Shale Gas Exploitation Research Articles

Viscoelastic Strains of Palaeozoic Shales under the Burger’s Model Description

This article presents the results of creep studies of Palaeozoic shales from the Baltic Basin in which the exploitation of shale gas in Poland was planned. Knowledge of instantaneous and long-term properties investigated in triaxial stress conditions is important from the point of view of exploitation techniques related to hydraulic fracturing. Rheological phenomena also play an important role in the analysis of the initial stress in shales, the knowledge of which is indispensable in the hydraulic fracturing process. The tests were carried out on samples representing four siltstone–claystone lithostratigraphic units occurring in the Baltic Basin. The studies and analyses were aimed at determining the character of creep in shales, selection of the appropriate rheological model for the analyzed rocks, and determination of the threshold of the linear creep under triaxial compression conditions. An original approach together with analysis results are presented here, which enable the separation and monitoring of shear and volume creep effects, and on this basis, the determination of the significance of the contribution of volume creep in the entire creep process. A relatively simple methodology for determination of the parameters of the Burgers model using this division is presented. The original value of the article is also due to the test results themselves and the parameter values of the analyzed model for triaxial creep of shales, which are not numerous in the literature. The investigations were performed at various loading levels in relation to the triaxial strength of the shales. Depending on the load, at its low values up to 0.7 (σ1 − σ3)max, creep had a determined character and did not show features of progressive creep. The linear creep threshold was also analyzed in this range. The loading level of 0.7 (σ1 − σ3)max was the limit of linear creep. Exceeding this load resulted in the loss of the linear character of creep, which in consequence lead to the subsequent third creep phase ending with rock damage. Parameters of the Burger’s model for gas shales from the Baltic Basin (northern Poland) were identified. There are significant differences in the behavior of shales depending on the lithostratigraphic unit from which the samples were collected. The mineral composition of the shales also influenced their behavior.

An equivalent thermo-hydro-mechanical model for gas migration in saturated rocks

Gas transport process in saturated rocks is gaining increasing attention in the environmental/energy geomechanical engineering fields, e.g., shale gas exploitation, carbon dioxide sequestration and nuclear waste disposal, etc., as it is closely associated with gas production rate, sealing capability of caprocks. To investigate gas migration behavior through saturated rocks which induces microcracking inside the material, an equivalent thermo-hydro-mechanical (THM) model is developed that considers coupled behavior of fractures. The fractured rock material is considered as the equivalent continuum, which consists of isotropic porous matrix and parallel fracture set. The mathematical formulations are constructed within the thermodynamic framework, where the THM constitutive relations are proposed. Compared to the model without considering fracture behavior, the equivalent model improves the calculation accuracy of about 60% against laboratory experiments (e.g., observed gas flow rate), which proves the model capability. Furthermore, the sensitivity analysis shows that highly-pressurized gas induced microcracking has significant influence on the coupled process, which greatly contributes to the observed gas flow phenomenon.

Upscaling the creep behavior of clay-rich and quartz-rich shales from nanoindentation measurements: Application to the Wufeng-Longmaxi shale, China

Creep of shale controls the stability of fault gouge in sedimentary rocks, aseismic deformation in the upper crust, and several engineering operations, such as proppant embedment, reservoir subsidence, and wellbore stability during unconventional shale gas exploitation. However, measuring the creep properties of shale is challenging due to the multi-scale structure of this rock. Here, our goal is to assess how creep parameters measured at the core sample scale depend on those measured at the nanoscale in shales. We selected three clay-rich and quartz-rich shale samples and conducted a series of experiments on shale samples at two scales: nanoindentation creep measurements on thin sections, and uniaxial creep tests on core samples. The creep parameters, including creep displacement (Δh), indentation creep (CIT), creep strain rate sensitivity (m), contact creep modulus (C), and viscoelastic parameters (E1, E2, η1, and η2) of the Burgers' model, were calculated by fitting the nanoindentation creep data of each single-phase component of shale matrix. From these data, we upscale the creep parameters (E1, E2, and C) of shale matrix to those of the bulk shale. Upscaled E2 and C values based on the Mori-Tanaka scheme are found to be relatively close to those measured in previous microindentation study on Longmaxi shale and are consistent with our measurements in uniaxial creep tests on core samples of clay-rich shale and quartz-rich shale. The upscaling value of the elastic parameters, E1, is almost twice larger than the results from uniaxial creep test. We interpret this difference as due to low elastic stiffness along grain contacts of the shale that nanoindentation measurements underestimate. Our results show a consistency of some creep parameters upscaled from nanoindentation data and those measured at a macroscopic scale on core samples.

