摘要 :
The pore sizes of unconventional reservoir rock, such as shale and tight rock, are on the order of nanometers. The thermodynamic properties of in-situ hydrocarbon mixtures in such small pores are significantly different from those...
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The pore sizes of unconventional reservoir rock, such as shale and tight rock, are on the order of nanometers. The thermodynamic properties of in-situ hydrocarbon mixtures in such small pores are significantly different from those of fluids in bulk size, primarily due to effect of large capillary pressure. For example, it has been recognized that the phase envelop shifts and bubble-point pressure is suppressed in tight and shale oil reservoirs. On the other hand, the stress-dependency is pronounced in low permeability rocks. It has been observed that pore sizes, especially the sizes of pore-throats, are subject to decrease due to rock deformation induced by the fluid depletion from over-pressurized tight and shale reservoirs. This reduction on pore spaces again affects the capillary pressure and therefore thermodynamic properties of reservoir fluids. Thus it is necessary to model the effect of stress-dependent capillary pressure and rock deformation on tight and shale reservoirs. In this paper, we propose and develop a multiphase, multidimensional compositional reservoir model to capture the effect of large capillary pressure on flow and transport in stress-sensitive unconventional reservoirs. The vapor-liquid equilibrium (VLE) calculation is performed with Peng-Robinson Equation of State (EOS), including the impact of capillary pressure on phase behavior and thermodynamic properties. The fluid flow is fully coupled with geomechanical model, which is derived from the thermo-poroelasticity theory; mean normal stress as the stress variable is solved simultaneously with mass conservation equations. The finite-volume based numerical method, integrated finite difference method, is used for space discretization for both mass conservation and stress equations. The formulations are solved fully implicitly to assure the stability. We use Eagle Ford tight oil formations as an example to demonstrate the effect of capillary pressure on VLE. It shows that the bubble-point pressure is suppressed within nano-pores, and fluid properties, such as oil density and viscosity, are influenced by the suppression due to more light components remained in liquid phase. In order to illustrate the effect of stress-dependent capillary pressure on tight oil flow and production, we perform numerical studies on Bakken tight oil reservoirs. The simulation results show that bubble-point suppression is exaggerated by effects of rock deformation, and capillary pressure on VLE also affects the reservoir pressure and effective stress. Therefore the interactive effects between capillary pressure and rock deformation are observed in numerical results. Finally, the production performance in the simulation examples demonstrates the large effect of large capillary pressure on estimated ultimate recovery (EUR) in stress-sensitive tight reservoirs.
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摘要 :
The pore sizes of unconventional reservoir rock, such as shale and tight rock, are on the order of nanometers. The thermodynamic properties of in-situ hydrocarbon mixtures in such small pores are significantly different from those...
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The pore sizes of unconventional reservoir rock, such as shale and tight rock, are on the order of nanometers. The thermodynamic properties of in-situ hydrocarbon mixtures in such small pores are significantly different from those of fluids in bulk size, primarily due to effect of large capillary pressure. For example, it has been recognized that the phase envelop shifts and bubble-point pressure is suppressed in tight and shale oil reservoirs. On the other hand, the stress-dependency is pronounced in low permeability rocks. It has been observed that pore sizes, especially the sizes of pore-throats, are subject to decrease due to rock deformation induced by the fluid depletion from over-pressurized tight and shale reservoirs. This reduction on pore spaces again affects the capillary pressure and therefore thermodynamic properties of reservoir fluids. Thus it is necessary to model the effect of stress-dependent capillary pressure and rock deformation on tight and shale reservoirs. In this paper, we propose and develop a multiphase, multidimensional compositional reservoir model to capture the effect of large capillary pressure on flow and transport in stress-sensitive unconventional reservoirs. The vapor-liquid equilibrium (VLE) calculation is performed with Peng-Robinson Equation of State (EOS), including the impact of capillary pressure on phase behavior and thermodynamic properties. The fluid flow is fully coupled with geomechanical model, which is derived from the thermo-poroelasticity theory; mean normal stress as the stress variable is solved simultaneously with mass conservation equations. The finite-volume based numerical method, integrated finite difference method, is used for space discretization for both mass conservation and stress equations. The formulations are solved fully implicitly to assure the stability. We use Eagle Ford tight oil formations as an example to demonstrate the effect of capillary pressure on VLE. It shows that the bubble-point pressure is suppressed within nano-pores, and fluid properties, such as oil density and viscosity, are influenced by the suppression due to more light components remained in liquid phase. In order to illustrate the effect of stress-dependent capillary pressure on tight oil flow and production, we perform numerical studies on Bakken tight oil reservoirs. The simulation results show that bubble-point suppression is exaggerated by effects of rock deformation, and capillary pressure on VLE also affects the reservoir pressure and effective stress. Therefore the interactive effects between capillary pressure and rock deformation are observed in numerical results. Finally, the production performance in the simulation examples demonstrates the large effect of large capillary pressure on estimated ultimate recovery (EUR) in stress-sensitive tight reservoirs.
