Integral equation solution of heat extraction‐induced thermal stress in enhanced geothermal reservoirs

During fluid injection in enhanced geothermal systems, thermo‐mechanical processes can play an important role. In fact, the phenomena of reservoir seismicity and the variation of injectivity with respect to injection water temperature can be attributed to the induced thermal stresses. In this paper, a three‐dimensional integral equation formulation is presented for calculating thermally induced stresses associated with the cooling of a fracture in a geothermal reservoir. By utilizing Green's function in the integral equation, the three‐dimensional heat flow and stresses in the reservoir are modelled without discretizing the reservoir. The formulation is implemented in a computer program for the solution of injection into an infinite fracture as well as for the injection/extraction in an arbitrarily shaped fracture. Copyright © 2005 John Wiley & Sons, Ltd.     

Semianalytical modeling on 3D stress redistribution during hydraulic fracturing stimulation and its effects on natural fracture reactivation

The hydraulic fracturing technique has been widely applied in many fields, such as the enhanced geothermal systems (EGS), the improvement of injection rates for geologic sequestration of CO₂, and for the stimulations of oil and gas reservoirs. The key points for the success of hydraulic fracturing operations in unconventional resources are to accurately estimate the redistribution of pore pressure and stresses around the induced fracture and predict the reactivations of preexisting natural fractures. The pore pressure and stress regime around hydraulic fracture are affected by poroelastic and thermoelastic phenomena as well as by fracture opening compression. In this work, a comprehensive semi-analytical model is used to estimate the stress and pore pressure distribution around an injection-induced fracture from a single well in an infinite reservoir. The model allows the leak-off distribution in the formation to be three-dimensional with the pressure transient moving ellipsoidically outward into the reservoir from the fracture surface. The pore pressure and the stress changes in three dimensions at any point around the fracture caused by poroelasticity, thermoelasticity, and fracture compression are investigated. With Mohr-Coulomb failure criterion, we calculate the natural fracture reactivations in the reservoir. Then, two case studies of constant water injection into a hydraulic fracture are presented. This work is of interest in the interpretation of microseismicity in hydraulic fracturing and in the estimation of the fracture spacing for hydraulic fracturing operations. In addition, the results from this study can be very helpful for the selection of stimulated wells and further design of the refracturing operations.     

Prediction and analysis of the stimulated reservoir volume for shale gas reservoirs based on rock failure mechanism

The ultra-low-permeability shale gas reservoir has a lot of well-developed natural fractures. It has been proven that hydraulic fracture growth pattern is usually a complex network fracture rather than conventional single planar fractures by micro-seismic monitoring, which can be explained as the shear and tensile failure of natural fractures or creation of new cracks due to the increase in reservoir pore pressure caused by fluid injection during the process of hydraulic fracturing. In order to simulate the network fracture growth, a mathematical model was established based on full tensor permeability, continuum method and fluid mass conservation equation. Firstly, the governing equation of fluid diffusivity based on permeability tensor was solved to obtain the reservoir pressure distribution. Then Mohr–Coulomb shear failure criterion and tensile failure criterion were used to decide whether the rock failed or not in any block on the basis of the calculated reservoir pressure. The grid-block permeability was modified according to the change of fracture aperture once any type of rock failure criterion was met within a grid block. Finally, the stimulated reservoir volume (SRV) zone was represented by an enhancement permeability zone. After calibrating the numerical solution of the model with the field micro-seismic information, a sensitivity study was performed to analyze the effects of some factors including initial reservoir pressure, injection fluid volume, natural fracture azimuth angle and horizontal stress difference on the SRV (shape, size, bandwidth and length). The results show that the SRV size increases with the increasing initial pore reservoir and injection fluid volume, but decreases with the increase in the horizontal principal stress difference and natural fracture azimuth angle. The SRV shape is always similar for different initial pore reservoir and injection fluid volume. The SRV is observed to become shorter in length and wider in bandwidth with the decrease in natural fracture azimuth angle and horizontal principal stress difference.     

Fracture roughness and thermal exchange: A case study at Soultz-sous-Forêts

Heat exchange during laminar flow in an open fracture is studied numerically on the basis of the Stokes equation in the limit of hydrothermal lubrication. We examine the influence of fracture roughness on hydraulic permeability and heat flux through the fracture sides when a cold fluid is injected into a homogeneous hot host rock. Spatial temperature fluctuations inside the fluid are studied assuming the temperature of the rock to be constant and the fracture aperture to be self-affine. An application to the case study at the deep geothermal reservoir of Soultz-sous-Forêts, France, is presented. Finally, a thermal model based on sparse spatial information of the geometrical aperture is successfully proposed to reproduce the response of the fracture.     

