On the physics‐based processes behind production‐induced seismicity in natural gas fields

Abstract

Induced seismicity due to natural gas production is observed at different sites worldwide. Common understanding states that the pressure drop caused by gas production leads to compaction, which affects the stress field in the reservoir and the surrounding rock formations and hence reactivates preexisting faults and induces earthquakes. In this study, we show that the multiphase fluid flow involved in natural gas extraction activities should be included. We use a fully coupled fluid flow and geomechanics simulator, which accounts for stress‐dependent permeability and linear poroelasticity, to better determine the conditions leading to fault reactivation. In our model setup, gas is produced from a porous reservoir, divided into two compartments that are offset by a normal fault. Results show that fluid flow plays a major role in pore pressure and stress evolution within the fault. Fault strength is significantly reduced due to fluid flow into the fault zone from the neighboring reservoir compartment and other formations. We also analyze scenarios for minimizing seismicity after a period of production, such as (i) well shut‐in and (ii) gas reinjection. In the case of well shut‐in, a highly stressed fault zone can still be reactivated several decades after production has ceased, although on average the shut‐in results in a reduction in seismicity. In the case of gas reinjection, fault reactivation can be avoided if gas is injected directly into the compartment under depletion. However, gas reinjection into a neighboring compartment does not stop the fault from being reactivated.

Fault reactivation mechanisms and dynamic rupture modelling of depletion-induced seismic events in a Rotliegend gas reservoir

“…This will result in a more complex state of stress on the fault, depending on the size of these lenses. Additionally, incorporating the different responses of gas and water in the reservoir generates a more complex stress state (Zbinden et al, 2017). Within the Slochteren Formation, compaction may be heterogeneous due to variations in porosity; this can also change the state of stress along the fault.…”

“…This is supported by the reservoir modelling and history matching, which suggest most faults are not sealing. However, if faults were (partially) sealing, the delayed pressure response would influence fault slip (Zbinden et al, 2017). Diffusion of pressure from the overburden and underburden formations and fault in response to the reservoir depletion would also influence slip.…”

“…Diffusion of pressure from the overburden and underburden formations and fault in response to the reservoir depletion would also influence slip. A decrease in pressure in the underburden fault would stabilise the fault, forming a barrier to propagating slip (Zbinden et al, 2017), opposite to the diffusion effect for injection (Buijze et al, 2015b). Depletion of the Ten Boer Member also influences the stress changes, bringing a larger area closer to failure.…”

Fault Stability Perturbation by Thermal Pressurization and Stress Transfer Around a Deep Geological Repository in a Clay Formation

“…Shear and normal stresses are calculated from the stress tensor according to the plane of weakness orientation relative to the principal stresses. A similar approach has been used extensively to model the response of fault zones subjected to fluid injection (Cappa & Rutqvist, ; Rinaldi, Jeanne, et al, ; Rinaldi, Rutqvist, & Cappa, ; Rutqvist et al, ) or extraction (Zbinden et al, ). Such a model can be further extended, to account for increasing complexity, being successfully applied also for a full 3‐D formulation (e.g., Rinaldi et al, ; Rutqvist et al, ), as well as for studying the ground surface response by dynamic rupture modeling (Cappa & Rutqvist, ; Rinaldi, Jeanne, et al, ) and with slip rate‐dependent friction evolution (Urpi et al, ).…”

Intergranular Clay Films Control Inelastic Deformation in the Groningen Gas Reservoir: Evidence From Split‐Cylinder Deformation Tests

“…Both effects are caused by the development of small vertical compaction strains (ε < 1%) in the reservoir as the effective stress increases during reduction of the pore pressure, typically by several tens of MPa (Morton et al, 2001;Mallman & Zoback, 2007). Reservoir compaction is generally assumed to be poroelastic (Bourne et al, 2014;Dempsey & Suckale, 2017;Zbinden et al, 2017), hence reversible, rate insensitive, and easily quantified (Wang, 2000). However, both field data (Santarelli et al, 1998) and experimental studies have shown that, even at the small reservoirs strains pertaining to hydrocarbon production, a significant part of the deformation behavior of sandstone reservoirs is inelastic (Bernabe et al, 1994;Schutjens et al, 1995;Hol et al, 2015Hol et al, , 2018Yale & Swami, 2017;Pijnenburg et al, 2018Pijnenburg et al, , 2019.…”