Depressurization Process Research Articles

Performance evaluation and economic feasibility of a PAFC-based multi-energy hub system in South Korea

In this study, a MW-scale fuel-cell-based multi-energy hub system is simulated to maximize microgrid flexibility. The system was designed to deliver electricity, heat, and H2 with a high energy efficiency, which can be realized through the integration of the exothermic phosphoric acid fuel cell and endothermic turboexpander generator modules. To extract additional useful work during the depressurization process in the natural gas supply chain, this multi-energy system is currently under construction on a pilot basis in South Korea. Based on a natural gas flow rate of 28,000kg/h, we analyze the performance of the system and its economic effects under different load demands and operating conditions. The system simulation results show that the total electrical power generated by the turboexpander generator and phosphoric acid fuel cell modules is approximately 3.395–3.539MWe, while the H2 yield is 19.94–40.00kg/h. In addition, an economic assessment of the multi-energy hub system is conducted based on the current natural gas, electricity, and H2 prices in South Korea. Assuming a life span of 20 years for the turboexpander generator and phosphoric acid fuel cell modules, the payback period is estimated to be between 7.09 and 10.05 years.

Study on the Flow Behavior of Wellbore Fluids of a Natural Gas Hydrate Well with the Combined Depressurization and Heat Injection Method

Natural gas hydrate (NGH) is a kind of clean energy with great potential because of its huge reserves. There are several effective methods for exploiting hydrate sediments such as depressurization, thermal excitation, inhibitor injection and displacement, etc. Among these methods, the combined depressurization and heat injection method is considered a very promising method, which solves the problem of insufficient heat supply during the depressurization process. In this paper, the mechanism of combined depressurization and heat injection exploitation of NGH is analyzed, and the multiphase flow models of the injection well and production well are established, respectively, for the parallel horizontal NGH well production system with this combined method. The multiphase flow laws of fluids in a wellbore were obtained, and the factors affecting the temperature and pressure distributions in the wellbore were analyzed. The results of this study show that gas and water are produced simultaneously in the process of exploitation with this combined depressurization and heat injection method. The electric submersible pump has a great influence on the flow of the fluids in the wellbore, and there are sudden skips of the temperature and pressure at the pump position. Increasing the depth and working frequency of the pump will reduce the risk of continuous discharge of water from the annulus. Increasing the injection rate and injection temperature can both improve the effect of heat injection. This study provides theoretical guidance for the combined extraction with depressurization and heat injection method and production optimization of NGH.

Experimental study on sand production and coupling response of silty hydrate reservoir with different contents of fine clay during depressurization

To further understand the characteristics of clay and sand production (hereafter collectively referred to as sand production) and to provide optimization designs of sand control schemes are critical for gas production from clayey silt natural gas hydrate reservoirs in the South China Sea. Thus-, gas-water-sand production behavoirs and coupling reservoir subsidence characteristics before, during, and after hydrate dissociation of the clayey silt hydrate reservoirs with different clay contents (5%, 10%, 15%, 20%, 25%, and 30%) have been studied through a self-developed experimental system. The results show that with the increase of clay content, the total mass of sand production first increases and then decreases, and it reaches maximum when the clayey content is 20%. The sand production is the lowest before hydrate dissociation and increases significantly during hydrate dissociation, which mainly occurs in the high-speed gas and water production stage at the beginning of hydrate dissociation. After hydrate dissociation, the sand production decreases significantly. During the whole depressurization process, the clay and free sand particles generally move to the sand outlet due to the fluid driving force and overlying stress extrusion. However, for conditions of high clay contents, those particles fail to pass through the sand control screen and gradually accumulate and block the screen by forming a mud cake, which greatly reduce the permeability of the screen and limite sand production as well as gas and water production. Our research lays a foundation for sand production prediction and sand control scheme selection during gas recovery from clayey silty hydrate reservoirs that greatly need to consider a balance between sand control and gas productivity.