Thermal recovery from a fractured medium in local thermal non‐equilibrium

Thermal recovery from a hot dry rock (HDR) reservoir viewed as a deformable fractured medium is investigated with a focus on the assumption of local thermal non‐equilibrium (LTNE). Hydraulic diffusion, thermal diffusion, forced convection and deformation are considered in a two‐phase framework, the solid phase being made by impermeable solid blocks separated by saturated fractures. The finite element approximation of the constitutive and field equations is formulated and applied to obtain the response of a generic HDR reservoir to circulation tests. A change of time profile of the outlet fluid temperature is observed as the fracture spacing increases, switching from a single‐step pattern to a double‐step pattern, a feature which is viewed as characteristic of established LTNE. A dimensionless number is proposed to delineate between local thermal equilibrium (LTE) and non‐equilibrium. This number embodies local physical properties of the mixture, elements of the geometry of the reservoir and the production flow rate. All the above properties being fixed, the resulting fracture spacing threshold between LTNE and LTE is found to decrease with increasing porosity or fluid velocity. The thermally induced effective stress is tensile near the injection well, illustrating the thermal contraction of the rock, while the pressure contribution of the fracture fluid is negligible during the late period. Copyright © 2012 John Wiley & Sons, Ltd.     

工质变物性对EGS热开采过程影响的数值模拟

增强型地热系统(EGS)热开采过程中循环工质的温度和压力会经历较大范围的变化,这会造成循环工质的热物性变化,从而影响流体工质的输运和岩石-流体热交换;数值模拟EGS热开采过程,预测EGS的寿命、出力等性能指标有必要考虑循环工质的热物性变化.笔者在EGS热开采过程三维数值模拟中考虑水和超临界二氧化碳的变物性,实现了热流双向耦合.针对水EGS分析了各物性变化对EGS采热性能的影响,并对变物性条件的水和超临界二氧化碳EGS的采热性能进行了对比研究.结果表明:工质在密度影响下开采寿命为9.0 a在密度和比定压热容共同影响下的开采寿命为7.5 a,说明密度和比定压热容越大则EGS开采寿命越短;在黏度系数影响下的开采寿命为18.0 a,说明黏度系数越大则EGS开采寿命越长;导热系数则对EGS采热性能无明显影响.注入压力一定的条件下以水为工质的EGS具有较长寿命,但相同时刻的质量流率和热开采率低于以临界二氧化碳为工质的EGS.     

A shear‐dilation‐based model for evaluation of hydraulically stimulated naturally fractured reservoirs

The role of shear dilation as a mechanism of enhancing fluid flow permeability in naturally fractured reservoirs was mainly recognized in the context of hot dry rock (HDR) geothermal reservoir stimulation. Simplified models based on shear slippage only were developed and their applications to evaluate HDR geothermal reservoir stimulation were reported. Research attention is recently focused to adjust this stimulation mechanism for naturally fractured oil and gas reservoirs which reserve vast resources worldwide. This paper develops the overall framework and basic formulations of this stimulation model for oil and gas reservoirs. Major computational modules include: natural fracture simulation, response analysis of stimulated fractures, average permeability estimation for the stimulated reservoir and prediction of an average flow direction. Natural fractures are simulated stochastically by implementing ‘fractal dimension’ concept. Natural fracture propagation and shear displacements are formulated by following computationally efficient approximate approaches interrelating in situ stresses, natural fracture parameters and stimulation pressure developed by fluid injection inside fractures. The average permeability of the stimulated reservoir is formulated as a function of discretized gridblock permeabilities by applying cubic law of fluid flow. The average reservoir elongation, or the flow direction, is expressed as a function of reservoir aspect ratio induced by directional permeability contributions. The natural fracture simulation module is verified by comparing its results with observed microseismic clouds in actual naturally fractured reservoirs. Permeability enhancement and reservoir growth are characterized with respect to stimulation pressure, in situ stresses and natural fracture density applying the model to two example reservoirs. Copyright © 2002 John Wiley & Sons, Ltd.     

