Synthetic Shale Research Articles

Robust superhydrophobic Poly(vinylidene fluoride) membranes with spherulitic surface morphology against wetting and scaling in membrane distillation

Membrane distillation (MD) offers a theoretically complete interception of salts, presenting a promising desalination pathway for freshwater supply. However, conventional hydrophobic membranes fail to desalinate water containing surfactants or supersaturated gypsum over prolonged periods. To address wetting and scaling challenges, we developed a superhydrophobization methodology for poly (vinylidene fluoride) membranes by in situ controlling spherulitic surface morphology through thermally induced phase separation. The major advantages of this one-step strategy lie in its fluorination-free, template-free, and nanoparticle-free. The as-prepared membranes exhibit a rough morphology and superhydrophobic characters with high air-trapping pockets. They demonstrate stable vapor fluxes and high salt rejections (>99 %) when used in desalination processes for saline water containing 0.6 mM sodium dodecyl sulfate, 0.03 mM cetyltrimethylammonium bromide, or gypsum solution with a saturation index of 1.1. Also, the outstanding anti-wetting and anti-scaling properties were demonstrated using synthetic shale gas wastewater. The formation of an air layer on the integrated spherulitic surface highlights the local kinetic barrier and slippery surface, effectively preventing the attachment of amphiphilic surfactants and mineral ions. Importantly, due to the membrane's structural integrity, the spherulitic surface displays high robustness and minimal impact from abrasion cycles and ultrasonication. This work exemplifies a facile strategy to engineer micro-/nano-spherulitic surface morphology for robust superhydrophobic membranes used in MD desalination.

Synthetic mixture of sand and shale: how conductor (shale) and saturation influence electrical characteristics

In the petroleum sector, spectral-induced polarization (SIP) can detect low-frequency electrical characteristics in rocks without causing any damage. Measurements from 1 mHz to 100 kHz were conducted at ambient temperature for synthetic shale sand combinations from Gabel El-Galala, Cairo, Egypt. Because of an increase in the relative conductor (shale) concentration despite a reduction in the relative insulator (sand) concentration, and the effect of increasing saturation, this is the first study to explain the electrical impact of shale sand combination. We will also look into its saturation and frequency to further model and analyze shale with electrical characteristics. Both conductivity and impedance decrease regarding sand's presence. The dielectric constant increases with increasing shale level and saturation below the percolation threshold but drops beyond it. Material rich in shale has lower impedance than material rich in sand. Concentrating and saturating the shale in the synthetic sand enhanced its electrical conductivity. Ionized liquid and the conductive minerals present in shale also contribute to this improvement. The mixture's electrical characteristics improved significantly as the frequency rose. Low-frequency analysis shows that the samples have high conductive and dielectric constants. These investigations could improve oil and gas recovery by illuminating the electrical characteristics of the reservoir rock.

Posterior regularization method for phase removal of shale nano-structure imaging in space domain

X-Ray computed tomography is a non-destructive method that is used, among many applications, to study the size, shape, 3D structures and interconnections of pores in shale. We use phase retrieval methods to deal with the “edge enhancement” effect caused by phase shift. The process of phase retrieval can be described by the transport-of-intensity equation (TIE). But this is an ill-posed problem. The existing methods focus on phase retrieval in the frequency domain. To tackle the ill-posedness, we propose a new method whose main idea is to solve this problem in space domain with a regularization technique. We study a synthetic shale model and simulate the projection data. Then we apply three methods to retrieve the phase: conventional method in frequency domain, direct solving method and iterative Tikhonov regularization method in space domain. Finally, we use the standard filtered back-projection (FBP) method to present the outcome. By analyzing the results, we find advantages of the new method: more stability and fewer artifacts under noise perturbations. The study shows that relative errors of the new method are nearly 1% of that of the traditional method based on frequency domain, and hence the new method is promising for the practical data processing.

