Shale Hydraulic Fracturing Research Articles

CT-based 3D reconstruction of the geometry and propagation of hydraulic fracturing in shale

Hydraulic fracturing is one of the primary techniques which has been widely implemented in order to allow for the stimulation of shale gas reservoirs. To accurately distinguish the extension characteristics and the spatial distribution of fractures during the hydraulic fracturing of the Longmaxi shale reservoirs in Changning, a self-developed, multifunctional, true triaxial experimental process (utilizing a fluid-solid coupling testing system) was used to simulate the horizontal well hydraulic fracturing. The CT technique was used to scan the samples prior to and following the conduction of the experiments; the CT scanning images of internal fractures were reconstructed and visualized via 3D reconstruction. The influence of in-situ stress, the displacement of the fracturing fluid upon the fracture morphology and the resultant fractural extensions caused by hydraulic fracturing were discussed. The study shows that the reconstructed 3D model based on the different CT values corresponding to the shale matrix, minerals and fractures can accurately characterize the internal structure and fracture distribution of shale. Shale hydraulic fracturing fractures can be divided into 3 categories: single transverse fractures, main arc fractures and complex fractures. The “fluctuation phenomenon” of a water pressure curve is related to the formation and extension of reservoir fractures, and is a discernible characteristic of complex fractures formed by the volume fracturing of shale. Stress differences and the stress difference coefficient are the governing factors in terms of the complexity of shale hydraulic fracturing networks. Displacement has a particular range of influence on the manifestation of fracture complexity; a low displacement may not result in a complete fracture, but too high a displacement tends to culminate a single fracture in the formation, which to a certain extent reduces fractural complexity.

A review of the current status of induced seismicity monitoring for hydraulic fracturing in unconventional tight oil and gas reservoirs

During the past decades, significant progress has been made in the development of induced seismicity monitoring for related human activities. Hydraulic fracturing and induced seismicity monitoring are operating procedures for safe and effective production of oil and gas from unconventional resources, particularly shales. Hydraulic fracturing can induce seismicity through fluid injection and disturbance of subsurface stress in tight reservoirs. Most seismic events associated with hydraulic fracturing exhibit magnitude of Mw ≤ 3 and are referred to as microseismicity, while a few larger-magnitude earthquakes (e.g. Mw > 3) could also be induced by reactivating pre-existing faults. Here, we review the current status of research concerning induced seismicity monitoring for shale hydraulic fracturing. Induced seismicity contains information relating to important subsurface characteristics, e.g. rock failure potential and seismogenic zones. Microseismic monitoring is essential for reservoir characterization, e.g. fracture geometry delineation and reservoir geomechanical analysis. It is carried out with advanced acquisition, processing, and interpretation techniques, while larger-magnitude earthquakes are mainly exploited for potential geohazard management and mitigation. Challenges and prospects associated with multi-disciplines for future research and applications of induced seismicity monitoring are identified, and it contributes to achieve safe and efficient unconventional (tight) oil and gas resource exploitation.

Investigation on Interference Test for Wells Connected by a Large Fracture

Pressure communication between adjacent wells is frequently encountered in multi-stage hydraulic fractured shale gas reservoirs. An interference test is one of the most popular methods for testing the connectivity of a reservoir. Currently, there is no practical analysis model of an interference test for wells connected by large fractures. A one-dimensional equation of flow in porous media is established, and an analytical solution under the constant production rate is obtained using a similarity transformation. Based on this solution, the extremum equation of the interference test for wells connected by a large fracture is derived. The type-curve of pressure and the pressure derivative of an interference test of wells connected by a large fracture are plotted, and verified against interference test data. The extremum equation of wells connected by a large fracture differs from that for homogeneous reservoirs by a factor 2. Considering the difference of the flowing distance, it can be concluded that the pressure conductivity coefficient computed by the extremum equation of homogeneous reservoirs is accurate in the order of magnitude. On the double logarithmic type-curve, as time increases, the curves of pressure and the pressure derivative tend to be parallel straight lines with a slope of 0.5. When the crossflow of the reservoir matrix to the large fracture cannot be ignored, the slope of the parallel straight lines is 0.25. They are different from the type-curves of homogeneous and double porosity reservoirs. Therefore, the pressure derivative curve is proposed to diagnose the connection form of wells.

