Impermeability is the property of a material to resist the absorption of liquid or gas. In nature there are metamorphic and igneous rocks that are impermeable and do not allow water to pass through. Gas reservoirs are generally trapped below impermeable rock formations which act as a trap. Shale rock also has low permeability and prevents the migration of gas to other regions. Horizontal directional drilling (HDD) and hydraulic fracturing make it economical to extract these gasses.
Trenchlesspedia Explains Impermeability
Shale, slate, mudstone and marble are some examples of impermeable rocks. When the individual grains are not well connected and there are no fractures in the rock, it makes the rock impermeable, thus preventing water or gas from passing through it. Some rocks, such as vesicular volcanic rocks like pumice, have high porosity but low permeability. The large pores seen are remnants of gas that became trapped in the lava when it cooled down, but these bubbles are not connected, thus making it impermeable. Gases from impermeable rocks such as shale are difficult to remove, but by using fracking techniques in which HDD is used, the rocks can be fractured to gain access to the reservoir.