Study on the evolution process of corrosion behavior under the sulfate reducing bacteria biofilm in shale gas gathering pipelines

Pipeline corrosion induced by sulfate-reducing bacteria (SRB) is particularly severe during the hydraulic fracturing technology in shale gas exploitation. To investigate the evolution of corrosion beneath the SRB biofilms, 49-day corrosion experiments were performed with and without SRB. The weight loss experiment demonstrated that the corrosion in the Blank group was mitigated by the corrosion product film. In contrast, the SRB group exhibited an initial inhibitory phase followed by an exacerbation. Scanning electron microscopy (SEM) revealed stratification in the Blank group, whereas SRB initially formed colonies and extracellular polymeric substances (EPS), eventually the biofilm evolved into a mixed layer with the corrosion product. 3D microscopy showed a relatively flat surface in the Blank group. Conversely, corrosion pits beneath SRB biofilm were circular and could reach a depth of 29.77 μm, significantly greater than in the Blank group. Over time, SRB-induced pits deepened from the edges to the center and widened to approximately 740 μm, accompanied by an increase in quantity. Energy Dispersive Spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS) analyses detected the presence of elements P, S, and FeS in the SRB group, serving as important indicators of SRB corrosion. Electrochemical experiments revealed significantly higher corrosion currents in the SRB group. The evolution of SRB corrosion consisted of three phases: initial, intermediate, and starvation phases. During the starvation phase, localized corrosion proved to be the most severe, with a corrosion rate of 1.28 mm/a. Therefore, the study of SRB-induced corrosion should be prioritized to study the localization phenomena.

Geological Characteristics and Challenges of Marine Shale Gas in the Southern Sichuan Basin

After more than 10 years of exploration, development, research, and practical efforts, China has opened up new perspectives for the commercial exploitation of marine shale gas. While high shale gas production is a main driver for energy security and economic development in China, there have been few attempts to systemically scientific analysis the challenges, prospect, development strategies, and goals for shale gas. Here, we present a detailed comparison of the differences in shale gas between the Sichuan Basin and North America from multiple dimensions, explain how and to what extent recent advances have been made, discuss the current challenges, and provide strategies to deal with these challenges. We demonstrate that a total of 13 graptolite zones developed in the Wufeng–Longmaxi Formations, achieved by representative cores from 32 coring wells and 7 outcrop profiles, can establish the chronostratigraphic framework in the Sichuan Basin, which leads to the potential impact of high-quality reservoir distribution and shale gas production. Shale gas is still faced with the challenges of complex underground and surface conditions, low single-well EUR, and immature deep development engineering technology. To circumvent these issues, here, we propose several strategies, including sweet-spot optimization, low-cost drilling techniques, and efficient fracturing technologies. Our results strengthen the importance of adopting fundamental theoretical research and practical and feasible development goals to realize more commercial discoveries of shale gas of diverse types and higher growth of shale gas reserves and production.

Shale oil and gas exploitation in China: Technical comparison with US and development suggestions

The shale oil and gas exploitation in China is technically benchmarked with the United States in terms of development philosophy, reservoir stimulation treatment, fracturing parameters, fracturing equipment and materials, oil/gas production technology, and data/achievements sharing. It is recognized that the shale oil and gas exploitation in China is weak in seven aspects: understanding of flow regimes, producing of oil/gas reserves, monitoring of complex fractures, repeated stimulation technology, oil/gas production technology, casing deformation prevention technology, and wellbore maintenance technology. Combined with the geological and engineering factors of shale oil and gas in China, the development suggestions of four projects are proposed from the macro- and micro-perspective, namely, basic innovation project, exploitation technology project, oil/gas production stabilization project, and supporting efficiency-improvement project, so as to promote the rapid, efficient, stable, green and extensive development of shale oil and gas industry chain and innovation chain and ultimately achieve the goal of “oil volume stabilizing and gas volume increasing”.