Experimental study of the dynamic fracture toughness of anisotropic black shale using notched semi-circular bend specimens

This paper investigates the dynamic fracture toughness anisotropy of black shale (BS). Notched semi-circular bend (NSCB) BS specimens are prepared with different loading angles (the angle between the stratification plane and the loading direction). NSCB specimens are dynamically loaded using a modified split Hopkinson pressure bar (SHPB) system. Formulae for calculating mode-I fracture toughness of transversely isotropic material are obtained by finite element method (FEM) in combination with the J-integral method. Dynamic fracture toughness of the NSCB sample is calculated using stress waves recorded in SHPB tests and the obtained formulae. The results show that the fracture toughness of specimens for all loading angles exhibits obvious loading rate dependence, with the fracture toughness increasing with the loading rate. Similar to the static case, the specimens with higher loading angle have larger fracture toughness values as compared with ones with lower loading angles under similar loading rates. We proposed two envelope lines to enclose the fracture toughness of specimens under different loading rate. The difference between two envelope lines decreases with the increase of the loading rate. The anisotropy of black shale diminishes under extreme high loading rate. Anisotropy index, defined as the ratio of the maximum to the minimum of fracture toughness, is equal to 1.75 under static loading and 1.52 under smaller dynamic loading. These conclusions may have significant implications to the design of hydraulic fracturing in shales, e.g., fracture fluids pumping could be considered as static loading and perforation can be considered as dynamic loading. The anisotropy of shales should be considered in both these cases.

Hack fracking for more methane

Microbial Ecology Microbes are thought to contribute to chemical processes that occur during hydraulic fracturing of shale. How these communities develop after injection of fracking fluid likely determines whether they are beneficial or detrimental. Borton et al. created laboratory models of the gas-well microbial community seeded with fluid from an active well. Microbes that might boost the methane output of wells proliferated in test conditions amended with glycine betaine, a metabolite produced to counteract the salinity of the well fluids. Amino acid fermentation reactions provided an energy source for the dominant microbes in the culture. The authors analyzed well fluid from 41 active wells in the eastern United States and confirmed production of metabolites that was consistent with the metabolic pathways identified in the laboratory. Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1800155115 (2018).

Acoustic Emission Response of Laboratory Hydraulic Fracturing in Layered Shale

Understanding the generation process of complex fracture network is essential for optimizing the hydraulic fracturing strategy in shale formations. In this study, laboratory fracturing was performed on shale specimens containing multiple bedding planes (BPs) combined with acoustic emission (AE) monitoring and computerized tomography scanning techniques. The injection pressure curve and the time dependency and hypocenter mechanisms of AE events in different stages were analyzed in detail. The relationship between AE spatial localization and hydraulically connected region were then further quantitatively discussed. Experimental results show that the characteristics of the pressure curve and AE response reflect well the hydraulic fracture (HF) growth behavior in layered shale. Shear events were detected around some weak BPs far away from the wellbore before the HF initiation. Stable injection pressure and a few AE events with low amplitude along the BP may indicate the stage of fluid leak-off. Numerous shear and tensile AE events and drastic pressure changes occur during the generation of a fracture network including breakdown in rock matrix and activation of multiple BPs. The shear instability of weak BPs caused by the stress perturbation during pressurization and HF growth tends to result in overestimation of the effective stimulated reservoir volume/hydraulically connected region.