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摘要 :
Gas injection has become the top choice for IOR/EOR pilots in tight oil reservoirs because of its high injectivity. The effects of nanoconfinement and geomechanics are generally considered as non-negligible, but its coupled effect...
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Gas injection has become the top choice for IOR/EOR pilots in tight oil reservoirs because of its high injectivity. The effects of nanoconfinement and geomechanics are generally considered as non-negligible, but its coupled effects and resulting flow and displacement are still not well understood for gas injection. We hence present a general compositional model and simulator to investigate the complicated multiphase and multicomponent behaviors during gas injection in tight oil reservoirs. This compositional model is able to account for vital physics in unconventional reservoirs, including nanopore confinement, molecular diffusion, rock-compaction, and non-Darcy flow. The MINC method is implemented to handle fractured media. The nanopore confinement effect is modeled by including capillarity in VLE calculations. The rock compaction effect is represented by solving the mean stress from a governing geomechanical equation which is fully coupled with the mass balance equations to ensure the numerical stability as well as a physically correct solution. The equations are discretized with integral finite difference method and then solved numerically by Newton's method. The simulator is validated against a commercial compositional software (CMG-GEM) before it is applied to simulate gas injection. Huff-n-puff with dry gas in Eagle Ford is investigated. The simulation result shows that if the reservoir pressure is much higher than the bubble point pressure, the nanopore confinement effect will have a minimal impact on the recovery factor (RF) for both the depletion and the first few cycles of gas huff-n-puff. Geomechanics is found to be an influencing factor on RF but not always in a detrimental way, as enhanced rock compaction drive could offset the reduction of permeability in certain scenarios. Gas huff-n-puff would improve the RF of each component compared with the depletion. The heavy component would first have a higher recovery than the light component at the first few cycles of huff-n-puff, but its RF will be outpaced by the light component when the gas saturation in the matrix surpasses the critical gas saturation. Lastly, considering the nanopore confinement effects would slightly reduce the RF of the light component but increase the RF of the heavy component after huff-n-puff when combined with the critical gas saturation effect in the matrix. This study presents a 3D multiphase, multicomponent simulator which is a practical tool for accurately modeling of primary depletion as well as gas injection IOR/EOR processes in unconventional oil reservoirs.This simulator is not only of great importance for assisting researchers to understand complex multiphase and multicomponent behaviors in tight oil production but also of great use for engineers to optimize gas injection parameters in field applications.
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摘要 :
The pore sizes of unconventional reservoir rocks,such as shale and tight rocks,are on the order of nanometers.The thermodynamic phase behavior of in-situ hydrocarbon mixtures in such small pores is significantly different from tha...