Machine-learning-based well production prediction under geological and hydraulic fracture parameters uncertainty for unconventional shale gas reservoirs

Shale gas production prediction under history-matching-based geomodel is crucial to achieve reliable assessment and economic management of unconventional shale resources, however, conventional history matching is generally performed through repeatedly running high-fidelity reservoir simulations and therefore presents intensive computation-cost in practical applications. Without the need of history matching step, this work presents an efficient and robust post-history production forecasting framework using a latent-space learning-based direct forecast approach, e.g., referred to as LS-LDFA. A novel dimensionality reduction method, e.g., convolutional autoencoder, is employed to regularize the multi-well time-series data by low-order representation within a latent space. Once the machine-learning proxy is trained offline, the online post-history production forecast can be efficiently achieved by input history data. This paper presents some comparative studies between LS-LDFA and model-based history matching. This approach is tested on two examples with an increasing complexity, e.g., a multi-fractured horizontal well and a naturally fractured reservoir model with multi-well-pad-production based on synthetic shale formation. The results confirm that the method achieves high robustness and computational efficiency simultaneously in comparison with the conventional history matching. The application of learning-based direct forecast approach can effectively fuse information from history data and thus support reliable decision-making and risk assessment.

A modified exponential decline method for shale gas reservoirs

Compared with the conventional reservoir, the production rate in shale gas reservoir declines rather quickly during the early production period and conventional decline rate analysis becomes not suitable for shale gas production. In this article, we build a four parameter empirical decline model considering the varying decline exponent and terminal decline rate and propose a simple but efficient calculation method. By analyzing the sharp-to-shallow decline trend, a sophisticated decline rate form was assumed, which takes the initial decline rate, terminal decline rate and the curvature of the decline rate curve into consideration. For shale gas reservoirs with short production time and little production data, the early production data may not reflect the real terminal decline rate. A modification for such cases is also proposed. Field examples from a synthetic shale gas reservoir, Barnett shale gas, Haynesville shale gas and Fuling shale gas were used to test our newly proposed model. [Received: October 27, 2017; Accepted: January 30, 2018]

A modified exponential decline method for shale gas reservoirs

Compared with the conventional reservoir, the production rate in shale gas reservoir declines rather quickly during the early production period and conventional decline rate analysis becomes not suitable for shale gas production. In this article, we build a four parameter empirical decline model considering the varying decline exponent and terminal decline rate and propose a simple but efficient calculation method. By analyzing the sharp-to-shallow decline trend, a sophisticated decline rate form was assumed, which takes the initial decline rate, terminal decline rate and the curvature of the decline rate curve into consideration. For shale gas reservoirs with short production time and little production data, the early production data may not reflect the real terminal decline rate. A modification for such cases is also proposed. Field examples from a synthetic shale gas reservoir, Barnett shale gas, Haynesville shale gas and Fuling shale gas were used to test our newly proposed model. [Received: October 27, 2017; Accepted: January 30, 2018]

Experimental investigation of mechanical compaction on the physical and elastic properties of synthetic shales

Three sets of synthetic shale samples with different clay minerals were prepared to study the effects of mechanical compaction stress on the physical and elastic properties of shales. The results suggest that the physical and elastic properties (i.e., porosity, density, acoustic velocity, etc.) of the mudstones vary greatly the compaction stress, and the type of clay minerals. For a given compaction stress, the synthetic shale with kaolinite exhibits the smallest density and a highest porosity, while the sample with smectite shows the highest density and the smallest porosity. Further, the velocity demonstrates a considerable dependence to compaction stress, both P- and S-wave velocities in different directions increase with the compaction stress. We also find that the velocity anisotropy parameters increase with the compaction stress, and the S-wave anisotropy is more sensitive to the compaction stress compared to the P-wave anisotropy. With respect to the mechanical properties, the dynamic Young's modulus increase with the compaction stress, while the Poisson's ratio decrease with the compaction stress. This can be attributed to the decrease in the porosity. The results also indicate that the mechanical properties display obvious anisotropic behaviours. Finally, strong correlations are observed between the porosity and the velocity anisotropy and mechanical anisotropy for all the prepared samples, providing a means of estimating seismic anisotropy from porosity in shales.

