Low Injection Pressure Research Articles

Experimental Study on EOR in Shale Oil Cores during Associated Gasflooding: A Case Study from Yanchang Formation, Ordos Basin

Summary A significant amount of associated gas has been produced from shale oil reservoirs in the Ordos Basin, northern China, in recent years, which has provided an opportunity for using low-cost, associated gas in enhanced oil recovery (EOR) projects. However, there are few other reports of EOR projects in shale oil reservoirs using associated gas, and a quantitative evaluation of the technique is needed. Therefore, we conducted associated gas and waterflooding experiments in shale oil samples at constant and gradually increasing injection pressure while monitoring the spatial distribution of movable and residual oil by means of nuclear magnetic resonance (NMR) technology. Before the injection experiments, we performed mercury intrusion tests and measured the NMR transverse relaxation time, T2, of fully saturated samples to characterize the pore-throat size distribution of rock samples. Furthermore, we established a novel and robust mathematical model based on a fractal description of the pore space and a capillary tube model to determine the lower limit of the pore radius of movable oil, rc, during gas- and waterflooding. We observed that the oil recovery factor at a low injection pressure (i.e., 0.6 MPa) during the associated gasflooding was lower than that during waterflooding under both constant pressure injection mode and gradually increasing pressure injection mode. However, the performance of associated gasflooding was greatly improved by increasing the injection pressure. High injection pressure indeed produced a higher oil recovery factor, thinner residual oil film thickness, and smaller rc during associated gasflooding than during waterflooding under both injection modes. These differences in behavior appear to be linked to dissimilarities in flooding mechanisms at high and low injection pressures. Our main conclusion is that associated gasflooding at high injection pressure (i.e., 6 MPa) has a better potential for enhancing the oil recovery factor than waterflooding in shale oil reservoirs.

CO2 Flow Characteristics in Macro-Scale Coal Sample: Effect of CO2 Injection Pressure and Buried Depth

Experimental studies have confirmed the permeability reduction of coal samples upon the adsorption of CO2. However, these studies were carried out under limited experimental conditions. In this study, CO2 flow behaviors in a macro-scale coal sample were numerically simulated using a coupled gas flow, mechanical deformation, and sorption-induced deformation finite element model. The simulation results show that the effect of the reduction of effective stress on the enhancement of permeability is greater than the negative effect of permeability reduction due to CO2 adsorption for low injection pressures. CO2 pressure development in the sample increases with increasing injection pressure due to the enhanced advection flux for sub-critical CO2 injections, while for super-critical CO2 injections, CO2 pressure development, as well as concentrations in the sample, decreases compared to sub-critical CO2 injections because of greater density and viscosity of super-critical CO2 as well as coal matrix swelling induced by the adsorption of super-critical CO2. Increasing axial stress (buried depth) obstructs CO2 migration in the sample due to the increased effective stress, and this effect is more influential for low injection pressures, which indicates that high CO2 injection pressures are preferred for CO2 sequestration in deep coal seams.

Experimental Research on Performance Development of Direct Injection Hydrogen Internal Combustion Engine with High Injection Pressure