Toxicity identification evaluation for hydraulic fracturing flowback and produced water during shale gas exploitation in China: Evidence from tissue residues and gene expression.

Hydraulic fracturing flowback and produced water (HF-FPW) from shale gas extraction processes is a highly complex medium with potential threats to the environment. Current research on ecological risks of FPW in China is limited, and the link between major components of FPW and their toxicological effects on freshwater organisms is largely unknown. By integrating chemical and biological analyses, toxicity identification evaluation (TIE) was used to reveal causality between toxicity and contaminants, potentially disentangling the complex toxicological nature of FPW. Here, FPW from different shale gas wells, treated FPW effluent, and a leachate from HF sludge were collected from southwest China, and TIE was applied to obtain a comprehensive toxicity evaluation in freshwater organisms. Our results showed that FPW from the same geographic zone could cause significantly different toxicity. Salinity, solid phase particulates, and organic contaminants were identified as the main contributors to the toxicity of FPW. In addition to water chemistry, internal alkanes, PAHs, and HF additives (e.g., biocides and surfactants) were quantified in exposed embryonic fish by target and non-target tissue analyses. The treated FPW failed to mitigate the toxicity associated with organic contaminants. Transcriptomic results illustrated that organic compounds induced toxicity pathways in FPW-exposed embryonic zebrafish. Similar zebrafish gene ontologies were affected between treated and untreated FPW, again confirming that sewage treatment did not effectively remove organic chemicals from FPW. Thus, zebrafish transcriptome analyses revealed organic toxicant-induced adverse outcome pathways and served as evidence for TIE confirmation in complex mixtures under data-poor scenarios.

Quantifying the Widths of Fault Damage Zones Based on the Fault Likelihood: A Case Study of Faults in the Fuji Syncline of the Luzhou Block, Sichuan Basin, China

Faults are critical to the preservation or destruction of shale gas concentration. The Lower Silurian Longmaxi Formation in the southern Sichuan Basin hosts relatively developed faults, which pose a huge challenge to the exploration and exploitation of shale gas. An urgent need to quickly determine the widths of fault damage zones (FDZs) arises in locating horizontal shale gas wells. In this study, FDZs were estimated using the fault likelihood. The results are as follows: (1) It is rational to constrain the FDZ width using a fault likelihood greater than 0.2. The six major NEE-trending faults in the Fuji syncline of the Luzhou block have complex structures and varying FDZ widths from about 240–1220 m. (2) The degree of influence of FDZs is negatively correlated with their distance from the faults. In other words, a greater distance from a fault is associated with a weaker influence and a smaller fault likelihood. (3) Based on the ratio of the fault throw to the FDZ width, we propose that the width of seismic-scale fault damages can be directly constrained using a ratio value of 3.5. This method is fast and accurate and can provide support for the evaluation of the shale gas preservation conditions and well placement in the Longmaxi Formation of the southern Sichuan Basin.

A New Method for Comprehensive and Quantitative Characterization of Shale Microfractures: A Case Study of the Lacustrine Shale in the Yuanba Area, Northern Sichuan Basin

