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Due to strong stress sensitivity resulted from unconventional tight formationsit is of practical interest to formulate a reasonable pressure drawdown plan to improve gas extraction recovery. The impact of water-shale interactions on the reservoir permeability was previously ignored in the managed pressure drawdown optimization. The controlled-pressure production dynamic analysis was mostly conducted using numerical simulation, lack of rigorous theoretical support. Hence in this paper, a theoretical production prediction model was proposed and verified with HIS RTA 2015by incorporating multiple pressure drawdown mechanisms and various non-linear gas flow process. The on-site production effects dominated by two different pressure drop methods was further compared, indicating that compared to depressurization production, the production reversion can occur in the controlled pressure production process and the EUR of single well can be increased by about 30% under the control of managed pressure drawdown approach. Finally, the pressure drawdown optimization strategy was carried out on the field test from the both production effect and economic benefits, which demonstrated that the best economic solution can generally be obtained in the early stage of production. The research results can be closely linked to the on-site production practice of shale gas wells, providing insights into designing optimized production strategy scheme. [ABSTRACT FROM AUTHOR]

Archie's parameters are substantial to be investigated in the evaluation of water saturation. Many researchers adhered to the opinion that Archie's equation can still be applied to water saturation calculation in shale if the accuracy of relative parameters could be improved. External conditions, such as temperature, confining pressure, water salinity, wettability, and displacement, may influence the determination of Archie's parameters. The aim of the study is to investigate the effect of salinity on Archie's parameters and their correlation with mineral composition and pore structure. The mineral contents and petrophysical properties were firstly acquired through X-ray diffraction (XRD) and basic measurements. Rock-electric experiments under different salinity were conducted on deep shale samples taken from Longmaxi (LMX) Formation in Luzhou (LZ). The results indicate that Archie's parameters of " a ," " b ," " m ," and " n " under actual brine salinity are assigned to 1.47, 1, 1.26, and 1.6, respectively. Our cementation factor is lower than that in other studies including (shaly) sandstones, carbonates, and shales, probably due to the extremely low porosity in study area. Salinity has a positive effect on cementation factor (m) and saturation exponent (n), suggesting the traditional assignment to Archie's parameters is inappropriate. It was concluded that the complicated pore structure and high porosity mainly associated with clay mineral may trigger the increased cementation factor. Further mathematical derivations confirmed that the rock resistivity is inextricable from pore system and establish a physical model. The paper provides a solid evidence base for further understanding and evaluation of water saturation in unconventional shale reservoirs, more significantly, innovatively unveils the research into Archie formula in deep shale reservoirs of southern Sichuan basin. [ABSTRACT FROM AUTHOR]

The shale gas productivity model based on shale gas nonlinear seepage mechanism is an effective way to reasonably predict productivity. The incomplete gas nonlinear effects considered in the current production prediction models can lead to inaccurate production prediction. Based on the conventional five-zone compound flow model, comprehensive gas nonlinearities were considered in the improved compound linear flow model proposed in the paper and a semianalytical solution for productivity was obtained. The reliability of the productivity model was verified by the field data, and then, the 20-year production performance analysis of the gas well was studied. Ultimately, the key influencing factors of the fracture control stage and matrix control stage have been analyzed. Research indicated the following: (1) the EUR predicted by the productivity model is higher than the EUR that the comprehensive nonlinear effects are not considered, which demonstrated that the various nonlinear effects cannot be neglected during the production prediction to ensure the greater calculation accuracy; (2) during the early production stage of shale reservoir, the adsorbed gas is basically not recovered, and the cumulative adsorption contribution rate does not exceed 10%. The final adsorption gas contribution rate is 23.28%, and the annual adsorption rate can exceed 50% in the 20th year, showing that free gas and adsorbed gas are, respectively, important sources of the early stage of production and long-term stable production; (3) the widely ranged three-dimensional fracturing reformation of shale reservoirs and reasonable bottom hole pressure in the later matrix development process should be implemented to increase the effective early production of the reservoir and ensure the earlier gas production process of the matrix development. The findings of this study can help for better ensuring the prediction accuracy of the estimated ultimate recovery and understanding the main influencing factors of the dynamic performance of gas wells so as to provide a theoretical reference for production optimization and development plan formulation of the shale gas reservoirs. [ABSTRACT FROM AUTHOR]

