How THM Changes in Layered Geological Systems Influence Stability of Fractured Networks

Various geo-energy applications such as geothermal energy, Carbon Capture and Storage (CCS) and underground heat storage are some of the technologies leveraged to realize Paris Agreement goals by cutting greenhouse gases emissions (GHG). However, these applications require in-depth understanding of the effect of coupled Thermal, Hydraulic, Mechanical and Chemical (THMC) processes that take place in the deep subsurface geological layers. This study focuses on the modeling of THM coupled processes in heterogeneous layered reservoirs.

When geological heterogeneities exist in the subsurface, each layer will have its own thermal, geomechanical and geological properties such as thermal conductivity, thermal expansion coefficient, porosity, permeability, Young’s modulus, Poisson’s ratio and so on. These variations in the properties add more complexity to the behavior of coupled THM processes of the system, which in turn requires sophisticated modeling approaches. To reduce complexity, the subsurface layers can be bundled into groups called “geomechanical facies” based on the mentioned material properties from THM characteristics perspective.

The work in this paper is based on key generic features of actual geo-energy applications where simulation modeling has been utilized to demonstrate how coupled THM processes are affected in heterogenous layers compared to homogenous layers. Hypothetical heterogenous layers have been divided into sets of distinct geomechanical facies. OpenGeoSys (OGS) open-source Finite Element based THMC code was utilized to build the run simulation models.

The results demonstrate that variations in the rock thermal, hydraulic and mechanical properties among different neighboring layers have a significant and individually different impact on stress mapping and distributions in addition to strain transfer to the surface. Geomechanical stability of the system parts that are more prone to failure such as fractures and faults were assessed using Factor of Safety (FOS) analysis which are based on stress distribution and rock mechanical properties. Results suggest that geological heterogeneity has more significant impact on hard rocks compared to soft rocks. The latter (i.e., soft rocks) have the ability to maintain sealing capability of caprocks because they are more ductile and have more room for further deformation prior to failure.

The simulation modeling results in this study contributes to the understanding of the key THM processes involved in heterogenous layered systems. Furthermore, this work provides valuable insights towards developing more generic design criteria and predictive models for various geo-energy applications which can be tailored and used in the design of the particular systems.