Hydrological properties of Rocks: Comprehensive Guide to Porosity, Specific Yield, and Storage Coefficient in Rocks

Hydrological properties of Rocks:

Hydrological properties are the characteristics of rocks and soils that affect the movement, storage, and distribution of water within the Earth’s crust. These properties include porosity, permeability, specific yield, specific retention, and storativity, which determine the capacity of geological materials to store and transmit water.

POROSITY (n)

The bulk volume of rock includes the grains or crystals as well as the contained void space. The volumetric portion of bulk rock that is not occupied by grains, crystals or natural cementing material is termed porosity. Thus, porosity n, is the ratio of void volume to bulk volume (grains plus void space). Porosity is of two types: primary porosity and secondary porosity. 

Primary porosity is the inherent characteristic that is developed during the formation of the rock, e.g. unconsolidated formations and sedimentary rocks have intergranular spaces. In basalts the primary porosity is due to gas cavities, the lava tubes and lava tunnels. The amount of pore space depends on the degree of compaction of the sediment (with compaction generally increasing with depth of burial), on the packing arrangement and shape of grains, on the amount of cementation, and on the degree of sorting. Typical cements are siliceous, calcareous or carbonate, or iron-bearing minerals. In consolidated rocks the openings are primarily present as fractures, at joints, along bedding planes and in the form of solution holes.  

Secondary porosity

Secondary porosity is developed due to subsequent processes such as fracturing, jointing, and solution activities. In the igneous and metamorphic rocks as well as the hard sedimentary rocks porosity is secondary in nature.  

The effective porosity ne, is the volume of pores that is available for transport of water, divided by the bulk volume. The effective porosity is slightly less than the porosity as part of the water that is molecularly bound to the pores or is stored in dead end pores does not participate in the flow. Porosity is a non-dimensional number, which could range from 0-1, it is often reported as a percentage. In the fractured rocks the porosity depends upon the size of the individual fractures, joints and other openings. 

Sedimentary rocks and soils generally have the highest porosities, mostly from primary, while secondary contributes to fracturing and dissolution. Igneous & metamorphic rock generally have low primary porosity, with sometimes-significant secondary porosity (fractures, especially in volcanic rocks). Rounded grains have smaller pore spaces between them in relation to irregularly shaped grains. Compaction results in reduction in porosity due to weight of overburden. 

SPECIFIC YIELD (Sy)   

In unconfined aquifers the water is released by gravity drainage in proportion to the amount of decline of the water table. The coefficient of storage of the water table aquifers is the specific yield of the desaturated material, given by:  

Sw = Sy + bS_s  

where: Sw = storage coefficient (water table aquifers) [-]  

b = height of the water table above the impermeable layer [m]  

Sy = specific yield of the aquifer [-] Usually Sy >> bS_s, thus Sw for all practical purposes is regarded as specific yield.  

The specific yield is defined as the ratio of the volume of water that a formation would yield by gravity to its volume. Thus, it effectively represents very closely the effective porosity. Table 2.2 gives some representative values of specific yield used in India. The coefficient of storage of the water table aquifer is expressed as a percentage of 0.01= 1%. When the values of the coefficient of storage and fluctuations of the water table or piezometric surface (∆h) are known, the quantity of water added or released from the aquifer (∆V) can be calculated as 

∆V = S_w∆h 

Specific Retention:

Specific retention S_r is defined as the ratio of volume of water retained by a rock against gravity to the total volume of the rock. Thus specific retention refers to the volume of water that remains in the rock mass after gravity drainage. Thus, 

Porosity=S_y + S_r   

Storage coefficient

The storage coefficient, also called storativity (S) of a confined aquifer is defined as the volume of water released or gained by a unit surface area of the aquifer for a fall or rise of potentiometric surface by unity. Water will be released with fall of the potentiometric surface and gained with the rise of the potentiometric surface. The storativity for a confined aquifer is equivalent to the specific yield of an unconfined aquifer. 

Storativity values range from .005 to .00005 and is dimensionless. Water is released from confined aquifers primarily by compression of the aquifer and expansion of water when pumped. During pumping, the confined aquifer remains fully saturated, only the pressure is reduced in the aquifer reflected by the decline of the potentiometric surface.