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 transfer and isostasy theory. These three mechanisms trigger or induce seismic instabilities at both local scale (D less-than-or-equal-to 5 km) and at regional scale (D greater-than-or-equal-to 20 km). The modeled changes in stress are small (less-than-or-equal-to 1 MPa), whatever the tectonic setting. Each mechanism corresponds to different production processes. ( 1) Local hydraulic fracturing due to fluid injection induces seismic-slip on cracks (M(L) less-than-or-equal-to 3) within the injected reservoir through decreasing the effective stress. (2) Pure fluid withdrawal causes pore pressure to decrease within the reservoir. It triggers adjustments of the geological structure to perturbations related to the reservoir response to depletion. Poroelastic mechanisms 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 upper crust. It can induce larger earthquakes (M(L) greater-than-or-equal-to 6) at greater distance from the hydrocarbon fields than the two other mechanisms. Due to the mechanical properties of the shallow rock matrices 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 hydraulic diffusivity (i.e., permeability) of the reservoir and surrounding rocks. Because small earthquakes (M(L) less-than-or-equal-to 3) indicate changes in stress and pore pressure, monitoring of seismicity is a means of assessing in situ reservoir behavior. The less constrained seismic response to hydrocarbon recovery is the possible connection between local fluid manipulations, triggered earthquakes and major regional earthquakes. Positive feedback mechanisms suggest that the region of seismic hazard changes is much larger than the area where hydrocarbons are extracted. These observations and models testify that fluid movement and pore pressure changes (increase or decrease) play important roles in the mechanics of earthquakes and in the triggering of natural earthquakes.