A large amount of fracturing fluid in hydraulic fracturing is imbibed into the shale fracture/matrix,which leads to significant uncertainty in gas recovery evaluation.The mechanism of imbibition impact on the gasewate...A large amount of fracturing fluid in hydraulic fracturing is imbibed into the shale fracture/matrix,which leads to significant uncertainty in gas recovery evaluation.The mechanism of imbibition impact on the gasewater flow is not well understood.In this study,systematic comparative experiments are carried out to simulate imbibition in fractured shale samples obtained from the Wufeng-Longmaxi shale reservoirs in China,and the imbibition effect in the fractureematrix system is qualitatively and quantitatively investigated.Nine cores are collected to measure their porosity and permeability using a helium porosimeter and nitrogen pulseedecay tests.Gas/liquid single-phase flow experiments are then carried out using methane and KCl solution,respectively.Subsequently,dynamic imbibition experiments are carried out on three samples in a visual cell.The gasewater interfacial tension,water imbibition amount,and displacement velocity are recorded.A single-phase gas/liquid flow test shows a high linear correlation between the fluid displacement velocity and pressure gradient in the fractured samples as the fracture is the main flow channel,dominantly determining the flow behavior.Moreover,the capillary force was introduced in the cross-flow term of the triple-medium model to characterize the imbibition effect,and a two-phase flow simulation model considering the fracturing fluid imbibition retention was developed,and the two-phase flow behavior by considering the imbibition effect of the fracturing fluid retention in the shale gas reservoir was analyzed.Valuable experiment data in this work are provided,which can be used to validate analytical equations for gas/water flow in the shale fractureematrix system.展开更多
Obtaining a comprehensive understanding of solute transport in fractured rocks is crucial for various geoengineering applications,including waste disposal and construction of geo-energy infrastructure.It was realized ...Obtaining a comprehensive understanding of solute transport in fractured rocks is crucial for various geoengineering applications,including waste disposal and construction of geo-energy infrastructure.It was realized that solute transport in fractured rocks is con-trolled by stochastic discrete fracture-matrix systems.However,the impacts and specific uncertainty caused by fracture network struc-tures on solute transport in discrete fracture-matrix systems have yet not been fully understood.In this article,we aim to investigate the influence of fracture network structure on solute transport in stochastic discrete fracture-matrix systems.The fluid flow and solute trans-port are simulated using a three-dimensional discrete fracture matrix model with considering various values of fracture density and size(i.e.,radius).The obtained results reveal that as the fracture density or minimum fracture radius increases,the corresponding fluid flow and solute transport channels increase,and the solute concentration distribution range expands in the matrix.This phenomenon,attrib-uted to the enhanced connectivity of the fracture network,leads to a rise in the effluent solute concentration mean value from 0.422 to 0.704,or from 0.496 to 0.689.Furthermore,when solute transport reached a steady state,the coefficient of variation of effluent concen-tration decreases with the increasing fracture density or minimum fracture radius in different scenarios,indicating an improvement in the homogeneity of solute transport results.The presented analysis results of solute transport in stochastic discrete fracture-matrix systems can be helpful for uncertainty management in the geological disposal of high-level radioactive waste.展开更多
Due to severe mass transfer limitations,the remediation efficiency of low-permeability contaminated sites often fails to meet expectations.Hydraulic fracturing technology has been utilized to enhance amendment deliver...Due to severe mass transfer limitations,the remediation efficiency of low-permeability contaminated sites often fails to meet expectations.Hydraulic fracturing technology has been utilized to enhance amendment delivery,but the influence of soil heterogeneity is commonly overlooked.To address this issue,this study develops a numerical model to simulate the enhanced transport of amendments,incorporating convection,diffusion,adsorption,and degradation processes.Within the model,random permeability fields are generated based on geostatistical methods to explore how soil heterogeneity affects amendment injection efficiency,distribution characteristics,and the underlying physical mechanisms.The results indicate that(1)soil heterogeneity significantly reduces the amendment injection efficiency,with stronger heterogeneity correlating to lower efficiency,(2)soil heterogeneity markedly alters the amendment distribution characteristics,leading to the formation of localized“nodes”,(3)the mechanism by which heterogeneity reduces injection efficiency involves decreasing the density of preferential flow paths in the soil,and(4)the adverse effects of heterogeneity can be mitigated by employing pressure compensation or adjusting well spacing.展开更多
