Poroelastic stressing and induced seismicity near the Lacq gas field, southwestern France

Abstract

Hundreds of shallow, small to moderate earthquakes have occurred near the Lacq deep gas field in southwestern France since 1969. These earthquakes are clearly separated from tectonic seismicity occurring in the Pyrenees, 25 km to the southwest. The induced seismicity began when the reservoir pressure had declined by ∼30 MPa. Repeated leveling over the field shows localized subsidence reaching a maximum of 60 mm in 1989. Segall (1989) suggested that poroelastic stressing, associated with volumetric contraction of the reservoir rocks, is responsible for induced seismicity associated with fluid extraction. To test this model, we compare the observed subsidence and hypocentral distributions with the predicted displacement and stress fields. We find that the relationship between average reservoir pressure drop and subsidence is remarkably linear, lending support to the linear poroelastic model. Displacements and stresses are computed based on a priori knowledge of the reservoir geometry, material properties, and reservoir pressure changes. The computed vertical displacements are found to be in excellent agreement with the subsidence observed from leveling. Stress perturbations accompanying gas extraction, computed using the same parameters, are found to be ∼0.2 MPa or less. Changes in Coulomb failure stress are computed assuming that slip occurs on optimally oriented planes. The predicted failure zones correlate very well with the spatial distribution of earthquakes if the perturbing stresses are small in comparison to the ambient regional deviatoric stresses and if the minimum regional compressive stress axis is vertical. Accurate determination of focal mechanisms of the induced events would allow a more rigorous test of the poroelastic model and could lead to important inferences about the crustal stress state.

Geomechanical models for induced seismicity in the Netherlands: inferences from simplified analytical, finite element and rupture model approaches

“…(1) We outline the main logic of geomechanical modelling approaches and insights developed in recent decades about induced seismicity related to hydrocarbon extraction (Segall, 1989;Roest & Kuilman, 1994;Segall et al, 1994;Simmelink et al, 2001;Van Wees et al, 2001Van Eijs et al, 2006;Suckale, 2009;Orlic & Wassing, 2013;Bourne et al, 2014;Van den Bogert, 2015). Subsequently, as an extension of existing modelling approaches we include (2) rupture models to evaluate the relationship between elastic stress change and seismic moment evolution.…”

Geomechanical response to seasonal gas storage in depleted reservoirs: A case study in the Po River basin, Italy

“…Moreover, existing faults crossing the reservoir might be reactivated, with a possible enhanced microseismic activity [e.g., Segall and Fitzgerald, 1998] and consequent upward gas migration [Segall and Grasso, 1991;Kouznetsov et al, 1994].…”

A hydromechanical approach to assess CO2 injection-induced surface uplift and caprock deflection

“…This approximation has been stated in Geertsma 13 and Segall et al 19 and is valid for the case in which the thickness of aquifer m is very small compared to its depth l to the caprock, for instance, in the application of surface subsidence due to depletion of a deep reservoir. Consequently, the assumption constrains the range of applications and is therefore not appropriate for the case in which the aquifer is situated just below or close to the caprock (l = m/2), a typical case for CO 2 storage that is one of the objectives of this study.…”

“…The caprock displacement due to an overpressure generation p of any form of distribution within the injection zone can be written with the aid of Green's function 19 :…”