| Budget Amount *help |
¥2,100,000 (Direct Cost: ¥2,100,000)
Fiscal Year 2004: ¥1,000,000 (Direct Cost: ¥1,000,000)
Fiscal Year 2003: ¥1,100,000 (Direct Cost: ¥1,100,000)
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| Research Abstract |
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.
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