Heat production from geothermal reservoirs is a typical heat transfer process involving a cold working fluid contacting a hot rock formation.Compared to the thermal-physical characteristics of water,supercritical CO_(...Heat production from geothermal reservoirs is a typical heat transfer process involving a cold working fluid contacting a hot rock formation.Compared to the thermal-physical characteristics of water,supercritical CO_(2)(scCO_(2))has a higher heat storage capacity over a wide temperature-pressure range and may be favored as a heat transfer fluid.Singularly characteristic of scCO_(2)-based heat extraction is that the hydraulic-thermal properties of the scCO_(2) vary dramatically and dynamically with the spatial pressure gradient during unsteady-state flow along fracture.This highly nonlinear behavior presents a challenge in the accurate estimation of heat extraction efficiency in scCO_(2)-based EGS.In this paper,a thermal-h ydraulic-mechanical(THM)coupled model is developed by considering deformation of the fractured reservoir,non-Darcy flow and the varying thermal-physical properties of scCO_(2).The proposed model is validated by matching the modeling temperature distribution with published data.The results show that during continuous injection of scCO_(2),the fracture first widens and then narrows,ultimately reopening over the long term.The sequential fracture deformation behaviors are in response to the combined impacts of mechanical compression and thermally-induced deformation.By controlling the injection parameters of the scCO_(2),it is found that the heat extraction rate is positively correlated to its pore pressure or mass flow rate.The heat extraction rate can be significantly enhanced,when the inlet temperature of scCO_(2) is below its critical temperature.As a result,the heat increment recovered per unit mass of scCO_(2) decreases as the hot rock is gradually cooled.Meanwhile,the heat increment recovered per unit mass of scCO_(2) decreases by increasing the inlet temperature of scCO_(2) or its mass flow rate,but increases as the outlet pressure rises.Furthermore,multi-linear regression indicates that controlling the inlet temperature of the scCO_(2) can significantly improve the thermodynamic efficiency of heat extraction.展开更多
Caprock is a water-saturated formation with a sufficient entry capillary pressure to prevent the upward migration of a buoyant fluid. When the entry capillary pressure of caprock is smaller than the pressure exerted b...Caprock is a water-saturated formation with a sufficient entry capillary pressure to prevent the upward migration of a buoyant fluid. When the entry capillary pressure of caprock is smaller than the pressure exerted by the buoyant CO2plume, CO2gradually penetrates into the caprock. The CO2penetration depth into a caprock layer can be used to measure the caprock sealing efficiency and becomes the key issue to the assessment of caprock sealing efficiency. On the other hand, our numerical simulations on a caprock layer have revealed that a square root law for time and pore pressure exists for the CO2penetration into the caprock layer. Based on this finding, this study proposes a simple approach to estimate the CO2penetration depth into a caprock layer. This simple approach is initially developed to consider the speed of CO2invading front. It explicitly expresses the penetration depth with pressuring time, pressure difference and pressure magnitude. This simple approach is then used to fit three sets of experimental data and good fittings are observed regardless of pressures, strengths of porous media, and pore fluids(water,hydrochloric acid, and carbonic acid). Finally, theoretical analyses are conducted to explore those factors affecting CO2penetration depth. The effects of capillary pressure, gas sorption induced swelling, and fluid property are then included in this simple approach. These results show that this simple approach can predict the penetration depth into a caprock layer with sufficient accuracy, even if complicated interactions in penetration process are not explicitly expressed in this simple formula.展开更多