<div>As a carbon-free power with excellent performance, the direct injection (DI) hydrogen-fueled internal combustion engine (H<sub>2</sub>-ICE) has the potential to contribute to carbon dioxide (CO<sub>2</sub>)-neutral on-road transport solutions. Aiming at high thermal efficiency, the influences of key factors on thermal efficiency over wide operating conditions of a turbocharging DI H<sub>2</sub>-ICE were investigated under the lean-burn strategy. And the nitrogen oxides (NO<sub>x</sub>) emission characteristics region was clarified in the high efficiency. The results confirm the optimal ignition strategy with the CA50 of 8–9 crank angle degrees after top dead center (°CA ATDC). The late-injection strategy manifests a significant advantage in brake thermal efficiency (BTE) compared with the early-injection strategy, and this advantage can be amplified by the increased load or injection pressure. The effects of injection (EOIs) pressure on BTE exhibit different laws at different EOIs. Under the early-injection strategy, the lower injection pressure improves BTE due to a more sufficient mixing. While under the late-injection strategy with strong mixture stratification, the high injection pressure conditions exhibit a higher BTE due to reduced compression work. In terms of air-fuel ratio, the BTE is improved monotonically with increased λ at low and medium loads. But there is an optimal λ value limited by the oxygen concentration at a high load. The late-injection strategies with high BTE perform a high level of NO<sub>x</sub> emissions, which confirms the strong trade-off relationship between the thermal efficiency and NO<sub>x</sub> emissions of H<sub>2</sub>-ICEs. A moderate late-injection strategy with an EOI of about 40°CA BTDC can significantly reduce the NO<sub>x</sub> emissions with a slight loss in BTE. The injection pressure shows different effects on NO<sub>x</sub> emissions in different EOI ranges, depending on the mixture distribution. In addition, ultra-lean burn and lower intake temperature are effective means to reduce NO<sub>x</sub> emissions without losing thermal efficiency.</div>

Multi-variable extremum seeking control for high temperature heat pump system incorporating double refrigerant injection

The use of refrigerant injection technology to enhance the heating efficiency of heat pumps with larger temperature lifts is becoming more common. Heat pumps with double refrigerant injection loops have better energy efficiency than traditional ones with single refrigerant injection loops. A multi-variable extremum seeking control (ESC) strategy for a high-temperature heat pump system with double refrigerant injection technology is proposed in this study. A Modelica-based dynamic simulation model of the considering system is established in Dymola, and a co-simulation with the proposed ESC strategy is built to validate the effectiveness of this control strategy to minimize power consumption. The strategy uses compressor power consumption as the feedback. The effective throttling area of the upper and lower EEVs is adjusted by a PI controller to manipulate the low and high-pressure vapor injection pressures. Three scenarios were simulated for control: stable, step curve, and freely variable heat source temperature conditions. The simulation results demonstrate that the multi-variable ESC has excellent convergence performance and confirm its effectiveness in searching for the optimal operating point.

Energy analysis and research on injection control parameter influence mechanism of air assisted spray system

Air assisted spray system is a two-phase flow spray technology, which consists of fuel injector and gas injector. High pressure gas is injected into the air assisted spray system to maintain the high pressure environment. When using, fuel is injected into the system to mix with high pressure gas, and then the mixture is sprayed out through gas injector to complete the atomization process. Because the air assisted spray system can achieve better atomization of high viscosity liquid under lower injection pressure (below 10 bar), it has attracted more attention recently. A geometric model of the air assisted spray system with a complete air assisted nozzle is established. Based on this model, working process of air assisted spray system is investigated by energy analysis method, and influence mechanisms of different injection control parameters are analyzed. As discovered, the energy provided by fuel injection promotes droplet breakup after interacting with gas inside nozzle. Simultaneously, droplet transfers kinetic energy to gas. Afterwards, kinetic energy of gas and droplet is consumed during their interaction, thereby weakening gas–liquid interaction and reducing droplet breakup. After opening needle valve of gas injector, gas can obtain more kinetic energy due to the pressure difference inside and outside the nozzle, which plays a dominant role in the gas–liquid interaction, and promotes the breakup of droplets. However, the kinetic energy of gas and droplets is subsequently transferred to ambient gas, thus weakening the gas–liquid interaction and hindering the breakup of droplets. The fuel injection duration indirectly affects the consumption of gas kinetic energy in gas–liquid interaction by affecting the total amount of droplets, and then affects the characteristics of air assisted spray. The fuel-gas injection interval directly affects the consumption of gas kinetic energy in gas–liquid interaction, and then affects the characteristics of air assisted spray. The influence of gas injection duration on the characteristics of air assisted spray is determined by the influence of the flow inside the nozzle on the external spray.

