In order to use micro-seismic data for understanding flow-pathway structure in subsurface, we considered in detail the process of pressure propagation to cause micro-seismicity in hydraulic stimulation. Then we found that pore pressure distribution along flow-pathway and its variation with time could be estimated by analyzing the data of micro-seismicity. Furthermore, the estimated pore pressure allows us to estimate the location of flow-pathways and the distribution of hydraulic conductivity along them. To do this, we assume an appropriate model of flow-pathway structure and adjust it as the pore pressure distribution computed by the model agrees well with that estimated from micro-seismicity. Finally, by a numerical experiment for one dimensional case, we demonstrated how we can optimize the model of flow-pathway according to the input data of pore pressure.
1. INTRODUCTION
For oil production, a network of natural fractures plays a role of flow-pathways to connect oil reservoirs with production wells. The fracture network is used also as heat exchanger in the advanced geothermal heat extraction system, i.e. the Hot Dry Rock system. Thus for designing and managing the oil-flow system and the heatexchanging system, it is important to know the flow-pathway structure [1-3]. We will present here a new concept to detect both of the location of flowpathways and the distribution of hydraulic conductivity along them by analyzing the data of micro-seismicity accompanying hydraulic stimulation.
2. PRESSURE PROPAGATION CAUSING MICRO-SEISMICITY IN HYDRAULIC STIMULATION
Hydraulic stimulation is carried out to improve the fracture connectivity and also to connect the fracture network with production / injection wells by creating new fractures and re-activating old fractures. In this operation, massive fluid is injected into subsurface rock through drilled wells. Then a number of micro-seismicity is commonly observed.
It is believed that the stimulation raises pore pressure in pre-existing fractures, the elevated pressure reduces friction between the fracture planes, and finally shear sliding occurs for optimally-oriented fractures to cause microseismicity. Based on such consideration, the detected source location of micro-seismicity has been used to estimate the location of flow-pathway. However, the source locations are usually distributed as a "cloud". The cloud-like distribution tells us just rough alignment of flow-pathways basically, and it does not allow us quantitative estimation of hydraulic conductivity along them while such kind of approach has been advancing year after year.
In order to detect more precise flow-pathway structure from micro-seismic data, let us consider in more detail the process of pressure propagation to cause micro-seismicity in hydraulic stimulation. We assume a model case that micro-seismicity occurs around the injection well as schematically shown in Fig. 1(a). As described above, there should exist one fracture at each location of microseismicity, and its sliding to cause micro-seismicity occurs due to the increase in pore pressure inside of the fracture. The additional pressure should be transferred to the fracture from the injection well through some flow-pathways.
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There may be treelike flow-pathways as shown in Fig. 1(b). Thus high pressure at the injection well propagates through major pathways, and it finally reaches fractures by way of relatively short branches connecting the fractures and the major pathways.