Effects of ‘soft-hard’ compaction and multiscale flow on the shale gas production from a multistage hydraulic fractured horizontal well

In this study, the shale gas production from a multistage hydraulic fractured shale gas reservoir was analyzed. Firstly, the reservoir was divided into single-porosity medium zone, dual-porosity medium zone, and hydraulic fracture zone. Then, a mathematical model was established for the multiscale gas diffusion and adsorption/desorption mechanisms in matrix, the gas flow in fractures and the interaction between gas flow and shale deformation. Thirdly, this mathematical model was numerically solved within the platform of Comsol Multiphysics and verified by the history matching of the gas production data from Barnett and Marcellus shale reservoirs. Finally, sensitivity study was conducted to identify key parameters to the efficiency of gas recovery enhancement. It is found that hydraulic fracture aperture and initial permeability of fractures are positively correlated with gas production rate. The ‘soft – hard’ compaction of fractures can significantly modify gas production rate but the modification varies with gas production stage. The deformation and flow properties of hydraulic fractures have significant impacts on gas production rate, particularly in the early stage.

Combined radiogenic (87Sr/86Sr, 234U/238U) and stable (δ88Sr) isotope systematics as tracers of anthropogenic groundwater contamination within the Williston BasinUSA

The Williston Basin has produced oil and gas from conventional structural and stratigraphic traps for more than 60 years. The advent of horizontal drilling and hydraulic fracturing of shale in the Devonian-Mississippian Bakken Formation has increased the production of oil in this basin from areas that partially overlap the prairie pothole region of North America. Massive scale of oil production in the Williston Basin increases the risk of accidental releases of coproduced formation water (brine) into the environment. The prairie pothole region is named for a multitude of small lakes and wetlands that provide critical habitat for waterfowl and other wildlife. The increased risks raise the importance of developing robust tracers that can identify the source(s) and quantity of contamination of this premier wetland ecosystem.Radiogenic 87Sr/86Sr and 234U/238U isotope tracers are widely used in hydrological studies to detect potential sources of groundwater contamination. Here, we used paired 87Sr/86Sr and δ88Sr values in wetland water, which is essentially shallow groundwater at the Goose Lake and Fuller sites in Montana and North Dakota where produced water contamination was previously established. We also analyzed brines from the Bakken Formation and Mississippian Charles Formation to evaluate potential sources of contamination. This is the first attempt to combine these isotope tracers to estimate a magnitude of groundwater contamination by the oil-field produced water. Moreover, because U+6 is soluble in water under subaerial oxidizing conditions and the U+4 content is orders of magnitude lower (pg/g levels) in reducing brines, the 234U/238U isotope tracer is insensitive to brine contamination. However, it elucidates potential variability of uncontaminated end-members and, therefore, helps to improve the accuracy of estimated degrees of brine contamination based on binary mixing relationships. In addition, U isotopes in groundwater have a potential for detecting some U-rich anthropogenic contaminants (e.g., phosphate fertilizers).Using these isotopic systematics we confirm previous conclusions that surface water and shallow groundwater in two studied sites are variably contaminated by produced water and estimate the degree of the contamination at ∼7% in a sample collected close to a brine tank and <0.5% in most other samples. The U isotope analyses in combination with the Sr isotope results help outline variable groundwater pathways and uncontaminated end-members. Also, U isotopes show no appreciable groundwater contamination from U-enriched P-fertilizers. Our study in the Williston Basin demonstrates the potential of using combined radiogenic (87Sr/86Sr, 234U/238U) and non-traditional stable (δ88Sr) isotopic systematics as tracers of anthropogenic contamination in surface and groundwater systems.

The dynamic elastic and mineralogical Brittleness of Woodford shale of the Anadarko Basin: Ultrasonic P-wave and S-wave velocities, XRD-Mineralogy and predictive models