The shale gas productivity model based on shale gas nonlinear seepage mechanism is an effective way to reasonably predict productivity. The incomplete gas nonlinear effects considered in the current production prediction models can lead to inaccurate production prediction. Based on the conventional five-zone compound flow model, comprehensive gas nonlinearities were considered in the improved compound linear flow model proposed in the paper and a semianalytical solution for productivity was obtained. The reliability of the productivity model was verified by the field data, and then, the 20-year production performance analysis of the gas well was studied. Ultimately, the key influencing factors of the fracture control stage and matrix control stage have been analyzed. Research indicated the following: (1) the EUR predicted by the productivity model is higher than the EUR that the comprehensive nonlinear effects are not considered, which demonstrated that the various nonlinear effects cannot be neglected during the production prediction to ensure the greater calculation accuracy; (2) during the early production stage of shale reservoir, the adsorbed gas is basically not recovered, and the cumulative adsorption contribution rate does not exceed 10%. The final adsorption gas contribution rate is 23.28%, and the annual adsorption rate can exceed 50% in the 20th year, showing that free gas and adsorbed gas are, respectively, important sources of the early stage of production and long-term stable production; (3) the widely ranged three-dimensional fracturing reformation of shale reservoirs and reasonable bottom hole pressure in the later matrix development process should be implemented to increase the effective early production of the reservoir and ensure the earlier gas production process of the matrix development. The findings of this study can help for better ensuring the prediction accuracy of the estimated ultimate recovery and understanding the main influencing factors of the dynamic performance of gas wells so as to provide a theoretical reference for production optimization and development plan formulation of the shale gas reservoirs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Revealing the effect of CO 2 on oil potential energy in nanopores. • CO 2 solubility and oil swelling factor explained within nanopores. • Influence of CO 2 on the recovery law of different components in oil. • Analysis of different water film thickness on CO 2 enhanced oil recovery. Understanding the mechanisms of CO 2 extraction or flooding are vital for enhancing oil recovery (EOR) in tight reservoirs. In this study, the CO 2 EOR mechanism in the displacement-affected area (DPAA) and diffusion-affected area (DFAA) of quartz nanopores were thoroughly investigated using molecular dynamics simulation techniques. First, the following two contents were mainly simulated, namely CO 2 flooding oil in the single/double nanopores of DPAA and CO 2 extraction oil in dead-end nanopores of the DFAA with and without the water film. Then, tight oil potential energy, threshold capillary pressure, CO 2 solubility, and oil swelling in nanopores were calculated to clarify the effects of CO 2 on oil transport. Moreover, different CO 2 injection/flowback rates and different water film thicknesses on dead-end nanopores on oil recovery were discussed. In the DPAA, the CO 2 solubility and the oil swelling factor gradually decreased with distance from the CO 2 -oil interface (Y = 0 nm), where the higher the injection rate, the more easily the CO 2 dissolved in the oil. However, the injection rate of CO 2 was inversely proportional to oil recovery. In addition, it took longer for the displacement efficiency in the 6 nm pore of double pores to reach the same displacement efficiency as in the single 6 nm pore. In the DFAA, the effect of flowback rate on the displacement efficiency of oil was relatively low. However, the thickness of the water film was a key factor that affected the oil displacement efficiency in the DFAA. [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]

