Induced microseismicity and reservoir growth at the Fjällbacka hot dry rocks project, Sweden

Abstract

Although still relatively uncommon, microseismic detection is accepted as a valuable technique in monitoring large-scale hydraulic injection or extraction from rock at depth. Even through microseismicity may potentially provide information on rock mass behaviour that is unavailable from other remote techniques, the interpretation of microsiesmic activity is often restricted to the analysis of their locations. This restricted usage is primarily due to our current limited understanding of fundamental mechanisms of microseismic energy release and their relations to the mechanics of rock deformation. In this paper, microseismic data will be used to investigate the mechanism of reservoir growth during viscous fluid injection into a jointed granitic rock mass at the Fjällbacka Hot Dry Rock geothermal energy site in Sweden. It will be demonstrated that using simple seismological and rock mechanics interpretation techniques it is possible to obtain a valuable insight into the behaviour of a jointed rock mass during fluid injection. It will be shown through the combined interpretation of microseismic fault plane solutions and in situ stress data, using the simple rock mechanics concept of peak shear strength, that reservoir growth took place through the shear failure of shallowly dipping natural joints due to sufficiently elevated pore fluid pressures.