# A Review of Evaporite Beds Potential for Storage Caverns: Uncovering New Opportunities
## 3 Properties and Geomechanics of Bedded Evaporites
### Physical Properties  
Density, porosity, and permeability are three key physical properties that impact the geomechanical behavior of salt caverns. Two definitions are provided in [132] for rock density: the density of rock constituents, which includes minerals and pore fluids, and the bulk density of rocks, which depends on the volume fraction and densities of the constituent minerals. The densities of halite, sylvite, and trona are 2.163, 1.987, and 2.170 (10\({}^{3}\) kg/m\({}^{3}\)), respectively. Additional mineral densities are available in [133]. If rock pores are fluid-filled, the density of the pore fluid can vary depending on the type of fluid. In terms of bulk rock density, the mineral volume fraction and density are used to calculate an estimate. Variations in density can occur within the same crystal or among different rock crystals, as noted in [134]. It is also important to consider fluid inclusion, which refers to small amounts of fluid trapped within crystals during rock deposition or diagenesis, as this can affect overall rock density [135]. In terms of the application of density in geomechanics, overburden stress (\(\sigma_{v}\)) can be calculated based on average density (\(\rho_{a}\)), the acceleration due to gravity (\(g\)), and depth (\(z\)), based on Equation (1) [136]. Several researchers have used this method to calculate overburden stress, which is also known as vertical stress [21; 46; 137]. \[\sigma_{v}=\rho_{a}gz \tag{1}\]  
Another important physical property that influences geomechanical characteristics is porosity. From a petroleum reservoir engineering perspective, porosity is defined as the pore volume within the rock, which indicates its storage capacity. In other words, porosity is the ratio of pore volume to the rock's bulk volume [138]. Rock salts are typically characterized by low porosity, making them suitable for cavern storage purposes [139; 140; 141]. A porosity range of 0.26-3.00% has been reported for rock salt [142]. Research on the porosity of potash and trona beds is limited. According to [143; 144], sylvite exhibits no porosity, whereas carallite can reach porosity levels of up to 65%. Trona beds are reported to have a porosity range of 0.47-2.35% [145]. Although these three evaporite deposits generally have low porosity, the porosity of insoluble interlayers, such as gypsum, mudstone, and anhydrite, is also critical to be considered to ensure cavern tightness [146; 147; 148; 149; 140; 147; 149; 140; 148]. An inverse relationship between porosity and density was determined in [132]. Regarding rock mechanics, the influence of porosity on rock mechanical properties can be determined through rock models [149]. For example, as porosity increases in rock salts, rock strength decreases, as demonstrated in [150]. Relationships between porosity and permeability are also discussed in [151]. The low permeability of rock salts (10\({}^{-17}\)-10\({}^{-21}\) m\({}^{2}\)) plays a crucial role in maintaining the integrity and tightness of salt caverns throughout their operational lifespan [139]. Generally, permeability is defined as a property of rock that indicates the ease with which fluids can flow through it [152]. Despite the inherently low permeability of undisturbed rock salts, changes in stress, pressure, and temperature during cavern construction, storage, and post-abandonment necessitate an investigation into how these parameters affect permeability. Studies focusing on the permeability of rock salts under such conditions are presented in [12; 151; 153; 154; 155]. Research has also considered the thermal, hydraulic, mechanical, and chemical coupling processes to understand rock salt permeability under these combined factors [148; 156; 157]. No reliable sources were found detailing permeability values for potash and trona beds. While seismic data, well logs, and experimental data are commonly used to determine the physical properties of various rocks, studies on evaporites are limited.