# A Review of Evaporite Beds Potential for Storage Caverns: Uncovering New Opportunities
## 3 Properties and Geomechanics of Bedded Evaporites
### Main Geomechanical Considerations in Bedded Evaporite Caverns
#### 3.3.2 Solution Mining  
According to the Solution Mining Research Institute (SMRI), solution mining involves injecting water into evaporite formations, through one or more wells, dissolving water-soluble minerals such as salt, potash, and trona, and producing the saturated brine via a pipe string [211]. Some researchers refer to this process as leaching [212; 213; 214; 215]. Solution mining can be performed using a single well or multiple wells. Single-well solution mining methods can follow either direct or reverse methods (Figure 12). Multiple-well solution mining is conducted using two or more vertical wells (Figure 13) or a combination of one horizontal (or inclined) and one or more vertical wells (Figure 14). During solution mining, the underground in situ stresses are disturbed, causing induced stresses around the wellbore and cavern. These induced stresses, coupled with the mechanical properties of the bedded salts and surrounding rocks, may lead to geomechanical issues such as wellbore or cavern instability. Figure 12: Single-well solution mining (**a**) direct circulation; (**b**) reverse circulation [131]. Well casing failure and cement integrity are the primary challenges in maintaining wellbore integrity. In the context of hydrogen storage, ref. [218] highlighted the corrosive effects of hydrogen on casing and cementing corrosion. Another problem described in [219] is flow-induced vibration within the leaching tubing, which leads to bending and damage of tubing. Additionally, salt deformation poses another risk, potentially causing casing collapse after drilling. Temperature differences between the top and bottom of the cavern  
Figure 14: Horizontal cavern with one inclined and one vertical well [217]. Figure 13: Solution mining with two vertical wells [216]. create differential creep rates and stresses, which can compromise casing and cement integrity by cracking the cement and creating pathways for fluid leakage [220]. Several methods have been developed to assess the wellbore integrity in bedded salt caverns. Factors affecting wellbore leakage were investigated in [221] and tightness tests, such as the Nitrogen Leak Test and Fuel-Oil Leak Test, were conducted. These tests showed that an effective well design can prevent leakage through the cemented well. Another study [222] categorized tightness tests into two groups: those assessing the integrity of the wellhead and open hole and those evaluating cavern integrity post-completion before gas injection. The studies in [223; 224] also examined drilling challenges in salt formations and their impact on well casing failure. Additionally, an ultrasonic cased-hole imager was proposed in [225] to evaluate cement quality. An ultrasonic cased-hole imager was proposed to evaluate the cement quality in [225]. The study in [226] concluded the presence of sediments in the cavern and contact of caverns with external aquifers as significant contributors to leakages. The effect of borehole spacing in solution mining on cavern geometry was modeled in [227], showing that roof displacement is primarily influenced by changes in borehole spacing. Once the well is drilled into the salt formation, the leaching process begins. According to [155], three main factors control leaching for producing large symmetric caverns: alternating injection and withdrawal, a high flow rate, and the use of an oil blanket. Both single-well [214; 228] and two-well [206; 217] leaching methods have been investigated. A comparison between these methods, conducted in [229], revealed that caverns using the single-well method are axisymmetric with circular cross-sections, while those produced using the two-well method are non-axisymmetric with ocular cross-sections. The experimental work in [230] showed that freshwater injection rates significantly affect salt dissolution rates and pipe settings within caverns. Furthermore, ref. [231] developed a technique to control fluid motion, brine distribution, wall dissolution rate, and pipe settings within caverns.