低渗透岩石的应力敏感性与孔隙结构三维重构研究

发布时间：2018-06-04 07:48

本文选题：低渗透岩石 + 渗透率应力敏感性　；参考：《中国矿业大学(北京)》2016年博士论文

【摘要】：低渗透岩石的物理力学性质是地下工程及基础设施建设、煤炭与油气资源开采、水力水电开发、核废料处置及能源储备、CO2地质封存等领域的研究热点。深入认识和理解低渗透岩石的物理力学性质是解决上述工程领域实际问题的核心与保障。这些工程的建设与实施过程中,由于开挖扰动或者实际工况影响,低渗透岩石所处的应力环境不断变化。低渗透岩石的物理力学响应随开采应力变化而不断演化。例如,地下硐室开挖导致原岩应力释放,在采动应力和初始地应力的共同作用下岩石中的微裂纹发育,导致围岩的物理力学性质,特别是渗透率随采动过程不断变化;深部煤与瓦斯共采时,需要考虑开采引起的煤层渗透率的变化规律,以便实施有效的抽采工艺;核废物处置工程中,需要考虑核废物衰变释放的热量导致的周围岩层物理力学性质的改变;低渗油气藏开采时,测量岩石样品的渗透率需要恢复样品所在地层的原始应力状态才能获得较准确的储层渗透率表征;CO2地质封存中,研究盖层密封性时需要考虑CO2上浮带来的盖层有效应力的变化对盖层岩石渗透率等物理力学性质的影响。因此,深入研究低渗透岩石物理力学性质随地层应力状态改变而变化的规律对于确定合理的工程建设设计与实施方案具有重要意义。定量研究和表征低渗透岩石渗透性质随应力的变化特征及演化规律是本文研究的重点。研究发现低渗岩石渗透率随有效应力的变化规律与常规岩石表现出了很大的不同。在较小的有效应力范围内,随着有效应力的增长,低渗透岩石表现出孔隙率变化不大但渗透率大幅下降的现象。渗透率甚至会下降几个数量级,国内学者将此现象称之为渗透率的压敏效应。为了描述与揭示低渗岩石压敏效应,国内外学者在宏观实验数据拟合以及在微观模型基础上开展数值分析两方面分别开展了相关研究,取得了不少进展。本文的研究在系统地收集了低渗岩石孔隙率和渗透率随有效应力变化的实验测量数据,并分析了当前学者用于描述低渗岩石孔隙率有效应力关系式和渗透率有效应力关系式后发现:这些关系式并不能准确描述整个应力范围内低渗岩石的孔隙率和渗透率的测量数据。并且这些关系式多半来自经验拟合,并不能准确反映岩石这些外在表象的内在物理规律,无法合理与准确地描述低渗透岩石的压敏效应。微观方面由于岩石内部孔隙、裂隙、节理等不连续结构的复杂性,大部分研究对这些不连续结构进行了过分简化。同时当前微观岩石模型难以准确表征受力后岩石内部不连续结构的变化。并且当前计算流体方法在处理岩石内部孔隙结构复杂边界条件时会遇到极大困难,使得模拟难以进行。本文针对上述问题,尝试从宏观和微观两个层面的研究出发,准确描述与定量表征低渗岩石渗透率和有效应力的关系,同时尝试结合宏观和微观分析得到的结论,对低渗岩石压敏效应外在表现的内在机理加以解释。在宏观方面,本文从实验测量数据出发,借助岩石“两部分胡克定律模型”,即two-parthooke’smodel(tphm)建立起了更符合实际物理规律的宏观理论关系式。tphm概念性地将岩石划分为“软”、“硬”两部分,并采用不同形式的胡克定律来描述软、硬两部分大不相同的变形规律。顾名思义,软的部分受力后产生相对自身尺寸来说较大的变形,采用自然应变关系来更准确描述;硬的部分变形规律描述与传统岩石小变形理论相似,采用工程应变关系近似描述。软硬两部分应力应变关系不同,导致它们对岩石孔隙率和渗透率的贡献大不相同。软硬两部分联合构成了岩石整体,采用这样的划分可以更准确地描述低渗岩石在应力作用下的渗透率变化规律。我们采用幂函数方程拟合了软的部分的孔隙率和渗透率的关系。拟合结果显示软的部分孔隙率和渗透率大致符合“立方定律”,这说明低渗岩石的渗透率压敏效应是由于岩石内部微裂纹闭合所导致的。本文建立起的宏观关系式具有如下优点:(1)宏观概念模型的建立考虑了岩石的非均质性。这为得到更合理的物理描述关系式打下了坚实的基础;(2)此宏观模型可以更准确地反映岩石在整个应力测试范围内物理力学性质的变化,尤其是可以表征岩石在较低有效应力范围内的非线性变化,例如较低有效应力范围内孔隙率的非线性降低和渗透率的大幅下降;(3)模型采用孔隙率作为中间桥梁,分析了低渗岩石受力后,由于不连续结构非均匀压缩变形导致的渗透率变化。相比直接分析渗透率随有效应力变化,更符合物理认知;(4)宏观模型关系式中各个参数具有明确的物理意义,而不是为了达到描述实验数据而进行的公式拟合;(5)此宏观模型可以给出符合物理定律的低渗岩石压敏效应描述与解释。在tphm理论框架下推导得到的一系列岩石孔隙率和渗透率随有效应力变化的关系式,可以更准确与合理地反映低渗岩石受力变化时的物理力学性质,同时可以对低渗岩石压敏效应加以合理解释。此外,我们应用宏观理论推导得到的关系式分析了页岩气开采产量递减曲线。作为tphm的一个应用,我们将页岩渗透率随有效应力变化的tphm关系式融合到了数值模拟软件中,采用comsol分析了页岩气产量递减曲线。通过模拟发现,对储层的合理改造是页岩气产量化开采的前提;储层气体压力的降低是页岩气产量下降的主要因素;储层岩石渗透率随有效应力的改变对页岩气产量递减曲线有着较大的影响。考虑储层岩石渗透率应力相关性的模型表现出了更迅速的产出速率递减,同时表现出了更长的生产年限。在微观方面,我们从岩石的孔隙结构特征出发,建立了可以准确表征岩石内部孔隙结构的数字岩心模型,并在此数字模型的基础上分析了岩石的渗透性质。首先,岩石渗透性质与岩石孔隙结构密切相关,准确和定量地表征孔隙结构十分重要。本研究中,我们通过ct扫描实验和计算机重构技术获得了岩石微观孔隙结构。作为CT扫描实验获得岩石孔隙结构的补充,重构方法在当前昂贵与耗时实验条件的限制下显得不可或缺。本文介绍了基于传统模拟退火算法改进得到的高效岩石孔隙结构模型重构算法。通过在重构过程中加入模拟岩石成岩的过程,改进了模型重构初期的执行效率和算法有效性;在重构算法中引入分形几何方法,更好地描述了孔隙结构复杂的几何形态;同时采用了新的系统迭代更新方式,提高了算法后期的执行效率。通过与CT扫描实验得到的参考模型相对比表明:重构模型与参考模型有较好的几何相似性,基本一致的几何统计特征,相似的小岛分形维数,相似的拓扑参数以及基本一致的单轴压缩力学响应。这些对比表明了重构算法的有效性。其次,由于当前实验条件的限制,难以通过实验直接获得低渗岩石内部不连续结构随应力变化的准确描述。因此利用岩石数字岩心模型开展相关研究,是完成这一任务必要和有效的方法。在微观模型的建立过程中,我们融合了宏观分析所得到的低渗岩石中微裂纹随有效应力增大而闭合的结论。在孔隙结构模型的基础上建立了岩石三维孔隙—微裂纹模型。通过在岩石三维孔隙结构模型的基础上增加随机构造的微裂纹,形成孔隙—微裂纹模型。分析中采用孔隙—微裂纹模型代表较低有效应力范围内低渗岩石内部的微观结构,采用孔隙模型代表较高有效应力范围内岩石内部的微观结构。这两组模型的建立,可以用来对比分析微观结构演化对岩石渗透率的影响。再次,我们采用格子玻尔兹曼方法,即Lattice Boltzmann Method(LBM),对比分析了两组模型内部流体的速度场分布,并换算得到了两个模型的渗透率。LBM可以直接利用具有复杂非连续结构的数字岩心模型进行渗透性质分析,相比传统计算流体力学方法难以考虑复杂边界具有天然的优势。通过对比计算发现,微裂纹的存在,大大增加了模型内部的有效流动通道。虽然微裂纹所占的孔隙体积比很小,但作为关键的流动通道,它们的存在大大提高了岩石的渗透性。在微观尺度上建立岩石内部孔隙及微裂纹结构,考虑由于应力改变导致孔隙结构的变化,并借助LBM方法研究结构内部流体的流动性质,可以直观定量地分析微观孔隙结构和岩石渗透性质之间的关系。通过LBM流动模拟,我们直观地展示了微裂纹对岩石渗透性质的巨大影响。总的来说,在岩石微观数字模型基础上开展的分析,可以帮助我们更深入地理解与认识岩石宏观物理力学性质外在表现背后的内在机理。本文从宏观和微观两个层面着手,详细地分析了低渗岩石渗透率的应力敏感性。在宏观描述方面,借助TPHM模型,建立了低渗岩石孔隙率、渗透率随有效应力变化的规律。在微观方面,建立了孔隙—微裂纹模型,借助LBM方法模拟了模型内部流体的流动规律,直观展示了微裂纹对模型渗透率的巨大贡献。分析表明低渗岩石内部的微裂纹在应力作用下产生相对自身来说较大的变形,这是低渗岩石渗透率应力敏感性外在表现的内在原因。