With the application of active control technology in vehicle, the performances of vehicle such as riding comfort, active safety and handling stability were greatly improved in recent years. However, most of these control systems were aimed to improve individual performance of vehicle respectively. In fact, the improvement of the overall vehicle dynamics performance not only depends on the cooperative work among these various control subsystems but also depends on the coupling interferences of the mechanical structure and control system of vehicle in the running process. In order to remove the coupling interferences between the mechanical structure and control system of vehicle chassis system and to further enhance the overall performance of vehicle, the method of simultaneous optimization of mechanical structure and controller parameters of vehicle chassis system based on Particle Swarm Optimization Algorithm is presented in this paper. According to the fundamental principle of vehicle dynamics, a half car mode of active suspension system and anti-lock braking systems established at first. Then the linear quadratic gauss controller of the active suspension system and sliding mode controller of anti-lock braking system are designed. Taking controllers designed for active suspension system and anti-lock braking system as bottom controllers, the upper coordinated control logic of the systems is put forward and the upper PID coordinated controller is designed on the basis of analyzing the coupling conflict between active suspension system and anti-lock braking system. Finally, a Particle Swarm Optimization Algorithm was adopted for simultaneous optimization of mechanical structure and controller parameters of vehicle chassis integrated control system, because the traditional design method of a vehicle system is always to design control parameters following structure parameters and it can not obtain the global optimal performances for the system. In order to verify the effectiveness of the algorithm, the simultaneous optimization program is developed based on Particle Swarm Optimization Algorithm in MATLAB environment while the mechanical structure and control parameters of chassis control system are set as optimization variables and the overall vehicle dynamics performance is set as objective function. Simulation result shows that the pitching angular acceleration of vehicle is reduced and vehicle riding comfort performance is improved after optimized. Braking distance and dynamic load of front and rear wheels of vehicle are also reduced significantly which indicating vehicle active safety is improved dramatically. The vehicle road test was also carried out based on integrated controller is development of anti-lock braking system and active suspension system using ARM7 when vehicle speed is 40 km/h under braking condition. The road test also shows that the dynamic load of front and rear wheels of vehicle are reduced by 34.20% and 34.10%, braking time and braking distance of vehicle were reduced by 2.31% and 4.50% respectively, the response of vehicle pitching angular acceleration at braking condition is decreased by 15.10% after optimized, and both vehicle active safety and riding comfort are improved at different levels. [ABSTRACT FROM AUTHOR]

Many factors need to be considered in the evaluation of tight conglomerate reservoirs, including the microscopic pore-throat structure, pore connectivity, lithology, porosity, permeability, and clay mineral content. The contents and types of clay minerals reflect the mineral evolution process during the deposition of the reservoir and can reflect the reservoir's physical properties to a certain extent. In this study, cores from the Baikouquan Formation in Mahu were used to comprehensively analyze the effects of the clay mineral content on the physical properties of a tight conglomerate reservoir, including field emission scanning electron microscopy (FE-SEM), casting thin section observations, X-ray diffraction (XRD), interface property testing, high-pressure mercury injection, low temperature N2 adsorption, and nuclear magnetic resonance (NMR)-movable fluid saturation testing. The results revealed that differences in different lithologies lead to differences in clay mineral content and pore structure, which in turn lead to differences in porosity and permeability. The interface electrification, adsorption, and specific surface area of the reservoir are positively correlated with the clay mineral content, which is mainly affected by the smectite content. As the clay mineral content increases, the proportion of nanoscale pore throats increases, and the core becomes denser. The saturation of the movable fluid controlled by the >50 nm pore throats in the tight conglomerate ranges from 8.7% to 33.72%, with an average of 20.24%. The clay mineral content, especially the I/S (mixed layer of Illite and montmorillonite) content, is negatively correlated with the movable fluid. In general, the research results clarified the relationship between the lithology and physical properties of clay minerals and the microscopic pore structure of the tight conglomerate reservoirs in the Baikouquan Formation in the Mahu area. [ABSTRACT FROM AUTHOR]