应力腐蚀和压力溶解引起的裂隙开度-刚度变化对THMM耦合过程影响的数值模拟

在使用Yasuhara等建立的裂隙开度的应力腐蚀和压力溶解模型的基础上,考虑裂隙闭合量对裂隙刚度的影响,针对一个假设的位于非饱和双重孔隙-裂隙岩体中且有核素泄漏的高放废物地质处置模型,拟定两种计算工况：（1）裂隙的刚度系数是裂隙闭合量的线性函数;（2）裂隙刚度为常数,进行热-水-应力-迁移耦合的二维有限元数值模拟,考察了岩体中的温度、裂隙开度的闭合速率、闭合量、孔（裂）隙水压力、地下水流速、核素浓度、裂隙刚度和正应力的变化、分布等情况。结果表明,两种工况岩体中的温度场、孔（裂）隙水压力、地下水流速、核素浓度无明显差别;裂隙闭合基本由应力腐蚀产生;在相同计算时间内两种工况的裂隙闭合量较为接近,工况1略大;工况1中离玻璃固化体越近,裂隙刚度值越高,并且在玻璃固化体附近的应力值较大,且集中程度较高。     

A numerical model simulating reactive transport and evolution of fracture permeability

A numerical model is presented to describe the evolution of fracture aperture (and related permeability) mediated by the competing chemical processes of pressure solution and free‐face dissolution/precipitation; pressure (dis)solution and precipitation effect net‐reduction in aperture and free‐face dissolution effects net‐increase. These processes are incorporated to examine coupled thermo‐hydro‐mechano‐chemo responses during a flow‐through experiment, and applied to reckon the effect of forced fluid injection within rock fractures at geothermal and petroleum sites. The model accommodates advection‐dominant transport systems by employing the Lagrangian–Eulerian method. This enables changes in aperture and solute concentration within a fracture to be followed with time for arbitrary driving effective stresses, fluid and rock temperatures, and fluid flow rates. This allows a systematic evaluation of evolving linked mechanical and chemical processes. Changes in fracture aperture and solute concentration tracked within a well‐constrained flow‐through test completed on a natural fracture in novaculite (Earth Planet. Sci. Lett. 2006, in press) are compared with the distributed parameter model. These results show relatively good agreement, excepting an enigmatic abrupt reduction in fracture aperture in the early experimental period, suggesting that other mechanisms such as mechanical creep and clogging induced by unanticipated local precipitation need to be quantified and incorporated. The model is applied to examine the evolution in fracture permeability for different inlet conditions, including localized (rather than distributed) injection. Predictions show the evolution of preferential flow paths driven by dissolution, and also define the sense of permeability evolution at field scale. Copyright © 2006 John Wiley & Sons, Ltd.     

Numerical Modeling and Spatial Moment Analysis of Thermal Fronts in a Coupled Fracture-Skin-Matrix System

In this study, the behavior of thermal fronts along the fracture is studied in the presence of fracture-skin in a coupled fracture-matrix system. Cold water is injected into the fracture, which advances gradually towards production well, while extracting heat from the surrounding reservoir matrix. The heat conduction into the fracture-skin and the rock-matrix from the high permeability fracture is assumed to be one dimensional perpendicular to the axis of fluid flow along the fracture. Constant temperature cold water is injected through an injection well at the fracture inlet. The fluid flow takes place along the horizontal fracture which ensures connectivity between the injection and production wells. Since the rock-matrix is assumed to be tight, the permeability of fracture-skin as well as the rock-matrix is neglected. The present study focuses on the heat flux transfer at the fracture-skin interface as against the earlier studies on fracture-matrix interface, and the sensitivity of additional heterogeneity in the form of fracture skin in a conventional fracture-matrix coupled system is studied. The behavior of thermal fronts for various thermal conductivity values of the fracture-skin and rock-matrix is analyzed. Spatial moment analysis is performed on the thermal distribution profiles resulting from numerical studies in order to investigate the impact on mobility and dispersion behavior of the fluid in the presence of fracture-skin. The presence of fracture skin affects the heat transfer significantly in the coupled fracture-matrix system. The lower order spatial moments indicate that the effective thermal velocity increases with increase in skin thermal conductivity and a significant thermal dispersion is observed at the inlet of the fracture owing to the high thermal conductivity of the fracture-skin at the early stages. Furthermore the higher spatial moments indicate that the asymmetricity increases with decrease in skin thermal conductivity unlike the case with half fracture aperture and fluid velocity and the kurtosis is maximum with higher skin thermal conductivity which implies enhanced heat extraction from the fracture-skin into the fracture. Results suggest that the amount of heat extraction by the circulating fluid within the fracture from the reservoir not only depends on the rock-matrix module of the reservoir characteristics but also the fracture-skin characteristics of the system and subsequently influence the reservoir efficiency.     

