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Abstract Shale gas is a potential substitute for the gradually depleted conventional oil and gas resources. Since studies on microscopic behavior of shale gas in nano−slits and the exact mechanical mechanisms behind it are still in urgent demand, this paper uses the model of graphite layers to describe microscopic details of shale gas occurrence behavior in organic−rich nano−slits from the viewpoint of molecular interactions by molecular simulation. "Dual adsorption mechanisms" and the consistency theory of gas distributions affected by wall effects under the condition of no overlapped wall force field are proposed to clarify the formation mechanisms of shale gas in the nano−slits. Results also show the critical channel width for all gases to be affected by the nano−scale effects is between 1 and 2 nm, and the regions (counted from the gas zone) for overlapped wall force field in 0.5 and 1 nm slits are about 0.32 and 0.42 nm, respectively. Furthermore, the effects of overlapped wall forces, channel size and pressure variations on gas aggregation and its movability etc., interaction mechanisms inside the nano−slits and their causalities, the development enlightenment for Knudsen layer (KL) and the whole gas have been explicitly depicted and clarified, which is expected to be a useful reference not only for shale gas evaluation and exploitation, but also for widespread research of gas occurrence phenomena in carbon−based materials in the field of industry. Highlights • Systematic interactions for partial shale gas and its relationship with microscopic gas behavior in nano−slits are studied. • "Dual adsorption mechanisms" and the consistency theory of gas distributions are proposed. • The enlightenment for gas development is presented from the viewpoint of microscopic mechanical mechanisms. [ABSTRACT FROM AUTHOR]

Predicting the production behaviors of shale gas wells is of great importance for further developing future unconventional hydrocarbon strategies. An accurate prediction production, as well as reliable shale gas production models, are required to fully understand the shale gas exploitation budget. However, a major problem with classical analytic methods is the insufficient accuracy of the existing models, the time-consuming collection of historical production data, and the costly computational expense. To minimize this problem, a combination of the exponential smoothing method, autoregressive integrated moving average (ARIMA) model, and long short-term memory (LSTM) model was proposed to provide robust support for the production behaviors of shale gas. In this paper, we employed shale gas well production data to establish a database for model training and optimized the predicted model. Hereby, we sought to evaluate the production data predicted by conventional analytical methods, the exponential smoothing method, the ARIMA model, and the LSTM model. Shortly afterward, we objectively compared the predicted results obtained by the novel LSTM model and traditional analytical methods, such as Arps, stretched exponential decline (SEPD), and the Duong model. Herein, we compared the computational cost between the LSTM model and traditional numerical simulation. The combined interpretation of the proposed model demonstrates that the LSTM model achieved scientific accuracy and outstanding results in both short-term and long-term predictions, and realized production prediction of the adjacent well, with excellent agreement with the real shale gas production and a low error, making it an effective tool in forecasting shale gas production. This assay could be used as a potential approach for evaluating deep learning in the petroleum industry and for predicting the future production of unconventional hydrocarbons. [Display omitted] • A framework for shale gas production potential estimation was developed. • A deep-learning-based time series analysis model was built to predict the short-term and longterm shale gas production, based on thousands of in-situ production data samples from three shale gas wells. • Computational cost of the shale gas production prediction was significantly reduced with proposed long short-term memory model. • Objectively compare the advantage and drawback forecasting production among conventional analytical methods, exponential smoothing algorithm, ARIMA, and LSTM model. • The study shed lights on the few-shot learning applications in shale gas production prediction. [ABSTRACT FROM AUTHOR]

Most shale gas production prediction models are established on the ideal production conditions (the constant pressure and constant production condition), deviating from the production performance characteristics of shale gas wells with variable production and pressure and leading to large error in unknown model parameter inversion of the actual shale gas wells, and obviously showing great limitations in characterizing the dynamic production performance and predicting EUR of shale gas wells for variable production-pressure. Furthermore, the material balance equation is more common in solving the variable production and pressure of shale gas wells issues, but it is only suitable for characterizing boundary flow. In this paper, a semi-analytical productivity prediction model of shale gas well for constant pressure was established and validated by the practical production data. Then the material balance pseudo time and linear superposition pseudo time were introduced respectively to modify the true pseudo time in the proposed productivity prediction model to demonstrate that the linear superposition pseudo time was proper to obtain the unknown fracture inversion parameters compared to material balance pseudo time during the variable pressure-production of actual shale gas wells. Finally, the Monte Carlo method in the IHS numerical software was adopted to analyze the impact of the fracture half-length uncertainty on the probability distribution of EUR, which showed that the P50 value is closer to the deterministic EUR value predicted by the analytical production prediction model. Research indicates that it is of certain theoretical reference to reduce the risk of production prediction during the production of shale gas wells and guide the optimization of development plans. • The production model is proposed to characterize the variable production and variable pressure issues. • The linear superposition time is more suitable to characterize the dynamic performance of shale gas wells. • The model parameters inversion and probability distribution of EUR is persuasive to reduce the risk of production effect evaluation. • The P50 value is closer to the true prediction result of EUR value. [ABSTRACT FROM AUTHOR]