[Abstract]:The physical and mechanical properties of low permeability rocks are the hot spots in the fields of underground engineering and infrastructure construction, coal and oil and gas resources exploitation, hydraulic and hydropower development, nuclear waste disposal and energy reserve, and CO2 geological sequestration. Understanding and understanding the physical and mechanical properties of low permeability rocks is the core of solving the practical problems in the engineering field. In the course of construction and implementation of these projects, the stress environment of low permeability rocks is constantly changing because of the influence of excavation disturbance or actual conditions. The physical and mechanical response of low permeability rocks evolves with the change of mining stress. For example, the excavation of underground chamber leads to the release of Yuan Yan stress, in the mining stress and the initial ground stress. The development of micro cracks in the rock under the joint action leads to the physical and mechanical properties of the surrounding rock, especially the change of permeability with the mining process. When the coal and gas are combined in the deep coal mining, the change law of the permeability of the coal seam caused by the mining should be considered so as to implement the effective extraction process. The physical and mechanical properties of the surrounding rock caused by the heat release are changed; when the low permeability oil and gas reservoirs are exploited, the permeability of the rock samples should be measured in order to obtain a more accurate reservoir permeability characterization. In the CO2 geological seal, the cover seal of the cover should be considered to be effective in the cover layer of the CO2 floatation. The influence of stress changes on the physical and mechanical properties of the rock permeability, so it is of great significance to study the change of the physical and mechanical properties of the low permeability rock with the change of the stress state of the ground. It is of great significance to determine the reasonable design and implementation of the engineering construction. It is found that the variation of permeability of low permeability rock with effective stress is very different from that of conventional rock. In the small effective stress range, with the increase of effective stress, the porosity of low permeability rock shows a little change in porosity and a large decrease in permeability. In order to describe and reveal the pressure sensitive effect of low permeability rocks, scholars at home and abroad have carried out a lot of research on two aspects of macro experimental data fitting and numerical analysis on the basis of microscopic models. In this paper, the experimental data of porosity and permeability variation of low permeability rocks are collected systematically, and the results of current scholars' use of effective stress relation of porosity and effective stress relation of permeability are analyzed. These correlations do not accurately describe the low permeability rock in the whole stress range. The measurement data of porosity and permeability of stone, and these correlations are mostly derived from empirical fitting, and can not accurately reflect the