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Horizontal well drilling and multi-stage hydraulic fracturing technologies are at the root of commercial shale gas development and exploitation. During these processes, typically, a large amount of working fluid enters the formation, resulting in widespread water-rock interaction. Deeply understanding such effects is required to optimize the production system. In this study, the mechanisms of water-rock interaction and the associated responses of shale fabric are systematically reviewed for working fluids such as neutral fluids, acid fluids, alkali fluids and oxidative fluids. It is shown that shale is generally rich in water-sensitive components such as clay minerals, acid-sensitive components (like carbonate minerals), alkali-sensitive components (like quartz), oxidative-sensitive components (like organic matter and pyrite), which easily lead to change of rock fabric and mechanical properties owing to water-rock interaction. According to the results, oxidizing acid fluids and oxidizing fracturing fluids should be used to enhance shale gas recovery. This study also indicates that an aspect playing an important role in increasing cumulative gas production is the optimization of the maximum shut-in time based on the change point of the wellhead pressure drop rate. Another important influential factor to be considered is the control of the wellhead pressure considering the stress sensitivity and creep characteristics of the fracture network. [ABSTRACT FROM AUTHOR]

Utilizing energy storage in depleted oil and gas reservoirs can improve productivity while reducing power costs and is one of the best ways to achieve synergistic development of "Carbon Peak–Carbon Neutral" and "Underground Resource Utilization". Starting from the development of Compressed Air Energy Storage (CAES) technology, the site selection of CAES in depleted gas and oil reservoirs, the evolution mechanism of reservoir dynamic sealing, and the high-flow CAES and injection technology are summarized. It focuses on analyzing the characteristics, key equipment, reservoir construction, application scenarios and cost analysis of CAES projects, and sorting out the technical key points and existing difficulties. The development trend of CAES technology is proposed, and the future development path is scrutinized to provide reference for the research of CAES projects in depleted oil and gas reservoirs. [ABSTRACT FROM AUTHOR]

A numerical model that simulates methane transport and adsorption in crushed shale was developed. This model uses the finite element method to solve the diffusion coefficient (D) and adsorption rate of methane in a spherical particle region, and it was calibrated by fitting experimental data on the adsorption process of methane in unsaturated crushed shale. The results show that the methane free concentration in shale reflected the strength of methane's diffusion ability. The D increased by 2.2–4.9 times for each 900 mol/m3 increment in the free concentration. The D was 1.43 × 10−11–1.23 × 10−9 m2/s as the boundary concentration ranged from 461 to 2766 mol/m3. Furthermore, the diffusion behavior can be divided into three stages during the adsorption–diffusion process. The increasing trend of the D was slow in the early and late stages of the adsorption–diffusion process, while the D obviously increased in the middle stage. The middle period accounted for 16.2–71.7% of the entire process. Moreover, the variation in the D along the radial axis that started at the center of the model and pointed to the model surface can be divided into two regions. The change in the D tended to be horizontal in a region close to the center, whereas in the other region, the D was positively correlated with the distance to the center. The horizontal area decreased as adsorption–diffusion proceeded, and the rate of shrinkage of this area can be described with an exponential equation. [ABSTRACT FROM AUTHOR]

During the drainage and production of coalbed methane (CBM) wells, the constant changes in stress and water saturation of reservoir restrict the dynamic change of the reservoir permeability. By carrying out stress sensitivity experiments with different water saturations in coal, the correlation between permeability and the coupling of effective stress and water saturation was analyzed. The water saturation sensitivity and stress sensitivity of reservoir were evaluated by the stress sensitivity index (S), permeability damage rate (PDR), and stress sensitivity coefficient (αk), and the change law of coal permeability under different stresses with different water saturations was revealed. The results showed that the coal reservoir permeability decreased with increasing stress following a negative exponential function and decreased nearly linearly with increasing water saturation. In addition, the coal water saturation sensitivity was positively correlated with effective stress, and the coal stress sensitivity was positively correlated with water saturation. Finally, a mathematical model for predicting coal permeability that considered the impacts of water saturation and effective stress was established, revealing the controlled mechanism affecting the permeability change. [ABSTRACT FROM AUTHOR]

