MECHANICS OF SEISMIC INSTABILITIES INDUCED BY THE RECOVERY OF HYDROCARBONS

We review earthquake distributions associated with hydrocarbon fields in the context of pore pressure diffusion models, poroelastic stress t ransfer and isostasy theory. These three mechanisms trigger or induce seismic instabilities at both local scale (D less-than-or-equal-to 5 k m) and at regional scale (D greater-than-or-equal-to 20 km). The model ed changes in stress are small (less-than-or-equal-to 1 MPa), whatever the tectonic setting. Each mechanism corresponds to different product ion processes. ( 1) Local hydraulic fracturing due to fluid injection induces seismic-slip on cracks (M(L) less-than-or-equal-to 3) within t he injected reservoir through decreasing the effective stress. (2) Pur e fluid withdrawal causes pore pressure to decrease within the reservo ir. It triggers adjustments of the geological structure to perturbatio ns related to the reservoir response to depletion. Poroelastic mechani sms transfer this stress change from the reservoir to the surrounding levels where M(L) less-than-or-equal-to 5 seismic instabilities occur either above or below the reservoir. (3) Massive hydrocarbon recovery induces crustal readjustments due to the removal of load from the uppe r crust. It can induce larger earthquakes (M(L) greater-than-or-equal- to 6) at greater distance from the hydrocarbon fields than the two oth er mechanisms. Due to the mechanical properties of the shallow rock ma trices involved, seismic slip triggered either by mechanism (1) or (2) , is a second-order process of the main elastoplastic deformation. For a minimum of 80% of commercially productive basins, most of the local deformation is reported as aseismic, i.e., there is no evidence for M (L) greater-than-or-equal-to 3 earthquakes. Nevertheless, the induced stresses vary as a function of time in a manner that depends on the hy draulic diffusivity (i.e., permeability) of the reservoir and surround ing rocks. Because small earthquakes (M(L) less-than-or-equal-to 3) in dicate changes in stress and pore pressure, monitoring of seismicity i s a means of assessing in situ reservoir behavior. The less constraine d seismic response to hydrocarbon recovery is the possible connection between local fluid manipulations, triggered earthquakes and major reg ional earthquakes. Positive feedback mechanisms suggest that the regio n of seismic hazard changes is much larger than the area where hydroca rbons are extracted. These observations and models testify that fluid movement and pore pressure changes (increase or decrease) play importa nt roles in the mechanics of earthquakes and in the triggering of natu ral earthquakes.