The recent advances in drilling and hydraulic fracturing technology rendered the Woodford shale of the Anadarko basin a prolific resource play producing gas and liquid hydrocarbons. Among the challenges facing improved economics, there is variable production from one well to another as well as from a completion stage to another due to the variability in facture gradient or brittleness index (BI) and the kerogen content. This investigation relies on laboratory ultrasonic P-wave and S-wave velocity in Woodford core samples to estimate BI based on the estimated elastic moduli. We modified a literature mineralogical brittleness index equation in order to take into account additional iron-rich minerals like pyrite, hematite, and iron oxides that are present in the Woodford shales, and also investigated the impact of total organic content (TOC) on the mineralogical brittleness index. Furthermore, a number of new reliable equations based on different elastic properties have been introduced in this study for the evaluation of the elastic BI, which is of significant importance in designing hydraulic fracturing in shale and tight reservoirs. In another panel of our study, the sensitivities of both mineralogical makeup of Woodford shale and elastic properties calculated –including density, acoustic impedance, Poisson's ratio, Young's modules, Lame's parameters– to the elastic brittleness index are examined. It was indicated that they have a positive effect on the prediction and can be employed in the respective predictive methods for the estimation of elastic BI. However, the analysis demonstrated that TOC has no considerable effect on mineral BI, at least in Woodford sample studied. A comparison between these newly developed predictive equations and the most commonly used methods for the prediction of elastic brittleness index available in the literature demonstrates that the new equations have a better performance in terms of both accuracy and simplicity. On comparing the BI prediction equation, with highest prediction quality “Method 1”, that was developed based on ultrasonic elastic moduli with a corresponding equation based on sonic well-logs elastic moduli, Method1 was proven applicable to estimating BI based sonic frequency elastic moduli.

Monitoring concentration and isotopic composition of methane in groundwater in the Utica Shale hydraulic fracturing region of Ohio.

Degradation of groundwater quality is a primary public concern in rural hydraulic fracturing areas. Previous studies have shown that natural gas methane (CH4) is present in groundwater near shale gas wells in the Marcellus Shale of Pennsylvania, but did not have pre-drilling baseline measurements. Here, we present the results of a free public water testing program in the Utica Shale of Ohio, where we measured CH4 concentration, CH4 stable isotopic composition, and pH and conductivity along temporal and spatial gradients of hydraulic fracturing activity. Dissolved CH4 ranged from 0.2μg/L to 25mg/L, and stable isotopic measurements indicated a predominantly biogenic carbonate reduction CH4 source. Radiocarbon dating of CH4 in combination with stable isotopic analysis of CH4 in three samples indicated that fossil C substrates are the source of CH4 in groundwater, with one 14C date indicative of modern biogenic carbonate reduction. We found no relationship between CH4 concentration or source in groundwater and proximity to active gas well sites. No significant changes in CH4 concentration, CH4 isotopic composition, pH, or conductivity in water wells were observed during the study period. These data indicate that high levels of biogenic CH4 can be present in groundwater wells independent of hydraulic fracturing activity and affirm the need for isotopic or other fingerprinting techniques for CH4 source identification. Continued monitoring of private drinking water wells is critical to ensure that groundwater quality is not altered as hydraulic fracturing activity continues in the region. Graphical abstract A shale gas well in rural Appalachian Ohio. Photo credit: Claire Botner.

Study on the Influence of Bedding Density on Hydraulic Fracturing in Shale

Natural fractures and weak bedding planes are important for forming complex fracture networks in shale reservoirs. There are many sets of shale layers with different bedding densities in Sichuan, China. In this paper, three typical drilling cores with different bedding densities were collected. The shale rock mechanical properties and fracture morphology are investigated. Bedding density affects the strength and deformation characteristics. A fracture characterization model is proposed, and the hydraulic fracturing effects are evaluated. It is shown that with decreasing bedding density, the peak strength and elastic modulus increase, and Poisson’s ratio decreases gradually. The effect of natural fractures on hydraulic fracturing is investigated by analyzing the expansion of hydraulic fracture. The research shows that natural fractures have a significantly induced effect on the initiation and propagation of hydraulic fracture. According to the results of physical simulation of hydraulic fracturing, three types fracture extension models are established: a strong extended complex fracture model, a weak bedding extended complex fracture model, and a natural fracture crack propagation model. Natural fractures with different bedding structural properties may result in different propagation types of hydraulic fracture.

