Temporal Distributions of Microearthquake Magnitudes During Fluid Injection in Rocks (Invited)
Abstract
Fluid injections in geothermal systems usually induce small earthquakes (-3 < M < 2). Occasionally, however, earthquakes with larger magnitudes occur. However, in the case of hydraulic fracturing of hydrocarbon reservoirs significant events occur never or extremely seldom. In the last case, in contrast to the former one, the structure of rocks is being actively destroyed and the fluid-rock interaction is strongly non-linear. What is the role of this non-linearity? We consider pore pressure relaxation in rocks as the main factor responsible for event triggering. The process of fluid-rock interaction can be strongly non-linear in the sense of a strong impact of the fluid injection on rock permeabilities. Such situations like a hydraulic fracturing or a shear-caused dilatation permeability enhancement are taken into account. We compare our theoretical results with various case studies from geothermal and hydrocarbon reservoirs. We show that under rather general conditions a probability of an event with a magnitude larger than a given one increases proportionally to the injected fluid mass. This rule seems to hold independently of the non-linearity of the fluid-rock interaction. We find that the process of (in generally non-linear) pore-pressure diffusion in a poroelastic medium with randomly distributed sub-critical cracks obeying a Gutenberg-Richter relation well explains our observations. The magnitude distribution is mainly inherited from the statistics of pre-existing fracture systems. Our observations support the hypothesis that the magnitude probability of a single given event is independent of the event number. The number of earthquakes with magnitudes greater than a given one also increases with the strength of the injection source and the tectonic activity of the injection site. We have also introduced a quantity, seismogenic index, helping to characterize the level of seismic activity corresponding to fluid injections into rock. Our formulation provides a way to estimate expected magnitudes of induced earthquakes. It can be used to avoid significant earthquakes by correspondingly planning fluid injections.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.S31E..01S
- Keywords:
-
- 4475 NONLINEAR GEOPHYSICS / Scaling: spatial and temporal;
- 7209 SEISMOLOGY / Earthquake dynamics;
- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting;
- 8123 TECTONOPHYSICS / Dynamics: seismotectonics