Microfractures can connect isolated pores within shale, significantly increasing the shale’s storage capacity and permeability, and benefiting shale gas exploitation. Therefore, the quantitative characteristics of microfractures are important parameters for shale reservoir evaluation. In this paper, taking the Jurassic Da’anzhai Member (J1z4) lacustrine shale in the Yuanba area of the northern Sichuan Basin as an example, we propose a method for comprehensive and quantitative characterization of shale microfractures that combines rock thin section (RTS) and scanning electron microscopy (SEM) observations. The different magnifications of RTSs and SEM images lead to the identification and characterization of microfractures of different scales using these two methods. RTSs are mainly used to characterize microfractures with widths larger than 10 μm, while SEM is mainly used to characterize microfractures with widths smaller than 10 μm. These techniques can be combined to comprehensively and quantitatively characterize microfractures of different scales in shale. The microfracture characterization results show that the average total porosity of the J1z4 shale is 4.46%, and the average microfracture surface porosity is 1.20% in the Yuanba area. The calculated average percentage of microfracture porosity to total porosity is 21.09%, indicating that the J1z4 shale reservoir space is dominated by pores and has the conditions for stable shale gas production and potential for shale gas exploration. However, the percentage of microfracture porosity to total porosity of shale near faults and fold zones approaches or exceeds 50%, which may lead to the loss of shale gas. The new method proposed in this study is also useful for quantitative characterization of shale microfractures in the Sichuan Basin and other basins.

Experimental study on brittle-to-ductile transition mechanism of lower Silurian organic-rich shale in south China

Brittle-ductile transition conditions and mechanisms are key scientific issues embedded in the reservoir deformation-related difficulties faced during shale gas exploration and exploitation. In this study, shale deformation experiments under different temperatures, confining pressure and differential stress were conducted using a NaCl solid confining pressure medium, taking the Lower Silurian marine shale from South China as an example. We aimed to clarify the characteristic textures and structures of brittle and ductile deformation in organic-rich shale and then, to reveal the conditions and mechanisms of the brittle-to-ductile transition under the deep in-situ environment of high-temperature, high-confining pressure and high-stiffness surrounding strata. Deformation characteristics of experimentally deformed shale showed significant differences between brittle and ductile deformed shale in terms of fracture development, failure combining form, deformation distribution homogeneity, shear angle and the structures and textures within intensely deformed zones. The micro-deformation structures of brittle deformed shale are characterized by the development of fractures and the simple mechanical crushing of shale blocks and mineral particles. The strong deformation zones of ductile deformed shale mainly show micro-deformation features, such as solid-state rheological deformation and accompanying fragmentation, rounding and directional arrangement of mineral particles. Comparison of control group experiments supported the conclusion that increasing confining pressure significantly improves the elastic modulus and compressive strength of shale and leads to a transition from large-area uniform deformation to narrow-band concentrated deformation. A confining pressure of 75–100 MPa is required for the brittle-to-ductile deformation transition of the target shale at 200 °C. An elevated temperature enhances shale deformation in the yield and creep stages. Finally, we revealed the progressive process and mechanism of brittle-to-ductile transition with the increase of confining pressure; that is, increasing confining pressure leads to a gradual transition of the shale micro-deformation mechanism, from brittle failures to ductile rheological deformation, by suppressing tension fracturing and enhancing friction.

Microseismic network sensitivity in case of no seismic activity

Underground human activities, such as mining, shale gas, and oil exploitation, waste-water disposal, or geothermal plants, can cause earthquakes; therefore, they are monitored by local seismic networks. An ideal seismic network has a triangulated grid, with spacing equal twice the minimal depth and no associated industry noise. In real cases, the network sensitivity is biased by stations placed near noisy roads, factories, or in a private garden, none located at optimal nodes. The sensitivity is also a function of the detection algorithm type and setting. The goal of this case study is to suggest a work-flow for network sensitivity calculation in case of no seismic activity. In other words: how small are the earthquakes that such seismic networks would detect if they were present? Such network sensitivity is a function of stations noise level, station-source geometry, and setting of the detection algorithm. A brief theory and work-flow description is followed by two real-case demonstrations from Czech Republic, Europe: first, a proof-test on a well-studied seismically active area of West Bohemia/Vogtland and second, an application to an uprising geothermal project in Litoměřice, where no seismic activity was detected in years of monitoring.

