【作者】 沈海超；

【导师】 程远方；

【作者基本信息】 中国石油大学 ， 油气工程力学， 2009， 博士

【摘要】 天然气水合物能量密度大、在全球范围分布广,被公认为是21世纪最具开发前景的重要能源。从经济可行性考虑,降压法被普遍认为是天然气水合物藏大规模开采的有效方法之一。天然气水合物藏降压开采过程中,固相水合物分解会引起储层物性、力学性质、孔隙压力以及温度等一系列变化,本研究将其统称为“水合物分解效应”;同时,该过程中岩石变形与孔隙流体渗流相互作用密切,因此,充分考虑“水合物分解效应”,开展水合物藏降压开采流固耦合数值模拟研究具有重要的理论和现实意义。本论文针对天然气水合物藏降压开采的相关问题开展流固耦合数值模拟研究。首先,根据质量守恒和能量守恒原理,综合考虑多孔介质中水合物分解热力学和动力学、气、水两相流以及“水合物分解效应”等因素,建立了天然气水合物藏降压开采相变渗流模型;在此基础上,根据流固耦合渗流理论,建立了考虑相态变化的渗透率各向异性天然气水合物藏降压开采气、水两相非等温流固耦合数学模型。与此同时,基于前人开展的疏松砂岩应力敏感性实验,考虑了储层物性参数与应力状态之间的关系,同时兼顾“水合物分解效应”影响,建立了水合物藏储层物性参数综合动态模型。对流固耦合模型控制方程进行离散,基于FEPG有限元平台,自主开发了天然气水合物藏降压开采气、水两相非等温流固耦合数值模拟软件,引入稳定性分析模块后,进一步开发了水合物藏降压开采近井储层稳定性分析软件。其次,利用天然气水合物藏降压开采气、水两相非等温流固耦合数值模拟软件,对水合物藏降压开采近井储层应力状态以及物性参数的变化规律进行了研究,结果表明:“水合物分解效应”、流固耦合作用及井眼效应是影响近井储层应力状态和物性参数的三个主要因素,其作用机理不同,影响程度各异;其中流固耦合作用对分解效应引起的储层物性参数变化趋势具有明显的抑制作用,必须引起重视。再次,利用水合物藏降压开采近井储层稳定性分析软件,对近井弱胶结、低强度、高孔渗分解区储层的稳定性进行了研究,建立了井壁不出砂的临界生产压差分析方法,结果表明:水合物分解区是整个储层稳定性较差的区域,其中井壁上最小水平主应力方向储层稳定性最差,是整个储层出砂的优先位置。影响因素分析进一步表明:“水合物分解效应”、生产压差大小、流固耦合作用以及水平地应力非均匀程度是影响近井储层稳定性的敏感因素,开井速度的影响相对较小。最后,本文对天然气水合物藏降压开采产能进行了流固耦合数值模拟研究,深刻剖析了流固耦合作用对水合物藏开采动态的影响机制。研究表明:尽管流固耦合作用提高了储层弹性驱动能,但岩石孔隙收缩导致储层渗储能力降低的效应占主导地位,因此,耦合模型较非耦合模型的产量预测值偏低。影响因素分析表明:水合物藏产气速率及累计产气量等随储层绝对渗透率增加而增大、随井底压力降低而增大、随地层温度升高而增大。更多还原

【Abstract】 By reason of extensive distributing and vast amount of quantity, natural gas hydrate is considered to be the most prospective energy in the 21st century. Considering the costs and feasibility, depressurization is the most economical and effective method for future large-scale natural gas hydrate production.During natural gas production from hydrate by depressurization, the petrophysical and mechanical parameters, the pore pressure and the temperature of hydrate reservoirs have been changing dynamically due to hydrate decomposition. These phenomena are named as gas hydrate decomposition effect in this paper. Furthermore, there is an intensive interaction between porous fluid flow and rock deformation during natural gas hydrate production. So it is of great theoretical and application importance to study on gas hydrate production with fluid-solid coupling numerical simulation.This paper focuses on the related problems during natural gas hydrate production by depressurization with the method of fluid-solid coupling numerical simulation.Firstly, a new phase change porous flow model for gas hydrate production by depressurization has been developed. The model includes mass and energy conservation theory, thermodynamics and kinetic of hydrate decomposition, gas-water two-phase flow and the gas hydrate decomposition effect. Based on this new model, a nonisothermal fluid-solid coupling numerical model has been developed for permeability anisotropy hydrate reservoirs. Moreover, several comprehensive dynamic models for hydrate reservoirs physical parameters have been developed. These models consider the gas hydrate decomposition effect based on former experimental research on stress sensitivity of unconsolidated sandstone reservoirs. After the discretization of the governing equations of the fluid-solid coupling model, a gas and water two-phase nonisothermal fluid-solid coupling numerical simulation software for gas hydrate production has been developed. By introducing the stability analysis module, the paper has developed a near well formation stability analysis software for hydrate reservoirs production by depressurization.Secondly, the dynamic variation and influence factors for the stress state and the physical and mechanical parameters of hydrate reservoirs have been analyzed with the self-developed fluid-solid coupling simulation software. The results show the gas hydrate decomposition effect, fluid-solid coupling and borehole effect are three main factors which have different mechanisms and influence level for stress state and parameters. The fluid-solid coupling has an inhibiting effect for variation of the physical and mechanical parameters due to gas hydrate decomposition effect, and more attention should be paid to the phenomena.Thirdly, the stability of near well formation of hydrate reservoirs has been analyzed using the self-developed stability analysis software, and this area has the features of weak consolidation, low strength and high poroperm due to hydrate decomposition. A method for critical drawdown pressure determination has been developed. The results show that the stability of the decomposition area is relatively poor, and borehole wall in the direction of the minimum horizontal principal stress is the preferred position for sand production. The influence factors analysis reveal that the gas hydrates decomposition effect, drawdown pressure; fluid-solid coupling and the nonhomogeneity degree of the principal stress are the most sensitive factors affecting the stability of the near well formation, and the influence from the pressure release velocity is relatively small.Finally, a fluid-solid coupling numerical simulation on the productivity of natural gas hydrate by depressurization has been performed. The results show that the decrease of the poroperm ability due to fluid-solid coupling is the dominant factor which affects the productivity of the natural gas hydrate, and the increase of the elastic drive energy by fluid-solid coupling is relatively small, so the predictive value of the productivity from fluid-solid coupling modeling is lower than that of the non-coupling modeling. The influence factors analysis reveals that the gas production rate and the cumulative gas increase with the increase of reservoir absolute permeability, with the decrease of bottom pressure, with the increase of reservoir temperature.更多还原