively leverages various types of data, including collected engineering design, production system, and production data, directly extracting the underlying patterns within the data through machine learning algorithms to establish a prediction model for gas production in plunger gas lift wells. Among these, the XGBoost algorithm stands out due to its robustness and numerous advantages, such as high accuracy, ability to effectively handle outliers, and reduced risk of overfitting. The results indicate that the XGBoost algorithm exhibits impressive performance, achieving an R2 (coefficient of determination) value of 0.87 for six-fold cross-validation and 0.85 for the test set. Furthermore, to address the “black box” problem (the inability to know the internal working structure and workings of the model and to directly understand the decision-making process), which is commonly associated with conventional machine learning models, the SHAP (Shapley additive explanations) method was utilized to globally and locally interpret the established machine learning model, analyze the main factors (such as starting time of wells, gas–liquid ratio, catcher well inclination angle, etc.) influencing gas production, and enhance the credibility and transparency of the model. Taking plunger gas lift wells in southwest China as an example, the effectiveness and practicality of this method are demonstrated, providing reliable data support for shale gas production prediction, and offering valuable guidance for actual on-site production.

Experimental research on biogenic methane generation pathway and microflora constraints based on in-situ gas-water-microbe

Biogenic coalbed methane (CBM), generated through the biodegradation process of coal organic matter, is a relatively clean and environment-friendly unconventional natural gas resource. To investigate the differences in microbial communities (bacteria and archaea) as well as their environmental influencing factors between the Permian Shanxi Formation (P1s) and Taiyuan Formation (P1t), we systematically collected in-situ CBM samples along with coalbed water and microbial samples from a representative biogenic CBM field – the Baode block locked in the Ordos basin in China. The relative content of biogenic CBM was quantitatively calculated while analyzing gas composition, stable isotope, ion content, and high-throughput sequencing analysis to discuss the generation pathway for biogenic CBM and environmental control factors affecting microbial community structure. The results indicate that P1s and P1t are mixed with early thermogenic gas and secondary biogenic gas genetic types within their respective CBMs. The average proportion for biogenic gas within current coal seams is approximately 62.24 % for P1s, while it reaches around 64.37 % for P1t. Water quality type primarily consists of Na-HCO3 type with total dissolved solids (TDS) measuring at approximately 1015.86 mg/L for P1s, whereas P1t exhibits Na-Ca-HCO3-Cl water type with TDS content reaching about 2314.85 mg/L. Combining dominant Methanobacterium species, along with Δ13C(CO2-CH4) ratios and δD(CH4) versus δD(H2O) indexes, represents the biogenic CBM primarily generated through the hydrogenotrophic methanogenesis pathway. The in-situ bacterial communities of P1s are mainly influenced by Ni, pH, and TDS. The archaeal communities exhibit higher sensitivity to environmental factors (such as Co, pH, Ni, and dissolved organic carbon) than bacteria. These findings hold significant implications for understanding the generation process of biogenic CBM in middle and low-rank coal areas.

Progress, Challenges, and Strategies for China’s Natural Gas Industry Under Carbon-Neutrality Goals

In recent years, the Chinese government has introduced a series of energy-saving, emission-reducing, and environmentally protective policies. These policies have gradually decreased the proportion of high carbon-emitting energy consumption, such as coal, in China’s energy structure. The proportion of natural gas consumption as a clean energy source has been increasing year by year. In the future, with the deepening decarbonization of the energy structure, the applied scope of natural gas utilization will expand, increasing demand. Therefore, this study first evaluated the development of China’s natural gas industry from the perspectives of development evolution, technological applications, and industry achievements. Secondly, based on the current situation of conventional and unconventional natural gas development, both resources and technological potential were analyzed. By taking several typical projects in the natural gas industry as examples, medium- and long-term prospects for natural gas development were planned and predicted. Building on this analysis, we employed the SWOT method to examine the development prospects of China’s natural gas industry and propose development goals. Finally, based on top-level design considerations and previous research analysis, suggestions and measures were proposed for technology implementation, regional layout, industrial chain collaboration, and support policies. These recommendations aim to provide planning support and management references for the development of China’s natural gas industry.

