基于微平面模型的深层页岩各向异性蠕变机理和模型研究

Anisotropic creep characteristics of deep shale have significant influences on the deep shale gas reservoir stimulation and production assessment. However, the anisotropic creep mechanism and theoretical model of deep shale are still unclear. It is necessary to study the creep behavior of deep shale. To accomplish this goal, the basic creep patterns of shale are identified by theoretical analysis firstly; the anisotropic tests are designed with considering the basic creep patterns and the storage situation of deep shale, then the effects of high temperature, stress and anisotropy on the shale creep mechanism are analyzed based on the experiments results. To model the anisotropic damage creep of deep shale in a mathematical way, a new microplane configuration named spherocylindrical microplane configuration with clear physical meaning is proposed. According to the configuration, spherocylindrical microplane constitutive model, which can describe both the inherently anisotropy and stress as well as heat induced damage of shale, is established. The generalized Kelvin chain and continuous relaxation spectrum, which consider the effects of temperature, are adopted to calculate the three dimensional anisotropic creep deformation of deep shale under complicated loading pattern, then this calculation method is coupled with the spherocylindrical microplane constitutive model to construct the anisotropic damage-creep model for deep shale. Finally, a simulation method is developed based on the new theoretical model. Different numerical models which consider the storage situation of deep shale are simulated to quantitatively evaluate the influences of creep on the crack closure and reservoir’s permeability. This project can provide theoretical support for assessing the shale gas production forecasting and economic analysis.

深层页岩各向异性蠕变特性对深层页岩气储层改造和产量预测均有重要影响。然而，对深层页岩蠕变机理的认知和理论模型的建立都严重落后于工程实践，有必要对此开展研究。因此，本项目首先分析确定页岩各向异性蠕变的三种基本模式，考虑深部页岩赋存环境，针对性的设计页岩各向异性蠕变实验，探索高温、高应力和各向异性对页岩蠕变的影响机制。在深层页岩损伤蠕变模型方面，提出物理意义明确的球-柱微平面构型，基于该构型建立能同时表征页岩层理构造和应力、温度引起的各向异性损伤的球-柱微平面模型；根据广义开尔文链和连续豫驰谱分析，建立考虑温度效应的深层页岩三维各向异性蠕变计算理论，进而将该理论与球-柱微平面模型耦合得到深层页岩各向异性损伤蠕变模型。发展基于新模型的数值模拟方法，定量评估不同深部赋存环境下页岩蠕变对储层裂纹闭合和渗透率的影响。该项目可为评价深层页岩气井产量预测和经济评估等提供理论支撑。

深层页岩各向异性蠕变特性对深层页岩气储层改造和产量预测均有重要影响。然而，对深层页岩蠕变机理的认知和理论模型的建立都严重落后于工程实践，有必要对此开展研究。因此，本项目开展了4类页岩各向异性蠕变实验：4种不同层理倾角下三轴压缩多级加载蠕变测试、4种不同层理倾角下直接剪切蠕变测试、2种层理倾角下长达480余天的长时蠕变测试、模拟不同赋存深度的高温高压蠕变测试。通过蠕变实验数据分析，发现页岩的各向异性对蠕变变形特征有重要的影响，蠕变变形和稳态蠕变率在层理倾角为45°时最大而在层理倾角为90°时最小。探明了页岩存在三种边界条件下的蠕变机制：(1)应力分量方向与层理面垂直、(2)应力分量方向与层理面平行、(3)纯剪应力作用下的页岩蠕变。证明了目前常用的用于描述蠕变变形的幂次函数是不合理的，这类函数一般无法描述初始蠕变变形，并且该类模型无法考虑页岩的各向异性。因此，提出了能够同时表征页岩层状构造特征和方向性非弹性变形的球-柱微平面构型；以新构型为基础，建立以页岩宏观各向异性损伤模型。基于新提出的三种页岩蠕变机制，建立了一般加载模式下的页岩三维各向异性蠕变计算方程。将页岩宏观各向异性损伤模型和蠕变计算方程耦合，最终建立了基于微平面理论的页岩各向异性损伤蠕变模型。通过对模型进行二次开发，对丁山地区不同赋存深度（对应不同温度和不同地应力）的页岩的裂缝闭合过程进行了分析，结果显示对于只有单层支撑剂支撑的裂缝，经历80天蠕变变形后，在蠕变的作用下该不光滑裂缝的将闭合，进而影响页岩气储层产气量。