Numerical modeling of fluid‐driven fractures in cohesive poroelastoplastic continuum

In this article, we investigate the main parameters that influence the propagation of a fluid‐driven fracture in a poroelastoplastic continuum. These parameters include the cohesive zone, the stress anisotropy, and the pore pressure field. The fracture is driven in a permeable porous domain that corresponds to weak formation by pumping of an incompressible viscous fluid at the fracture inlet under plane strain conditions. Rock deformation is modeled with the Mohr–Coulomb yield criterion with associative flow rule. Fluid flow in the fracture is modeled by the lubrication theory. The movement of the pore fluid in the surrounding medium is assumed to obey the Darcy law and is of the same nature as the fracturing fluid. The cohesive zone approach is used as the fracture propagation criterion. The problem is modeled numerically with the finite element method to obtain the solution for the fracture length, the fracture opening, and the propagation pressure as a function of the time and distance from the pumping inlet. It is demonstrated that the plastic yielding that is associated with the rock dilation in an elastoplastic saturated porous continuum is significantly affected by the cohesive zone characteristics, the stress anisotropy, and the pore pressure field. These influences result in larger fracture profiles and propagation pressures due to the larger plastic zones that are developing during the fracture propagation. Furthermore, it is also found that the diffusion process that is a major mechanism in hydraulic fracture operations influences further the obtained results on the fracture dimensions, plastic yielding, and fluid pressures. These findings may explain partially the discrepancies in net pressures between field measurements and conventional model predictions. Copyright © 2012 John Wiley & Sons, Ltd.     

The influence of pumping parameters in fluid‐driven fractures in weak porous formations

In this work, we investigate the main pumping parameters that influence a fluid‐driven fracture in cohesive poroelastic and poroelastoplastic weak formations. These parameters include the fluid viscosity and the injection rate. The first parameter dominates in the mapping of the propagation regimes from toughness to viscosity, whereas the second parameter controls the storage to leak‐off dominated regime through diffusion. The fracture is driven in weak permeable porous formation by injecting an incompressible viscous fluid at the fracture inlet assuming that the fracture propagates under plane strain conditions. Fluid flow in the fracture is modeled by lubrication theory. Pore fluid movement in the porous formation is based on the Darcy law. The coupling follows the Biot theory, whereas the irreversible rock deformation is modeled with the Mohr–Coulomb yield criterion with associative flow rule. Fracture propagation criterion is based on the cohesive zone approach. Leak‐off is also considered. The investigation is performed numerically with the FEM to obtain the fracture opening, length, and propagation pressure versus time. We demonstrate that pumping parameters influence the fracture geometry and fluid pressures in weak formations through the viscous fluid flow and the diffusion process that create back stresses and large plastic zones as the fracture propagates. It is also shown that the product of the propagation velocity and fluid viscosity, µv that appears in the scaling controls the magnitude of the plastic zones and influences the net pressure and fracture geometry. These findings may explain partially the discrepancies in net pressures between field measurements and conventional model predictions for the case of weak porous formation. Copyright © 2014 John Wiley & Sons, Ltd.     

热-水-应力耦合影响的有限元分析

为了考虑应力拉压和压力（化学）溶解对裂隙开度的综合影响,对所建立的双重孔隙-裂隙介质热-水-应力耦合模型中裂隙开度的计算模型作了改进。通过一个假定的高放废物地质处置库算例,就岩体裂隙开度变化的3种工况,分析了岩体中的温度、孔（裂）隙水压力、地下水流速和主应力的变化、分布情况。结果显示：3种工况的计算域中温度场基本相同;孔（裂）隙水渗流场形态相似,但其量值有一定差别;工况1的裂隙开度在应力和压力（化学）溶解的共同作用下闭合量最大,负孔（裂）隙水压力增值最高;核废物的释热效应明显地改变了岩体自重应力场的水平分量,但对其垂直分量影响较小。     

考虑饱和度修正的应力腐蚀和压力溶解作用热-水-应力耦合过程有限元分析

针对笔者开发的双重孔隙-裂隙介质热-水-应力（T-H-M）耦合二维有限元分析程序中使用的应力腐蚀和压力溶解模型,引入水饱和度修正因子。以一个假设的位于含水地层中的高放废物地质处置模型,拟定3种初始裂隙水饱和度的计算工况（Sw20=1.0、0.8、0.2）进行热-水-应力耦合的数值模拟,考察岩体中的温度、裂隙开度的闭合速率、闭合量、孔(裂)隙水压力、地下水流速和应力的变化、分布等情况。结果显示,随着初始裂隙水饱和度的由高值到低值,应力腐蚀和压力溶解产生的闭合速率从大变小,裂隙开度由初始值趋于残余值,粗糙面接触率由初始值趋于其名义接触率的时间也增加,裂纹应力强度因子的下降亦变慢;近场的裂（孔）隙水压力的变化、分布及其流速矢量场的形态有明显的不同;3种工况的岩体中的应力量值及分布的差别不大。     