Experimental Study on the Adhesive Fuel Features of Inclined Wall-Impinging Spray at Various Injection Pressure Levels in a Cross-Flow Field

The wall-impingement phenomenon significantly impacts mixture formation, combustible performance, and pollutant release in DISI engines. However, there is insufficient knowledge regarding the behavior of fuel adhesion. Thus, here, we examine adhesive fuel features at various injection pressure levels (5 and 10 MPa) in a cross-flow field (0 to 50 m/s). The RIM optical method was employed to track the expansion and distribution of fuel adhesion. As a result, adhesive fuel features such as area, mass, thickness, and lifetime were assessed. Postprocessing image analysis reveals that fuel adhesion was consistently thinner at the edge region. With increased injection pressure, the cross flow led to a rise in the fuel-adhesion area and mass; however, small changes in pressure did not affect adhesive thickness. Adhesive thickness significantly decreased in the cross flow, indicating enhanced evaporation potential. Furthermore, lifetime prediction was conducted to quantitatively evaluate the impact of cross flow and injection pressure upon fuel adhesion, which could be calculated by examining the decreasing trend in adhesive area. Results show that the lifetime was dramatically reduced with higher cross-flow velocity, and slightly decreased with lower injection pressure. Under injection pressure of 10 MPa, the adhesive lifetime in the cross-flow field of 50 m/s was reduced by 77.5% compared with the static flow field (0 m/s). The experimental results provide corresponding guidance for low-carbon fuel utilization and emission reduction in DISI engines.

Time-resolved low-pressure air-assisted spray performance and unsteadiness evaluation

The main advantages of air-assisted spray are its high-quality atomization at low injection pressures and insensitivity to the viscosity of atomized liquid. In this study, the droplet size and velocity of a low-pressure intermittent air-assisted spray were studied by using phase Doppler anemometry, and the effects of liquid fuel injection duration on time-resolved spray microscopic characteristics and spray unsteadiness were analyzed. Droplet size-velocity joint probability density functions were employed to characterize the droplet diameter-velocity distribution as well as the probability range. A comparison of the droplet Weber number with an empirical critical value indicates that atomized droplets hardly undergo secondary shear breakup. Based on the ideal spray theory of Edwards and Marx, an improved algorithm is proposed with the concept of iterative rejection of inter-particle arrival times to quantify the unsteadiness of air-assisted sprays by eliminating the dependence of the calculation results on droplet sampling data. The results show that intermittent air-assisted spray is an inherently unsteady process that can be influenced by fuel injection duration and spatial location, while independent of the droplet size. In addition, the spray unsteadiness exhibits noteworthy variations at different spray stages segmented by droplet velocity vs time. The relation between the potential internal gas–liquid two-phase status determined by fuel injection duration and the spray performance is elaborated.

Dynamic evolution of low-viscosity fuel particle distribution driven by constant flow

The effects of mass concentration and injection pressure on the atomization characteristics of low-viscosity fuel spray are studied in a constant-volume chamber. Microscopic spray parameters are measured by laser diffraction at different axial and radial positions downstream of the nozzle. The results show that the atomization effect is inhibited linearly with the increase of mass concentration. The increase of injection pressure promotes the droplet breakup. However, the trend gradually weakens and becomes more noticeable at high concentrations. Comparing with the concentration, the influence of the injection pressure on the atomization characteristics is dominant. Although low concentration and high injection pressure can promote the droplet breakup, they also increase the probability of droplet collision, resulting in droplet aggregation. This is more evident in low-viscosity fuels. The droplet size increases in the axial direction owing to the aggregation. However, the diameter decreases in the radial direction owing to the outward deflection of small droplets caused by air turbulence and entrainment. In addition, the high-velocity airflow significantly promotes the droplet breakup near the nozzle and spray axis regions and inhibits the aggregation effect. However, the lower-viscosity fuels keep smaller droplet sizes and better atomization in the whole spraying process, which is easier to realize than the higher-viscosity fuels. Overall, low concentration, high injection pressure, and low viscosity of fuel have beneficial effects on the droplet breakup. This is very important for improving the atomization effect of fuel.

