Abstract
Abstract. A critical issue concerning geomechanical safety for UGS (underground gas storage) in compartmentalized reservoirs is fault reactivation. Indeed, the displacement (land subsidence, land upheaval) and the stress fields caused by the seasonal injection and production of CH4 into and from deep reservoirs is peculiar. The need of improving our understanding of compartmentalized reservoir behavior and to define safe bounds for the pressure fluctuation in order to prevent undesired land movements and induced seismicity is becoming even more important. This also in view of the expected energy transition when large amount of green energy will potentially be stored and recovered through UGS of compressed air or hydrogen. In this framework, an in-depth modelling investigation has been carried out for the typical UGS geological setting and operations in The Netherlands. The specific goals of the study are the following: (i) explaining the possible mechanisms responsible for seismic events unexpectedly recorded during UGS phases; (ii) understanding which are the critical factors (e.g. the geological configuration, the geomechanical properties, and the reservoir operations) that increase the probability of fault reactivation during the various UGS stages; and (iii) advancing possible guidelines for safe UGS operations. This contribution summarizes the main outcomes obtained by the modelling simulations: the combinations of factors causing fault reactivation during primary production (PP) are also more prone to generate fault failure during cushion gas injection (CG) and UGS. In fact, fault activation during PP leads to a stress redistribution and a new (deformed) “equilibrated” configuration that is newly loaded, in the opposite direction, when the pressure variation changes the sign because of CG and/or UGS. Finally, the various combinations have been ranked to highlight the conditions where the fault system is most likely reactivated during CG and UGS operations: the initial stress regime of the system, the geomechanical properties of the fault, and dislocation of the reservoir compartments are the major influencing drivers to fault instability.
Highlights
Induced seismicity is become a major issue in fluid production from and injection into deep formations
The main goal of the study is (i) to understand which the mechanisms are responsible for this “unexpected” seismicity, and (ii) provide some considerations about the possibility of safe UGS activities, i.e. seasonally storing gas into and producing gas from UGS fields reducing the risk of re-activating the faults bounding and/or crossing the reservoir
The main interest is focused on the interface elements (IE) solution and, on the stress conditions and eventually sliding of the discontinuity surfaces
Summary
Induced seismicity is become a major issue in fluid production from and injection into deep formations. Apart from fracking, where low-permeability formations are intentionally fractured to increase productivity, (micro-) seismicity are an inevitable consequence of a successful project (e.g., Farahbod et al, 2015), several seismic events caused by “conventional” removal or injection of diverse fluids are listed in specific databases, such as HiQuake (https: //inducedearthquakes.org/, last access: 1 March 2020), and related publications (e.g., Foulger et al, 2018). Seismicity induced by fluid removal from faulted formations is somehow “expected” when the extracted volumes or the associated pressure decline overcome a certain bound (Fig. 1a). Seismicity has been reported when large pressure declines cause significant differential compaction between produced and unproduced blocks, i.e. large increase of the shear stress on the sealing faults between the compartments (van Eck et al, 2006).
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