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

在描述储层裂缝时，最关键的因素是初始裂缝孔径、裂缝尺寸和界面刚度。为了估计这三个参数，本方法比较了从现场实验中获得的峰值频率与理论分析充满流体的裂纹的动力学问题所提供的期望。在轴对称变形模式下，裂纹被建模为硬币形裂纹，流体从其周边泄漏。分析是借助边界积分方程法进行的。在第一个财政年度，基本性质的影响因素，如粘度，压缩性，杨氏模量等，进行了调查，强调周围的裂纹损伤区的影响。在第二个财政年度，该方法被应用到两个示范点，以检查其适用性。首先，将该方法应用于东北大学东八满台HDR试验场。在试验现场，有一个水循环地下系统，由两个威尔斯井穿过水力压裂产生的主要裂缝。利用附近井的钻井噪声作为人工震源，激发裂缝内的流体。发现裂缝半径约为40 m。孔径取决于储层压力。当井口地层压力小于2 MPa时，井口压力差约为0.3mm;当井口地层压力大于2 MPa时，井口压力差约为0.5mm。变化非常剧烈;在2 MPa下，它似乎几乎是逐步变化的。其次，将该方法应用于澳大利亚HDR场地观测到的地震同相轴。该事件约4000米深。衰减非常弱，信号持续超过100秒。估计其半径和孔径分别为160 m和17 mm。界面刚度非常小。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.