Modeling 3D thermal fracture propagation by transient cooling using virtual multidimensional internal bonds

Thermal fracturing can play an important role in development of unconventional petroleum and geothermal resources. Thermal fractures can result from the nonlinear deformation of the rock in response to thermal stress related to cold water injection as well as heating. Before the rock reaches the final failure stage, material softening and bulk modulus degradation can cause changes in the thermo‐mechanical properties of the solid. In order to capture this aspect of the rock fracture, a virtual multidimensional internal bond‐based thermo‐mechanical model is derived to track elastic, softening, and the failure stages of the rock in response to the temporal changes of its temperature field. The variations in thermo‐mechanical properties of the rock are derived from a nonlinear constitutive model. To represent the thermo‐mechanical behavior of pre‐existing fractures, the element partition method is employed. Using the model, numerical simulation of 3D thermal fracture propagation in brittle rock is carried out. Results of numerical simulations provide evidence of model verification and illustrate nonlinear thermal response and fracture development in rock under uniform cooling. In addition, fracture coalescence in a cluster of fractures under thermal stress is illustrated, and the process of thermal fracturing from a wellbore is captured. Results underscore the importance of thermal stress in reservoir stimulation and show the effectiveness of the model to predict 3D thermal fracturing. Copyright © 2016 John Wiley & Sons, Ltd.     

壁面局部接触裂隙岩体水流-传热的一种理论模型及计算分析

核废物地质处置、地热开发、石油开采等工程领域都可能涉及稀疏裂隙岩体中的水流-传热过程。现有的裂隙岩体水流-传热理论模型和计算方法基本上都是以平行光滑壁面裂隙模型为基础的,没有考虑裂隙的壁面局部接触对水流、水-岩热交换以及岩体传热的影响。针对粗糙壁面裂隙水流过程,阐述了基于Stokes方程的Reynolds润滑方程及Hele-Shaw裂隙模型,采用MATLAB软件中的PDE工具求解,并与Walsh的等效水力开度公式进行对比;分析壁面局部接触裂隙水流-传热与填充裂隙水流-传热的相似性,提出了瞬时局部热平衡假设的适用条件,并在裂隙局部接触体传热满足Biot数条件的前提下,计算分析裂隙局部接触体与水流之间的局部热平衡时间及其影响因素;在裂隙局部接触体与水流之间满足瞬时热平衡假设的前提下,利用填充裂隙水流-传热的解析解,计算了壁面局部接触裂隙水及两侧岩石的温度分布,并分析了裂隙局部接触面积率、裂隙开度、裂隙水平均流速对岩石温度和裂隙水温度的影响特征,结果表明：（1）在设定条件下,由于裂隙局部接触体与裂隙水流之间的热交换,裂隙水流对其两侧岩石温度的影响范围随接触面积率的增大而减小,裂隙两侧岩石对裂隙水流温度的影响程度随接触面积率的增大而增大;（2）裂隙开度和裂隙水流速对岩石温度和裂隙水温度的影响方式的影响是一致的,即由于裂隙水流量随裂隙开度和裂隙水流速的增大而增大,裂隙水流对其两侧岩石温度的影响范围随裂隙开度和裂隙水流速的增大而增大,裂隙两侧岩石对裂隙水流温度的影响程度随裂隙开度和裂隙水流速的增大而减小。     

