Accurate prediction of shale gas production is important for the government to formulate energy policies. China’s shale gas production is characterized by limited data and unclear influencing factors. Grey prediction models hold advantages in solving problems with inadequate data and insufficient information. However, existing grey models for shale gas production prediction have drawbacks of insufficient characteristic expression and parameter optimization. Therefore, a new grey model with four parameters is proposed in this work. Firstly, the power index, accumulative order, and background value coefficient in the new model are optimized using the PSO algorithm. Then, based on an in-depth analysis of the model structure and tests using random samples, the new model’s compatibility and adaptability are demonstrated to be better than other similar grey models. Thirdly, the new model is applied to simulate China’s shale gas production. The results show that the comprehensive error of the new model is only 1.3626%, lower than that of the SGGM(1,1) and UGM(1,1) models, which are 1.6211% and 1.6575% respectively. Lastly, using the new model, China's shale gas production is predicted, which is expected to reach 752 hundred million cubic meters by 2030. After analyzing the rationality of the result, relevant suggestions are put forward. 

Prediction, analysis and suggestions of shale gas production in China based on a new grey model with four parameters

The shale of the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the Xichang Basin is the main replacement horizon for the shale gas exploration being conducted in the Sichuan Province, except for the Sichuan Basin. The fine identification and classification of the types of shale facies are important for shale gas exploration and development evaluation. However, the lack of systematic experimental studies on rock physical characteristics and micro-pore structures leads to a lack of physical evidence for the comprehensive prediction of shale sweet spots. Therefore, the present study used different means, such as core observation, total organic carbon content (TOC), helium porosity measurement, X-ray diffraction analysis, and mechanical properties analysis, in combination with the analysis of the whole rock mineral composition and characteristics of shale, for the identification and classification of the lithofacies of the shale layer, the systematic petrology and hardness measurement of the shale samples with different lithofacies, and discussion of the dynamic and static elastic properties of the shale samples and the control factors. It was revealed that nine types of lithofacies existed in the Wufeng Formation— the Long11 sub-member in the Xichang Basin, among which moderate organic carbon content–siliceous shale facies, moderate organic carbon content–mixed shale facies, and high-organic carbon content–siliceous shale facies were the best lithofacies with the optimum reservoir conditions, providing sufficient space for shale gas accumulation. The siliceous shale facies mainly developed organic pores and fractures, and the pore texture was excellent overall. The mixed shale facies mainly developed intergranular pores and mold pores, with a preference toward pore texture. The argillaceous shale facies mainly developed dissolution pores and interlayer fractures, and the pore texture was relatively poor. The geochemical characteristics of the organic-rich shale samples with TOC > 3.5% revealed that the sample was composed of microcrystalline quartz grains as the rock support framework, while the intergranular pores were located between the rigid quartz grains, which exhibited hard pores in the analysis of their mechanical properties. In the relatively organic-poor shale samples with TOC < 3.5%, the quartz source was mainly terrigenous clastic quartz, and the sample was composed of plastic clay minerals as the rock support skeleton, while the intergranular pores were located between argillaceous particles, which exhibited soft pores in the analysis of their mechanical properties. The difference in the rock fabric of the shale samples resulted in an ″initial increase followed by a decrease″ trend of velocity with quartz content, with the organic-rich shale samples exhibiting low velocity–porosity and velocity–organic matter content change rate, and the two kinds of rocks were easier to distinguish in the correlation diagram of the combined elastic parameters such as the P-wave impedance–Poisson ratio and the elastic modulus–Poisson ratio. The samples dominated by biogenic quartz exhibited greater hardness and brittleness, while the samples dominated by terrigenous clastic quartz exhibited lower hardness and brittleness. These results could serve as a basis for logging interpretation and seismic ″sweet spot″ prediction of high-quality shale gas reservoirs in Wufeng Formation–Member 1 of the Longmaxi Formation.

https://pubs.acs.org/doi/pdf/10.1021/acsomega.3c01307

Lithofacies Types and Physical Characteristics of Organic-Rich Shale in the Wufeng–Longmaxi Formation, Xichang Basin, China