Experimental and mechanism investigation on flowability and wax deposition of waxy crude oil with dissolved CH4 by pressurized laboratory apparatus

In offshore oilfields, crude oils often contain dissolved gas, and are unavoidably transported under pressure. However, the flowability and wax deposition of crude oils with dissolved gas are rarely investigated. In this paper, Methane (CH4) was used as a similar substitute for the dissolved gas in crude oils, and a series of pressurized laboratory apparatuses were developed to study the influencing mechanism of dissolved CH4 on the flowability and wax deposition of Changqing waxy crude oil. It was found that with the increase of the dissolved CH4 pressure, the crude oil flowability was improved, and the wax deposition rate was inhibited. However, the wax appearance temperature (WAT), the average wax content of wax deposits, and the mass of wax diffused into wax deposits all increased accordingly. This was mainly attributed to the influence of dissolved CH4, which can effectively increase the intermolecular distance between wax molecules, decrease the intermolecular forces, and weaken the strength of wax crystals to overlap each other. The improvement of rheological properties, on the one hand, promotes the diffusion of wax molecules, which leads to the increase in the mass of wax diffused into wax deposits; on the other hand, it increases the wax content required for the formation of wax deposits according to the dynamic gelation mechanism, thus resulting in the decrease of wax deposition rate. Also, the wax stripping effect during the deposition and depressurization processes cannot be ignored. The research results have guiding significance for the optimal design of the gathering and transportation pipeline network and the calculation of wax deposition.

Characteristics of In Situ Stress and Reservoir Pressure in Deep Coal Seams and Their Influences on Reservoir Depletion: A Field Case Study

The production of coalbed methane resources is greatly dependent on the reservoir depletion process. To improve the depressurization efficiency of deep coalbed methane (DCBM) reservoirs, the fluid production characteristics and implications of in situ stress and reservoir pressure for reservoir depletion in the eastern margin of the Ordos Basin are analyzed. The results indicate that the duration of the single-phase water production stage is overall long, with the gas declining stage occurring too early. The daily gas production volume of DCBM wells is only a few hundred m3. For DCBM reservoirs, it is considered that in situ stress, initial reservoir pressure, and critical desorption pressure significantly affect the fluid production and depressurization process. A high in situ stress in deep coal seams is likely to damage the flow conductivity of fracture systems due to the increase in effective stress during reservoir depletion. Based on the in situ stress characteristics of deep coal seams and their implications for reservoir depletion, we propose that the depressurization of DCBM reservoirs during gas production can be promoted via a new method of stress release. Slots or cavities created in coal seams by high-pressure hydraulic jet can provide space for stress release. This novel depressurization method could help improve the gas production efficiency of DCBM reservoirs.

Phase Behavior and Reversibility Mechanisms of Asphaltene Precipitation for High-Pressure High-Temperature CO2–Oil Systems

Net-zero carbon emission pledges by 2050 are prompting researchers to investigate alternatives to store CO2 safely. Subsurface CO2 storage faces many challenges, especially when the geological space is a depleted oil reservoir. A crucial problem is the newly formed asphaltenes during CO2 injection that could limit or even block the rock-effective zone for gas storage. In this work, a novel pressure–volume–temperature cell and a solid detection system are used to measure the phase behavior, asphaltene onset pressure, and asphaltene precipitation. Results show four different equilibrium phases during the isothermal depressurization processes. Likewise, asphaltene precipitation increases with CO2 fractions and decreases with temperature, reaching its maximum at the bubble point. More importantly, the redissolution of asphaltene particles back into the solution is observed, revealing the formation process reversibility at certain conditions (25 mol % CO2 at 60, 90, and 120 °C; 35 mol % CO2 at 90 and 120 °C). The Peng–Robinson equation of state model is employed to model the fluid phase, while the solid phase equilibrium is modeled with a solid model. A new correlation for the asphaltene molar volume is proposed, which predicts the experimental data with an AARD of 0.7%.

