An Analytical Probabilistic Approach for Fault Stability Analysis During Fluid Injection: The SAFE Tool Methodology

The increasing interest in CO₂ storage and geothermal projects leads Oil & Gas companies to develop tools and methodologies to support the operation design phase. SAFE (Stability Analysis Fault Eni) is an in-house developed code based on an analytical probabilistic approach to provide a fast preliminary assessment of fault stability for fluid injection strategies in reservoirs or aquifers. The method allows to identify: i) the impact of the uncertain parameters affecting the fault stability; ii) possible critical cases for which a refinement of the analyses is required, e.g. 3D Finite Element Model focused on critical faults. Beyond a summary of the theoretical framework of the fault reactivation process, the paper will highlight the assumptions and the computation workflow of the developed tool, as well as the output data resulting from the analysis.

In the present days, the attention for the climate change and the environment protection are the engines that guide civil and industrial development, through regulations aiming to reduce global warming, wastes and pollution while promoting health and safety. To this aim, CO₂ storage in deep underground layers will be of paramount importance to reach the world net-zero emissions by 2050. Amongst all the known phenomena related to fluid injection, fault destabilization is one of the main effects to be evaluated with a geomechanical study for the reservoir exploitation. In fact, fault reactivation with induced seismicity has been observed for wastewater disposal in the US (Ellsworth, 2013; McGarr et al., 2015; Rubinstein and Mahani, 2015) where the earthquake rate increase since 2001 is partially due to the injection of the water resulting from hydraulic fracturing operations in nearby disposal wells. Likewise, CO₂ disposal (Espinoza and Santamarina, 2011) or geothermal projects (Buijze et al., 2019) may generate seismic events. Moreover, the thermal effect due to the injection of a fluid with lower temperature than the formation temperature leads to additional stress variation that may reduce the fault resistance (Rutqvist, 2012; Kim and Hosseini, 2014). Geochemical effects might also be an issue, due to possible rock properties degradation in presence of reactive fluids such as CO₂ saturated water. This aspect is still not fully understood, and results are sometime contradictory, i.e. some papers evidence effects of CO₂ on the rock mechanical properties, other show little or almost no effects (e.g. Bateman et al., 2013; Rathnaweera et al., 2015, 2017, Menezes, 2019).