Multichannel Sandstone Thin Sections Identification Based on Improved DeepLab V3 Plus Neural Network.

As an unconventional natural gas resource, tight sandstone gas is primarily stored in the minuscule pores between rocky sand grains. A thorough understanding of the pore structure characteristics of tight sandstone reservoirs is essential for formulating an extraction plan and enhancing the efficiency of gas field development. The pore structure and mineral composition in the sandstone can be directly observed by thin sections. Nevertheless, previous approaches for the automated identification of sandstone thin sections exhibit certain limitations including slow identification, low accuracy, and challenges in the recognition of particle sizes. To achieve more accurate and convenient mineral component identification, this study introduces a multichannel identification method built upon the enhanced DeepLab V3 Plus model. Initially, all 224 × 224 × 3 cross-polarized light (CPL) and orthogonal polarized light (XPL) sandstone thin sections were amalgamated into 224 × 224 × 6 multichannel (six channels) images. Subsequently, multiple networks were employed to train the three polarized data sets, and the optimal semantic segmentation architecture and data set were selected through filtering. Following that, embedding the attention mechanism into the semantic segmentation network enhanced the identification accuracy. Ultimately, mineral sizes were calculated to enable more precise classification and naming of sandstone thin sections. The results show that the new method outperforms in terms of recognition accuracy, achieving 89.8% for Mean PA and 81.2% for Mean IOU. The novel approach's enhanced level of detailing enables more precise identification of mineral composition and pore structure, a crucial aspect in evaluating reservoirs and predicting oil and gas production. It can also provide new insights into identifying and categorizing other thin sections with similar compositions.

Multi-phase retrieval of methane hydrate in natural sediments by cryogenic x-ray computed tomography.

In this study, we observed natural methane (CH4) hydrate sediments, which are a type of unconventional natural gas resources, using x-ray computed tomography (CT). Because CH4 hydrates are formed by hydrogen bonding of water molecules with CH4, material decomposition becomes challenging when CH4 hydrates coexist with liquid or solid water in natural sediments. Tri-contrast (absorption, refraction, and scattering) imaging was performed via diffraction enhanced x-ray CT optics using monochromatic synchrotron x rays. The quantitative characterization of the contrast changes successfully enabled the decomposition of CH4 hydrates coexisting with frozen seawater (ice) in natural sediments obtained from the Okhotsk Sea. This study reveals complementary structural information about the microtexture and spatial relation among CH4 hydrates, ice, and pores by utilizing the distinct physical properties of x rays when passing through the materials. These results highlight the exceptional capabilities of high-resolution multicontrast x-ray tomography in materials science and geoscience applications.

An efficient numerical simulation of coupled thermo-hydro-mechanical processes in deep shale gas reservoirs

Shale gas is an important unconventional natural gas resource, and it is an important target of exploration and development in recent years. Deep shale gas reservoirs are high-temperature, high-pressure, and high-stress with complex gas–water relationships, and its occurrence and flow mechanisms are still unclear, making the simulation of deep shale gas reservoirs still a challenging problem. Aiming at the key problems faced in the development of deep shale gas reservoirs, this paper establishes a flow-stress-temperature field coupled mathematical model and numerical model, studies the thermo-hydro-mechanical sequential decoupling method to solve the coupled mathematical model, and forms a multi-field coupled simulation technology which provides theoretical support for the development of deep shale gas. Based on the thermo-hydro-mechanical coupling calculation module, this paper conducts research on the influence of thermo-hydro-mechanical multi-field coupling parameters on the production and development performance, and it further verifies the practicability of our proposed model in real reservoirs.