In Situ U—Pb Dating and Trace Element Analysis of Garnet in the Tongshanling Cu Polymetallic Deposit, South China

The grossular–andradite garnet is an ideal mineral for indicating the formation age of skarn, which also pretends to constrain skarn processes because of its higher REE (rare earth elements) content. The Tongshanling deposit is a medium-sized reduced skarn Cu–W–Pb–Zn deposit associated with a highly evolved I-type granodiorite intrusion in the Nanling metallogenic belt, South China. Different mineral assemblages, microscopic characteristics, and BSE images distinguish two kinds of garnets in the prograde and retrograde skarn stages. The garnet grains from the prograde skarn stage have a U–Pb isochron age of 165.4 ± 3.8 Ma (MSWD = 0.7) and that from the retrograde skarn stage have a U–Pb isochron age of 159.5 ± 1.7 Ma (MSWD = 1.8), implying that the thermal metamorphism and hydrothermal metasomatism mainly occurred in the middle Upper Jurassic. The total amount of rare earth elements (∑REE+Y) in the garnet gradually decreased and the REE patterns shifted from enriched HREE with Eu negative anomaly to HREE-depleted with Eu positive anomaly. The decreasing U content and increasing Eu anomaly in the retrograde skarn stage indicate a redox environment change from oxidation to reduction. However, garnet from different elevations within the same stage (+90 m, +5 m, −35 m, −200 m, and −400 m) exhibit similar REE patterns, despite weak cooling and significant depressurization processes confirmed by fluid inclusion microthermography. As a result, the REE content and patterns are dominated by the REE species of parent fluids, which are changed over time by symbiotic REE-enriched mineral precipitation and the redox environment, while being slightly affected by the fluid pressure. Grossular garnets, rich in U and REEs, and found in reduced skarn deposits, can constrain chronology and reveal the spatio-temporal zonal characteristics.

Joint-production characteristics of typical marine composite reservoir with water-saturated methane hydrates and higher-pressure gas by depressurization

Marine natural gas hydrate is water-saturated and usually accompanied with higher pressure underlying gas, and this typical composite reservoir is greatly economical for production. Yet, the experimental simulation on the typical marine composite reservoir is hard, and its joint-production studies are quite rare. In this work, the same 7.5 MPa methane hydrate reservoirs with an approximately 20% saturation underlain with four different pressures (7.1, 8.0, 8.1 and 9.0 MPa) gas were remolded. During the depressurization process from the top of hydrate layer, the water-saturated hydrate layer can allow the overpressure of underlying gas as highest as 6.0 MPa because of the presence of the hydrate seal. The early production stage is independent and lasts for several minutes until the hydrate seal is broken due to the dissociation of some hydrates, and then the massive inflow of underlying gas into hydrate layer induces the dissociation pause or even reformation of hydrates in return. After that, the production of underlying gas become synchronous and in a same depressurization process with the hydrate reservoir. As a result, the joint-production process of water-saturated methane hydrate and higher-pressure gas can be divided into independent and synchronous stages. In addition, the decisive factors of underlying gas and hydrate production processes are gas seepage and hydrate dissociation, respectively, and the production rate of underlying gas is much faster than that of hydrate reservoir. It is therefore suggested that the key of improving joint-production efficiency of the composite reservoir is to accelerate the hydrate dissociation. This work is of great significance for the guidance of spot production of marine natural gas hydrate with higher-pressure underlying gas.

Numerical simulation on pressure evolution process of liquid hydrogen storage tank with active cryogenic cooling

Inevitable evaporation process occurring in liquid hydrogen storage may bring about safety risks associated with over-pressure and hydrogen discharge. Using a cryogenic refrigerator to compensate for the heat leakage is expected to eliminate the risk of hydrogen vent, realizing long-term lossless storage. Presently, active lossless storage technology applied on civil fields has rarely been studied. This paper presents a numerical study on the dynamic evolution process within an active cooling system of a liquid hydrogen tank. The accuracy of the model is validated with an average deviation of 0.47%. During the self-pressurized process, the maximum temperature difference is more than 26 K in the ullage zone. After cooling capacity being introduced, a three-stage depressurization process appears. Minor cooling capacity that is less than heat leakage can still realize pressure management. Further analyses show two operating modes similar to gravity heat pipes and thermal redistributors occurring in the ullage and the liquid zone, respectively. This work suggests the effectiveness of active cooling on controlling the ullage pressure, which may be applicable to future hydrogen refueling stations and intercity hydrogen transportation.