The existing temporary plugging evaluation methods cannot simulate the pumping process and plugging process. To quantitatively evaluate the plugging effect of the knot temporary plugging agent, based on the high-pressure delivery system of the temporary plugging agent and the plunger pump system, it carried on the ground simulation test. The results showed that the knot temporary plugging agent had an excellent plugging performance. The numerical simulation method calculated the flow field, and the velocity field and pressure field with the different number of perforations and different displacements were analyzed. When the number of perforations was few, the total flow rate, the flow rate through the perforation, and the negative pressure were significant. But the suction force on the temporary plugging agent was too large, which may lead to the failure of the temporary plugging agent. For 12 perforations, when the flow was small, the negative pressure at the perforation was too small to suck the temporary plugging agent into the perforation. For 12 perforations, when the flow was 2 m3/min, the maximum speed of the perforation was 47.5 m/s, and negative pressure of 1.25 MPa was formed inside the perforation relative to the wellbore. The negative pressure could effectively suck in the temporary plugging agent. The research results can provide a theoretical basis for the optimization of the fracturing temporary plugging process. [ABSTRACT FROM AUTHOR]

Lost circulation in fractured strata during drilling incurs additional costs and leads to difficulties in promoting drilling safety and efficiency. Plugging-while-drilling is a feasible means to address lost circulation in fractured formations. The particle size distribution (PSD) of plugging particles is determined empirically; therefore, it is often not matched with the fracture sealing requirement. This study investigates the lost circulation mechanism of fractured strata and identifies a calcite particle-based material as the preferred plugging agent. The plugging mechanism and the design of PSD and particle concentration are demonstrated. Based on the concentration, a plugging-while-drilling technique was developed for fractured strata. The results show that calcite particles tend to form the filling layer at the fracture inlet, which cuts off the leakage of the drilling fluid into the fracture, eliminating the drilling fluid pressure applied on the fracture surface. Thus, a stable sealing for the fractured formation is achieved, and the pressure-bearing capacity of the borehole wall is increased. The result also reveals the optimal mixing and concentration models for calcite particles with various diameters. The plugging technique based on calcite particles for fractured formations is applied in a field experiment. The results confirm that the technique can improve the chance of lost circulation prevention and thief-zone plugging in fractured strata and remarkably reduce both the event quantity of lost circulation and the volume of circulation loss. The findings of this research lay a theoretical basis to address lost circulation in fractured formations and, thus, have important practical significance. [ABSTRACT FROM AUTHOR]

The understanding of water adsorption and desorption behavior in the shale rocks is of great significance in the reserve estimation, wellbore stability and hydrocarbon extraction in the shale gas reservoirs. However, the water sorption behavior in the shales remains unclear. In this study, water vapor adsorption/desorption isotherms of the Longmaxi shale in the Sichuan Basin, China were conducted at various temperatures (30 °C, 60 °C) and a relative pressure up to 0.97 to understand the water sorption behavior. Then the effects of temperature and shale properties were analyzed, and the water adsorption, hysteresis, saturation and capillary pressure were discussed. The results indicate that water adsorption isotherms of the Longmaxi shale exhibit the type II characteristics. The water molecules initially adsorb on the shale particle/pore surfaces at low relative pressure while the capillary condensation dominates at high relative pressure. Temperature favors the water sorption in the shales at high relative pressure, and the GAB isotherm model is found to be suitable for describe the water adsorption/desorption behavior. The high organic carbon and full bedding are beneficial to water adsorption in the shales while the calcite inhibits the behavior. There exists the hysteresis between water adsorption and desorption at the whole relative pressure, which suggests that the depletion of condensed water from smaller capillary pores is more difficult than that from larger pores, and the chemical interaction contributes to the hysteresis loop for water sorption. The capillary pressure in the shales can be up to the order of several hundreds of MPa, and thus the desorption of water from the shales may not be as easy as the water adsorption due to the high capillary pressure, which results in water retention behavior in the shale gas reservoirs. These results can provide insights into a better understanding of water sorption behavior in the shale so as to optimize extraction conditions and predict gas productivity in the shale gas reservoirs. [ABSTRACT FROM AUTHOR]