Effects of Different Bedding Angles and Impact Angles on the Dynamic Tensile Properties of Thermal-Treated Shale After Cooling Treatment

This study comparatively analyzed the geological, geochemical, reservoir, and gas-bearing characteristics of the lower Cambrian marine shale in the Middle and Lower Yangtze regions. The main factors controlling the gas-bearing properties of the shales were identified, and the favorable and unfavorable conditions for shale gas accumulation are discussed. The results show that the organic carbon contents and thermal evolution degree of the organic matter in the lower Cambrian marine shale in the Lower Yangtze area were higher than those generally found in the Middle Yangtze area. The brittle mineral composition of the Middle Yangtze area was typically low silicon and high calcium, whereas the Lower Yangtze area was characterized by high silicon and low calcium. The development of micropores in the Lower Yangtze area was poorer than in the Middle Yangtze area, with the organic pores being particularly underdeveloped. The adsorption capacity of shale in the Lower Yangtze area was obviously higher than in the Middle Yangtze area. It was considered that the organic carbon content, thermal evolution degree, and molecular structure of kerogen were the main factors that controlled the adsorption properties of the shale. In addition, the Lower Yangtze area suffered a stronger tectonic transformation and frequent magmatic activity, and the preservation conditions were inferior to those in the Middle Yangtze area. 

Comparative Study of the Characteristics of Lower Cambrian Marine Shale and Their Gas-Bearing Controlling Factors in the Middle and Lower Yangtze Areas, South China

In unconventional reservoir engineering, such as coalbed methane and shale gas extraction, fracture behavior is pivotal in gas accumulation, migration, and extraction, acting as a primary channel for gas flow. Current research inadequately addresses the quantitative impact of fracture roughness on gas extraction. In this study, we introduce a novel interdisciplinary model that quantitatively characterizes shale fracture roughness and correlates it with shale permeability. This model comprehensively considers factors affecting shale extraction, including fracture roughness, in situ stress, reservoir deformation, and adsorption-desorption dynamics. It provides a thorough analysis of how fracture roughness influences gas seepage, extraction efficiency, and reservoir stability under various physical conditions. Our validated findings reveal that fracture roughness significantly affects shale permeability, stress responses, and displacement. As the fracture roughness coefficient χ increases from 0.2 to 1.0, the maximum reduction in gas pressure within the hydraulically fractured region is 4.7%, while the maximum increase in reservoir stress is 2.8%. In shale reservoirs near extraction well, particularly in hydraulically fractured zones, the maximum decrease in shale gas pressure is 11.1%, and the maximum increase in stress is 3.2%, which offer a groundbreaking approach for optimizing extraction rates and ensuring project safety in the industry.

Optimizing unconventional gas extraction: The role of fracture roughness

Shale gas accumulation patterns in China

Geothermal resources are composed of rock and water, in which water-rock interaction (WRI) has a great influence on the physical properties of thermal reservoirs. The Red River Fault is abundant in different kinds of geothermal resources and the main thermal reservoirs are sandstones and limestones. The study has used the water chemistry, X-ray diffraction (XRD), N2 adsorption, mercury injection, scanning electron microscopy (SEM), physical parameters, and WRI simulation experiments to study the physical properties changes in thermal reservoirs. The pores of thermal reservoirs are mainly intragranular and intergranular pores, in which sandstones are dominated by micropores and mesopores while limestones are dominated by macropores. Through the WRI simulation experiment, the reservoirs have generated many dissolution pores and microfractures, of which the pore-throat connectivity has deteriorated. The changes of physical parameters can be divided into two stages with opposite trends, in which the first stage is controlled by temperature and pressure while the second stage is influenced by dissolution and precipitation. The study has suggested that the change stage of physical properties should be clarified to update the resource quantity dynamically in the development process and the components of fracturing fluids should be studied to achieve the optimal improvement of physical properties and reduce the adverse impact on pore-throat connectivity before the development engineering. 

Influences and implications of water-rock interaction on physical properties of geothermal reservoirs along the Red River Fault

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Shale gas accumulation patterns in China

Influences and implications of water-rock interaction on physical properties of geothermal reservoirs along the Red River Fault