Modeling of pressure control in a cryogenic tank by a thermodynamic vent system considering flash evaporation

Thermodynamic vent system (TVS) has been recognized as a promising method for pressure control in cryogenic propellant tanks in microgravity environments. Prior simulations basically treated the liquid in the tanks quasi-static during the depressurization process via a venting operation. However, experimental evidence suggests that the liquid could be metastable and superheated at that moment. Therefore, improved models are needed for a more accurate simulation of the transient liquid-vapor phase change and heat transfer during the TVS depressurization. A new model that considers flash evaporation effect has been developed in the present work. The predicted pressure agrees with the experimental data better than those by conventional models. It is found that the flashing phenomenon plays a significant role in the depressurization, and behaves more intensely in the lower fill level cases. The flashing duration is mainly determined by the width of the pressure control band. The model has also been used to analyze the efficiency of the TVS in reducing propellant loss. It is found that the ineffectiveness of the heat exchanger and the unbalance of heating and cooling to the ullage in a full cycle are the main causes for the efficiency degradation of the TVS. Either increasing heat exchanging area, reducing orifice diameter of the throttling device or narrowing the pressure control band helps to improve the efficiency.

CH4 hydrate production coupled with CO2 sequestration and hydrate restoration employing depressurization assisted by CO2-N2 injection at marine conditions

Natural gas hydrates (NGHs) buried below the seafloor are globally abundant, energy-dense, and essential to the world’s future energy mix. The combination of depressurization and CO2-N2 injection processes for NGHs recovery has been identified as an environmental method to harvest energy, CO2 sequestration, and to maintain the mechanical stability of sediment. In this regime, partial CH4 was produced by depressurization followed by CO2-N2 injection to form CH4-CO2-N2 mixed hydrate (Mix-H). In this study, a series experiments were designed to examine the key factors in the depressurization process (i.e. bottom hole pressure BHP and CH4 production ratio) on the subsequent processes of Mix-H formation, CO2 sequestration and hydrate restoration. It was observed that Mix-H formation occurred in all cases, but the final amount of Mix-H were all lower than the initial amount of MH before depressurization. Particularly, the formation of CO2/N2 hydrate in Mix-H is positively correlated with the amount of remaining CH4 in the combined phases of gas and hydrate after depressurization. The formation of CH4 hydrate in Mix-H during the reformation process occurred in five cases when MH dissociation ratio (DMH) reaches above 45.1 % during depressurization. MH was only observed to dissociate continuously after CO2/N2 injection when DMH is low at 30.8 %, which indicates possible “CH4-CO2 replacement reaction”. Overall, increasing CH4 production at the same BHP decreases the CO2 sequestration and hydrate restoration. We first verified the feasibility of the combination method for synergistic CH4 recovery and CO2 sequestration in marine conditions. The experimental results provide possible guidance on the optimal design of the coupled processes and shed light on the relationship between CH4 recovery and CO2 sequestration that need to be wholistically balanced.

High-speed imaging of degassing kinetics of CO2–water mixtures

The exsolution of gas molecules from gas–liquid mixtures plays a significant role in a wide range of applications from industrial processes such as metal casting to subsurface flow of oil or geothermal waters. This study aims to improve the understanding of the conditions under which free gas bubbles start forming in CO2–water mixtures. The bubble point pressure was determined under various different conditions like the temperature and initial pressure of the mixture along with other parameters such as the bubble growth rate. A series of depressurization experiments at high pressure and temperature (up to 100 bar and 100 °C) is performed using a pressure cell that allows for visual monitoring of the degassing process. Bubble formation during the depressurization process is recorded using a high-speed camera paired with a uniform light source along with a pressure transducer and thermocouple. Image analysis allows for the determination of the bubble point pressure and rate of bubble formation. For CO2 in its gaseous state and at moderate temperatures, decent agreement between experimental results and the theoretical bubble point pressure is found, although significant deviations are observed at elevated temperatures. More pronounced differences in bubble point are observed for mixtures starting out at high pressures where CO2 is a supercritical fluid, which lead to lower than expected bubble point pressures.