Study on the nonlinear coupling seepage model of shale reservoir with discrete networks

Gas shale is rich in natural fractures, in which the percolation mechanisms are complicated. Consequently, it’s the key scientific problem of accurately evaluating shale gas production to figure out the law of flow in fractured shale reservoirs. In this paper, a multi-continuum/discrete fracture model was built to describe the shale reservoir space after hydraulic fracturing, and a nonlinear coupled seepage flow mathematical model is established based on the adsorbed gas and free gas coupling, including the nonlinear non-equilibrium desorption and surface diffusion of adsorbed gas, viscous flow and Knudsen diffusion mechanism of free gas. The model believes that the surface diffusion of adsorbed gas occurs on the surface of the organic matter, and non-equilibrium desorption occurs between the adsorbed gas and the pores of the inorganic matrix. There are viscous flow and Knudsen diffusion in the pores of the inorganic matrix. But there is only viscous flow in natural and hydraulic fractures. Through the numerical simulation results of a fractured horizontal well, the time-space evolution process of the net desorption rate of the adsorbed gas in the hydraulic fractured shale reservoir was revealed. It was found that the region of the maximum net desorption rate gradually spread from the artificial network to the matrix. Through the analysis of multiple pressure systems, it was found that the diffusion of free gas in natural fractures, free gas in inorganic matrix pores and adsorbed gas in organic matter lagged successively. Through the flow analysis of natural fracture field, it is found that the flow patterns, outside the envelope of discrete fracture network, appear in succession with time: elliptical radial flow, linear flow and non-linear flow caused by primary fracture interference. This study reveals the percolation mechanism of fractured shale reservoirs and provides a scientific basis for the exploitation of the shale gas reservoir.

Assessing Residential Exposure Risk from Spills of Flowback Water from Marcellus Shale Hydraulic Fracturing Activity.

Identifying sources of concern and risk from shale gas development, particularly from the hydraulic fracturing process, is an important step in better understanding sources of uncertainty within the industry. In this study, a risk assessment of residential exposure pathways to contaminated drinking water is carried out. In this model, it is assumed that a drinking water source is contaminated by a spill of flowback water; probability distributions of spill size and constituent concentrations are fit to historical datasets and Monte Carlo simulation was used to calculate a distribution of risk values for two scenarios: (1) use of a contaminated reservoir for residential drinking water supply and (2) swimming in a contaminated pond. The swimming scenario did not produce risks of concern from a single exposure of 1 h duration, but 11 such 1-h exposures did produce risks of 10−6 due to radionuclide exposure. The drinking water scenario over a 30-year exposure duration produced cancer risk values exceeding 10−6 for arsenic, benzene, benzo(a)pyrene, heptachlor, heptachlor epoxide, pentachlorophenol, and vinyl chloride. However, this extended exposure duration is probably not realistic for exposure by a spill event. Radionuclides produced risks in the residential drinking water scenario of 10−6 in just 8 h, a much more realistic timeline for continual exposure due to a spill event. In general, for contaminants for which inhalation exposure was applicable, this pathway produced the highest risks with exposure from ingestion posing the next greatest risk to human health followed by dermal absorption (or body emersion for radionuclides). Considering non-carcinogenic effects, only barium and thallium exceed target limits, where the ingestion pathway seems to be of greater concern than dermal exposure. Exposure to radionuclides in flowback water, particularly through the inhalation route, poses a greater threat to human health than other contaminants examined in this assessment and should be the focus of risk assessment and risk mitigation efforts.

An Experimental Investigation of Hydraulic Fracturing in Shale Considering Anisotropy and Using Freshwater and Supercritical CO2

The process of hydraulic fracturing makes use of a liquid to fracture reservoir rocks for the exploitation of unconventional resources. Hence, it is vital to understand the processes that produce the fracture networks that occur during hydraulic fracturing. A shale reservoir is one of the largest unconventional resources and it displays obvious anisotropic characteristics due to its inherent sedimentary structures. The viscosity and flow ability of the fracturing fluid plays an important role in this process. We conducted a series of hydraulic fracturing tests on shale cores (from the southern Sichuan Basin) using freshwater and supercritical CO2 (SCO2) as fracturing fluids to investigate the different modes of fracture propagation. The pump pressure curves that we obtained during the fracturing experiment show how the shale responded to each of the fracturing fluids. We examined the influence of the anisotropic characteristics on the propagation of hydraulic fractures by conducting a series of hydraulic fracturing experiments on the shale cores using different bedding orientations. The bedding orientation of the shale had a profound influence on the fracture propagation when using either freshwater or a SCO2 fluid. The breakdown pressure of the shale core was affected not only by the bedding orientation but also by the fracturing fluid. A macroscopic observation of the fractures revealed different fracture geometries and propagation patterns. The results demonstrated that the anisotropic structures and the fracturing fluids could influence the path of the hydraulic fracture.