Seepage Characteristics of Shale Fracture and the Effect of Filling Sand under Normal Stress

As a new type of unconventional natural gas resource, shale gas plays a vital role in energy supply. In order to deeply understand the shale fracture seepage characteristics, filled and unfilled fracture seepage tests were carried out on shale samples with different fracture surface topography with the use of self-developed shale fracture seepage testing equipment. The fitting formula that the seepage discharge decreased as a negative exponential function with the increase in normal stress was obtained in the unfilled fracture seepage test; the fitting coefficient had a good logarithmic and exponential function relationship with joint roughness coefficient JRC and surface development interface area ratio Sdr. Meanwhile, the modified cubic law was obtained by adding the correction coefficient. The correction coefficient had an exponential relationship with the anisotropy value Ka. Compared with the unfilled one, the fracture surface topography had little effect on the seepage discharge when it was filled. The experimental results show the effects of water head difference, fracture surface topography, particle size, and thickness of filling sand on shale fracture seepage characteristics under different normal stress conditions, which have a certain significance for improving the efficiency of shale gas production.

Research Progress and Prospects on Microbial Response and Gas Potential in the Coal Gasification Process.

As an essential unconventional natural gas resource, China's coalbed methane resources are only commercially exploited in a few areas, such as the Qinshui Basin and the Ordos. The rise of coalbed methane bioengineering makes it possible to realize the conversion and utilization of carbon dioxide through microbial action and the carbon cycle. According to the metabolic behavior of the underground microbial community, if the coal reservoir is modified, it may stimulate the microorganism to continuously produce biomethane to prolong the production life of depleted coalbed methane wells. This paper systematically discusses the microbial response to promoting microbial metabolism by nutrients (microbial stimulation), introducing exogenous microorganisms or domestication of in situ microorganisms (microbial enhancement), pretreating coal to change its physical or chemical properties to improve bioavailability, and improving environmental conditions. However, many problems must be solved before commercialization. The whole coal reservoir is regarded as a giant anaerobic fermentation system. Some issues still need to be solved during the implementation of coalbed methane bioengineering. Firstly, the metabolic mechanism of methanogenic microorganisms should be clarified. Secondly, it is urgent to study the optimization of high-efficiency hydrolysis bacteria and nutrient solutions in coal seams. Finally, the research on the underground microbial community ecosystem and biogeochemical cycle mechanism must be improved. The study provides a unique theory for the sustainable development of unconventional natural gas resources. Furthermore, it provides a scientific basis for realizing the carbon dioxide reuse and carbon element cycle in coalbed methane reservoirs.

Advances in Cryogenic Fracturing of Coalbed Methane Reservoirs with LN2

Coalbed methane (CBM) is a significant unconventional natural gas resource existing in matrix pores and fractures of coal seams and is a cleaner energy resource compared to coal and crude oil. To produce CBM, stimulation operations are required, given that the coal permeability is generally too low. Hydraulic fracturing is the most widely used technology for reservoir stimulation; however, there are a few challenging issues associated with it, e.g., huge water consumption. In the past decade, the use of liquid nitrogen (LN2) as a fracturing fluid has been intensively studied for stimulating CBM reservoirs, achieving considerable progress in understanding fracturing mechanisms and optimizing fracturing techniques. This paper presents a thorough review of experimental design and observations, modeling procedures and results, field applications, and published patents. Existing studies are divided into five different groups for discussion and comparison, including immersion tests, injection tests, jet drilling tests, numerical modeling, and field applications. Based on the comprehensive evaluation of the outcomes, it is obvious that cryogenic fracturing using LN2 is a promising eco-friendly fracturing technique that can effectively enhance coal rock permeability to increase the production of CBM.

Analysis of factors affecting microwave heating of natural gas hydrate combined with numerical simulation method

As a clean and abundant unconventional natural gas resource, natural gas hydrate (NGH) holds the characteristics of safety, high efficiency and sustainable exploitation, which helps to alleviate the energy shortage of China, reduce the foreign-trade dependence of oil and gas, and ensure the national energy security. Microwave heating is a significant method that has been used in natural gas hydrate exploration. By using the microwave heating, the NGH in the reservoir formations would be heated, decomposed and stimulated thanks to taking advantage of microwave heating's unique characters: efficiency, high speed, clean and pollution-free. This paper established the temperature under microwave heating gas hydrate distribution theory model, and by using the finite element method for simulating temperature field of microwave heating gas hydrate, this paper analyzed the natural gas hydrate in the microwave field temperature distribution in the influencing factors. Microwave has a significant heating effect on the hydrate reservoir in the immediate vicinity of wellbore, and it is not affected by the initial conditions of reservoir. The temperature can rise to above 50°C within 1 h which is higher than the phase equilibrium temperature at the time of hydrate decomposition and is helpful to improve the decomposition rate of hydrate. The frequency is set at 915 MHz, and the feed port has a spiral arrangement with a length of 10 mm, which greatly expands the microwave heating range.