Gas production from marine gas hydrate reservoirs using geothermal-assisted depressurization method

Natural gas production from marine gas hydrate reservoirs has become attractive to the oil and gas industry in recent years. It is still a great challenge to recover natural gas from hydrate reservoirs efficiently mainly due to sand production and wellbore collapse problems associated with the production scheme of depressurization. The thermal recovery method has not been proven economical due to the high cost of energy consumption. This study focuses on using geothermal energy to assist the depressurization process so that well pressure drawdown can be reduced and thus sand production and wellbore collapse problems can be mitigated. The authors investigated the transfer of heat energy from a natural geothermal zone to a marine gas hydrate reservoir and its effect on gas well productivity using analytical models. The result of our investigation shows that the initial well productivity can be significantly improved using geothermal energy more than 10-fold. This work provides engineers with an analytical tool for the feasibility analysis of using geothermal energy to improve well performance in gas hydrate reservoirs. Cited as: Mahmood, M. N., Guo, B. Gas production from marine gas hydrate reservoirs using geothermal-assisted depressurization method. Advances in Geo-Energy Research, 2023, 7(2): 90-98. https://doi.org/10.46690/ager.2023.02.03

Fractal Analytical Solutions for Nonlinear Two-Phase Flow in Discontinuous Shale Gas Reservoir

The paths of a two-phase flow are usually non-linear and discontinuous in the production of shale gas development. To research the influence mechanism between shale gas and water, several integer two-phase flow models have been studied but few analytical solutions have been obtained on shale gas and water pressure. This study first developed a local fractional mathematical model for gas and water two-phase flow in shale gas production. The model thus created considers the effects of capillary pressure, the fractal dimension of the flow pipe, and the discontinuity of the flow path. Second, the local fractional traveling wave method and variational iteration method were applied to this model for the development of iterative analytical solutions. Both shale gas and water pressure were analytically derived. Third, the depressurization process of the shale gas and water was analyzed, and a parametric study was conducted to explore the impacts of fractional dimension, entry capillary pressure, and travel wave velocity on shale gas pressure. Finally, our conclusions are drawn, based on the results of these studies.

Heat utilization efficiency analysis of gas production from hydrate reservoir by depressurization in conjunction with heat stimulation

The heat utilization efficiency of heat injection is a key factor for the available heat stimulation method in natural gas hydrate, and the heat transfer characteristics during natural gas production from hydrate reservoir require further analysis. In this work, based on the laboratory tests by the Pilot Hydrate Stimulator (PHS), the heat transfer process of hydrate decomposition in the PHS was investigated, and the real-time decomposition rate and the energy utilization efficiency of the hydrate decomposition were quantitatively studied. The experimental results showed that the energy consumption of hydrate decomposition was mainly obtained from the sensible heat hydrate bearing sediments and the heat transfer from the environment in the depressurization process. In the initial stage of heat injection, the energy consumption of hydrate decomposition mainly came from the heat contained in the injected hot water and the heat transfer through the boundaries. In the later stage of heat injection, all of the energy consumption for hydrate decomposition was obtained from heat injection. In this work, the heat transfer coefficient between the reservoir and the environment was first proposed to quantify the heat transfer through the boundary. The results showed that the heat transfer coefficient presented large fluctuations during the initial stage of the depressurization and the period when the reservoir temperature reached the environment temperature in the heat injection process. Meanwhile, the coefficient remained stable at the later stage of heat injection. These results indicated that the change in gas-water hydrate inside the reservoir and the change in reservoir temperature affected the heat transfer coefficient between the reservoir and the environment. Furthermore, the energy utilization efficiency and gas production rate were optimized for hydrate production when the reservoir temperature was controlled within the range close to the environmental temperature. These results may be of great significance for heat injection control during field production from hydrate reservoirs.