Mixture formation characteristics and feasibility of methanol as an alternative fuel for gasoline in port fuel injection engines: Droplet evaporation and spray visualization

Under the background of energy saving and emission reduction, methanol is considered an ideal alternative fuel for gasoline because of its renewability, high combustion efficiency and low emissions, while an issue is the cold start of methanol port fuel injection engines. To comprehensively understand the underlying mechanisms and the feasibility of methanol as a substitute of gasoline, a suspension method was first used to investigate the evaporation characteristics of methanol and gasoline droplets at low temperatures. Then an optical intake port was constructed based on the outer contour modification strategy and an optical distortion correction algorithm was proposed. The effects of inlet temperature, inlet flow rate and injection pressure on spray characteristics of methanol and gasoline were comparatively investigated. In addition, the influence of valve temperature on the characteristics of spray impingement on a valve was studied. The results showed that the difference between the evaporation rates of methanol and gasoline was small at 60–90 °C. The difference became obvious when the temperature was below 60 °C or above 90 °C. Reducing the ambient humidity may reduce this difference and improve the cold start performance of methanol engines. A higher inlet temperature slightly increased the spray area. A higher inlet flow rate and injection pressure significantly increased the spray area, but also caused serious wet wall problem. From the point of view of spray in the port, methanol was a feasible substitute for gasoline in the presence of inlet flow and at low injection pressure of 6 bar. The valve temperature should exceed 200 and 230 °C for methanol and gasoline, respectively to form vapor films that prevent fuel adhesion onto the valve. However, methanol may not be a suitable alternative fuel for gasoline when spray directly impacted on the valve in port fuel injection engines due to its much smaller spray area.

Numerical study of the mechanisms of the longitudinal pulsed detonation in two-dimensional rotating detonation combustors

A numerical study of the longitudinal pulsed detonation (LPD) is conducted in the present paper. The occurrence mechanism of the LPD, called shock wave amplification by coherent energy release, is verified preliminarily in this study. To be specific, upstream propagating shock waves, which originate from the outlet, induce a specific gradient of reactant distribution, and then detonation waves are ignited and evolve along the gradient in close succession. It is worth noting that the occurrence of LPD does not mean that the LPD will necessarily be sustained. The low injection pressure ratio PR (i.e., the ratio of inlet pressure to outlet pressure) = 1.3 is found to be conducive to the sustenance of the LPD instability in the baseline model. A lower PR (PR ≤ 1.2) or a slightly higher PR (1.4 ≤ PR ≤ 1.8) shall lead to an unstable detonation or quenching of detonations, while a much higher PR (PR > 1.8) contributes to the formation of stable canonical rotating detonation waves. In addition, the combustion regimes of five combustors of different heights at different PR are explored. As the combustion chamber height increases, the PR of the sustainable LPD is nearly linearly increasing, and its operating frequency decreases gradually. The calculation formula between the sustainable LPD propagating frequency and the natural acoustic resonance frequency of the combustor is employed and discussed, but in consideration of its imperfection, further investigation is required.

Investigations of string cavitation and air back suction during injection duration in the scaled-up diesel fuel injection nozzle