Abstract: Low permeability reserves occupy a large proportion in the remaining reserves all over the world. So it is urgent to research intensively the law of the seepage in the low permeability reservoirs. Fluid flow in the low permeability reservoirs deviates from the classic Darcy''s law and instead conforms to the one of nonlinear seepage. According to this characteristic, a nonlinear seepage numerical simulation software has been developed. Unlike most mature commercial numerical simulation softwares at the present stage, the influence caused by the nonlinear seepage is specially considered. In this paper, a nonlinear seepage mathematical model is proposed. In addition, on the basis of practical field and laboratory experiment data, an ideal model of diamond inverted nine spot well pattern is also established. Under the same reservoir condition, the nonlinear simulation, conventional Darcy simulation and the simulation of threshold pressure gradient are also conducted. The simulation result shows that: in the low permeability reservoirs, nonlinear seepage dominates in a large scale of formation and the nonlinear simulation result shows excellent agreement with the production data. Therefore taking the nonlinear seepage into account is more suitable to reflect the percolation mechanism and development performance of low permeability reservoirs. [Copyright &y& Elsevier]

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]

The shale gas productivity model based on shale gas nonlinear seepage mechanism is an effective way to reasonably predict productivity. The incomplete gas nonlinear effects considered in the current production prediction models can lead to inaccurate production prediction. Based on the conventional five-zone compound flow model, comprehensive gas nonlinearities were considered in the improved compound linear flow model proposed in the paper and a semianalytical solution for productivity was obtained. The reliability of the productivity model was verified by the field data, and then, the 20-year production performance analysis of the gas well was studied. Ultimately, the key influencing factors of the fracture control stage and matrix control stage have been analyzed. Research indicated the following: (1) the EUR predicted by the productivity model is higher than the EUR that the comprehensive nonlinear effects are not considered, which demonstrated that the various nonlinear effects cannot be neglected during the production prediction to ensure the greater calculation accuracy; (2) during the early production stage of shale reservoir, the adsorbed gas is basically not recovered, and the cumulative adsorption contribution rate does not exceed 10%. The final adsorption gas contribution rate is 23.28%, and the annual adsorption rate can exceed 50% in the 20th year, showing that free gas and adsorbed gas are, respectively, important sources of the early stage of production and long-term stable production; (3) the widely ranged three-dimensional fracturing reformation of shale reservoirs and reasonable bottom hole pressure in the later matrix development process should be implemented to increase the effective early production of the reservoir and ensure the earlier gas production process of the matrix development. The findings of this study can help for better ensuring the prediction accuracy of the estimated ultimate recovery and understanding the main influencing factors of the dynamic performance of gas wells so as to provide a theoretical reference for production optimization and development plan formulation of the shale gas reservoirs. [ABSTRACT FROM AUTHOR]

Geochemical data have been used as the most effective proxies for determining palaeoclimate and palaeoenvironmental conditions. In this article, we conduct a systematic review on a variety of geochemical data of sediments that are considered proxies for tracing palaeoclimate and palaeoenvironmental information. Comprehensive studies have been carried out on the palaeoproductivity, redox conditions, palaeoclimate, and the acquired data on the enrichment mechanism of organic matter have been summarized. The general mechanisms of the geochemical data regarded as the palaeoclimate and palaeoenvironmental archives include the characteristics of mineral composition, major and trace elements, and rare earth elements. The palaeoproductivity index, including the Ba and P, indicate that the organic‐rich shale was deposited in a high‐productivity water column. The trace elements revealed that the Longmaxi units recorded variable redox conditions from anoxic to aerobic. The elements V, U, Th, Ni, and Co together with the U/Th, V/Cr, Ni/Co, and V/(V + Ni) are essential indicators for defining the redox conditions. The NASC‐normalized REE + Y distribution indicates that the Longmaxi Formation was slightly altered by detrital materials in reduced and acidic environments. The Sr/Cu and MgO/CaO all reveal that the palaeoclimate was gradually transformed from humid to dry heat. The Sr/Ba ratio implies that the water depth fluctuates frequently at the early stage of sedimentation. All geochemical data integrated with the sedimentary characteristics show that anoxic conditions, high palaeoproductivity, and relatively humid palaeoclimates contributed to the accumulation of organic‐rich shale. We propose that jointly utilizing various geochemical and sedimentary indices can expand the outlook of the palaeoenvironment and provide a better understanding of the organic matter accumulation mechanisms. [ABSTRACT FROM AUTHOR]