Investigation of thermal performance in dual‐fuel units

AbstractNumerical studies were carried out to investigate the thermal performance of the muzzle flow with strong shock discontinuities and gas–gas heat transfer mechanism in the base bleed unit (BBU) of an aerodynamic vehicle under the combined effects of muzzle flow and depressurization process. The process was modeled using the Navier–Stokes equations with chemical kinetics and interior ballistic equations. The results show that the muzzle phenomena with large temperature gradients and different wave structures appear after the supersonic vehicle enters into the free‐flight stage. The BBU also experiences a strong depressurization process with the release of high‐temperature propellant gas at 2200 K. However, the igniter combustion gas at a higher temperature of 3050 K in the BBU develops downstream and upstream repeatedly under the combined effects of the base wave and depressurization process, which makes the energy conversion process in the unit more complicated. With the projectile leaving the control of the muzzle flow, the BBU is full of the igniter combustion gas and the energy in the unit is uniformly distributed gradually. The high‐temperature area, at approximately 2750 K, propagates downstream along the axial and radial directions simultaneously until it finally dominates the entire unit. The maximum temperature of the combustion gas acting on the propellant surface is always 2740 K and the maximum heat flux remains approximately 1250 W/cm2 at later periods.

Visual study on methane hydrate formation and depressurization-induced methane hydrate dissociation processes in a micro-packed bed reactor

Natural gas hydrate has become one of the most promising green alterative energies. The safe and efficient exploitation of natural gas hydrate is of importance. Depressurization method has been considered as one of the most feasible hydrate exploitation approaches. The methane hydrate dissociation by depressurization has been widely investigated by using a variety of reactors or simulators. However, the micro-packed bed reactor has been rarely used for the hydrate-related studies. The packing materials in the micro-packed bed reactor are useful for simulating the porous condition of sediments. The optical transparency of the microreactor can provide the direct visualization of phase behavior, morphology, interfacial phenomenon and fluid migration in pore spaces. Thus, a high-pressure transparent micro-packed bed reactor recently developed was used to investigate the methane hydrate formation and depressurization-induced methane hydrate dissociation behaviors in porous media in this study. The hydrate formation results identify the different patterns of methane hydrate formation caused by the various methane bubble dispersion conditions, such as size and distribution in pores. The depressurization-induced methane hydrate dissociation behavior was investigated by continuously extracting fluids after the methane hydrate formation in the micro-packed bed reactor. The hydrate reformation phenomenon was observed during the early stage of depressurization process. This was triggered by the methane escaping out of the collapsed hydrate shell. The impact of fluid extraction rate of depressurization was studied. The results indicate higher fluid extraction rate leading to more severe hydrate shell collapsing and significant gas migration, resulting in more significant methane hydrate reformation. The hydrate dissociation process and dramatic fluid migration phenomena in the pore spaces were also visualized during the later stage of depressurization.

Experimental evaluation of enhanced oil recovery in unconventional reservoirs using cyclic hydrocarbon gas injection