Reservoir damage caused by guar gum fracturing fluid and slick water seriously affects the subsequent oil and gas production. However, the invasion characteristics and retention mechanisms of fracturing fluids in the ...Reservoir damage caused by guar gum fracturing fluid and slick water seriously affects the subsequent oil and gas production. However, the invasion characteristics and retention mechanisms of fracturing fluids in the fracture-matrix zone are still unclear. In this work, a microscopic model reflecting the characteristics of the fracture-matrix zone was designed. Based on the microfluidic experimental method, the process of fracturing fluid invasion, flowback and retention in the fracture-matrix zone was investigated visually and characterized quantitatively. The factors and mechanisms affecting fracturing fluid retention in the fracture-matrix zone were analyzed and clarified. The results indicated that in the invasion process, the frontal swept range of slick water was larger than that of the guar gum fracturing fluid, and the oil displacement efficiency and damage rate were lower than those of the guar gum fracturing fluid under the same invasion pressure. With the increase in invasion pressure, the damage rate of slick water increased from 61.09% to 82.77%, and that of the guar gum fracturing fluid decreased from 93.45% to83.36%. Before subsequent oil production, the invaded fracturing fluid was mainly concentrated in the medium-high permeability area of the fracture-matrix zone. The main resistance of slick water was capillary force, while that of the guar fracturing fluid was mainly viscous resistance. The fracturing fluid retention was most serious in the low permeability region and the region near the end of the fracture.The experimental and numerical simulation results showed that increasing the production pressure difference could improve the velocity field distribution of the fracture-matrix zone, increase the flowback swept range and finally reduce the retention rate of the fracture fluid. The retention mechanisms of slick water in the fracture-matrix zone include emulsion retention and flow field retention, while those of the guar gum fracturing fluid include viscous retention and flow field retention. Emulsion retention is caused by capillary force and flow interception effect. Viscous retention is caused by the viscous resistance of polymer, while flow-field retention is caused by uneven distribution of flowback velocity.展开更多
基金supported by Open Fund of State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development(G5800-18-ZS-KFGY010)。
文摘A large amount of fracturing fluid in hydraulic fracturing is imbibed into the shale fracture/matrix,which leads to significant uncertainty in gas recovery evaluation.The mechanism of imbibition impact on the gasewater flow is not well understood.In this study,systematic comparative experiments are carried out to simulate imbibition in fractured shale samples obtained from the Wufeng-Longmaxi shale reservoirs in China,and the imbibition effect in the fractureematrix system is qualitatively and quantitatively investigated.Nine cores are collected to measure their porosity and permeability using a helium porosimeter and nitrogen pulseedecay tests.Gas/liquid single-phase flow experiments are then carried out using methane and KCl solution,respectively.Subsequently,dynamic imbibition experiments are carried out on three samples in a visual cell.The gasewater interfacial tension,water imbibition amount,and displacement velocity are recorded.A single-phase gas/liquid flow test shows a high linear correlation between the fluid displacement velocity and pressure gradient in the fractured samples as the fracture is the main flow channel,dominantly determining the flow behavior.Moreover,the capillary force was introduced in the cross-flow term of the triple-medium model to characterize the imbibition effect,and a two-phase flow simulation model considering the fracturing fluid imbibition retention was developed,and the two-phase flow behavior by considering the imbibition effect of the fracturing fluid retention in the shale gas reservoir was analyzed.Valuable experiment data in this work are provided,which can be used to validate analytical equations for gas/water flow in the shale fractureematrix system.
基金support from research grants provided by the National Natural Science Foundation of China(Grant Nos.42302303 and 42277128)the Zhejiang Provincial Natural Science Foundation of China(Grant No.ZCLQ24D0201).