Continuous TDEM for monitoring shale hydraulic fracturing

Monitoring and delineating the spatial distribution of shale fracturing is fundamentally important to shale gas production. Standard monitoring methods, such as time-lapse seismic, cross-well seismic and micro-seismic methods, are expensive, timeconsuming, and do not show the changes in the formation with time. The resistivities of hydraulic fracturing fluid and reservoir rocks were measured. The results suggest that the injection fluid and consequently the injected reservoir are characterized by very low resistivity and high chargeability. This allows using of the controlled-source electromagnetic method (CSEM) to monitor shale gas hydraulic fracturing. Based on the geoelectrical model which was proposed according to the well-log and seismic data in the test area the change rule of the reacted electrical field was studied to account for the change of shale resistivity, and then the normalized residual resistivity method for time lapse processing was given. The time-domain electromagnetic method (TDEM) was used to continuously monitor the shale gas fracturing at the Fulin shale gas field in southern China. A high-power transmitter and multi-channel transient electromagnetic receiver array were adopted. 9 h time series of Ex component of 224 sites which were laid out on the surface and over three fracturing stages of a horizontal well at 2800 m depth was recorded. After data processing and calculation of the normalized resistivity residuals, the changes in the Ex signal were determined and a dynamic 3D image of the change in resistivity was constructed. This allows modeling the spatial distribution of the fracturing fluid. The model results suggest that TDEM is promising for monitoring hydraulic fracturing of shale.

The effect of heterogeneity on hydraulic fracturing in shale

Modelling hydraulic fracturing processes in shale oil and gas development remains a major challenge for industrial applications. The omission of heterogeneity inherent in the shale matrix may be one of the causes for the divergence between simulated fractures and those in reality. To elucidate the effects of heterogeneity on hydraulic fracturing, a workflow is developed for generating heterogeneous fields of hydraulic and geomechanical properties based on relatively easily available compressional wave velocity and shear wave velocity. After that, the workflow is implemented in our thermo-hydro-mechanical simulator for hydraulic fracturing. With the integrated simulator, the following are investigated: the effect of heterogeneity and the interactive effects of heterogeneity with other factors, i.e., in-situ stress gradient and stress shadow, the significance of different levels of heterogeneity, and the impact of various injection rates. The simulation results showed that these factors could have either synergetic or competitive effects under different circumstances. Also, different levels of heterogeneity result in different patterns of fractures’ geometry. According to our study, a heterogeneity field with a large coefficient of variation (CV) and medium correlation length (CL) turns out to develop fractures of the most complex patterns. Moreover, the high injection rate leads to neutralization of the influence of heterogeneity through the averaging effect of intense fracturing. The study leads to better understandings in the mechanism of hydraulic fracturing process under complicated factors such as heterogeneity, interactive effects, fracturing strategies, and both modes of rock failure. This study may shed light on future research on hydraulic fracturing and possible applications in practice as well as some improvements in designing hydraulic fracturing simulator.

A new numerical investigation of cement sheath integrity during multistage hydraulic fracturing shale gas wells