应力腐蚀和压力溶解作用下双重孔隙岩体中 THMM耦合过程的有限元分析

在使用Yasuhara等建立的裂隙开度的应力腐蚀和压力溶解模型的基础上,将溶质浓度场引入笔者已开发的双重孔隙-裂隙介质热-水-应力耦合的二维有限元分析程序中,针对一个假设的位于非饱和岩体中且有核素泄漏的高放废物地质处置库,拟定2种计算工况：（1）裂隙开度随应力腐蚀和压力溶解而变化（基岩的孔隙率亦是应力的函数）;（2）裂隙开度和基岩的孔隙率均为常数,进行热-水-应力-迁移耦合的数值模拟,考察了岩体中的温度、裂隙开度的闭合速率、闭合量、孔（裂）隙水压力、地下水流速、核素浓度和应力的变化、分布等情况。结果主要显示：应力腐蚀引起的闭合速率要高于压力溶解引起的闭合速率6个数量级,且两种因素产生的闭合速率随时间先增加后减小,并趋于稳定;当考虑应力腐蚀和压力溶解时,近场的负裂隙水压力上升很高;工况1中裂隙开度和孔隙率减小,使得相应的渗透系数降低,故该工况的裂隙和孔隙中核素浓度较工况2为高;由于不计入负的孔（裂）隙水压力对应力平衡的影响,2种工况的岩体中的应力量值及分布基本相同。     

化学刺激法提高花岗岩类岩石裂隙渗透性的实验研究

增强型地热系统(EGS)用于通过人工形成地热储层的方法从深部低渗透性岩体中开采地热能;国际上常采用水力压裂辅以化学刺激的方法改造EGS 储层以提高其渗透率。本文以采自青海共和盆地的花岗闪长岩样品为对象,选用3种不同化学刺激剂(氢氧化钠、盐酸和土酸),在3组不同注入流速条件下开展了系统化学刺激实验。结果表明:注入盐酸和土酸后样品渗透率均有提高,且采用土酸时渗透率提高幅度明显大于盐酸;但注入氢氧化钠后,样品渗透率反而降低。在3类化学刺激剂中,土酸对长石类矿物的溶蚀能力最强,而氢氧化钠溶液对石英的溶蚀能力最强,但氢氧化钠溶液在溶解岩石样品裂隙表面矿物后极易形成非定形态二氧化硅或非定形态铝硅酸盐蚀变矿物并阻塞裂隙,反而对化学刺激效果造成负面影响。总体来看,土酸是青海共和盆地干热岩体的最佳化学刺激剂。在中等注入速度(3 mL·min^-1)条件下,土酸对岩石样品的溶蚀程度就可达到最高;在此基础上进一步降低流速,则可能使溶解组分更易从液相中沉淀而充填于样品裂隙,导致样品渗透率有所下降。     

塔里木盆地和田河气田奥陶系裂缝应力敏感性研究

为研究碳酸盐岩储层应力敏感性及其对气藏产能和气井见水的影响,选取塔里木盆地和田河气田奥陶系钻井岩心, 开展了应力敏感性研究,结合实验结果及数学推导建立了裂缝开度和有效应力之间的力学关系模型,最终借助有限元方 法,对持续开发过程中碳酸盐岩储层裂缝开度的空间变化规律进行了模拟分析。结果表明：裂缝开度的变化随围压或有效 应力变化呈非线性关系;随着围压或有效应力的升高,裂缝开度不断减小,0~0.8 MPa之间减小速度快,岩样应力敏感性 强,到2.6 MPa或5.4 MPa后变化趋于稳定,说明高应力环境下,岩样的应力敏感性减弱,塑性变形特征明显;随着围压的 逐渐降低,裂缝开度不断增大,但不能恢复至初始情况;未来若干年内随着快速开采,和田河气田有效应力增大,裂缝闭 合,产能下降;随着生产井关闭或开采速度放缓,气水界面之上有效应力仍然降低,裂缝闭合,界面之下有效应力增大, 裂缝开启,底水上升,破坏气藏生产。建议在裂缝性碳酸盐岩气藏开发政策的制定过程中,一方面要开展裂缝分布精细研 究,分析裂缝应力敏感性主控因素,同时要制定合理的开发方式和开采速度,确保气藏稳产、高产。     

FDEM-TH3D: A three-dimensional coupled hydrothermal model for fractured rock

This paper proposes a three-dimensional coupled hydrothermal model for fractured rock based on the finite-discrete element method to simulate fluid flow and heat transport. The 3D coupled hydrothermal model is composed of three main parts: a heat conduction model for the rock matrix, a heat transfer model for the fluid in the fractures (including heat conduction and heat convection), and a heat exchange model between the rock matrix and the fluid in the fractures. Four examples with analytical solutions are provided to verify the model. A heat exchange experiment of circulating water in a cylindrical granite sample with one fracture is simulated. The simulation results agree well with the experimental results. The effects of the fracture aperture, fluid viscosity, and pressure difference on the heat exchange between the fluid and rock are studied. Finally, an application concerned with heat transport and fluid flow in fractured rock is presented. The simulation results indicate that the 3D fully coupled hydrothermal model can capture the fluid flow and temperature evolution of rocks and fluids.