No abstract

Although it is generally accepted that earthquakes occur along preexisting faults, the distribution of earthquake locations is often so smeared that the underlying fault or joint structures along which the earthquakes occur cannot be inferred from visual inspection of location plots. We present a statistical method for identifying fault or joint planes within what may otherwise appear to be an amorphous earthquake location set. The method takes all the hypocenters in an event set three at a time in order to determine the strikes and dips of all possible planes within the event set. A procedure for correcting for the shape of the region in which the earthquakes occur is applied. After correction, the orientation (one or a few) that is seen most often is taken as that of the zone of preexisting fault(s) or joint(s). We applied the method to a set of hypocenters determined for microearthquakes that accompanied a hydraulic injection into crystalline rock. The method was able to resolve successively five statistically significant orientations (planes) along which most of the microearthquakes occurred. The first two planes determined by the method are parallel to one nodal plane from each of the two most commonly found fault plane solutions. One of the two planes intersects the injection well bore at a location where water is known to have entered the rock during the injection. The planes identified thus coincide with the major fluid paths during the hydraulic injection.

Hydraulic fracturing is a common procedure to increase the permeability of a reservoir. It consists in injecting high‐pressure fluid into pilot boreholes. These hydraulic tests induce locally seismic emission (microseismicity) from which large‐scale permeability estimates can be derived assuming a diffusion‐like process of the pore pressure into the surrounding stimulated rocks. Such a procedure is applied on six data sets collected in the vicinity of two geothermal sites at Soultz (France) and Ogachi (Japan). The results show that the method is adequate to estimate large‐scale permeability tensors at different depths in the reservoir. Such an approach provides permeability of the medium before fracturing compatible with in situ measurements. Using a line source formulation of the diffusion equation rather than a classical point source approach, improvements are proposed for accounting in situation where the injection is performed on a well section. This technique applied to successive fluid‐injection tests indicates an increase in permeability by an order of magnitude. The underestimates observed in some cases are attributed to the difference of scale at which the permeability is estimated (some 1 km3 corresponding to the seismic active volume of rock compared to a few meters around the well for the pumping or pressure oscillation tests). One advantage of the proposed method is that it provides permeability tensor estimates at the reservoir scale.

Experiments on CO 2 -water-rock interaction at hydrothermal temperatures have been performed to investigate dissolution and precipitation phenomena, including Ca extraction from rocks that might occur during CO 2 sequestration into geothermal fields. Distilled water samples were exposed to a CO 2 atmosphere at a temperature of 25°C and pressures up to 6 MPa. The resulting solutions were then reacted with granodiorite samples from the Ogachi hot/dry rock field and labradorite, at 200°C and 120°C respectively. The calcium concentrations in the solutions that had reacted with CO 2 were twice those with N 2 instead of CO 2 . Combined with the results of thermodynamic calculations, these observations indicate that calcium can be released from rocks (silicates) easily and might be removed as CaCO 3 and/or CaSO 4 during CO 2 sequestration into geothermal fields.