【作者】 张莉；

【导师】 吴必胜；

【作者基本信息】 清华大学 ， 土木工程， 2024， 博士

【摘要】 天然气水合物储量大、能量密度高,是未来战略替代能源之一,而现有试采工程无法满足商业化开采要求。水合物分解涉及多相流、热传递、相变和力学变形等多物理场耦合,对分解过程精确预测才能保障安全、高效、可持续开采。本文沿“基础理论-数值模型-计算方法-工程应用”主线,对水合物分解力学行为展开研究:（1）基于热力学基本原理和Clausius-Duhem不等式推导得到描述天然气水合物分解过程中储层力学变形的热-孔-弹塑性理论。针对天然气水合物分解过程,构建了欧拉形式和拉格朗日形式下的质量守恒、动量守恒、能量守恒和动能定理的一般方程,针对拉格朗日形式得到了多孔介质的弹性和弹塑性本构关系,并在关联和非关联塑性势理论基础上推导得到一般形式的硬化弹塑性刚度矩阵。（2）研发了一个考虑多相流、多组分和相变的水合物分解三维热-流-力-化全耦合模型及其模拟器DEHydrate。基于前面提出的水合物分解理论,考虑了固液气多相流、热传递、相变和固体变形多物理场,解决了位移场和渗流场的不同阶问题,实现了渗流场和力学场全耦合求解;研发了全隐式有限元模拟器,用于预测分解时储层压强、温度、位移、应力、饱和度以及分解速率等力学行为演变规律。（3）利用DEHydrate模拟器比较了南海第二次试采储层水平井和直井开采行为,发现水平井时水合物分解量和产气量远大于直井（3倍左右）,但井筒产气量低于直井,直井时产气量主要源于储层本身气体;水平井时储层横向和纵向应力差值随时间逐渐增大,储层稳定性降低,直井时井周储层稳定性逐渐加强。（4）针对南海神狐海域SH7站点,对比了基于理想弹塑性和线弹性本构预测的水合物分解行为,发现了不同本构理论预测的井筒产气效率相对差值为0～2%,对水合物的分解速率基本没有影响;相比于线弹性本构,基于理想弹塑性本构预测的井筒上侧和右侧横向有效应力σ_zz^E偏大,相对差值达21%,而井筒下侧纵向有效应力σ_zz^E偏大,相对差值在5%左右,应力偏大值都体现在最小主应力方向。（5）通过研究含多裂缝水合物储层降压分解时储层力学行为,发现和相同宽度井孔相比裂缝可提高水合物分解速率两到三个量级;在小时间尺度,较小裂缝间距使得裂缝间低压效应叠加,单裂缝分解速率相对较高,而在大时间尺度,最优裂缝间距相对较大;较小裂缝间距限制储层横向和纵向的变形程度,并且降低应力变化程度,在裂缝上部表现为应力集中,而在裂缝下部表现为应力消散。更多还原

【Abstract】 Natural gas hydrate(NGH)is one of the most promosing alternatives to conventional energy sources due to a great deal of reserve and high energy density.However,the existing NGH extraction does not meet the requirements for commercial development.The NGH dissociation is a complicated multi-physics process involving multiphase flow,heat transfer,phase change and mechanical deformation in the hydrate bearing sediments(HBSs).An accurate predicting of the mechanical behavior of HBSs during NGH disscoation is crucial for ensuring safe and efficient NGH extraction.Following the route of“fundamental theory-numerical modeling-computational methods-engineering applications”,the present thesis carries out a series of studies on the fully coupled thermo-hydro-mechanical-chemical(THMC)mechanical behaviors of the HBSs during NGH dissociation,as follows:(1)Based on thermodynamic principles and the Clausius-Duhem inequality,a comprehensive thermo-poro-elastoplastic theory is proposed to describe the mechanical deformation of HBSs considering the NGH dissociation.The general equations for mass conservation,momentum balance,and energy conservation,as well as kinetic energy theorem in both Eulerian and Lagrangian forms are derived for the NGH dissociation process.Utilizing the Lagrange saturation and considering the interaction between the solid-liquid-gas interfaces,the poroelastic and elastoplastic constitutive models for porous media bearing NGH are developed.Furthermore,the stress-strain relationships are derived to address hardening plasticity,encompassing both associative and non-associative plastic potentials,where a generalized stiffness matrix for the hardening elastoplastic modulus is obtained.(2)A fully coupled strongly nonlinear three-dimensional THMC model along with its computational simulator,i.e.DEHydrate,is developed,considering multiphase flow,multicomponent and phase change during NGH dissociation.Based on the proposed thermo-poro-elastoplastic theory for NGH dissociation,the model accounts for multiphysics behaviours including solid-liquid-gas multiphase flow,heat transfer,NGH phase change and mechanical deformation during NGH dissociation.The multiphase flow and heat transfer are simulated by low-order elements while the solid deformation is calculated by high-or low-order elements,thus achieving full coupling of the seepage and geomechanics.An implicit finite element simulator is developed to predict the evolution of mechanical behaviors such as pressure,temperature,displacement,stress,multiphase saturation and NGH dissociation rate during NGH dissociation.(3)Employing the DEHydrate simulator,the NGH dissociation behavior under horizontal and vertical wellbore layouts is compared for the second trial exploitation HBSs in the Shenhu area,South China Sea.With the horizontal wellbore layout,the NGH dissociation and gas production are about 3 times larger,but the gas collection from the wellbore is smaller than that with the vertical wellbore layout,as the gas collected in the vertical wellbore layout mainly originates from the gas already stored in reservoir.With horizontal well layout,the difference in horizontal and vertical stresses of HBSs gradually increases over time,leading to a decrease in HBSs stability,while HBSs stability around the wellbore gradually strengthens with vertical wellbore layout.(4)The NGH dissociation behaviors predicted from the ideal elasto-plastic and linear elastic constitutive models are compared for the SH7 site in the Shenhu area,South China Sea.It is found that the relative difference in the gas collection from the wellbore predicted by different constitutive models is 0-2%,which has little effect on the NGH dissociation.Compared with the linear elastic constitutive model,the predicted horizontal effective stress on the upper and right sides of the wellbore based on the ideal elastic-plastic constitutive model is larger,with a relative difference of 21%,while the vertical effective stress on the lower side of the wellbore is larger,with a relative difference of about 5%,both of them are the local minimum principal stress.(5)The NGH dissociation by depressurization method from a single horizontal well intercepted by multiple fractures created via hydraulic fracturing is investigated numerically.It is found that the fracture can significantly increase the NGH dissociation rate by two to three orders of magnitude compared to perforation with the same width.At a short time,a small fracture spacing strenghthens the depressurization effect between fractures,thus increasing the dissociation rate each fracture.At a large time,the optimal fracture spacing is relatively large.Smaller fracture spacings reduce the horizontal and vertical displacements of HBSs and stress changes,with stress concentration observed in the upper part and stress dissipation in the lower part of the fracture.更多还原