internal physical laws of these external representations of rock, and can not describe the pressure sensitivity effect of low permeability rocks reasonably and accurately. Most of these studies simplify these discontinuous structures. At the same time, the current micro rock model is difficult to accurately characterize the changes in the discontinuous structure of the rock interior. And the current computational fluid method will encounter great difficulties when dealing with complex boundary conditions of the inner pore structure of the rock, which makes it difficult to simulate. From the two aspects of macroscopic and microscopic studies, we try to describe and quantitatively describe the relationship between permeability and effective stress of low permeability rocks. At the same time, we try to explain the internal mechanism of the external performance of low permeability rock pressure sensitivity effect combined with the conclusions obtained by macro and micro analysis. According to the data, the rock "two part Hooke's law model", that is, two-parthooke 'Smodel (tphm), set up a macroscopic theoretical relation type which is more in line with the actual physical law,.Tphm conceptually divides the rock into "soft", "hard" two parts, and uses different forms of law to describe the large and different deformation of the soft and hard two parts. Law. As the name suggests, the soft part produces larger deformation relative to its own size after the force, which is more accurately described by the natural strain relation; the hard part deformation law is similar to the traditional rock small deformation theory, and is approximately described by the engineering strain relation. The relationship between the two parts of the soft and hard strain and the strain and strain is different, which leads to the rock holes. The contribution of the gap rate to the permeability is different. The two parts of the soft and hard joint constitute the rock whole. By this division, the permeability change law of the low permeability rock can be more accurately described. We use the power function equation to fit the relationship between the porosity and the permeability of the soft part. The fitting results show the soft part hole. The clearance rate and permeability are roughly conformed to the "cubic law", which indicates that the pressure sensitivity effect of low permeability rocks is caused by the closure of micro cracks within the rock. The macroscopic relation formula established in this paper has the following advantages: (1) the establishment of the macroscopic conceptual model takes into account the heterogeneity of the rock. This is a more reasonable physical description relationship. The formula has laid a solid foundation; (2) this macro model can more accurately reflect the changes in the physical and mechanical properties of the rock in the range of the whole stress test, especially the nonlinear changes that can characterize the rock in the lower effective stress range, such as the nonlinear reduction of porosity in the lower effective stress range and the significant decrease in permeability. 