Detection of Flow-pathway Structure based on Pore Pressure Distribution Estimated from Microseismic Events

In the hydraulic stimulation, a number of microseismic events are commonly observed. It is believed that injected fluid flows through subsurface pre-existing fractures, and the fluid flow raises pore-pressure in those fractures, and finally the raised pore-pressure triggers microseismic events. In the previous work of Osada et al. (2005), we have considered carefully such a scenario and come up with a method to estimate pore-pressure distribution in a rock formation by analyzing the observed data of microseismic events. On the other hand, pore pressure distribution along flow-pathways should change according to the location of flow-pathways and the distribution of hydraulic conductivity along them. Therefore, it could be possible to estimate flow-pathway structure as it gives a good explanation of the pore pressure distribution. Based on this idea, in the present work we propose a new method to estimate the flow-pathway structure. We assume an appropriate model of flow-pathway structure and optimize it as the pore pressure distribution estimated numerically by the model agrees well with the pore pressure distribution estimated from microseismic events by the method proposed in the previous work. Then we accept the most optimized model as the answer to represent the actual flow-pathway structure. We demonstrated how we can optimize the model of flow-pathway structure according to synthetic pore pressure distributions in ID and 2D cases. Finally we applied this method to Soultz field in France and estimated the subsurface flow-pathway structure. The results were in good agreement with the flow logs carried out in the injection well.