Quantifying the Effects of Cementation on Hydromechanical Properties of Granular Porous Media Using Discrete Element Models

A series of Discrete Element Method (DEM) models were used to quantify the effects of cementation on the specific storage of clastic materials. Each model simulates a biaxial test of cemented sandstone. The amount of cementation is quantified by a bond to grain ratio (BGR). The BGR is the number of bonds (the bonds represent the cement) divided by the number of grains in each model. Higher BGRs correlate with more cementation. BGRs of .71 and 1.23 correlates to approximately 7 and 19 percent cementation as constrained by observations of natural sandstone samples. Three naturally representative BGRs (0.5, 1.85, and 2.33) are used in the DEM models. The changes in the constitutive behaviors of the DEM models, including stress and strain relations, which result from variations in BGR are used to calculate theoretical elastic and inelastic properties. The resulting bulk moduli are used as input parameters for 2D poroelastic models. The poroelastic models couple fluid flow and solid elastic deformation equations and are able to address the implications of changes in micro-mechanical properties on large (i.e. continuum) scale fluid removal and deformation. The poroelastic models utilized in this research simulate 100 days of constant fluid withdrawal from a basin scale aquifer. At the pumping well, BGR 0.5 results in a decline of hydraulic head of 14.97 meters and 11.4 mm of surface displacement. BGR 1.85 results in a decline of hydraulic head of 16.63 meters and 1.96 mm of surface displacement. Finally, BGR 2.33 results in a decline of hydraulic head of 16.70 meters and 1.56 mm of surface displacement. In summary, DEM models help identify and quantify the effects of key micro-mechanical properties of rocks (i.e. cement) on large-scale coupled fluid-solid mechanics.