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The pore sizes of unconventional reservoir rocks,such as shale and tight rocks,are on the order of nanometers.The thermodynamic phase behavior of in-situ hydrocarbon mixtures in such small pores is significantly different from that of bulk fluids in the PVT cells,primarily due to effect of large capillary pressure.For example,it has been recognized that the phase envelop shifts and bubble point pressure is suppressed under subsurface condition in tight oil reservoirs.On the other hand,it has been observed that the pore sizes,especially the sizes of pore-throats,are subject to change due to rock deformation induced by the fluid depletion from over-pressurized unconventional reservoirs.As the fluids are being produced from the pore space,the effective stress on reservoir rock increases,resulting in reduction of the pore and pore-throat sizes.This reduction on pore spaces again affects the fluid flow through impacts on the thermodynamic phase behavior,as well as stress induced changes in porosity and permeability.Thus a coupled flow-geomechanics model capturing in-situ reservoir phase behavior is in general necessary to model tight and shale reservoir performance. In this paper,we propose a multiphase,multidimensional compositional reservoir model,fully coupling fluid flow with geomechanics for tight and shale reservoirs.The fluid flow model is a compositional model,based on general mass conservation law for each component,incorporating both Darcy flow and molecular diffusions.The geomechanical model is derived from the thermo-poro-elasticity theory ex- tended to multiple porous and fractured media systems;mean normal stress as the stress variable is solved simultaneously with mass conservation equations.The vapor-liquid equilibrium(VLE)calculation is performed with Peng-Robinson Equation of State(EOS)including the effects of capillary pressure on phase behaviors.The finite-volume based numerical method,integrated finite difference method,is used for space discretization for both mass conservation and stress equations.The formulations are solved fully implicitly to assure the stability. This compositional model integrates key subsurface behaviors of unconventional shale reservoirs,such as rock compaction effect,stress-induced changes of rock properties,and stress-dependent capillary effects on VLE.We take the Eagle Ford tight oil as an example to illustrate the effects of stress-dependent capillary pressure on VLE and in-situ fluid properties.This model can be generally applied to both dew-point(gas condensate)and bubble-point(tight oil)systems of tight and shale reservoirs.Eventually it could improve the forecast accuracy for long-term production rate and recovery factors of unconven tional petroleum reservoirs.
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摘要 :
The pore sizes of unconventional reservoir rocks,such as shale and tight rocks,are on the order of nanometers.The thermodynamic phase behavior of in-situ hydrocarbon mixtures in such small pores is significantly different from tha...
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The pore sizes of unconventional reservoir rocks,such as shale and tight rocks,are on the order of nanometers.The thermodynamic phase behavior of in-situ hydrocarbon mixtures in such small pores is significantly different from that of bulk fluids in the PVT cells,primarily due to effect of large capillary pressure.For example,it has been recognized that the phase envelop shifts and bubble point pressure is suppressed under subsurface condition in tight oil reservoirs.On the other hand,it has been observed that the pore sizes,especially the sizes of pore-throats,are subject to change due to rock deformation induced by the fluid depletion from over-pressurized unconventional reservoirs.As the fluids are being produced from the pore space,the effective stress on reservoir rock increases,resulting in reduction of the pore and pore-throat sizes.This reduction on pore spaces again affects the fluid flow through impacts on the thermodynamic phase behavior,as well as stress induced changes in porosity and permeability.Thus a coupled flow-geomechanics model capturing in-situ reservoir phase behavior is in general necessary to model tight and shale reservoir performance. In this paper,we propose a multiphase,multidimensional compositional reservoir model,fully coupling fluid flow with geomechanics for tight and shale reservoirs.The fluid flow model is a compositional model,based on general mass conservation law for each component,incorporating both Darcy flow and molecular diffusions.The geomechanical model is derived from the thermo-poro-elasticity theory ex- tended to multiple porous and fractured media systems;mean normal stress as the stress variable is solved simultaneously with mass conservation equations.The vapor-liquid equilibrium(VLE)calculation is performed with Peng-Robinson Equation of State(EOS)including the effects of capillary pressure on phase behaviors.The finite-volume based numerical method,integrated finite difference method,is used for space discretization for both mass conservation and stress equations.The formulations are solved fully implicitly to assure the stability. This compositional model integrates key subsurface behaviors of unconventional shale reservoirs,such as rock compaction effect,stress-induced changes of rock properties,and stress-dependent capillary effects on VLE.We take the Eagle Ford tight oil as an example to illustrate the effects of stress-dependent capillary pressure on VLE and in-situ fluid properties.This model can be generally applied to both dew-point(gas condensate)and bubble-point(tight oil)systems of tight and shale reservoirs.Eventually it could improve the forecast accuracy for long-term production rate and recovery factors of unconven tional petroleum reservoirs.