An integrated approach to identify the source apportionment of potentially toxic metals in shale gas exploitation area soil, and the associated ecological and human health risks

Public awareness of the potential environmental risks of shale gas extraction has increased in recent years. However, the status and environmental risks of potentially toxic metals (PTMs) in shale gas field soil remain unclear. A total of 96 topsoil samples were collected from the first shale gas exploitation area in China. The sources of nine PTMs in the soils were identified using positive matrix factorization and correlation analysis, and the ecological and human health risks of toxic metals from different sources under the two land use types were calculated. The results showed that mean pollution load index (PLI) values for farmland (1.18) and woodland (1.40) indicated moderate pollution, As, Cd and Ni were the most serious contaminants among all nine PTMs. The following four sources were identified: shale gas extraction activities (43.90%), nature sources (31.90%), agricultural and traffic activities (17.55%) and industrial activities (6.55%). For ecological risk, the mean ecological risk index (RI) values for farmlands (161.95) and woodlands (185.27) reaching considerable risk. The contribution ratio of shale gas extraction activities for farmlands and woodlands were 5.70% and 8.90%, respectively. Regarding human health risk, noncarcinogenic risks for adults in farmlands and woodlands were negligible. Industrial activities, agricultural and traffic activities were estimated to be the important sources of health risks. Overall, shale gas extraction activities had little impact on the ecological and human health risk. This study provides scientific evidence regarding the soil contamination potential of shale gas development activities.

Virtual Special Issue of Recent Advances in Shale Gas Exploration and Exploitation

Virtual Special Issue of Recent Advances in Shale Gas Exploration and Exploitation

Does shale gas exploitation contribute to regional sustainable development? Evidence from China

Shale gas is a growing contributor to China's natural gas production. It is seen as a key component of the country's energy transition. As a major shale gas producing region, Chongqing has large shale gas production and high expectations for its future development. However, it is unclear how regional sustainability will be affected by this shale gas boom. To help answer this question, this paper establishes a sustainability assessment framework covering three economic-social-environmental dimensions and 21 specific indicators; obtains a composite sustainability index (CSI) of districts and counties in the Chongqing region based on the Entropy Weight Method (EWM); and then empirically analyses the impacts of shale gas development in Fuling and neighbouring districts by using the Synthetic Control Method (SCM). Our results show that the level of sustainable development in Chongqing has a fluctuating increase, with a rapid rise in the CSI for the sample area, but shale gas development activities do not contribute significantly. Further analysis shows that the CSI is relatively stable when weights are changed and indicators are substituted, and the scale of the shale gas industry may limit its impact on sustainable development. The synthetic control results for sub-dimensional sustainability indices are also not significant. However, the potential for sustainability levels in the environmental dimension and in region adjacent to shale gas field to be affected by shale gas development is greater. Developers and governments can focus on assessing these two aspects if shale gas production is larger.

A rapid approach to evaluating ground surface conditions for shale gas extraction in mountainous areas

Ground surface conditions, regarding the engineering, eco-environmental, and economic constraints, are crucial for shale gas extraction in the mountainous areas. A novel approach has thus been proposed to perform a rapid and integrated evaluation of ground surface conditions on a regional scale. The southeastern margin of the Sichuan Basin, China’s primary shale gas exploitation target, serves as an example to demonstrate the execution of this approach. Critical metrics for ground surface condition evaluation, including geomorphology, land utilization, water supply, and transportation conditions, were obtained from published GIS and remote sensing data. Different ratings were assigned to facilitate a quantitative evaluation of the metrics, and the composite index was calculated based on the ratings. The study area was first filtered using a composite index threshold determined from the calculated values at the existing well sites. The contiguous regions with the sufficient dimensions within the screened area were further selected as the final potential targets. The overall assessment recommended that only 10.99%, 7.61%, and 4.86% of the study area be deemed marginally, moderately, and highly suitable for potential shale gas extraction sites, respectively. These results suggest that this approach can enable efficient and accurate ground surface condition evaluations beyond the actual field surveys, while excluding the most unsuitable areas for further shale gas extraction site selection.