Review of Unconventional Natural Gas Liquefaction Processes

Liquefied natural gas (LNG) has become an important part in the energy industry on account of its high energy density, low carbon emission, and convenient transportation. In recent years, with the discovery of unconventional natural gas resources, the situation of the world’s natural gas liquefaction plants using conventional natural gas as feedstock is changing. Unlike traditional LNG plants, unconventional natural gas liquefaction processes require special considerations in design and manufacturing. This review summarizes and analyzes the characteristics and differences of several typical unconventional natural gas liquefaction processes compared to traditional natural gas liquefaction processes, including coalbed methane liquefaction, synthetic natural gas liquefaction, LNG-FPSO, and PLNG (pressurized liquefied natural gas). Moreover, a state-of-the-art review of the recent progress on design and optimization of unconventional natural gas liquefaction processes is presented.

A novel multi-path sand-control screen and its application in gravel packing of deepwater horizontal gas wells

Gravel packing is a promising sand control method for natural gas wells in deepwater environments. However, premature screenout, wellbore shrinkage, and packer installationlimit gravel packing efficiency in long-reach horizontal wells. In this paper, we introduce a novel multi-path screen that provides alternative pathways once the circulating annulus is blocked. The basic geometric parameters of the screen are given first. Then, the frictional pressure loss in each circulating unit during packing is evaluated. Finally, the application of this screen in a long-reach horizontal well in the northern South China Sea is discussed. The application results demonstrate the applicability of multi-path screens in complex downhole completion conditions. The multi-path screen described here may widen the application ranges of gravel packing techniques to meet the exploitation demand for unconventional natural gas resources, such as marine natural gas hydrate.

Geochemical Characteristics and Gas Source Contributions of Noble Gases of the Sulige Large Tight Gas Field of Upper Paleozoic in Ordos Basin, China

Tight gas is the fastest developing unconventional natural gas resource, becoming the principal part for gas reserves and production growth in China. The Sulige gas field is the largest gas field and also the typical low porosity and low permeability tight sandstone gas field discovered in China, with an annual natural gas output exceeding 300 billion and cumulative output exceeding 290 billion, playing an important role in ensuring national energy provision, helping China’s energy transformation, and promoting green, low-carbon, environmental protection and high-quality development. Based on sample collection and laboratory analysis, natural gas compositions including hydrocarbons, non-hydrocarbons, light hydrocarbons, and noble gases of the Sulige gas field are systematically analyzed, their genetic identifications are identified, and finally gas source originations and contribution proportions are comprehensively discussed from the perspectives of noble gases and hydrocarbon gases. The main achievements are as follows: 1) natural gases in the Sulige gas field are mainly alkane gases, with high methane content, high drying coefficient, low heavy hydrocarbon contents, low non-hydrocarbon gas contents of CO2 and N2, and relatively low noble gas contents. The helium content is relative 2 order of magnitude higher than the atmospheric value, while neon, argon, krypton, and xenon are relatively about 1–2 orders of magnitudes lower than the atmospheric values. 2) The carbon and hydrogen isotopes of alkanes are generally positive sequence distributions with some part inversion. The 3He/4He values are mainly distributed in magnitude of 10−8, the 40Ar/36Ar is ranged from 506 to 1940, the 129Xe is relative loss, and the 132Xe is relative surplus. 3) Natural gases in the Sulige gas field are typical coal-formed gases generated from a humic organic mother material with maturity from high mature to over mature according to C7 and C8 light hydrocarbons and alkane carbon isotopes. Noble gases are typical crustal genesis, mainly originating from the radioactive decay of crustal source materials. 4) The gas source correlations of noble gases and alkane gases and their quantitative evaluations on source contributions show that natural gases in the Sulige gas field are originated from Carboniferous-Permian coal measure source rocks in Ordos Basin, mainly contributed by coals and supplemented by mudstones, accounting for 55–60% and 40–45%, respectively.