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摘要 :
A sharp initial decline in the production rate is being experienced in many shale gas plays.One reason is the closure of natural micron and micro fractures.The natural fractures,which widely exist in the over-pressurized source ro...
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A sharp initial decline in the production rate is being experienced in many shale gas plays.One reason is the closure of natural micron and micro fractures.The natural fractures,which widely exist in the over-pressurized source rock,react sensitively to the change of subsurface stress.The change in stress can be caused by the decrease of gas pressure during production.These fracture closure or re-open phenomena have significant effects on reservoir permeability and gas production. In this paper,a fully coupled geomechanics and multiphase fluid flow model is presented to accurately simulate the fields of stress and fluid flow in shale gas reservoirs.Several relationships between fracture closure and applied stress are incorporated in this model,based on the experimental data from literatures. Therefore,the stress dependency of shale natural fractures is quantified and modeled in its full complexity. The natural fractures in this model are characterized as stiff,self-propped,and prone to closure.It represents an extension of our earlier“hybrid-fracture model”(DFN for hydraulic fractures,double- porosity for natural fractured domain inside the SRV,and single porosity or dual-continuum outside the SRV).
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摘要 :
A sharp initial decline in the production rate is being experienced in many shale gas plays.One reason is the closure of natural micron and micro fractures.The natural fractures,which widely exist in the over-pressurized source ro...
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A sharp initial decline in the production rate is being experienced in many shale gas plays.One reason is the closure of natural micron and micro fractures.The natural fractures,which widely exist in the over-pressurized source rock,react sensitively to the change of subsurface stress.The change in stress can be caused by the decrease of gas pressure during production.These fracture closure or re-open phenomena have significant effects on reservoir permeability and gas production. In this paper,a fully coupled geomechanics and multiphase fluid flow model is presented to accurately simulate the fields of stress and fluid flow in shale gas reservoirs.Several relationships between fracture closure and applied stress are incorporated in this model,based on the experimental data from literatures. Therefore,the stress dependency of shale natural fractures is quantified and modeled in its full complexity. The natural fractures in this model are characterized as stiff,self-propped,and prone to closure.It represents an extension of our earlier“hybrid-fracture model”(DFN for hydraulic fractures,double- porosity for natural fractured domain inside the SRV,and single porosity or dual-continuum outside the SRV).
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A major concern in development of fractured reservoirs in Enhanced Geothermal Systems (EGS) is to achieve and maintain adequate injectivity, while avoiding short-circuiting flow paths. The injection performance and flow paths are ...
