深部煤系地层复合相态CO2渗流特性与致裂机理

Coalbed methane (CBM) resources in deep coal in China are abundant, and the amount of CBM buried deep in 1200~2000 m is 18.47×1012 m3. However, as the deep CBM reservoirs generally have the characteristics of high temperature, high ground stress, strong plastic deformation and low permeability, the conventional hydraulic fracturing theory is difficult to work, so it is very urgent to explore new theory and technology on coal fracturing and increasing permeability. In this project, on the base of the characteristics of high density, low viscosity and strong diffusivity of supercritical CO2, fracturing for increased production of CBM in deep coal in terms of supercritical CO2 fracturing and the relative research are to be conducted. The dimorphic switching and percolation characteristics of supercritical CO2 in coal deeper than 1000 meters, coal damage and fracture characteristics in coupling of temperature-stress-supercritical CO2 and the effect of coal-rock interface on cracks propagation in supercritical CO2 fracturing are to be studied in terms of a combination of testing, theoretical analysis and numerical simulation. The percolation characteristics of mixed phase carbon dioxide and the evolution mechanism of its effect on coal structure under high stress and high temperature are to be revealed. Additionally, a seepage model of CO2 in mixed dimorphic, a damage-fracture visco-plastic model of deep coal-rock in supercritical CO2 fracturing and the cracks initiation and propagation principles of coal-rock are to be established. Eventually, cracks initiation and propagation criterion and crack-net formation mechanism of deep coal-rock in supercritical CO2 fracturing are to be ascertained, which can provide important theoretical foundations for promoting efficient exploitation of CBM in deep coal measure strata by supercritical CO2 fracturing.

我国深部煤层气资源丰富，埋深1200~2000m的煤层气资源量达184700亿m3，而深部煤层气储层具有地温高、地应力大、塑性变形强、及渗透性低的特点，传统水压致裂增渗理论难以凑效，急需探索新的致裂理论与技术。本项目针对ScCO2“高密度、低粘度、易扩散”的特点，拟开展ScCO2压裂深部煤层增产煤层气基础研究。通过实验、理论分析与数值模拟等方法，对千米以深条件下煤岩体中ScCO2相态转化与渗流特性、温度-应力-ScCO2耦合作用下煤体损伤断裂特性、煤岩界面对ScCO2压裂裂缝扩展影响等内容进行研究。在揭示高地温与高地应力条件下，复合相态CO2渗流特性及其对煤体结构损伤演化机理的同时，建立深部煤系地层复合相态CO2渗流模型、ScCO2压裂煤岩体损伤断裂粘塑性模型、及裂缝起裂扩展准则，最终探明ScCO2压裂深部煤岩体缝网形成机理，为深部煤系地层ScCO2致裂促进煤层气高效开采提供重要理论基础。

由于深部煤层气储层具有地温高、地应力大、塑性变形强、及渗透性低的特点，传统水压致裂增渗理论难以凑效，探索新的致裂理论与技术是深部煤储层煤层气开采面临的关键问题。本项目围绕ScCO2“高密度、低粘度、易扩散”的特点，开展高温高压条件下煤体ScCO2相态转化及渗透特性、温度-应力-ScCO2耦合作用下煤体损伤断裂特性及机理、ScCO2压裂煤体起裂与裂缝扩展规律、煤岩界面对ScCO2压裂裂缝穿层扩展影响规律、及弹塑性煤体压裂裂缝扩展模型及模拟等内容研究，为深部煤系地层ScCO2致裂促进煤层气高效开采提供理论基础。经过四年的努力，主要研究成果如下： .（1）揭示了CO2驱替煤层CH4过程中，煤体CO2/CH4混合气体渗透特性及其与多元气体浓度比例的相关性，建立了煤体多元混合气体渗透率模型，发现了气体运移过程中ScCO2相变对煤体渗透率的削弱作用，提出了由不同CO2相态主控的三种煤体CO2渗透率演化机制。.（2）发现了ScCO2对煤体脂肪族化合物的强萃取能力，阐明了ScCO2浸蚀作用对煤体微观结构损伤机理，揭示了温度-应力-ScCO2耦合作用下，不同阶煤多尺度结构的时变劣化特征，探明了ScCO2浸蚀对煤体I和II型断裂韧度的影响规律。.（3）发现了煤岩体ScCO2压裂的起裂压力低、产生裂缝复杂，分叉裂缝显著的特点，掌握了不同界面强度煤岩裂缝扩展贯通形态特征，提出了SCCO2压裂裂缝跨界面穿层扩展的临界差应力与界面粘聚力的影响关系。 .（4）揭示了不同阶煤裂纹尖端过程区内黏聚力与相对位移间的非线性力学行为，构建了基于黏聚型裂纹本构关系的煤岩压裂韧性破坏模型，表征了煤岩松软的力学特性及其裂纹尖端过程区对弹塑性煤体压裂过程的影响规律。.（5）提出了表征煤体塑性破坏的粘聚性单元，建立了基于B-K混合断裂能准则的黏聚型裂纹扩展本构模型，发展了弹塑性煤体压裂裂缝扩展的有限元-离散元模拟方法，揭示了不同非均质程度的煤储层在压裂过程中复杂裂缝形态的演化规律。.在本项目资助下，共发表学术论文16篇，其中SCI、EI收录论文15篇；国际会议EI论文1篇，申请国家发明专利10项，其中授权4项；培养博士后1名、博士生7名（毕业3名）、硕士生6名（毕业4名）；获得山西省技术发明一等奖1项（排名第一），受邀在多个国内外会议上做学术报告，2020年和2022年分别主办第二届和第三届原位改性采矿国际会议。