Experimental investigation of the effects of clay content and compaction stress on the elastic properties and anisotropy of dry and saturated synthetic shale

Anisotropy in shales is an important issue in exploration and reservoir geophysics, and it has been proven extremely difficult to correlate anisotropy in natural shale by means of a single variable (in this case, clay content or compaction stress) because of the influence of multiple factors, such as water content, total organic carbon content, and complex mineral compositions. Thus, we used quartz, kaolinite, calcite, and kerogen extract as the primary materials to construct two sets of synthetic shale samples, each with a different clay content by weight and a different compaction stress. Ultrasonic experiments were conducted to investigate the anisotropy of velocity and mechanical properties in dry and saturated samples of our synthetic shales. The results reveal that the velocities decrease with clay content by weight and increase with compaction stress and that these changes are significant at low compaction stress. The velocity anisotropy of the samples increases with clay content and compaction stress due to the increasing alignment of the clay platelets. S-wave anisotropy is more sensitive to the clay content or compaction stress than P-wave anisotropy. The dynamic Young’s modulus [Formula: see text] of the samples decreases with clay content and increases with compaction stress, whereas Poisson’s ratio [Formula: see text] increases with clay content and decreases with compaction stress. Young’s modulus perpendicular to the symmetry axis is always larger than that parallel to the symmetry axis, but Poisson’s ratio perpendicular to the symmetry axis may be larger or smaller than that parallel to the symmetry axis, which indicates that mechanical properties have obvious anisotropic behavior. The elastic properties and anisotropy are also affected by fluids; the values of elastic and mechanical anisotropy parameters in saturated samples are significantly lower than those in dry samples.

Adsorption of Lithium from Shale Gas Produced Water Using Titanium Based Adsorbent

Lithium is a valuable metal that has been recovered from synthetic shale gas produced water using the titanium-based adsorbent H2TiO3 in this study. The maximum adsorptive capacity of lithium obtained was 2.58 mmol/g after adsorption–desorption tests, and the recovery rate of Li+ was much higher than those of the other cations in the produced water when used with a pH buffer. To enhance the adsorptive capacity and selectivity of lithium, the precipitation process using sodium carbonate was applied prior to adsorption. More than 96% of the divalent cations such as barium, calcium, and strontium were precipitated. From the precipitation–adsorption–desorption test, the lithium desorption capacity obtained for the produced water was 3.61 mmol/g, with a higher selectivity compared to the other cations. Thus, adsorption using a titanium-based adsorbent might be a promising method for lithium recovery from shale gas produced water, when used in conjunction with precipitation.

Application of fast analytical approach and AI optimization techniques to hydraulic fracture stage placement in shale gas reservoirs

In the last decades, natural gas from unconventional reservoirs has become a major portion of total gas supply due to advances in horizontal well drilling and multi-stage hydraulic fracturing as well as reduction of operational costs and capital expenditures. However, hydraulic fracturing technique is a still costly and resource intensive production strategy that requires optimal planning to conform to the best safety practices and to obtain the highest returns on investments. Thus, the proposed fast and reliable hydraulic fracture (HF) placement optimization technique based on physics and analytical equations is the key tool to balance between gas production costs and anticipated revenues.In this study, we develop an analytical model in which the modified Wattenbarger slab model with the pseudo-pressure approach are integrated into the Net Present Value (NPV) as the objective function. We consider four decision variables including number of HF stages, HF spacing, HF half-length, and wellbore spacing and use three stochastic gradient-free optimization methods (i.e., genetic algorithm (GA), differential evolution (DE), and particle swarm optimization (PSO)) to optimize the objective function on a synthetic shale gas reservoir model with the Barnett Shale properties. To verify the accuracy of the obtained optimal solutions, we conduct four trials for each stochastic optimization method with 100 generations and the population size of 20. The results show that the best overall value of the NPV found by PSO are 1.7% and 7.6% higher than those obtained by DE and GA, respectively. Moreover, PSO has the fastest convergence rate (in 50 generations), saves at least 10% of the computational time in comparison to those required by other methods, and results in the same optimal solution in all trials. Finally, considering bilinear flow at the early stages of the production period, nonlinear flow at the late production time, and gravitational effects in the analytical model are still open areas for future research in this field.