[Display omitted] • The CO 2 recovery from different types of quartz nanopores was discussed. • The higher the amount of CO 2 dissolved could improve the flow velocity of oil and gas in nanopores. • There were three flow modes in the oil–water two-phase flow process: WDOWF, WFOWF and ODOWF. • The effect of CO 2 on the distribution and flow of oil–water depends on the flow modes of oil–water. Multiphase flow in nanoporous media is ubiquitous in various fields, including geophysics, physical chemistry, and bioengineering. Tight sandstone reservoirs have relatively well-developed nanoscale pores. Understanding the mechanism of multiphase flow in nanoporous media is crucial for enhanced oil recovery. In this study, the pore distribution characteristics of tight sandstone cores were visually observed and analyzed through Wood's metal injection experiments and scanning electron microscopy. Subsequently, molecular dynamics simulations were used to establish a multiple-pore model with/without bound water. The mainstream channel (MC) was selected as the research pore, and multiple groups of molecular dynamics models with different oil–gas–water ratios were established to simulate multiphase flow. The results showed that the micro-nano-scale pore throats of the tight sandstone were well developed, necessitating a study of fluid flow characteristics within the nanopores. In the process of CO 2 injection production, the bound water could occupy dead-end pores (DEP) and small channels (SC), which was beneficial to the oil recovery. When there was no bound water, the MC mainly relied on the CO 2 displacement for oil recovery, while the DEP of the MC, the DEP of the SC, and the SC mainly relied on oil expansion and CO 2 extraction. In the MC nano-confined fluid flow process, the larger the proportion of CO 2 was, the lower the viscosity of the oil was, and the more easily it could flow. However, when water was present, three flow modes formed: water drop oil–water flow (WDOWF), water film oil–water flow (WFOWF), and oil drop oil–water flow (ODOWF), among which ODOWF had the greatest influence on the flow of bound fluid in the nanopores. The injection of CO 2 could improve the flow in the WDOWF and WFOWF modes and had no positive effect on the ODOWF. [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]

• The mass transfer experiment of CO 2 -oil system was studied. • The oil phase swelling effect was characterized through the modified PR-EOS equation. • Diffusion coefficient of CO 2 and swelling factor of oil was calculated by using a variety of methods. • Effects of CO 2 on the potential energy and self-diffusion coefficient of oil. The oil volume expansion and CO 2 diffusion are one of the main mechanism of CO 2 -enhanced oil recovery (CO 2 -EOR). This paper established a series of experiments, numerical simulations, and molecular dynamics (MD) simulations to describe mass transfer behaviors between oil–gas phases. Expressly, the CO 2 diffusion coefficient and light oil swelling factor are signification parameters to quantify and analyze these behaviors. In detail, the CO 2 diffusion coefficient and the oil swelling factor were obtained from the traditional pressure decay method and the advanced MD simulation. Synthetically, the pressure decay method and MD simulation results were mutually verified. The results showed that the equilibrium pressure was proportional to the mole fraction of CO 2 and inversely proportional to the mole fraction of light oil. The equilibrium time was proportional to the mole fraction of CO 2 and light oil. At the temperature of 333.15 K and the initial pressure of 7.5 MPa, the CO 2 diffusion coefficient in light oil was positively correlated with the relative molar proportion of CO 2 , while the light oil swelling factor was vice versa. In addition, the closer to the CO 2 -light oil interface, the greater the light oil's potential energy and self-diffusion coefficient, and the stronger the transport ability. [ABSTRACT FROM AUTHOR]