Study on the Imbibition Damage Mechanisms of Fracturing Fluid for the Whole Fracturing Process in a Tight Sandstone Gas Reservoir

Tight sandstone gas is a significant unconventional natural gas resource, and has been exploited economically mostly through the application of hydraulic fracturing technology in recent decades. However, formation damage occurs when fracturing fluid percolates into the pores inside sandstones through imbibition driven by capillary pressure during fracturing operations. In this work, the formation damage resulting from the whole operation process composed of fracturing, well shut-in and flowback, and the degree of damage at different moments were investigated through core flow experiments and the low-field Nuclear Magnetic Resonance (NMR) technique. The results show that imbibition damage occurs starting from the contact surface between the formation and the fracturing fluid, which penetrates into an increasingly deep position with time down to a certain depth. The T2 spectra of NMR at different moments indicates that fracturing fluid initially enters the small pores, followed by the large pores due to the larger capillary pressure in the former. Thus, the sandstone cores with low permeability incur a higher degree of damage due to their stronger capability of retaining fracturing fluid compared to high-permeability cores. The front position of the fracturing fluid imbibition at different moments, along with the degree of damage, were characterized through the one-dimensional encoding processing of the NMR signal. These results underlie the effective strategy to relieve formation damage resulting from imbibition during hydraulic fracturing operations.

Lithology prediction method of coal-bearing reservoir based on stochastic seismic inversion and Bayesian classification: a case study on Ordos Basin

Abstract The Upper Palaeozoic coal measures in the Ordos Basin are rich in unconventional natural gas resources; however, the reservoir heterogeneity is relatively strong, which majorly restricts the accuracy of lithology prediction. Stochastic seismic inversion can synthesize the geological–geophysical information and establish high-resolution reservoir models based on geostatistical theory to obtain good inversion results; moreover, it can characterize reservoir lithology and fluid-bearing properties. The present study aimed to propose a high-precision lithology classification method for coal-bearing strata in the Ordos Basin, using Bayesian classification and stochastic seismic inversion. Initially, the reservoir geological model was established on the basis of the sequential Gaussian simulation algorithm, and stochastic seismic inversion was performed in combination with rock physics analysis, thereby obtaining a high-resolution elastic parameter data volume. Thereafter, the probability density function (PDF) and the probability density confusion matrix (PDF confusion matrix) were introduced to quantitatively analyse the ability of sensitive elastic parameters to distinguish lithology. Eventually, a logging lithology-fluid classification template was established based on the Bayesian classification technology; furthermore, a 3D lithology-fluid prediction was completed in combination with the inversion results. The prediction results are in good accordance with the logging data, which verifies the feasibility and effectiveness of the method.

Effects of Regional Differences in Shale Floor Interval on the Petrophysical Properties and Shale Gas Prospects of the Overmature Niutitang Shale, Middle-Upper Yangtz Block

