基于混沌理论煤层气井压裂孔裂隙分形演化与渗流特征研究

During the fracturing of the coal-bed gas wells, the evolution of the natural pore and crack forms the fracture network system, which has much difference with the evolution of the unitary fracture. It's very meaningful to distinctly describe the research on the features of the fracture system. This paper develops the description of the features of the pore and crack network system in the coal layer by using the chaos theory and the fractal theory to establish the model of the actual stress of the rock and the mechanical model of the surrounding rock of the well bore; to study the dynamic fractal feature of the pore and the microcrack of the coal during the process of the hydraulic fracture, to establish the research on the evolution of the coal body during the hydraulic fracture and the formation mechanism of the crack by incorporating the chaos theory. To analyze the branch process of the rock body during the fracturing and the formation rule of the crack; to develop the permeability of the coal body after the fracturing and the research on the analysis, the diffusion and the seeping characters of coal seam gas. This paper takes the fractals and the chaos of the coal layer as the entry point to develop the research on the formation of the crack network system of the rock body during the fracturing, which has basically revealed the physics connotation of the formation of the crack network system in the coal layer during the hydraulic fracturing and provides the reliable scientific references for the knowledge and the recognition of the essence of the formation of the crack system after the fracturing of the coal-bed gas wells and the further exploitation of the coal seam gas. This theme belongs to the frontier themes at home and abroad, which deserves important academic value.

煤层气井压裂煤岩天然孔裂隙演化形成裂缝网络系统与单一裂缝扩展差别很大。清晰地描述裂缝系统特征研究意义重大。本项目提出运用混沌、分形理论开展煤岩层孔裂隙网络系统的特征描述，建立岩石有效应力模型以及井眼围岩的力学模型；研究水力压裂过程煤岩孔隙与微裂缝动态分形特性，结合混沌理论建立水力压裂煤岩体演化及裂缝形成机理研究。分析压裂岩体裂缝分岔过程与裂缝形成规律；开展压裂后煤岩体的渗透性与煤层气解析、扩散、渗流特性研究。该项目以煤岩层分形与混沌作为问题的切入点，开展水力压裂岩体裂缝网络系统形成研究，从根本上揭示了水力压裂煤岩层裂缝网络系统形成的物理内涵，对了解和认识煤层气井压裂后裂缝系统形成本质与煤层气的进一步开采均提供了可靠的科学依据。本课题研究属国内外前沿课题，具有重要的学术价值。

煤岩发育大量的割理裂隙等结构弱面，具有强度低、弹性模量低、泊松比高、易碎等性质，与非煤岩石力学特性存在较大差别，准确地描述煤岩裂缝系统特征研究意义重大。.本项目考虑了煤层特殊的孔隙系统和裂隙系统特征，提出采用基于分形理论描述煤岩层复杂的孔裂隙系统的方法，建立了煤岩孔裂隙系统的分形描述方法，对煤岩层割理维数运用盒维数方法进行了测定；对外生裂隙运用L-系统进行了形态上耦合描述，反推其分形维数。基于分形理论建立了煤层气井井眼围岩的损伤力学模型，得出了井眼围岩力学分布的规律，给出了井眼围岩不同区域的分形损伤特征。.建立了具有分形特征的煤岩等效弹性模量和泊松比三维数值模型，并开展力学实验。研究表明，煤层裸眼完井井壁周向应力与煤岩基质和割理力学特征密切相关，煤岩基质弹性模量和泊松比、面割理与水平最大主应力方向夹角、面割理分布密度以及端割理分布密度均影响井壁应力分布状况，并且在面割理与水平最大主应力方向相同时，压裂裂缝最容易起裂，面割理间距增加，裸眼井压裂所需的裂缝起裂压力增大，不同取心方向煤岩力学参数差异明显，存在明显各向异性特征。.基于突变理论与能量守恒原理建立了煤层气井压裂煤岩体损伤破裂与裂缝分形演化模型，得到了煤岩体破裂与裂缝扩展演化的标准形式。研究了煤岩体压裂裂缝演化系统相空间重构的方法，得出了相空间重构的两个参数：延迟时间与嵌入维数的计算方法。.引入了煤岩体压裂裂缝演化系统混沌特征量：关联维数、最大Lyapunov指数和Kolmogorov熵，计算裂缝数目，尖端径向应力以及尖端周向应力的关联维数、最大Lyapunov指数和Kolmogorov熵，判别裂缝演化的混沌特征，计算结果表明，基于分形理论的裂缝演化行为具有混沌特征。.开展水力压裂微裂缝形成机理试验研究，利用提取的裂缝网络结构的混沌特征，分析水力压裂裂缝分布特性，结合混沌理论建立了裂缝分岔演化后煤岩体孔裂隙的渗透张量计算模型与煤层气的解析、扩散及渗流模型。研究结果表明，割理走向对裂缝形态的影响，水平主应力差对裂缝形态及压裂曲线的影响明显。.本项目基于非线性动力学方法揭示了煤层气井水力压裂裂缝演化的根本内涵，对了解和认识煤层气井压裂后裂缝系统展布和煤层气的高效开采提供了可靠的科学依据，研究成果对于国内外煤层气资源的开发及煤矿井下瓦斯抽放具有一定的指导作用和现实意义。