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A major concern in development of fractured reservoirs in Enhanced Geothermal Systems (EGS) is to achieve and maintain adequate injectivity, while avoiding short-circuiting flow paths. The injection performance and flow paths are dominated by the permeability distribution of fracture network for EGS reservoirs. The evolution of reservoir permeability can be affected by rock deformation, induced by the change in temperature or pressure around the injector, and chemical reactions between injection fluid and reservoir rock minerals. Thus the change in permeability due to geomechanical deformation and mineral precipitation/dissolution could have a major impact on reservoir long-term performance. A coupled thermal-hydrologicalmechanical- chemical (THMC) model is in general necessary to examine the reservoir behavior in EGS. This paper presents a numerical model, TOUGH2-EGS, for simulating coupled THMC processes in enhanced geothermal reservoirs. This simulator is built by coupling mean stress calculation and reactive geochemistry into the existing framework of TOUGH2 (Pruess et al., 1999) and TOUGHREACT (Xu et al., 2004a), the well-established numerical simulators for geothermal reservoir simulation. The geomechanical model is fully-coupled as mean stress equations are solved simultaneously with fluid and heat flow equations. After solution of the fluid, heat, and stress equations, the flow velocity and phase saturations are used for reactive geochemical transport simulation in order to sequentially couple reactive geochemistry at each time step. We perform coupled THMC simulations to examine a prototypical EGS reservoir for permeability evolution at the vicinity of the injection well. The simulation results demonstrate the strong influence of rock deformation effects in the short and intermediate term, and long-term influence of chemical effects. It is observed that the permeability enhancement by thermalmechanical effect can be counteracted by the chemical precipitation of minerals, initially dissolved into the low temperature injected water. We analyze the sensitivity of temperature of injected water on the coupled geomechanical and geochemical effects, and conclude that the temperature of injected water could be modified to maintain or even enhance the reservoir permeability and the injection performance.
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摘要 :
A major concern in development of fractured reservoirs in Enhanced Geothermal Systems (EGS) is to achieve and maintain adequate injectivity, while avoiding short-circuiting flow paths. The injection performance and flow paths are ...
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A major concern in development of fractured reservoirs in Enhanced Geothermal Systems (EGS) is to achieve and maintain adequate injectivity, while avoiding short-circuiting flow paths. The injection performance and flow paths are dominated by the permeability distribution of fracture network for EGS reservoirs. The evolution of reservoir permeability can be affected by rock deformation, induced by the change in temperature or pressure around the injector, and chemical reactions between injection fluid and reservoir rock minerals. Thus the change in permeability due to geomechanical deformation and mineral precipitation/dissolution could have a major impact on reservoir long-term performance. A coupled thermal-hydrologicalmechanical- chemical (THMC) model is in general necessary to examine the reservoir behavior in EGS. This paper presents a numerical model, TOUGH2-EGS, for simulating coupled THMC processes in enhanced geothermal reservoirs. This simulator is built by coupling mean stress calculation and reactive geochemistry into the existing framework of TOUGH2 (Pruess et al., 1999) and TOUGHREACT (Xu et al., 2004a), the well-established numerical simulators for geothermal reservoir simulation. The geomechanical model is fully-coupled as mean stress equations are solved simultaneously with fluid and heat flow equations. After solution of the fluid, heat, and stress equations, the flow velocity and phase saturations are used for reactive geochemical transport simulation in order to sequentially couple reactive geochemistry at each time step. We perform coupled THMC simulations to examine a prototypical EGS reservoir for permeability evolution at the vicinity of the injection well. The simulation results demonstrate the strong influence of rock deformation effects in the short and intermediate term, and long-term influence of chemical effects. It is observed that the permeability enhancement by thermalmechanical effect can be counteracted by the chemical precipitation of minerals, initially dissolved into the low temperature injected water. We analyze the sensitivity of temperature of injected water on the coupled geomechanical and geochemical effects, and conclude that the temperature of injected water could be modified to maintain or even enhance the reservoir permeability and the injection performance.
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Considering that the frequency under fixed demand cannot meet the travel need of passengers, this paper studies the frequency optimization of modern trams based on stochastic demand. In this paper, the contradiction between passen...
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Considering that the frequency under fixed demand cannot meet the travel need of passengers, this paper studies the frequency optimization of modern trams based on stochastic demand. In this paper, the contradiction between passenger travel cost and operation cost is considered. According to the arrival characteristics of passenger flow under different demands, we propose a frequency model under fixed demand. Then, with the condition of the arrival of passenger flow obeying Poisson distribution, an optimization model based on stochastic demand is proposed. Taking the modern tram line of a certain city as an example, MATLAB is applied to simulate the model with the improved genetic simulated annealing algorithm. By simulation calculation, the departure frequencies under different demands are given. The results show that the departure frequency based on stochastic demand can better meet the demand of actual passenger flow, and minimize the cost of passengers and operators.
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