The lower Cambrian Niutitang/Qiongzhusi shale gas in the Middle-Upper Yangtz Block had been regarded as a very promising unconventional natural gas resource due to its high total organic carbon, great thickness, and large areal distribution. However, no commercial shale gas fields have yet been reported. From the northwest to the southeast there are considerable differences in the sedimentary environments, lithology, and erosive nature of the underlying interval (the floor interval) of the Niutitang shale. However, systematic research on whether and how these regional differences influence shale petrophysical properties and shale gas preservation in the Niutitang shale is lacking. A comparison of Niutitang shale reservoirs as influenced by different sedimentary and tectonic backgrounds is necessary. Samples were selected from both the overmature Niutitang shales and the floor interval. These samples cover the late Ediacaran and early Cambrian, with sedimentary environments varying from carbonate platform and carbonate platform marginal zone facies to continental shelf/slope. Previously published data on the lower Cambrian samples from Kaiyang (carbonate platform), Youyang (carbonate platform marginal zone) and Cen’gong (continental shelf/slope) sections were integrated and compared. The results indicate that the petrophysical properties of the floor interval can affect not only the preservation conditions (sealing capacity) of the shale gas, but also the petrophysical properties (pore volume, porosity, specific surface area and permeability) and methane content of the Niutitang shale. From the carbonate platform face to the continental shelf/slope the sealing capacity of the floor interval gradually improves because the latter gradually passes from high permeability dolostone (the Dengying Formation) to low permeability dense chert (the Liuchapo Formation). In addition, in contrast with several unconformities that occur in the carbonate platform face in the northern Guizhou depression, no unconformity contact occurs between the Niutitang shale and the floor interval on the continental shelf/slope developed in eastern Chongqing Province and northwestern Hunan Province. Such regional differences in floor interval could lead to significant differences in hydrocarbon expulsion behaviour and the development of organic pores within the Niutitang shale. Therefore, shale gas prospects in the Niutitang shales deposited on the continental shelf/slope should be significantly better than those of shales deposited on the carbonate platform face.

Microbial communities reveal impacts of unconventional oil and gas development on headwater streams

The demand for natural gas has led to the development of techniques used to access unconventional oil and natural gas (UOG) resources, due to the novelty of UOG, the potential impacts to freshwater ecosystems are not fully understood. We used a dual pronged approach to study the effects of UOG development on microbial biodiversity and function via a laboratory microcosm experiment and a survey study of streams with and without UOG development within their watersheds. The microcosm experiment simulated stream contamination with produced water, a byproduct of UOG operations, using sediment collected from one high water-quality stream and two low water-quality streams. For the survey study, biofilm and sediment samples were collected from streams experiencing varying levels of UOG development. In the microcosm experiment, produced water decreased microbial aerobic and anaerobic CO2 production in the high water-quality stream sediment but had a positive effect on this microbial activity in the lower water-quality stream sediments, suggesting habitat degradation alters the response of microbes to contaminants. Results from the stream survey indicate UOG development alters stream water temperature, chemistry, sediment aerobic and anaerobic CO2 production, and microbial community biodiversity in both sediments and biofilms. Correlations among UOG associated land use, environmental, and microbial variables suggest increases in light availability and sediment delivery to streams, due to deforestation and land disturbance, impact stream microbial communities and their function. Consistent changes in the relative abundance of bacterial taxa suggest microorganisms may be good indicators of the environmental changes associated with UOG development. The observed impacts of UOG development on microbial community composition and carbon cycling could have cascading effects on stream health and broader ecosystem function.

Pore Structures of the Lower Permian Taiyuan Shale and Limestone in the Ordos Basin and the Significance to Unconventional Natural Gas Generation and Storage

In the Ordos Basin, multiple sets of coal seams, organic-rich shale, and limestone are well developed in the Permian Taiyuan Formation, which are favorable targets for collaborative exploration of various types of unconventional natural gas resources, including coalbed methane, shale gas, and tight gas. In this study, core samples from the Permian Taiyuan Formation in the eastern margin of the Ordos Basin were used to carry out a series of testing and analysis, such as the organic matter characteristics, the mineral composition, and the pore development characteristics. In the shale of the Taiyuan Formation, the total organic carbon (TOC) content is relatively high, with an average of 5.38%. A thin layer of black shale is developed on the top of the Taiyuan Formation, which is relatively high in TOC content, with an average of 9.72%. The limestone in the Taiyuan Formation is also relatively high in organic matter abundance, with an average of 1.36%, reaching the lower limit of effective source rocks (>1%), being good source rocks. In the shale of the Taiyuan Formation, various types of pores are well developed, with relatively high overall pore volume and pore-specific surface area, averaging 0.028 ml/g and 13.28 m2/g, respectively. The pore types are mainly mineral intergranular pores and clay mineral interlayer fractures, while organic matter-hosted pores are poorly developed. The limestone of the Taiyuan Formation is relatively tight, with lower pore volume and pore-specific surface area than those of shale, averaging 0.0106 ml/g and 2.72 m2/g, respectively. There are mainly two types of pores, namely, organic matter-hosted pores and carbonate mineral dissolution pores, with a high surface pore rate. The organic matter in the limestone belongs to the oil-generation kerogen. During thermal evolution, the organic matter has gone through the oil-generation window, generating a large number of liquid hydrocarbons, which were cracked into a large number of gaseous hydrocarbons at the higher mature stage. As a result, a large number of organic matter-hosted pores were generated. The study results show that in the Ordos Basin, the shale and limestone of the Permian Taiyuan Formation have great potential in terms of unconventional natural gas resources, providing a good geological basis for the collaborative development of coal-bearing shale gas and tight limestone gas in the Taiyuan Formation.

