Drainage by gravity

The saturated paste method provides an estimate of salt in soil at normal field moisture equivalents. Saturation percentage is about four times the 15-atmosphere moisture equivalent or permanent wilting point (irreversible plant desiccation), and approximately two times the 0.33 atmosphere moisture equivalent or field capacity (moisture held in soil against drainage by gravity). Soil with a saturated paste EC of... 

Figure 13.11 Drainage by gravity approximated by drainage from a vertical slab...

This is a very fast drainage rate and if it continued the foam would collapse within seconds. However, the assumption of a constant lamella thickness is, of course, not correct for more than the initial stage. As Uquid drains the lamella thins and the rate of drainage by gravity rapidly decreases (see the lamella thiekness dependency in Equation 13.5). Later on the main destabilization mechanism can be gas diffusion. 

The Plateau borders in a foam form a network into which liquid from the films collects, and from which liquid is drained away by gravity. This drainage process, which is important in foam beds used for mass transfer or separation, has been modeled by several groups, starting with Haas and Johnson (1967 see Gururaj et al. 1995 for a recent review). 

When gas is used as the pressure maintenance agent, it is usually injected into a zone of free gas (i.e., a gas cap) to maximize recovery by gravity drainage. The injected gas is usually natural gas produced from the reservoir in question. This, of course, defers the sale of that gas until the secondary operation is completed and the gas can be recovered by depletion. Other gases, such as N2, can be injected to maintain reservoir pressure. This allows the natural gas to be sold as it is produced. 

Drainage The process of removing surplus ground or surface water either by artificial means or by gravity flow. 

As far as supply is concerned, the capacity of a material to yield water is of greater importance than its capacity to hold water. Even though a rock or soil may be saturated, only a certain proportion of water can be removed by drainage under gravity or pumping, the remainder being held in place by capillary or molecular forces. The ratio of the volume of water retained, /wn to that of the total volume of rock or soil, V, expressed as a percentage, is referred to as the specific retention, 3,. ... 

The specific yield, Sy, of a rock or soil mass refers to its water-yielding capacity, attributable to gravity drainage as occurs when the water table declines. It is the ratio of the volume of water, after saturation, that can be drained by gravity, V(,d. to the total volume of the... 

However, In confined aquifers, water Is not yielded simply by gravity drainage from the pore space because there is no falling water table and the material remains saturated. Hence, other factors are involved regarding yield, such as consolidation of the aquifer and expansion of groundwater consequent upon lowering of the piezometric surface. Therefore, much less water is yielded by confined than unconfined aquifers. 

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