Dynamic Characteristics of a Large Fluid Filled Crack for Pollution Free Use of the Earth's Crust

在表征储层裂缝时，最关键的因素是初始裂缝开度、裂缝大小和界面刚度。为了估计这三个参数，本方法将现场试验得到的峰值频率与理论分析充液裂纹动力学问题所提供的期望进行了比较。在轴对称变形模式下，裂纹被模拟为周边有流体泄漏的硬币状裂纹。利用边界积分方程法进行了分析。在第一个财政年度，研究了粘性、可压缩性、杨氏模量等因素影响的基本性质，强调了裂缝周围破损区的影响。在第二个财政年度，该方法被应用于两个示范点，以检验其适用性。首先，将该方法应用于东北大学东八里台HDR试验场。在试验场，有一个水循环地下系统，由两口井穿过水力压裂产生的主要裂缝组成。利用附近一口井的钻井噪声作为人工震源，激活裂缝内的流体。结果表明，裂纹半径约为40m。孔径取决于储集层压力。井口处油藏压力小于2 Mpa时约为0.3 mm，大于2 Mpa时约为0.5 mm。这种变化非常剧烈；在2兆帕时，它似乎几乎是一步一步地变化。其次，将该方法应用于澳大利亚HDR现场观测的一次地震事件。这场地震大约有4000米深。衰减非常弱，信号持续超过100秒。半径估计为160米，口径估计为17毫米。界面刚度很小。这表明裂缝几乎完全打开。

In characterizing reservoir cracks, most crucial factors are initial crack aperture, crack size and interface stiffness. In order to estimate these three parameters, the present method compares the peak frequencies obtained from field experiments with the expectation provided by analyzing theoretically a dynamic problem of a fluid-filled crack. The crack is modeled as a penny shaped crack under an axisymmetric deformation mode with fluid leakage from its periphery. The analysis is performed with the aid of Boundary Integral Equation Method. In the first fiscal year, the basic nature of the influences of factors, such as viscosity, compressibility, Young's modulus and so on, is investigated, emphasizing the effect of damaged zone surrounding the crack. In the second fiscal year, the method is applied to two demonstration sites to examine the applicability. Firstly, the method is applied to Higashi-hachimantai HDR test site, Tohoku University. In the test site, there is a water circulation subsurface system consisting of two wells crossing through a predominant crack created by hydraulic fracturing. Drilling noise of a nearby well is employed as artificial seismic source to activate the fluid within the crack. It is found that the radius of the crack is about 40m. The aperture is dependent on the reservoir pressure. It is about 0.3mm for the reservoir pressure less than 2MPa at wellhead, whereas it is about 0.5mm for the reservoir pressure larger than 2MPa. The change is very sharp; it seems to change almost step wisely at 2MPa. Secondly, the method is applied to a seismic event observed at Australian HDR site. The event is about 4000m deep. The attenuation is very weak and the signal continues over 100sec. The radius and the aperture are estimated to be 160m and 17mm, respectively. The interfacial stiffness is very small. This suggests that the crack is almost completely open.