Hydro-Mechanical coupled analysis of gas injection in clay-based materials using different element types

This study investigates the impact of various factors on HM-coupled geological media through a multi-category analysis of the so-called Heat and Gas Fracking model (HGFRAC) and the so-called Gas Threshold Pressure Test (GTPT). The HGFRAC model consists of six different types of benchmark exercises. It is important to note that different types of elements and integration methods used in finite element simulations can influence the obtained results. Comparing the results of quadrilateral and triangle elements, it is observed that the quadrilateral element, due to its bi-linear gradient characteristic, produces a more stable stress field compared to the linear triangle element. The computational efficiency of the HGFRAC model has been improved due to the introduction of the selective integration method for quadrilateral elements. This is because the standard integration method lacks stability and is prone to locking effects, which leads to convergence problems. Additionally, sensitivity analyses have been performed on the fluidity parameter of the clay material, which controls the viscoplastic deformations. Further analysis of the response of quadrilateral elements, using the GTPT axisymmetric model, reveals that the integration method on Gauss points may cause convergence issues. These issues can be resolved by introducing the nodal point integration method, the selective integration method or a combination of both. In general, when dealing with HM-coupled gas injection problems, the use of selective integration enhances stability and cost-efficiency in calculations. Although the integration method on nodal points can resolve convergence issues in the GTPT model, it is worth noting that the calculation results can still be affected by locking effects.