Coupled Flow and Geomechanical Modeling of Fluid Production and Injection in the Cavone Oil Field, Northern Italy: an Assessment of the Potential for Induced Seismicity

Abstract:

There has been a recent increase in the number of earthquakes reported in proximity of active oil and gas fields. In particular, the occurrence of a sequence of damaging earthquakes in May 2012 near the Cavone oil field, in Northern Italy, raised the question of whether these earthquakes might have been triggered, or, if not, if future activities might trigger other damaging events. Production and injection of fluids in the underground reservoirs are known to be capable of triggering seismicity by inducing slip on seismogenic faults. However, the effects of injection and production on fault stability in real fields are not always intuitively obvious, and require the development of new-generation coupled flow-geomechanical models that capture the effect of multiphase poromechanics on faults. We study, by way of numerical modeling and simulation, the potential for induced seismicity at the Cavone field. Using a coupled flow and geomechanics model of the field that honors reservoir geology and historical well schedule, we simulate oil production and water injection in the field for a period of three decades leading up to the earthquake sequence. We calculate the change in Coulomb stress on the bounding Mirandola fault, which sourced the May 29, 2012 M 5.8 earthquake. This quantity varies in space and evolves in time with changing pore pressure and total stress in the reservoir. A novel and important aspect of our work is the identification of a potential instability mechanism for a bounding fault at the edge of a reservoir experiencing pressure depletion. The discontinuity in pore pressure across the fault means that there is a discontinuity in effective normal stress and that, therefore, the Coulomb failure criterion must be evaluated locally on both sides of the fault. We track the evolution of the Coulomb stress at the earthquake hypocenter and compare it with the regional tectonic stressing rate to conclude in favor of tectonic origin of the earthquake. In addition, analysis of the locations of aftershocks of the May 2012 sequence shows a lack of seismicity in the area where the stressing rates from contraction of the reservoir are largest. This observed lack of seismic activity within 1–2 km from the reservoir suggests that fluid production and injection from the Cavone field was not an important driver for the observed seismicity.