油页岩原位热解物理力学特性演化及其对地下水污染基础研究

The technology of oil and gas exploitation by injecting steam to oil shale in situ, which can realize high-efficient mining by using hydraulic fracturing and proppant in underground oil shale terrane to increase permeability and injecting superheated steam to pyrolyze oil shale, is an emerging technology. For the problem that this technology could influence the groundwater environment caused by the changes of oil shale and its roof and floor rocks’ physical and mechanical properties, in this scientific research project, Jimusaer oil shale mining area in Xinjiang is taken as the study area, basing on the occurrence characteristics of oil shale, and the influence on quality and occurrence environment of groundwater is studied according to the occurrence characteristics of oil shale. It is obviously that this study has significantly practical value. Theoretical analysis, experimental research, numerical simulation and similar model experiment will be plan to use, and rock mechanics, seepage flow mechanics, structural mechanics, and heat-flow-solid-chemistry coupling theory will be applied in this project to study the change rule of oil shale and its roof and floor rocks’ elastic modulus and coefficient of thermal conductivity changing with temperature and time, the change law permeability changing with the confining pressure, the variation mechanism of oil shale’s elastic modulus, the transportation law of pollutants in mining area’s groundwater. In this scientific research item, corresponding mathematical models also will be set up and put into simulation verification, measures to prevent and control pollution and some advice on process optimization will be put forward as well. It is obviously that the project’s implementation will provide theoretical basis for prevention and control of mining area’s groundwater pollution and improvement for technology of oil shale pyrolysis in situ.

“油页岩原位注蒸汽开采油气”技术，是在地下油页岩层中实施水力压裂和支撑剂支撑，增加油页岩渗透性，注过热蒸汽热解并驱油气，实现高效开采。针对该工艺会改变油页岩及其顶底板岩石弹性模量、渗透性等物理力学特性，进而影响采区地下水环境的问题，本项目以新疆吉木萨尔油页岩矿区为研究区，依据其油页岩赋存特征，研究过热蒸汽原位热解油页岩对采区地下水赋存环境以及地下水水质的影响，具有明显的实用价值。项目拟采用理论分析、实验研究、数值模拟、相似模型试验等手段，应用岩石力学、渗流力学、结构力学和热-流-固-化耦合等理论，研究油页岩及其顶底板岩石弹性模量、导热系数等随温度和时间的变化规律，渗透率随围压的变化规律，油页岩弹性模量变化机理，采区地下水中污染物迁移规律，建立相应数学模型并模拟验证，提出污染防治措施和工艺优化建议。为防治采区地下水污染和改进油页岩原位热解技术提供理论依据。

本研究以油页岩原位热解为工艺背景，对油页岩原位热解造成的地下水污染问题开展基础性研究，即研究热解过程中油页岩孔隙率、弹性模量、渗透率变化规律与变化机理，并基于机理建立定量模型，为后期污染物扩散运移提供基础本构模型。在此基础上，开展污染物运移数值模拟，研究污染物迁移分布规律，并从环保和经济角度，对油页岩原位热解工艺提出建议。结论如下：.1）结合热解过程中油页岩物理力学特性变化机理和油页岩热解反应动力学，初步建立了热解过程中油页岩孔隙率定量计算模型和油页岩弹性模量定量计算。模型计算结果与实验结果吻合良好，能较准确地计算热解过程中油页岩孔隙率和弹性模量，误差较小。.2）热解过程中油页岩物理力学特性发生变化主要是由油母质热解造成的，此外，油页岩骨架在高温条件下的损伤劣化也是造成油页岩弹性模量变小的一个重要原因。.3）随着温度的上升，油页岩孔隙率增大速率呈现加快趋势，油页岩弹性模量衰减速率逐渐加快，衰减幅度逐渐增大。.4）建立了原位热解过程中油页岩孔隙率定量模型和油页岩渗透率定量模型，通过模拟计算发现：相同温度与热解时间条件下，油页岩孔隙率与渗透率受压力影响不大，变化很小；压力和温度相比较而言，油页岩孔隙率与渗透率主要受温度影响，随温度上升而快速增大。.5）在油页岩原位热解开采完毕20a后，即便是在最不利情形下，由于热解后油页岩渗透率仍然很低，各污染物仍主要集中于热解油页岩层中，其在上部潜水层与底部细砂岩层中的运动迁移量很小，污染物浓度低，地下水水质基本不受影响。.6）油页岩原位热解工艺绿色、高效、环保，基本不会给采区地下水带来污染问题。地下水pH会随热解温度的升高而逐渐增大，呈碱性，在600℃时甚至呈强碱性。尽管高温热解速率快，但高温所需的能量成本高，且更易破坏地下水体环境，使地下水体呈强碱性。因此，过度地追求热解速率不可取，结合生产效率、注热成本和环境效应，建议油页岩热解温度不宜过高，以400℃-500℃为宜。