文摘Obtaining a comprehensive understanding of solute transport in fractured rocks is crucial for various geoengineering applications,including waste disposal and construction of geo-energy infrastructure.It was realized that solute transport in fractured rocks is con-trolled by stochastic discrete fracture-matrix systems.However,the impacts and specific uncertainty caused by fracture network struc-tures on solute transport in discrete fracture-matrix systems have yet not been fully understood.In this article,we aim to investigate the influence of fracture network structure on solute transport in stochastic discrete fracture-matrix systems.The fluid flow and solute trans-port are simulated using a three-dimensional discrete fracture matrix model with considering various values of fracture density and size(i.e.,radius).The obtained results reveal that as the fracture density or minimum fracture radius increases,the corresponding fluid flow and solute transport channels increase,and the solute concentration distribution range expands in the matrix.This phenomenon,attrib-uted to the enhanced connectivity of the fracture network,leads to a rise in the effluent solute concentration mean value from 0.422 to 0.704,or from 0.496 to 0.689.Furthermore,when solute transport reached a steady state,the coefficient of variation of effluent concen-tration decreases with the increasing fracture density or minimum fracture radius in different scenarios,indicating an improvement in the homogeneity of solute transport results.The presented analysis results of solute transport in stochastic discrete fracture-matrix systems can be helpful for uncertainty management in the geological disposal of high-level radioactive waste.
基金supported by the National Natural Science Foundation of China(Grant Nos.42227804 and 42402279)the Natural Science Foundation of Shanghai(Grant No.24ZR1467500).
文摘Due to severe mass transfer limitations,the remediation efficiency of low-permeability contaminated sites often fails to meet expectations.Hydraulic fracturing technology has been utilized to enhance amendment delivery,but the influence of soil heterogeneity is commonly overlooked.To address this issue,this study develops a numerical model to simulate the enhanced transport of amendments,incorporating convection,diffusion,adsorption,and degradation processes.Within the model,random permeability fields are generated based on geostatistical methods to explore how soil heterogeneity affects amendment injection efficiency,distribution characteristics,and the underlying physical mechanisms.The results indicate that(1)soil heterogeneity significantly reduces the amendment injection efficiency,with stronger heterogeneity correlating to lower efficiency,(2)soil heterogeneity markedly alters the amendment distribution characteristics,leading to the formation of localized“nodes”,(3)the mechanism by which heterogeneity reduces injection efficiency involves decreasing the density of preferential flow paths in the soil,and(4)the adverse effects of heterogeneity can be mitigated by employing pressure compensation or adjusting well spacing.
基金The financial support from the National Natural Science Foundation of China(Nos.41772154 and 42102338)Natural Science Foundation of Shandong Province(Nos.ZR2019MA009 and ZR2020QE115)SDUST Research Fund of China(No.2018TDJH102)。
文摘Heat production from geothermal reservoirs is a typical heat transfer process involving a cold working fluid contacting a hot rock formation.Compared to the thermal-physical characteristics of water,supercritical CO_(2)(scCO_(2))has a higher heat storage capacity over a wide temperature-pressure range and may be favored as a heat transfer fluid.Singularly characteristic of scCO_(2)-based heat extraction is that the hydraulic-thermal properties of the scCO_(2) vary dramatically and dynamically with the spatial pressure gradient during unsteady-state flow along fracture.This highly nonlinear behavior presents a challenge in the accurate estimation of heat extraction efficiency in scCO_(2)-based EGS.In this paper,a thermal-h ydraulic-mechanical(THM)coupled model is developed by considering deformation of the fractured reservoir,non-Darcy flow and the varying thermal-physical properties of scCO_(2).The proposed model is validated by matching the modeling temperature distribution with published data.The results show that during continuous injection of scCO_(2),the fracture first widens and then narrows,ultimately reopening over the long term.The sequential fracture deformation behaviors are in response to the combined impacts of mechanical compression and thermally-induced deformation.By controlling the injection parameters of the scCO_(2),it is found that the heat extraction rate is positively correlated to its pore pressure or mass flow rate.The heat extraction rate can be significantly enhanced,when the inlet temperature of scCO_(2) is below its critical temperature.As a result,the heat increment recovered per unit mass of scCO_(2) decreases as the hot rock is gradually cooled.Meanwhile,the heat increment recovered per unit mass of scCO_(2) decreases by increasing the inlet temperature of scCO_(2) or its mass flow rate,but increases as the outlet pressure rises.Furthermore,multi-linear regression indicates that controlling the inlet temperature of the scCO_(2) can significantly improve the thermodynamic efficiency of heat extraction.