In the process of drilling for oil and gas, plugging materials are used to effectively plug the formation pores and microfractures and thus ensure the stability of the wellbore. In this work, SiO2 expansible composite microspheres (SECMs) were prepared by inverse suspension polymerization using SiO2, vinyltrimethoxysilane, acrylamide, and 2-acrylamido-2-methyl propane sulfonic acid as raw materials. The results of infrared spectroscopy, laser particle size, thermogravimetric analysis, and scanning electron microscopy investigations show that the SECMs are homogeneous. They are spherical in shape with a median particle size of 19.38 μm and a particle size distribution that is relatively uniform. They also have good water absorption and expansion capacity. After 24 h, the average particle size of the SECMs was 34.4 μm, which was 77% higher than the average particle size before water absorption. The SECMs were further investigated by adding them to a low-solid water-based drilling fluid containing 4 wt% bentonite. Plugging tests carried out using the drilling fluid showed that the SECMs can significantly reduce the filtration rate of the drilling fluid. A 0.6 wt% dose of the SECMs led to a FLAPI value of 13 mL. This dropped to 11 mL after aging at 150 °C for 16 h. This shows that the SECMs have a good resistance to high temperatures and that the soft shells of the microspheres deform at high temperature which improves the sealing effect in irregularly shaped pores. [ABSTRACT FROM AUTHOR]

The rapid development of mechanical systems' miniaturization has expanded the demand for higher precision and performance micro formed parts. For improving the forming height and quality of formed parts, and studying the influences of the stress annealing and imprinting times on the forming as well as surface roughness (Sa) of the formed parts, this paper proposed a composite process of imprinting times and stress annealing on warm laser shock imprinting (WLSI) to realize the precision forming of the aluminium foil. In this research, the WLSI experiments were carried out with the imprinting temperature, imprinting times, and stress annealing treatment as variables. Subsequently, the forming height, surface roughness, and surface oxidation of the formed parts were measured, the loading–displacement curve were monitored, the transient deformation process induced by WLSI was divided four stages (deformation stage, rebound stage, vibration stage, and stable stage), and the residual stress distribution of the formed parts were simulated and analyzed by ABAQUS. Finally, the influences of the above three factors on the formed parts were discussed and explicated, and these results showed that the imprinting times and stress annealing can improve effectively the forming effect of WLSI. [ABSTRACT FROM AUTHOR]

Two laser-induced shock wave pressures, 4.5 and 6.5 GPa, were applied to punch LC4CS aluminum sheet respectively, and the influence of different pressures on fracture behaviors was investigated. The code ANSYS/LS-DYNA, dynamic finite element software, was employed to investigate the sheet fracture behaviors during the punching process. The experimental results display that the punching quality manufactured by higher peak pressure of shock wave is better than that by lower one. The finite element method visualizes the punching process, including sheet deformation, cracks growth, and plug flying away. The computational analysis results reveal that the time to punch the sheet with higher peak pressure of shock wave is shorter than that with lower one, and the edge of punched hole resulted from the higher peak pressure is smoother than that from the lower one, which are consistent well with the experimental results. [ABSTRACT FROM AUTHOR]

Porosity and permeability stress-sensitive behavior of sandstone is investigated through porosity and permeability measurements under hydrostatic compression. An empirical logarithm model is applied to the evaluation of stress sensitivity. Mercury injection, casting thin sections and scanning electron microscope are adopted to discuss the microscopic controlling factors and mechanisms of stress-sensitive behavior of sandstone. Disparities between laboratory-scale and field-scale data are also discussed. Experimental results show that both porosity and permeability decrease with increasing effective stress, and permeability is more sensitive to stress. The evolution of porosity and permeability with increasing effective stress follows the logarithmic model and can be classified into three stages: rapid decline, moderative decline and stable-slight decline stage. Stronger permeability stress sensitivity is observed in specimens with lower initial porosity and permeability, which is fundamentally controlled by the size and shape of void spaces as well as the content and distribution of soft minerals. While porosity stress sensitivity is observed to be discrete. Pore throat with small aspect ratio and irregular shape, fractures and minerals with lower elasticity modulus lead to the increase in permeability stress sensitivity of sandstone. The stress sensitivity characteristics are essentially the result of the dissimilar deformation responses of pores and framework grains under effective stress. Stress sensitivity evaluated in the laboratory usually does not completely represent the stress sensitivity in the field, due to stress release, drilling induced fractures and the difference of experimental and in situ conditions. [ABSTRACT FROM AUTHOR]