Shale gas resources across the world: A future source of energy

Unconventional natural gas resources are very important for ensuring energy security. It is a concern that conventional natural gas resources will be unable to meet growing supply requirements. This concern has prompted a scramble to secure long-term natural gas supplies. In a high oil and gas price environment, unconventional resources are viewed as important and economically attractive components of future natural gas supplies. Shale gas is natural gas produced from shale formations. Gas shales are organic-rich shale formations. Shale gas is a dry gas composed of methane. Various factors which have contributed to its rapid development are mainly advancements in horizontal drilling, hydraulic fracturing, and, perhaps most importantly, the rapid increase in natural gas prices in the last several years as a result of significant variation in supply and demand. India has a high potential for shale reserves. According to the available data, a comprehensive shale gas pilot project carried out in the Damodar Valley Basin has made an initial gasin- place estimate of 300 to 2,100 trillion cubic feet (TCF) in Indian shale gas basins, which is around 300 times higher than the Krishna Godavari Dhirubhai 6 (KG-D6) basin, by far the largest gas field in the country. In this paper, a brief review of shale gas reserves across the world and their government policies has been explained.

Fractal Characteristics and Significance of Different Pore Types of the Wufeng-Longmaxi Formation, Southern Sichuan Basin, China, Based on N2 Adsorption and Image Analysis.

Shale gas is an important unconventional natural gas resource, and its reservoirs have pores with strong heterogeneity, which have an important effect on the adsorption and migration of shale gas, but the specific mechanism is still unclear. To further clarify the pore structure characteristics of shale gas reservoirs and the mechanism of their influence on CH4 adsorption capacity, marine shale samples from the Wufeng–Longmaxi formation of wells N1, N3, and N10 in Changning block, southern Sichuan Basin, China, were selected for total organic carbon (TOC), X-ray diffraction (XRD), N2 gas adsorption (N2-GA), CH4 gas adsorption (CH4-GA), and field emission scanning electron microscopy (FE-SEM). The Frenkel–Halsey–Hill (FHH) model and Slit Island Analysis (SIA) were used to calculate the fractal dimension of the pore system and different types of pores, and their relationship and influence on CH4 adsorption capacity were also discussed. The results show that the fractal dimension could reflect the complexity and heterogeneity of pores. According to the FHH model, fractal dimensions of the surface and structure of the pore system (D1 and D2, D1 < D2) were obtained, and the pore structure was more complex than the pore surface. According to SIA, the surface fractal dimensions of four types of reservoir space (DDP, DOP, DIP, and DMF) decrease progressively, and their main body is 2.60–2.80, 2.40–2.65, 2.20–2.40, and 2.05–2.30. Organic pores and intergranular pores are the most abundant, and so D1 is mainly related to DOP and DIP. In high-TOC samples, D1 is close to DOP, while in low-TOC samples, D1 is close to DIP. The complexity of the pore surface, D1, and specific surface area have a positive correlation, and with the increase of pore surface complexity, methane adsorption capacity could be significantly improved. Therefore, D1 may be used as a characterization parameter of CH4 adsorption capacity, which could provide some evidence to further clarify the adsorption mechanism of shale gas.