Editorial: Advances in exploration and exploitation of deep and ultra-deep shale oil and gas

Editorial: Advances in exploration and exploitation of deep and ultra-deep shale oil and gas

An improved transport model in shale nanopores based on advection diffusion method

As an unconventional oil and gas resource, shale gas plays a very important role in global energy supply. A breakthrough in efficient exploitation of shale gas is how to accurately describe the percolation mechanism of shale gas in nano-pores. In this article, a new shale gas apparent permeability model is established on the basis of advection diffusion model and considering adsorption stress effect and gas viscosity correction. On this basis, the influence factors of apparent permeability, the comparative difference between stress effect or not and viscosity correction or not, and the contribution of gas flow mechanism are studied. The results show that: the apparent permeability decreases with the increase of pressure, increases with the increase of pore radius, and increases with the increase of temperature; when the pore radius increases, the difference caused by stress correction becomes obvious; when the pressure decreases, the difference caused by viscosity correction becomes obvious. The results can provide some new ideas and methods for the study of gas flow mechanism in microscopic nano-pores.

Experimental Research on Supercritical Carbon Dioxide Fracturing of Sedimentary Rock: A Critical Review

AbstractSupercritical carbon dioxide (ScCO2) fracturing has great advantages and prospects in both shale gas exploitation and CO2 storage. This paper reviews current laboratory experimental methods and results for sedimentary rocks fractured by ScCO2. The breakdown pressure, fracture parameters, mineral composition, bedding plane angle and permeability are discussed. We also compare the differences between sedimentary rock and granite fractured by ScCO2, ultimately noting problems and suggesting solutions and strategies for the future. The analysis found that the breakdown pressure of ScCO2 was reduced 6.52%–52.31% compared with that of using water. ScCO2 tends to produce a complex fracture morphology with significantly higher permeability. When compared with water, the fracture aperture of ScCO2 was decreased by 4.10%–72.33%, the tortuosity of ScCO2 was increased by 5.41%–70.98% and the fractal dimension of ScCO2 was increased by 4.55%–8.41%. The breakdown pressure of sandstone is more sensitive to the nature of the fracturing fluid, but fracture aperture is less sensitive to fracturing fluid than for shale and coal. Compared with granite, the tortuosity of sedimentary rock is more sensitive to the fracturing fluid and the fracture fractal dimension is less sensitive to the fracturing fluid. Existing research shows that ScCO2 has the advantages of low breakdown pressure, good fracture creation and environmental protection. It is recommended that research be conducted in terms of sample terms, experimental conditions, effectiveness evaluation and theoretical derivation in order to promote the application of ScCO2 reformed reservoirs in the future.

Numerical Modeling of Natural Fracture Distributions in Shale

AbstractThe production efficiency of shale gas is affected by the interaction between hydraulic and natural fractures. This study presents a simulation of natural fractures in shale reservoirs, based on a discrete fracture network (DFN) method for hydraulic fracturing engineering. Fracture properties of the model are calculated from core fracture data, according to statistical mathematical analysis. The calculation results make full use of the quantitative information of core fracture orientation, density, opening and length, which constitute the direct and extensive data of mining engineering. The reliability and applicability of the model are analyzed with regard to model size and density, a calculation method for dominant size and density being proposed. Then, finite element analysis is applied to a hydraulic fracturing numerical simulation of a shale fractured reservoir in southeastern Chongqing. The hydraulic pressure distribution, fracture propagation, acoustic emission information and in situ stress changes during fracturing are analyzed. The results show the application of fracture statistics in fracture modeling and the influence of fracture distribution on hydraulic fracturing engineering. The present analysis may provide a reference for shale gas exploitation.

Removal and recycling of the organic pollutants from the oily hazardous wastes by cyclone deoiling technology

Oily hazardous wastes generated from the shale gas exploitation has long been a major environmental threat, yet traditional treatment techniques are suffered by high energy consumption and secondary pollution. To address this challenge, we developed an optimized Cyclone Deoiling Technology that directly separated the oily compounds from drilling wastes and recycled into diesel fuel products with high purity. Instead of destructing the hazardous wastes, the oily components were extracted from porous solid substrates by coupling revolution-rotation movement. By doing so, as low as 0.34 kWh/kg energy consumption was achieved, which is 70% lower than that of heating approaches, and the recovery rate was over 98%. Using our prototype facility as testbed, the oil content in solid wastes was dramatically reduced from 7.7% to 0.27% after treatment, qualifying the environmental standard of 0.3%. We believe the technology is a promising technique facilitating the sustainable development of the oil and gas industry.