CO 2 -enhanced oil recovery (CO 2 -EOR) technology has shown great application potential in the development of tight reservoirs. Since the proportion of nano-pores in tight oil reservoirs exceeds 77%, it is necessary to further study the occurrence state of crude oil in nano-pores. In this paper, the molecular dynamics simulation was applied to study the adsorption and diffusion behaviors of multi-component crude oil in quartz (hydrophilic) nano-pores. Besides, the density discretization method was used to investigate the occurrence characteristics of crude oil, the proportion of movable fluid in nano-pores, and the effects of CO 2 and polar molecules (C 3 H 6 O) on the adsorption characteristics of crude oil. The simulation results showed that the adsorption state of crude oil in the quartz nano-pores was in the form of 4 adsorption layers with a thickness of 0.45 nm each. In nano-pores, the oil molecules with longer molecular chains were more likely to aggregate and adsorb on the quartz surface. Among them, polar oil molecules have the strongest adsorption capacity on the quartz surface. Moreover, the crude oil potential energy and the self-diffusion coefficient gradually increased from the vicinity of the quartz wall and tended to be stable in the free layer. Meanwhile, the content of movable crude oil gradually augmented with the increasement of nanometer pore width and temperature. Furthermore, the pressure had little effect on the density distribution of crude oil in the pores. In contrast, the temperature had a more significant effect on the density distribution of the adsorption layer. At last, due to the more substantial adsorption capacity of CO 2 on the rock surface, the crude oil adsorbed initially on the rock surface would be stripped off by CO 2 , converting from irreducible oil to moveable oil. [Display omitted] • Distribution profile of adsorbed and free crude oil in various nanoslits. • The self-diffusion coefficient and potential energy of crude oil at different positions. • Temperature has a great influence on the occurrence state of crude oil. • Effects of different proportions of CO 2 on the occurrence state of crude oil in nanoslits. [ABSTRACT FROM AUTHOR]

CO 2 huff and puff (HNP) is one of the most effective methods to improve tight oil recovery after the primary depletion process. The seepage mechanisms between CO 2 and crude oil are complicated in porous media during CO 2 HNP process. Therefore, in this paper, the CO 2 HNP process of Chang-7 tight oil reservoir, Ordos Basin, China, was studied by nuclear magnetic resonance (NMR) technology, microscopic observation and numerical simulation. Experimentally, using NMR technology and microscopy methods, the distribution characteristics of residual oil during CO 2 HNP process were measured intuitively. Numerically, a group of core-scale and field-scale simulations considering molecular diffusion and asphaltene precipitation were established to further verify and elongate the experimental results. The results show that at the initial state, the crude oil in the tight core was mainly distributed in nanopores, sub-micro-nanopores and sub-micropores, where the oil content exceeded at least 73% in these pores. During CO 2 HNP process, the oil recovery was more pronounced for the 1st and 2nd rounds than for 3rd to 5th rounds. Notably, even if the cores with more nano-pores were more favorable for the 4–5th CO 2 HNP rounds, the oil molecules in nanopores were still difficult to be available. Moreover, the CO 2 sweep scope could be divided into displacement affected region and diffusion affected region. CO 2 could effectively drive the crude oil in the displacement affected region. While the oil could be successfully displaced by dissolved gas flooding in the diffusion affected region only under the appropriate conditions. Meanwhile, the core-scale numerical models confirmed that it was necessary to consider molecular diffusion and asphaltene precipitation factors, which would make the simulation results in line with the experiment. In terms of the ultimate oil recovery, the field-scale model only considering the diffusion (2.456%) > the model both considering the diffusion and asphaltene (2.436%) > the model without considering the diffusion and asphaltene deposition (2.412%) > the model only considering the asphaltene deposition (2.388%). • The oil were difficult to be available in nanopores due to the nano-constraint effect. • The CO2 displacement area was associated with diffusion and asphaltene precipitation. • SP-T2 spectrum and microscopic image were applied to obtain CO2 sweep scope. • A group of simulations were established to analyse the effect of CO2 sweep scope. [ABSTRACT FROM AUTHOR]