Ultrasonic velocity and mechanical anisotropy of synthetic shale with different types of clay minerals

Clay minerals are the most abundant materials in shale. Their presence significantly influences the elastic behavior of reservoir rocks as a function of mineral type, volume, and distribution, and their orientation controls the shale’s intrinsic anisotropic behaviors. Thus, knowing the elastic properties of shale with different types of clay minerals is imperative for fully understanding the seismic properties of the reservoir. However, it is extremely difficult to measure the elastic properties of natural shale by means of a single variable (in this case, the type of clay), due to the influences of multiple factors, including water, total organic carbon content, complex mineral composition, and so on. Thus, we use quartz, clay (kaolinite, illite, and smectite), carbonate, and kerogen extract as the primary materials to construct synthetic shale with different types of clay. Ultrasonic experiments were conducted to study the anisotropy of velocity and mechanical properties (Young’s modulus and Poisson’s ratio) in dry synthetic shale samples as a function of applied axial stress. The results show that the velocity of samples increases with applied pressure and the rate of velocity increase is higher at low pressures. Similarly, the dynamic Young’s modulus and Poisson’s ratio increase with applied pressure; [Formula: see text] is always larger than [Formula: see text], but [Formula: see text] may be larger or smaller than [Formula: see text]. Furthermore, velocity anisotropy and mechanical anisotropy decrease with the increase of stress and are sensitive to stress and lithology. The closure of large aspect-ratio pores (and/or microcracks) seems to be a dominant mechanism controlling the change of anisotropy. Finally, the changes in mechanical anisotropy under applied stress are larger compared with the changes in velocity anisotropy, indicating that mechanical properties are more sensitive to the changes in rock property.

Compaction trends of full stiffness tensor and fluid permeability in artificial shales

Summary We present a methodology and describe a set-up that allows simultaneous acquisition of all five elastic coefficients of a transversely isotropic (TI) medium and its permeability in the direction parallel to the symmetry axis during mechanical compaction experiments. We apply the approach to synthetic shale samples and investigate the role of composition and applied stress on their elastic and transport properties. Compaction trends for the five elastic coefficients that fully characterize TI anisotropy of artificial shales are obtained for a porosity range from 40 per cent to 15 per cent. A linear increase of elastic coefficients with decreasing porosity is observed. The permeability acquired with the pressure-oscillation technique exhibits exponential decrease with decreasing porosity. Strong correlations are observed between an axial fluid permeability and seismic attributes, namely, VP/VS ratio and acoustic impedance, measured in the same direction. These correlations might be used to derive permeability of shales from seismic data given that their mineralogical composition is known.

Benchmarking EUR estimates for hydraulically fractured wells with and without fracture hits using various DCA methods

Various decline curve analysis (DCA) methods can be applied to forecast the production performance of hydrocarbon wells, including horizontal wells stimulated with hydraulic fracturing. Yet, which method is more preferable remains in doubt. The objective of this study is to evaluate various DCA methods by history matching and hindcasting both synthetic and field production data in order to assess for each method the reliability of production forecasts and the estimated ultimate recovery (EUR). Five DCA methods have been evaluated because of their computational simplicity and broad application. These methods are the Modified Hyperbolic Decline model (MHD), Duong model, Logistic Growth Analysis Model (LGM), and the Power Law Exponential (PLE, with D∞=0 and D∞=∞) Decline models. Each method was evaluated using three data sets: synthetic shale oil production rates based on a CMG reservoir model using Eagle Ford reservoir characteristics, data from producing Eagle Ford wells and from Austin Chalk wells. The hindcast method was applied to the Eagle Ford synthetic wells to establish the EUR deviation between the synthetic reservoir model production forecast and the various DCA methods. The weighted residuals of history matched production rates for Eagle Ford indicate that the MHD and Duong models give the lowest matching errors (as low as 1.32%) and the highest EUR estimations (as high as 3,447,609 stb). The PLE model (with D∞≠0) gives the most conservative EUR estimation, and also the least reliable history matching method (on our Eagle Ford data sets), which therefore is the least preferable DCA method. For the Austin Chalk wells in our study, all methods generate fairly similar EUR predictions with similar matching errors (from 13% to 15%) so that the DCA method preference becomes indifferent. This study also touches upon the relationship between DCA results when well interference occurs due to various hydraulic fracture hits. Shale wells that primarily contain hydraulic fractures and few natural fractures (Eagle Ford) give the most reliable forecasts using the MHD and Duong DCA methods. However, wells in the Austin Chalk, a naturally fractured reservoir, show little difference between the forecast accuracy from the various DCA methods.

