7/28/2019 00-Mechanics of Seismic Instabilities Induced

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Mechanics of seismic instabilities induced bythe recovery ofhydrocarbons

Grasso, J.-R.

Pure and Applied Geophysics PAGEOPH, Volume 139, Issue 3-4, pp. 507-534

We review earthquake distributions associated with hydrocarbon fields in thecontext of pore pressure diffusion models, poroelastic stress transfer andisostasy theory. These three mechanisms trigger or induce seismic instabilities atboth local scale ( D5 km) and at regional scale ( D20 km). The modeledchanges in stress are small (1 MPa), whatever the tectonic setting. Eachmechanism corresponds to different production processes. (1) Local hydraulicfracturing due to fluid injection induces seismic-slip on cracks ( M L3) within theinjected reservoir through decreasing the effective stress. (2) Pure fluid

withdrawal causes pore pressure to decrease within the reservoir. It triggersadjustments of the geological structure to perturbations related to the reservoirresponse to depletion. Poroelastic mechanisms transfer this stress change fromthe reservoir to the surrounding levels where M L5 seismic instabilities occureither above or below the reservoir. (3) Massive hydrocarbon recovery inducescrustal readjustments due to the removal of load from the upper crust. It caninduce larger earthquakes ( M L6) at greater distance from the hydrocarbonfields than the two other mechanisms. Due to the mechanical properties of theshallow 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 localdeformation is reported as aseismic, i.e., there is no evidence for ML3earthquakes. Nevertheless, the induced stresses vary as a function of time in amanner that depends on the hydraulic diffusivity (i.e., permeability) of thereservoir and surrounding rocks. Because small earthquakes ( M L3) indicatechanges in stress and pore pressure, monitoring of seismicity is a means ofassessing in situ reservoir behavior. The less constrained seismic response tohydrocarbon recovery is the possible connection between local fluidmanipulations, triggered earthquakes and major regional earthquakes. Positive

feedback mechanisms suggest that the region ofseismic hazard changes is muchlarger than the area where hydrocarbons are extracted. These observations andmodels testify that fluid movement and pore pressure changes (increase ordecrease) play important roles in themechanics of earthquakes and in thetriggering of natural earthquakes.

Keywords: Induced seismicity, faulting, fluid, hydrocarbon, pore pressure

DOI:10.1007/BF00879949

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