The significant amount of gas produced from unconventional reservoirs in the US in recent years has reduced market prices for hydrocarbon gas and created an opportunity for using low-cost gas in enhanced oil recovery (EOR) projects. In addition to its availability in large quantities and at low cost, injecting produced gas back into the reservoir prevents flaring the gas, reducing environmental concerns associated with greenhouse gas emissions to the atmosphere. Furthermore, while CO2 has shown tremendous success as an EOR injectant in huff-and-puff operations in tight liquid-rich shales, the lack of sufficient pipeline infrastructure supplying low-cost CO2 as well as operational issues (e.g., corrosion) has forced field operators to consider other gas options, such as produced gas, to substitute for or mix with CO2.While limited, experimental and numerical studies conducted on produced gas huff-and-puff in unconventional resources have shown promising results and highlighted the great potential of this technique for enhanced oil recovery from ultra-tight shale resources. The objective of the current work is to investigate the feasibility of produced gas huff-and-puff operations in one of the major oil-producing shale formations in North America further by performing several flow experiments using various mixtures of hydrocarbon gases. Three reservoir-condition core-flooding experiments are carefully designed and executed on shale core plugs in the absence and presence of proppant-filled fractures. After establishing initial two-phase brine-oil conditions in the core samples, hydrocarbon gas is injected into the medium through a huff-and-puff approach. The cycle of injection-soaking-production is repeated multiple times in each experiment. The results show that the use of hydrocarbon gas in huff-and-puff operations has great potential similar to CO2 for enhanced oil recovery from tight shale rocks and can lead to remarkable oil recovery. The hydrocarbon gas mixture in the fracture penetrates the matrix due to pressure and concentration gradients during the injection and soaking periods and forces the oil to leave the matrix during the depressurization process. It is observed that the rate of oil production is high in the first cycle and decreases in subsequent cycles as the rate of mass transfer and the amount of oil remaining in the matrix decrease. The average oil production due to the cyclic gas injection process in an intact and fractured composite rock sample was more than 48 and 21%, respectively. The results show that even though the ultratight formations could be heavily fractured, a proper cyclic gas injection EOR scheme could potentially improve oil recovery from the shale oil reservoirs, significantly.

Implication of garnet zoning in high pressure- ultrahighpressure eclogite from Changning-Menglian suture zone, Bangbing area, Southeast Tibetan Plateau

Major and trace element zonation patterns were determined in ultrahigh-pressure eclogite garnets from the Changning-Menglian suture, Bangbing area. Several stages of eclogite metamorphism are recorded by garnet zonation. Cores of garnets are generally darker and inclusion-rich, while rims are brighter and inclusion-poor. Garnets were analyzed for the major element by EPMA and trace element by LA-ICP-MS. The results show that both major elements and rare earth elements in garnet have typical compositional bands. The major element zonation recorded the process of increasing temperature and pressure and the metamorphism of the eclogite from the core to rim, and the rare earth element zonation recorded different mineral chemical reactions during the formation of the garnet and its three stages of growth. Eclogites completely imply the process of rapid exhumation, cooling, and depressurization after short-term peak metamorphism. The contents of some trace elements (such as Y and Sr) in garnet are controlled by major elements and mineral combinations. The formation of new minerals in the process of retrograde metamorphism affects the distribution of trace elements.

Impact of Coal Orthotropic and Hydraulic Fracture on Pressure Distribution in Coalbed Methane Reservoirs

Coalbed methane (CBM) shows tremendous in situ reserves, attracting a great deal of research interests around the world. The efficient development of CBM is closely related to the dynamic pressure distribution characteristics in the coal seam. As the dominant component of the geological reserve for CBM, the adsorption-state gas will not be exploited until the local coal pressure becomes less than the critical desorption pressure. Therefore, although the CBM reserve is fairly large, the production performance is generally limited, with a poor understanding of the dynamic pressure field during the CBM production. In this work, in order to address this issue properly, the coal’s inherent properties, the coal’s orthotropic features, as well as artificial hydraulic fracturing are considered, all of which affect pressure propagation in the coal seam. Notably, to the current knowledge, the impact of coal’s orthotropic features has received little attention, while the coal’s orthotropic features are formed during a fairly long geological evolution, changing the dynamic pressure field a lot. Numerical simulation is performed to shed light on the pressure propagation behavior. The results show that (a) coal’s orthotropic features mitigate the depressurization process of CBM development; (b) the increasing length of a hydraulic fracture is helpful for efficient decline in the average formation pressure; and (c) there exists an optimal layout mode for multi-well locations to minimize the average pressure. This article provides an in-depth analysis upon pressure distribution in CBM reservoirs under impacts of coal orthotropic feature and hydraulic fractures.