Characterization of a Geothermal Reservoir Crack Based on the Analysis of Dynamic Response of a Crack Filled with Fluid

本项目的目标是建立一种描述充满流体的地热储层裂缝的几何和物理特征的方法。首先，我们提出了一个考虑流体渗流的弹性介质中充液裂纹的模型。上、下表面部分接触，沿裂纹方向存在界面刚度，同时考虑了流体的粘性。结果表明，在没有局部接触的情况下，当粘性和渗透率较小时，本征角频率高度依赖于长宽比(裂纹尺寸与孔径之比)。然而，在存在部分接触的情况下，本征角频率高度依赖于界面刚度。当渗透率达到通常地热场中地层的渗透率时，渗透率和粘度的影响变得更大。为了更清楚地了解渗透率和流体粘度的影响，我们分析了岩石为刚性的情况，发现存在一个使压力响应最大的流体粘度的最佳值。流体粘度的最佳取值主要取决于地层的渗透率。反之，根据地层渗透率选择粘度最优的流体，即可获得最大压力响应。最后，在分析结果的基础上，利用新引入的目标函数，建立了估计裂纹长宽比、初始裂纹开度和界面刚度的反问题。

The objective of the present project is the establishment of a method for characterizing the geometrical and physical characteristics of a fluid-filled geothermal reservoir crack. Firstly, we have proposed a model for a fluid-filled crack within an elastic medium with permeation of fluid. The upper and lower surfaces are in partial contact to each other, thus there exists interface stiffness along the crack, and also the fluid viscosity is taken into account. It has been revealed that, in the case that the partial contact is absent, the eigen angular frequencies are highly dependent on the aspect ratio (the ratio between the crack size and aperture), when the viscosity and permeability are small. However, in the case that the partial contact is present, the eigen angular frequencies are highly dependent on the interfacial stiffness. When the permeability is large as that of a formation in a usual geothermal field, then the effects of the permeability and viscosity become stronger. In order to get clear understanding of the effects of permeability and fluid viscosity, we have analyzed the case that the rock is rigid and have found that there exists an optimum value of fluid viscosity which gives the largest pressure response. The optimum value of fluid viscosity is mostly determined by the permeability of the formation. Inversely speaking, it is possible to get the maximum pressure response by choosing the fluid with the optimum value of viscosity depending on the formation permeability. Finally, on the basis of the results of the analysis, we have set up the inverse problem to estimate the aspect ratio of the crack, the initial crack aperture and the interfacial stiffness by using a newly introduced objective function.