Volume fracturing of shale gas wells has been observed to lead to the failure of cement sheath and to sustained casing pressure (SCP). This paper presents a new numerical investigation aimed at understanding the failure mechanism of the cement sheath during volume fracturing. A wellbore temperature model was established to obtain the required input parameters for the dynamic temperature boundary. The numerical model considers the coupling of casing, cement sheath, and formation rock to calculate the radial and tangential stresses in the cement sheath. In addition to the cement mechanical properties, the influencing factors considered include the casing pressure, fracturing fluid displacement, and initial temperature. The cement sheath integrity was evaluated using the Mohr-Coulomb failure criterion. The results show that the temperature of cement sheath changes drastically during fracturing. The radial and tangential stresses in the cement sheath change continuously with time. Lowering the internal wellbore pressure can effectively reduce the radial and tangential stresses in the cement sheath. Reducing the fracturing fluid displacement can significantly lower the radial and tangential stresses in the cement sheath. Increasing the initial fracturing fluid temperature will cause the radial stress of cement sheath to increase and the tangential stress to decrease. The use of cements with low Young's modulus values can significantly reduce the radial and tangential stresses in the cement sheath. The use of cements with low Poisson's ratio values can lower the tangential stress. The results obtained using the model were verified by field data that demonstrated no occurrence of casing pressure when using a cement with a low elastic modulus, as suggested by the model.

Identification and Comparative Mammalian Cell Cytotoxicity of New Iodo-Phenolic Disinfection Byproducts in Chloraminated Oil and Gas Wastewaters

Hydraulic fracturing wastewaters discharged to surface water have led to elevated bromide and iodide levels, as well as enhanced formation of brominated trihalomethanes, haloacetic acids, haloacetonitriles, and iodo-trihalomethanes at downstream drinking water treatment plants, in chlorinated effluent from wastewater treatment plants, and in controlled laboratory studies. This enhanced formation of brominated and iodinated disinfection byproducts (DBPs) raises concerns regarding human health, because they are much more toxic than chlorinated DBPs. This study represents the first nontarget, comprehensive analysis of iodinated DBPs formed in chloraminated produced waters associated with hydraulic fracturing of shale and conventional gas formations. Fifty-six iodo-phenolics were identified, comprising three homologous series of mono-, di-, and tri-iodinated phenols, along with two new classes of DBPs: iodomethylphenols and iododimethylphenols. Four iodo-phenolics (2-iodophenol, 4-iodophenol, 2,4,6-triiodophe...

Seismicity Induced by Hydraulic Fracturing in Shales: A Bedding Plane Slip Model

AbstractPassive seismic monitoring of microseismic events induced in oil or gas reservoirs is known as microseismic monitoring. Microseismic monitoring is used to understand the process of hydraulic fracturing, which is a reservoir stimulation technique. We use a new geomechanical model with bedding plane slippage induced by hydraulic fractures within shale reservoirs to explain seismicity observed in a typical case study of hydraulic fracturing of a shale gas play in North America. Microseismic events propagating from the injection point are located at similar depths (within the uncertainty of their locations), and their source mechanisms are dominated by shear failure with both dip‐slip and strike‐slip senses of motion. The prevailing dip‐slip mechanisms have one nearly vertical nodal plane perpendicular to the minimum horizontal stress axis, while the other nodal plane is nearly horizontal. Such dip‐slip mechanisms can be explained by slippage along bedding planes activated by the aseismic opening of vertical hydraulic fractures. The model explains the observed prevailing orientation of the shear planes of the microseismic events, as well as the large difference between seismic and hydraulic energy.

Experiments and analysis on hydraulic sand fracturing by an improved true tri-axial cell

In this paper, a hydraulic sand fracturing physical modelling experimental equipment is designed, in which different pumping stages are considered, and pad fluid, sand-carrying fluid, and displacing fluid are continuously injected into the core following certain ratio. Then, hydraulic sand fracturing experiments are conducted separately in the natural coal and shale specimens using this equipment. The characteristics of proppant migration and distribution and their influences on the initiation and propagation of hydraulic fractures are investigated. The testing results show that fracture morphologies after hydraulic fracturing in coal and shale have significant differences. Hydraulic fractures in the coal are affected by cleats, while the progation of hydraulic fractures in shale is mainly influenced by natural fractures. Almost all the proppants are distributed around the wellbore. The migration distance of proppants is limited. When there are natural fractures around the wellbore, proppants can plug the natural fractures. Secondary fractures will occur in the shale, and multiple fractures are formed. The practice shows that based on the restoration of field fracturing process, this equipment can be used to analyze the initiation and propagation of hydraulic fractures in sand fracturing.