Abstract: Alkaline agents have an appeal for enhanced oil recovery because of their low cost and favorable performance. In this paper, sodium carbonate (Na2CO3) and sodium hydroxide (NaOH) are used as the alkaline chemicals, at the same Na2O content, to investigate the oil/water interfacial reactions between the Daqing crude oil and the alkaline solutions. Moreover, oleic acid or the mixture of ethyl acetate and phthalic acid diethyl ester were added into the crude oil, respectively, to facilitate the direct observation of the interfacial reactions and to compare the functional effectiveness of alkalis. The results showed that: Na2CO3 reacted slowly and partly with the acid components in crude oil, while NaOH did it very fast and completely. Interestingly, Na2CO3 is better than NaOH in lowering the oil/water interfacial tension (IFT), due to its buffer effect. These help the optimum formulation design of flooding alkali, which should also be of great importance for tertiary oil recovery. [Copyright &y& Elsevier]

Recent years have witnessed a surging wave of shale reservoirs in unconventional resources and gas production are expected to mount a new summit historically in 2020. The petrophysical characterization of shale gas reservoir, however, is still in a dilemma, especially the fluid saturation evaluation. In this paper, an extensive document survey improved the comprehension of current situation of the problem, accessible resolutions, and previous outcomes as a reference. The resolution consists of laboratory method, electrical resistivity method, special techniques and non-electro conventional method. The laboratory method conventionally has acceptable accuracy but can only be a standard to check the applicability of other methods due to its expensive and time-consuming traits. Electrical method, based on Archie's law, is still a prevailing approach to calculate water saturation of shale accepted by most researchers. Recognized as a non-destructive method, Nuclear magnetic resonance (NMR) is sufficiently powerful to depict the nanometer pore structure and estimate the water saturation in shale quantitively. Non-electro conventional method conveys empirical equations mainly with porosity, permeability, clay content and other properties. To summarize, this review strives to supply comprehensive information on the currently existing methods and experiences conducted on characterizing fluid saturation in unconventional shale reservoirs. • Estimation of water saturation in shale is classified into four categories. • Comprehensive description on the existing methods is proposed. • An expanded literature survey of previous outcomes is provided as a reference to shed light on future investigation. [ABSTRACT FROM AUTHOR]

Nuclear magnetic resonance (NMR) has emerged as an essential method for appraising gas-shales, quantifying their petrophysical properties, classifying their fluid types, and discerning their productivity. Comprehensive measurement campaigns, as well as reliable thermal-heating experiments, are required to fully understand the NMR mechanism of shale and to further develop future gas-shale exploitation strategies. NMR signatures from individual matrix components, which comprise shale, are investigated to gather data to better estimate its petrophysical parameters. Subsequently, we perform T 1 and T 2 measurements of kerogen, solid bitumen, and clay minerals. Next, we analyze the temperature effect on the NMR signal interpretation of different compositions of gas-shale samples in detail, coupled with the position of clay minerals and solid bitumen from the T 1 -T 2 mapping. A shale plug sample was quantified by the NMR T 1 -T 2 relaxation signals under initial conditions and different temperature conditions of 110 °C, 250 °C, 450 °C, and 650 °C. The fluid volume significantly decreased as the temperature increased. The organic matter maturity changed at 450 °C and 650 °C. According to a thermal-heating experiment with powdered clays, we obtained the hydroxyl group and crystal water signals and determined the critical temperature for different dried types of water in clay mineral. It is reasonable to infer that deconvoluting the entire shale sample into different components can further determine the NMR relaxation mechanisms. The ultimate goal of this project was to develop a holistic methodology to understand the NMR mechanisms of isolated components and to assess the positions of kerogen, solid bitumen, illite, kaolinite, smectite, and chlorite from T 1 -T 2 mapping. This work contributes to the fundamental understanding of shale formations by quantifying the NMR relaxation mechanisms of various constituents in shale and provides implications for shale rock evaluations. • To investigate the T 1 -T 2 correlation spectrum of isolated components of the shale. • Accurately quantifying the relaxation mechanism of the specific constituents in clay mineral. • To complement the position of bitumen and clay mineral in T 1 -T 2 mapping of gas-shale. • To imply temperature affects on the relaxation times on kerogen, bitumen, and clay mineral. [ABSTRACT FROM AUTHOR]