基金the financial support from the Creative Research and Development Group Program of Jiangsu Province(2014-27)the National Science Fund for Distinguished Young Scholars(Grant No.51125017)the Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD2014)
文摘Caprock is a water-saturated formation with a sufficient entry capillary pressure to prevent the upward migration of a buoyant fluid. When the entry capillary pressure of caprock is smaller than the pressure exerted by the buoyant CO2plume, CO2gradually penetrates into the caprock. The CO2penetration depth into a caprock layer can be used to measure the caprock sealing efficiency and becomes the key issue to the assessment of caprock sealing efficiency. On the other hand, our numerical simulations on a caprock layer have revealed that a square root law for time and pore pressure exists for the CO2penetration into the caprock layer. Based on this finding, this study proposes a simple approach to estimate the CO2penetration depth into a caprock layer. This simple approach is initially developed to consider the speed of CO2invading front. It explicitly expresses the penetration depth with pressuring time, pressure difference and pressure magnitude. This simple approach is then used to fit three sets of experimental data and good fittings are observed regardless of pressures, strengths of porous media, and pore fluids(water,hydrochloric acid, and carbonic acid). Finally, theoretical analyses are conducted to explore those factors affecting CO2penetration depth. The effects of capillary pressure, gas sorption induced swelling, and fluid property are then included in this simple approach. These results show that this simple approach can predict the penetration depth into a caprock layer with sufficient accuracy, even if complicated interactions in penetration process are not explicitly expressed in this simple formula.
基金supported by the National Natural Science Foundation of China (No. 51874330, 51974341)the Fundamental Research Funds for the Central Universities (No. 20CX06070A)the Opening Fund of Shandong Key Laboratory of Oilfield Chemistry and the Fundamental Research Funds for the Central Universities(No. 19CX05006A)。
文摘Reservoir damage caused by guar gum fracturing fluid and slick water seriously affects the subsequent oil and gas production. However, the invasion characteristics and retention mechanisms of fracturing fluids in the fracture-matrix zone are still unclear. In this work, a microscopic model reflecting the characteristics of the fracture-matrix zone was designed. Based on the microfluidic experimental method, the process of fracturing fluid invasion, flowback and retention in the fracture-matrix zone was investigated visually and characterized quantitatively. The factors and mechanisms affecting fracturing fluid retention in the fracture-matrix zone were analyzed and clarified. The results indicated that in the invasion process, the frontal swept range of slick water was larger than that of the guar gum fracturing fluid, and the oil displacement efficiency and damage rate were lower than those of the guar gum fracturing fluid under the same invasion pressure. With the increase in invasion pressure, the damage rate of slick water increased from 61.09% to 82.77%, and that of the guar gum fracturing fluid decreased from 93.45% to83.36%. Before subsequent oil production, the invaded fracturing fluid was mainly concentrated in the medium-high permeability area of the fracture-matrix zone. The main resistance of slick water was capillary force, while that of the guar fracturing fluid was mainly viscous resistance. The fracturing fluid retention was most serious in the low permeability region and the region near the end of the fracture.The experimental and numerical simulation results showed that increasing the production pressure difference could improve the velocity field distribution of the fracture-matrix zone, increase the flowback swept range and finally reduce the retention rate of the fracture fluid. The retention mechanisms of slick water in the fracture-matrix zone include emulsion retention and flow field retention, while those of the guar gum fracturing fluid include viscous retention and flow field retention. Emulsion retention is caused by capillary force and flow interception effect. Viscous retention is caused by the viscous resistance of polymer, while flow-field retention is caused by uneven distribution of flowback velocity.