The flow inside the diesel fuel injector nozzle is directly associated with the subsequent jet atomization and fuel–air mixture performance, and finally affects the combustion efficiency and emission. In this study, high-speed microscope imaging technology is applied to capture the complex multiphase flow and near-field spray during the process of fuel injection. Besides normal optical experiments for nozzle internal flow and spray, the submerged jet experiment for comparison is also carried out to investigate the phenomenon of air back suction. Combined with the simulation of a three-component multiphase model, the difference between air and cavitation vapor is analyzed. Results show that the submerged jet experiment will not change the internal flow field but can eliminate the influence of air. The shaded area in the hole obtained by the shadow method may be the sucked air or vapor generated by string-type cavitation phase change. Both can coexist in the hole and are closely related to the swirling intensity and the spray structure, respectively. When the needle lift is low, the internal flow field has larger swirling intensity resulting in a kind of hollow spray. The low-pressure region exists in the vortex core, nearly at the axis of the nozzle orifice. The ambient air will be sucked into the orifice to form an air column under the action of pressure difference, which is also easy to occur under low injection pressure. When the needle lift is high, the weak swirl intensity leads to hole-to-hole string cavitation which has weak cavitation intensity and obvious periodicity. At this time, the results of non-submerged and submerged jet experiments are basically the same. In addition, the cavitation bubble near the orifice outlet was observed, for the first time, to move upstream under the action of the pressure difference inside and outside the orifice in the submerged jet experiment. It can be predicted that in the non-submerged jet experiment, the ambient air will be sucked into the orifice to form an air column.

Recovery efficiency and characteristics of long core by CO2 flooding in low permeability sandstone reservoirs

CO2 flooding is an economical and efficient enhanced oil recovery technology, however, it is difficult for conventional short core experiments to provide accurate parameters for the later on-site optimization scheme. In this paper, on the basis of long core flooding experiments and applying NMR technology, we quantitatively evaluated the recovery efficiency and remaining oil distribution characteristics by CO2 in low-permeability sandstone reservoirs from a microscopic perspective, and explored the potential of CO2 flooding in low-permeability sandstone reservoirs. Results showed that with the CO2 injection pressure increase, the recovery efficiency of the long core increased and the ultimate recovery was 65.3%. Recovery efficiency of short cores at the inlet, middle and outlet decreases successively, but the change range was not large, and all of them are around the recovery efficiency of long core. In addition, at low injection pressure, almost all CO2 went into the larger pores to replace oil. As increased pressure, the oil started to be produced from the smaller pores, but at 22 MPa, the recovery efficiency in larger pores (76.88%-83.38%) was still higher than that of smaller pores (68.73%-72.74%). Which provided a guide for optimizing the CO2 enhanced oil recovery method in the field.

MODIFICATION OF AMULTIHOLE INJECTOR TO A SINGLE-HOLE INJECTOR AND SPRAY CHARACTERISTICS OF THE MODIFIED INJECTOR BY TWO DIFFERENT IMAGING TECHNIQUES

In the present days, almost all the diesel engines are fitted with multihole injectors. Spray characterization of the injectors by different optical measurement techniques is very important to under-stand the air-fuel mixing and combustion procedure inside the cylinder. Instead of characterization of the dense sprays from a real multihole diesel injector, often a single plume is used to study the spray parameters. In the present work, spray characteristics are studied for a multihole injector with all nozzle holes open and with a single hole open conditions. Experiments have been conducted in a constant-volume, high-pressure, and high-temperature chamber with various injection pressures and at various ambient conditions. A high-pressure common rail injector with six holes is considered for the study. The penetration and cone angle of the spray from each hole are measured, and the hole-to-hole variation is studied. It has been observed that hole-to-hole variations with respect to spray penetration are prominent in low chamber pressures and injection pressures at the initial stage of injection, though they are minimal at later stages of injection. After that, five holes of the injector are blocked by microwelding, and the remaining single plume was studied and the results compared with the original multiplume spray. The study shows that the penetration length and cone angle of the single plume match well with that of the multiplumes at all chamber pressures and injection pressures. Later, the spray from the modified single-hole injector was characterized by two different imaging techniques, Mie scattering and diffused backlit imaging (DBI), to compare the penetration length and cone angle measured by the two imaging techniques. It is seen that the differences in penetration lengths by two imaging techniques are negligible, whereas the cone angles are greater in the case of backlit shadow imaging, particularly at higher temperatures.