Treatment and reuse of shale gas wastewater: Electrocoagulation system for enhanced removal of organic contamination and scale causing divalent cations

The present study explores the feasibility of using the electrocoagulation (EC) process for the treatment and reuse of wastewater produced during shale gas recovery by hydraulic fracturing. The electrocoagulation process has been evaluated for the removal of suspended solids, total organic carbon (TOC) and scale (hardness) causing divalent cations, which, if untreated, can clog the gas well. Experiments were performed with actual shale gas wastewater (ASWW), synthetic shale gas wastewater prepared with low concentration of dissolved salts (SSWW – LDS) and synthetic shale gas wastewater prepared with a high concentration of dissolved salts (SSWW – HDS).EC is found to be effective for removing TOC and hardness from both the actual and synthetic shale gas wastewaters. The electric energy required per unit mass (EEM) for removal of TOC for ASWW, SSWW – LDS and SSWW – HDS are 243, 102 and 70kWh/kg respectively. The EEM for removal of hardness for ASWW, SSWW – LDS and SSWW – HDS are 303, 104 and 25kWh/kg respectively. The high conductivity of SSWW – HDS helps in achieving higher currents and hence the lower reported EEM values for SSWW – HDS. Also, under alkaline conditions, the performance of EC increases significantly. Combination of aeration with EC is also found to increase the performance of EC, especially for wastewater containing high concentrations of chloride ions.

Lithium recovery from shale gas produced water using solvent extraction

Shale gas produced water is hypersaline wastewater generated after hydraulic fracturing. Since the produced water is a mixture of shale formation water and fracturing fluid, it contains various organic and inorganic components, including lithium, a useful resource for such industries as automobile and electronics. The produced water in the Marcellus shale area contains about 95 mg/L lithium on average. This study suggests a two-stage solvent extraction technique for lithium recovery from shale gas produced water, and determines the extraction mechanism of ions in each stage. All experiments were conducted using synthetic shale gas produced water. In the first-stage, which was designed for the removal of divalent cations, more than 94.4% of Ca2+, Mg2+, Sr2+, and Ba2+ ions were removed by using 1.0 M di-(2-ethylhexyl) phosphoric acid (D2EHPA) as an extractant. In the second-stage, for lithium recovery, we could obtain a lithium extraction efficiency of 41.2% by using 1.5 M D2EHPA and 0.3 M tributyl phosphate (TBP). Lithium loss in the first-stage was 25.1%, and therefore, the total amount of lithium recovered at the end of the two-step extraction procedure was 30.8%. Through this study, lithium, one of the useful mineral resources, could be selectively recovered from the shale gas produced water and it would also reduce the wastewater treatment cost during the development of shale gas.

Application of three different water treatment technologies to shale gas produced water

Shale gas produced water is a hypersaline wastewater that is generated during the shale gas development process called as a hydraulic fracturing. The produced water contains many substances including inorganic salts, organic compounds, and particulates. The treatment process of the produced water is mainly composed of four parts: oil and water separation, removal of suspended solids, removal of organics, and salts removal. This study focuses on the total dissolved salts removal through applying three different desalination techniques – membrane distillation (MD), reverse osmosis (RO), and evaporative crystallization (EC). The experiments were conducted using synthetic shale gas produced water to understand the changes of chemical properties in permeate and concentrate. In the permeate solution, MD and EC showed more than 99% of salts removal efficiency for all ions, but RO showed relatively low efficiency. In the concentrate solution, the concentrations of all ions varied according to the type of ions and applied treatment methods.

Removal of Radium from Synthetic Shale Gas Brines by Ion Exchange Resin

Abstract Rapid development of hydraulic fracturing for natural gas production from shale reservoirs presents a significant challenge related to the management of the high-salinity wastewaters that return to the surface. In addition to high total dissolved solids (TDS), shale gas-produced brines typically contain elevated concentrations of radium (Ra), which must be treated properly to prevent contamination of surface waters and allow for safe disposal or reuse of produced water. Treatment strategies that isolate radium in the lowest volume waste streams would be desirable to reduce disposal cost and generate useful treatment by-products. The present study evaluates the potential of a commercial strong acid cation exchange resin for removing Ra2+ from high-TDS brines using fixed-bed column reactors. Column reactors were operated with varying brine chemistries and salinities in an effort to find optimal conditions for Ra2+ removal through ion exchange. To overcome competing divalent cations present in the b...