3) the model adopts the porosity as the intermediate bridge, and analyzes the permeability change caused by the inhomogeneous compression deformation of the discontinuous structure after the stress of the low permeability rock. Compared with the direct analysis of the permeability with the effective stress change, it is more consistent with the physical cognition. (4) each parameter in the macroscopic model relation has a clear physical meaning, not for the purpose of achieving it. The formula fitting for the experimental data is described. (5) this macro model can give the description and interpretation of low permeability rock pressure sensitivity effect in accordance with the law of physics. A series of rock porosity and permeability derived in the tphm theory framework can be more accurately and reasonably reflected in the stress variation of low permeability rocks. At the same time, the pressure sensitive effect of low permeability rock can be explained reasonably. In addition, we analyze the production decline curve of shale gas production by using the relational formula derived from macro theory. As an application of tphm, we fuse the shale permeability with the tphm relation of effective stress change to the numerical simulation software. The production decline curve of shale gas is analyzed by COMSOL. Through the simulation, it is found that the rational transformation of the reservoir is the precondition for shale gas production. The reduction of reservoir gas pressure is the main factor of the decline of shale gas production, and the permeability of reservoir rock has a great influence on the decline curve of shale gas production with the change of effective stress. The model considering reservoir rock permeability stress correlation shows a more rapid decline in output rate and longer production years. On the microcosmic aspect, we set up a digital core model which can accurately characterize the pore structure of rock, based on the pore structure characteristics of rock, and based on this digital model. The permeability properties of rock are analyzed. First, the permeability of rock is closely related to the pore structure of rock. It is very important to accurately and quantitatively characterize the pore structure. In this study, we obtained the micro pore structure of rock through the CT scanning experiment and the computer reconstruction technique. As a supplement to the pore structure of the rock as a CT scanning experiment, the reconfiguration side of the rock is obtained. The method is indispensable under the restriction of expensive and time-consuming experimental conditions. This paper introduces the reconstruction algorithm of high efficient rock pore structure model based on the improvement of the traditional simulated annealing algorithm. By adding the rock formation process during the reconstruction process, the efficiency and effectiveness of the model reconfiguration are improved; The fractal geometry method is introduced to describe the geometric shape of the complex pore structure, and the new method of updating the system is adopted to improve the efficiency of the later period of the algorithm. The comparison of the reference model obtained from the CT scanning experiment shows that the reconstruction model has a better geometric similarity and is basically the same as the reference model. The geometric statistical features, similar fractal dimensions of small islands, similar topological parameters and the basic uniform uniaxial compression mechanical response. These comparisons show the effectiveness of the reconstruction algorithm. Secondly, it is difficult to obtain the exact description of the internal discontinuous structure of low