Experimental research on the performance of hollow and annular rotating detonation engines with nozzles

• The performance of hollow and annular RDEs with nozzles are compared. • The hollow RDE can reduce the injection pressure loss. • The hollow chamber brings a higher performance loss. Rotating detonation engines (RDEs), which are hoped to achieve significant performance improvements in propulsion systems, have received much attention in recent years. The annular RDE and hollow RDE are two of the major configurations of which the structure is very similar to each other. However, there has been a lack of experimental research discussing the differences in their performance. In the present study, experiments on hollow and annular RDEs are conducted. Four configurations in total are tested at ambient conditions: hollow and annular combustion chambers with two exit throat areas. The gaseous methane and gaseous oxygen are used as propellants. The results reveal that the c*-efficiency of the two hollow combustion chamber configurations is on average 0.7 and 0.79, which is lower than the 0.75 and 0.84 of annular chambers. However, it is exhibited that the hollow RDE can ensure high detonation efficiency at a lower injection pressure ratio. From this perspective, the hollow RDE can be considered a more promising configuration for practical application because it provides a new approach to reducing injection pressure loss. In addition, the analysis of nozzle indicates that the Laval nozzles overperform the aerospike nozzles, which goes against previous studies. The mechanism that accounts for this phenomenon needs further investigation.

Numerical investigation of AdBlue film formation and NH3 conversion in generic SCR system using Eulerian stochastic fields method

In this paper, the Eulerian stochastic fields (ESF) method in a LES framework is applied to a generic selective catalytic reduction (SCR) configuration in order to retrieve seamlessly the effect of turbulence–chemistry interaction on the NH3-conversion and AdBlue film formation. The ESF method is based on the transport equation of a joint scalar filtered density function. The injection of AdBlue and relevant spray dynamics are modeled by the Lagrange-particle description where a computational parcel represents a cluster of real AdBlue droplets. Owing to the low injection pressure, weak atomization of AdBlue leads to intense droplet wall-impingement and film formation on the SCR duct wall and/or on the mixer elements, if present. A 2-D thin film approach is adopted to model the film formation and dynamics in combination with multi-regimes droplet–wall interaction model. After verification tasks against the reference data for simple gas phase reactive cases, the verified ESF method is coupled with a one-equation based LES turbulence model together with the Lagrange particle-tracking which is combined with the 2-D thin film approach to simulate the AdBlue injection and the film formation in the generic SCR configuration. At first, the assessment of the adopted LES mesh is carried out in terms of the so-called LES quality of index to determine the optimal mesh resolution. Next, to ensure the convergence with respect to the number of required ESF and mesh resolution, SCR simulations are performed by using various (2, 4, 6, 8, 12 and 16) stochastic fields and also with refined mesh. Thereby, the sensitivity of the predictive capability of the numerical tool is evaluated for the production of NH3 and HNCO with and without ESFs. This clearly indicates the importance of an accurate description of the turbulence–chemistry interaction to retrieve reliably the conversion of NH3. Finally, the film dynamics especially the evolution of the film thickness described with six required ESF and without ESF is compared with experimental data from the generic SCR configurations. To further demonstrate the potential of suggested numerical approach, a detailed numerical analysis is provided in terms of spray-impingement dynamics, scalar uniformity index, droplet life time and characteristics of the droplets prone to form solid deposits in the monolith channel.

Comparative study on the spray and combustion characteristics between diesel and kerosene underlow-temperature combustion (LTC)mode conditions