The desorption of adsorbed gas and its diffusion in nano-channels are important mechanisms for the late stage of shale gas development. However, there is no commonly accepted theory to reveal the laws of these mechanisms in sustaining appreciable shale gas production after relatively high flow period. Experimental simulation of shale gas production and its sound theoretical deduction are capable of providing useful in-situ development guidance. In this paper, experiments are carried out to simulate the depletion production performance of horizontal shale gas wells and to explore the production laws and scientific explanations. By establishing the resemblance between the physical experiment and the in-situ development, the high matching of experimental results and real production data is realized, making the reliable prediction of gas production performance happen. It is found by the experimental results that the production of shale gas wells in the middle and late stages is dominated by a special gradual release mechanism, which is named "unsteady intermittent coordination mechanism" in this work. This special mechanism is mainly restricted by the gas mobility in shale nanopores and the supply and transport capacity in the pore-fracture system. In view of adsorption/desorption, the late-stage production characteristics are largely dependent on the adsorption and desorption processes, and the unsteady mechanism of adsorbed gas desorption is preliminarily revealed from the microscopic viewpoints of desorption laws at different pressures and in different pores. Interestingly, the desorption itself is predicted to be unsteady in essence. In terms of mobility, the transport of desorbed gas in single channels and pore networks undergoes a slow cycle of molecular accumulation and release due to the presence of wall confinement in the microscopic pores. After flowing into the fracture system, the gas will be quickly diverted to the production well. Meanwhile, the fracture system cannot be supplemented in time by gas in the micro-nano pores. To sum up, a qualitative understanding and scientific explanation of the "unsteady intermittent coordination mechanism" of the adsorbed gas desorption and transport in the late stage of shale gas development are realized. • The high matching of results is realized by establishing the resemblance between experiment and in-situ shale gas production. • The "unsteady intermittent coordination mechanism" of the adsorbed gas desorption and transport in shale gas is proposed. • According to the dynamic law and internal mechanism of shale gas production, the special mechanism is explained. • The intermittent mechanism of the macroscopic output of desorbed gas was explored based on the transport mechanism of desorbed gas between different pores and from the matrix pores to fractures. [ABSTRACT FROM AUTHOR]

As an unconventional natural gas resource, shale gas is attracting increasing attention worldwide because of its potential to strengthen global energy security and reduce carbon emissions. The shale gas flow in nanopores is one of the main concerns affecting its development process. However, due to the strong nonlinearity and complexity of gas flow at the nanoscale, there is no satisfactory consensus on a physically sound flow mechanism scheme and its corresponding coupling method. Furthermore, although the integration method using specific functions has been proposed to facilitate the consideration of various pore sizes in shale matrix, the real shale experiments are rarely involved to realize this integration method with definitely determined parameters. In this study, the concept of "wall-associated diffusion" is firstly utilized to clarify the relationship among several flow mechanisms, and thereby a comprehensive flow mechanism scheme differing from common research is proposed, which physically considers both the division of mechanical mechanisms in nanopores and partition of flow space. Besides, the deficiencies in the mathematical models of viscous flow and several diffusion processes for flow description are illustrated. Using the molecular collision frequency expressions, a new coupling coefficient-based flow model in nanopores is derived to eliminate the aforementioned deficiencies by strengthening the correspondence between the flow models and Knudsen number (Kn), which is applicable in the full flow regime scope and avoids segment processing. Furthermore, based on the pore size distribution experiments of the real shale samples from a gas field, the fitting parameters needed for the macroscopic form of the above coupled model are obtained, so as to establish an integration method-based unified model for gas flow prediction in shale matrix. Results show that the prediction accuracy of the new model for the two tested shale samples is at least 2.2 times higher than several typical models and is also 1.9 and 7.5 times more accurate than the model without coupling coefficients, respectively. And the application of the proposed model to the flow experiment on another shale sample at higher pressure levels is in accordance with these observations. Interestingly, as temperature increases, the flow rate decreases with minor amplitudes. Under the conditions studied, the pressure change can cause over 10 times variation of the flow rate, revealing measures should be taken to maintain relatively high reservoir pressures to guarantee appreciable gas production from matrix if the other factors remain unchanged. In addition, the difference between the average flow shape in the whole rock and the gas motion in the pore of mean radius is revealed. Sounder in theoretical bases and better in application effects, the proposed model is expected to be of practical significance for guiding shale gas reservoir development. • A comprehensive flow mechanism scheme differing from com-mon re-search is proposed. • A new coupled flow model with deduced coupling coefficients is established. • A unified model, i.e. the macroscopic form of the new coupled model, is obtained. [ABSTRACT FROM AUTHOR]

Highlights • Systematic nano-scale effects are characterized by virtue of clearly defined layers. • The main reason for the variation law of Knudsen layer thickness is revealed. • The role of density gullies as "lubricant layers" is shown. • The role of gas-gas interactions in controlling molecular motion is evidenced. • Nano-scale effects on gas movability are displayed. Abstract As an unconventional natural gas resource, shale gas plays a significant role in satisfying increasing global energy demands. Because of the less costs and better repeatability of molecular simulation for nano-scale research, it's becoming a nice way to study the distribution and motion characteristics of shale gas, which is the key to get insights into the nano-scale effects in shale matrix. In this paper, pressure-dependent equilibrium molecular simulation of shale gas in organic-rich nano-slit is conducted, where the gas is divided into clearly defined layers and the structural and motion characteristics of different parts are detailedly analyzed, making systematic observations and descriptions of nano-scale effects happen. By virtue of this, the variation law of shale gas in the nano-slit and the mechanical theory behind it are proposed. Results also reveal the main reason for the roundabout ascension variation of Knudsen layer (KL) thickness, the role of density gullies as "lubricant layers" to enhance the diffusion in adsorption layers, the evidence of the role of gas-gas interactions in controlling molecular motion in addition to gas-wall interactions, nano-scale effects on gas movability with pressure from the viewpoint of gas development, etc. The physical phenomena caused by nano-scale effects and their variations with pressure in organic-rich nano-slit have been explicitly clarified, which is expected to be a useful reference for shale gas evaluation and exploitation. [ABSTRACT FROM AUTHOR]