Creation of synthetic samples for physical modelling of natural shale

ABSTRACTNatural shale samples, particularly well‐preserved, drilled core samples, are extremely difficult to obtain for laboratory research. Multiple tests must be carried out on one sample, and some samples are disposed after destructive tests. Therefore, rarity and non‐reusability of samples strongly restrict shale studies. In this study, based on statistical data from the world's major shale block, a new type of synthetic shale was physically constructed via a process of interfusion, stuffing, and compaction using quartz, clay, carbonate, and kerogen as the primary materials, according to statistical data from the world's major shale blocks. Further evaluation of the synthetic shale involved the use of scanning electron microscopy imagery and analysis of its anisotropic characteristics in comparison with natural shale. The synthetic shale had a laminated microstructure similar to natural shale, and its velocity anisotropy corresponded to Thomsen's anisotropy of a transversely isotropic medium. The results of tests for homogeneity and repeatability indicated that the construction process was stable and that several identical synthetic samples, which were satisfactorily similar to natural shale, could be produced for both iterative and destructive tests. The composition of each mineral, as well as the density, porosity, permeability, and anisotropy of the samples, were all variable. Therefore, a series of synthetic samples could be obtained with properties set to meet the requirements of petrophysical experimentation. Moreover, gas or oil saturation was also considered in the construction of the synthetic shale, meaning that the characteristics of gas or oil saturation (or the complete range of data from dry to saturated samples) could be tested using the synthetic shale.

Effect of pretreatment on fouling propensity of shale gas wastewater in membrane distillation process

AbstractMembrane distillation (MD) has potential to concentrate produced water from the hydraulic fracturing process, leading to reduced disposal cost for shale gas wastewater. However, it is anticipated that MD suffers from severe membrane fouling if it is directly applied to treat such wastewater. Accordingly, this study explored the techniques for pretreatment of shale gas wastewater to mitigated fouling in MD process. Microbubble treatment and filtration were considered in lab-scale experiments. A synthetic shale gas wastewater was prepared based on the compositions of the real produced water and flowback water.Experimental results showed that the microbubble pretreatment could be successfully applied for MD treatment of the synthetic shale gas wastewater. Without pretreatment, flux decline in MD system was found to be very high (>90% decrease within 600 min). After the pretreatment using microbubbles, the flux maintained constant even after 700 min. This was attributed to the removal of colloidal par...

Effect of Confinement on Pressure/Volume/Temperature Properties of Hydrocarbons in Shale Reservoirs

Summary Shale reservoirs play an important role as a future energy resource of the United States. Numerous studies were performed to describe the storage and transport of hydrocarbons through ultrasmall pores in the shale reservoirs. Most of these studies were developed by modifying techniques used for conventional reservoirs. The common pore-size distribution of the shale reservoirs is approximately 1 to 20 nm and in such confined spaces that the interactions between the wall of the container (i.e., the shale and kerogen) and the contained fluids (i.e., the hydrocarbon fluids and water) may exert significant influence on the localized phase behavior. We believe this is because the orientation and distribution of fluid molecules in the confined space are different from those of the bulk fluid, causing changes in the localized thermodynamic properties. This study provides a detailed account of the changes of pressure/volume/temperature properties and phase behavior (specifically, the phase diagrams) in a synthetic shale reservoir for pure hydrocarbons (methane and ethane) and a simple methane/ethane (binary) mixture. Grand canonical Monte Carlo (GCMC) simulations are performed to study the effect of confinement on the fluid properties. A graphite slab made of two layers is used to represent kerogen in the shale reservoirs. The separation between the two layers, representing a kerogen pore, is varied from 1 to 10 nm to observe the changes of the hydrocarbon-fluid properties. In this paper, the critical properties of methane and ethane as well as the methane/ethane mixture phase diagrams in different pore sizes are derived from the GCMC simulations. In addition, the GCMC simulations are used to investigate the deviations of the fluid densities in the confined space from those of the bulk fluids at reservoir conditions. Although not investigated in this work, such deviations may indicate that significant errors for production forecasting and reserves estimation in shale reservoirs may occur if the (typical) bulk densities are used in reservoir-engineering calculations.