Laser shock forming (LSF) technology employs shock waves to form sheet metal into three-dimensional complex parts, and has application potential in manufacturing sheet metal parts. In this paper, the forming of 2024 aluminum alloy sheet with LSF was investigated through numerical and experimental methods. The numerical model was established with the commercial code ABAQUS/Explicit. The formed conical cup was obtained from the simulation, and validated by the experiment. With the verified numerical model, the deformation behaviors, including deformation velocity, sheet thickness variation and strain distribution, were studied. In addition, the influence of different shock wave pressures on the forming precision was also investigated. The experimental and numerical results show that the metal sheet loaded by shock wave can take the shape of the mold, and the non-uniform thickness is distributed in the formed cup. The investigations also display that there exists reverse deformation at the central region of deforming sheet owing to severe collision during LSF. In order to obtain formed part with better quality, an appropriate pressure of applied shock waves is required. [ABSTRACT FROM AUTHOR]

Laser shock forming (LSF) is a sheet plastic forming technology, which employs laser-induced shock waves to make sheet metal duplicate a desired shape of the mold. In this paper, a finite element analysis (FEA) model was developed to simulate dynamic forming process with the commercial finite element code ABAQUS/Explicit, and a series of dynamic deformation behaviors of the metal sheet shaped into conical cup at the end of different periods of time were displayed and discussed in detail. The springback of conical cup and the distribution of residual stress were analyzed with ABAQUS/Standard. All these investigations could provide insight into the physics process of the ultra-fast deformation. The LSF experiment was further conducted to verify the results predicted by FEA. The experiment results are well consistent with the numerical predicted data, which validates the FEA model. It indicates that FEA can be used to simulate the forming process and optimize its parameters. [ABSTRACT FROM AUTHOR]

A review of Namibian shorebird densities over two decades and two additional visits to the coastal diamond-mining areas at Elizabeth Bay, southern Namibia, were undertaken to assess the long-term influence of mining activity on density of shorebirds (Charadrii) and particularly threatened African Black Oystercatchers Haematopus moquini and Damara Terns Sterna baleanarum. Oystercatcher numbers remained relatively stable over 25 years, with some recent declines relative to previous surveys along the beach, while that of other shorebirds showed dramatic declines. Density of shorebirds fell 4.3-fold from 220 birds/km to 41 birds/km, relative to similar (pre-mining) surveys 25 years previously. Over the last nine years birds have declined from 12 to nine species. Control sites on nearby sandy beaches over the same (nine-year) period showed no such declines. From the mid-1970s (pre-mining), Damara Terns also declined from 20 breeding pairs to 2–7 breeding pairs between 1996 and 2002; they have remained stable elsewhere in Namibia. Mining activity does not directly impact the known tern nesting sites, but reduced tern numbers have occurred since 1.5 million m3 of fine sediments have been deposited annually into the bay. Observed foraging success of terns at Elizabeth Bay in 2002 was lower than recorded elsewhere in southern Africa, and independent data indicates that the abundance and biomass of their surf zone fish prey is lower in Elizabeth Bay than the nearest control site. Fish availability may also be reduced because most fish occurred within the sediment plume. The decline of the mollusc-eating component of shorebirds can also be explained by the disappearance of the White Mussel and other shellfish favoured by oystercatchers. I conclude that releasing sediment into the bay is detrimental to coastal avifauna and simple conservation measures are given that could reverse and allow further (experimental) study of the reasons for the trends found. [ABSTRACT FROM AUTHOR]

Water is an inherent component in coal masses. The presence of moisture has a significant impact on the mechanical properties of gas-bearing coal and plays a key role in the development of coal and gas outburst disaster. However, how moisture affects the mechanical behavior of gas-bearing coal is poorly understood. In this research, a series of triaxial compression and acoustic emission (AE) tests were performed on gas-bearing coal under different water treatment conditions. The results show that moisture reduces the change in surface energy of the coal by methane adsorption due to the predominance of water adsorption. Therefore, the influence of moisture is more prominent than that of methane and results in a reduction in strength and enhancement in the gas softening coefficient with increasing moisture content. Simultaneously, AE activity of gas-bearing coal is significantly inhibited by moisture, and two failure types can be determined based on the AE results around peak stress: a sudden rupture type for dry and low-water-bearing coal and a stable rupture type for water-saturated coal. Moreover, a meso-statistical damage model is proposed, which agrees well with the experimental results. An analysis indicates that the moisture in coal can reduce the surface energy and weaken the bonds among coal matrix particles, which not only changes the strength of gas-bearing coal but also restrains the energy release during failure; thus, moisture promotes the development of damage in gas-bearing coal. [ABSTRACT FROM AUTHOR]