Kerosene, an intermediate component of wide distillation fuel, has drawn increasing attention for low-temperature combustion mode due to its compromise of evaporation and ignition characteristics between diesel and gasoline. This study focuses on the comparison of spray and combustion characteristics between diesel and kerosene under low-temperature combustion mode conditions based on visualization tests in a constant volume chamber. Different injection pressures, ambient temperatures, and oxygen concentrations were tested at a constant ambient density of 16.67 kg/m3. The image processing results reveal that the liquid penetration distance of kerosene shows a maximum reduction of 33.5% compared to diesel with a smaller cone angle and spray area at 160 MPa and 800 K. At the same time, the ignition delay of kerosene is 0.9 ms longer than that of diesel, and the injection and combustion processes of kerosene are separated, making it much more similar to the premixed combustion. An increase in injection pressure improves the spray penetration distance but exerts little influence on the flame lift-off length and ignition delay for both fuels. Kerosene presents a lower spatially integrating natural luminosity, especially at low injection pressure. The liquid spray penetration distance and the ignition delay of kerosene are more sensitive to the increase in ambient temperature. However, the higher ambient temperature presents a greater effect on the SINL of diesel. With the decrease of ambient oxygen concentration, the ignition delay, flame lift-off length, spatially integrating natural luminosity, and soot oxidation rate of both fuels decrease obviously. Especially at 15 vol.% ambient oxygen, kerosene presents a much weaker luminosity. These results imply that kerosene exhibits a very similar trend to diesel in terms of spray and combustion characteristics. Moreover, kerosene has a stronger potential to reduce soot formation than diesel with a better fuel-air mixing process under low-temperature combustion mode conditions.

Pore- and Core-Scale Mechanisms Controlling Supercritical Cyclic Gas Utilization for Enhanced Recovery under Immiscible and Miscible Conditions in the Three Forks Formation

Tight formations such as the Three Forks Formation in the Williston Basin have very low primary recovery factors (typically 10% or less), which potentially leave billions of barrels stranded in the reservoir. According to the results of gas enhanced oil recovery (EOR) pilot tests that have been conducted in the Bakken Formation, cyclic gas injection appears to be a promising strategy for increasing oil recovery in unconventional reservoirs. Despite the pilot tests and the reported results in Bakken shale, Middle Bakken, and Three Forks Formations, little is known about the mechanisms of cyclic gas injection and the behavior between the injected gases and Three Forks reservoir fluids. In this paper, a series of experiments are applied under different constraints. The parameters that are examined include the effect of soaking time under immiscible and miscible conditions, the effect of fluid state (vapor, supercritical), injection pressure, selected gases (CO2, ethane, and propane), target formations (Upper Three Forks (UTF) and Middle Three Forks (MTF)), and the effect of pore size distribution on the recovery factor. During the Huff-n-Puff process, the nuclear magnetic resonance (NMR) technique was used to analyze the microscopic oil production and the micro residual oil distribution before and after CO2 injection. The results showed that the soaking time enhanced the recovery factor dramatically in both formations under and near the minimum miscibility pressure (MMP) of CO2, while the soaking time was less efficient in cases with injection pressures above the MMP. Propane proved to be the most effective gas at low injection pressure, followed by ethane, with CO2 being the least effective. On increasing the pressure, the performance of ethane and CO2 increased due to the sufficient change in the ethane and CO2 densities. Additionally, according to the NMR tests in UTF, micropores, mesopores, and small pores were predominant in oil extraction during the first cycle. After a few Huff-n-Puff cycles, oil in micropores becomes the primary source of oil extraction. However, in MTF, the dominant pore distributions were mesopores, small pores, and fewer micropores than in UTF. The findings in this study provide new insight to better understand the mechanisms of gas HnP enhanced recovery in the Three Forks Formation, which is of great significance for the efficient hydrocarbon exploitation and greenhouse gas utilization in shale reservoirs.

An integrated multi-scale model for CO2 transport and storage in shale reservoirs