permeability rock with the stress change due to the restriction of the current experimental conditions. Therefore, using the rock digital core model to carry out the related research is a necessary and effective method to accomplish this task. In the process of establishing the microscopic model, we fuse the conclusion that the micro crack in the low permeability rock is closed with the increase of effective stress. On the basis of the pore structure model, the three-dimensional pore of the rock is established. Micro crack model. By adding the micro crack of random structure on the basis of the three-dimensional pore structure model of rock, the pore micro crack model is formed. In the analysis, the pore micro crack model represents the micro structure inside the low permeability rock under the lower effective stress range, and the pore model represents the inner rock within the high effective stress range. The establishment of the two sets of models can be used to compare and analyze the influence of microstructure evolution on rock permeability. Again, we use the lattice Boltzmann method, Lattice Boltzmann Method (LBM), to compare and analyze the velocity distribution of the internal fluid in the two groups of models, and convert the permeability.LBM of the two models. The analysis of the permeability of the digital core model with complex discontinuous structures is a natural advantage compared with the traditional computational fluid mechanics method which is difficult to consider complex boundary. It is found that the existence of micro cracks greatly increases the effective flow channel inside the model. Although the pore volume of the micro crack accounts for the pore volume. It is very small, but as the key flow channel, their existence greatly improves the permeability of rock. Establishing the pore and micro crack structure of rock inside the micro scale, considering the change of the pore structure due to the stress change, and using the LBM method to study the flow properties of the internal fluid in the structure can be used to analyze the micropores quantitatively and quantitatively. The relationship between gap structure and rock permeability. Through LBM flow simulation, we intuitively show the great influence of micro cracks on rock permeability. In general, the analysis based on the micro numerical model of rock can help us to understand and understand the external performance of rock macroscopic physical and mechanical properties more deeply. In this paper, the stress sensitivity of low permeability rock permeability is analyzed in detail from two aspects of macro and micro levels. In the macroscopic description, with the help of TPHM model, the porosity of low permeability rocks and the law of permeability change with effective stress are established. In the microcosmic aspect, the pore micro crack model is built, and the LBM method is used to simulate the model. The flow law of the internal fluid in the model shows the great contribution of the micro crack to the permeability of the model. The analysis shows that the micro cracks in the low permeability rocks produce relatively large deformation under stress, which is the intrinsic reason for the external performance of the permeability stress sensitivity of low permeability rocks.
【学位授予单位】：中国矿业大学(北京)
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TU45

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