Shale reservoirs have gradually been recognized as promising potential candidates for CO2 geological storage due to their wide geographic distribution and enormous storage potential. CO2 sequestration in shale is a complex multi-scale process with spatial sizes ranging from nano-scale to kilometer-scale. In this study, an integrated multi-scale model, considering CO2 adsorption, dissolution, slip flow, and diffusion, is developed to describe CO2 transport and storage in shale reservoirs with a multi-stage fractured horizontal well. The computationally efficient semi-analytical solution of the multi-scale model is obtained via Laplace transformation and Pedrosa’s substitution. Based on the established model, a workflow for evaluating the CO2 storage capacity of shale reservoirs is proposed. Taking the Longmaxi shale reservoir in the Sichuan Basin as an application case, the results show that the storage capacity can reach up to 29.97 × 108 m3 at a high constrained injection pressure. In addition, sensitivity analysis suggests that CO2 storage capacity is strongly influenced by the adsorption index of clay minerals, kerogen content, solubility coefficient, and inter-porosity flow coefficient of kerogen and inorganic matrix. Most reservoir parameters have negligible effects on storage capacity at low injection pressures, yet significant effects at high injection pressures. More specifically, the CO2 storage capacity can increase by 6.8 folds when the constrained injection pressure is increased from 5.5 to 8.5 MPa. The findings obtained in this study further expand the description of CO2 transport in the shale formation, laying the foundation for highly efficient CO2 sequestration in shale reservoirs.

A simple preparation method of rotaxane cross-linker based on γ-cyclodextrin and its application in hydrogel materials

Currently, preparation of rotaxane cross-linker (RC) requires at least two-step or more complex chemical reactions. Therefore, a simple and efficient method is proposed to prepare RC based on γ-Cyclodextrin (γ-CD), which requires only one-step chemical reaction. This means that it is more convenient to apply RC to prepare hydrogels with pulley effect. The structures of RC are characterized by 1 H NMR, 2D NOESY NMR, DOSY NMR, XRD and thermal gravimetric analysis. And the effects of reaction conditions on the yield of RC are studied. RC and traditional cross-linker (N, N’-Methylenebis(acrylamide), MBA) are, respectively, can be copolymerized with acrylamide (AM) to prepare hydrogels to compare the swelling ratios and deformation properties of hydrogels. Furthermore, the effect of mineralization degrees on the swelling ratios of hydrogels and the effect of hydrogels prepared with different content of cross-linker on deformation properties are explored. After absorbing water for 200 h, the swelling ratio of slip-ring hydrogel 0.5% (% is the weight percentage of cross-linker to AM) is 79.05, while that of traditional hydrogel 0.5% is 8.8. The elongation at break of traditional hydrogel 0.6% (211%) is much smaller than that of slip-ring hydrogel 0.6% (6467%) and the compression modulus of traditional hydrogel 0.6% (0.20 MPa) are greater than that of slip-ring hydrogel 0.6% (0.11 MPa). Thus, the swelling ratios and deformation properties of hydrogel prepared by RC are better than that of hydrogel prepared by traditional cross-linker. Lastly, the seepage properties of hydrogel dispersions in porous media are also investigated. The hydrogel dispersion prepared by RC has lower injection pressure and higher residual resistance coefficient than that of traditional cross-linker.

Thermal performance analysis of coaxial borehole heat exchanger using liquid ammonia

The non-water working fluid is crucial to evaluate and optimize the thermal performance of the coaxial borehole heat exchanger (CBHE). An unsteady heat transfer model is built to evaluate the thermal performance of CBHE using different working fluids. Liquid ammonia, among other eight fluid candidates, is selected as an effective non-water working fluid because of lower injection pressure (5 MPa), a lower pressure loss (2.45 MPa), a better thermal performance (Outlet temperature and thermal power are 0.9 °C and 27.17 kW higher than that of water, respectively), a higher coefficient of performance (7.34) and a low cost than other fluid. Moreover, when a double-layered insulation pipe structure insulates 88.9% of the top of the central tube, the thermal power can be 8.41 times that of an uninsulated case. A high flow rate (23 m³/h) can provide a thermal power 15.35 times that of the 1 m³/h flow rate, but with an extra pressure loss of 3.38 MPa. Moreover, the effect of cement sheath, pipe size, injection temperature, injection pressure, and periodic shut-in operation is analyzed and optimized by analyzing the engineering and economic factors. This paper provides a theoretical basis for the future application of CBHE.