Aquifer specific yield

In an unconfined aquifer the permeability relates to the specific yield. This term express the ability for the aquifer to be drained and is clearly related to the grain size. The specific retention reflects the capillary forces that tend to retain water close to the grains. How these parameters relate to each other is shown in Figure 38. 

Figure 38. The relation of porosity, specific yield, specific retention and grain size in an unconfined aquifer...

Uddin et al. (2008b) conducted several depressurization simulations for the Mallik 5L-38 well. Their results showed that the Mallik gas hydrate layer with its underlying aquifer could yield significant amounts of gas originating entirely from gas hydrates, the volumes of which increased with the production rate. However, large amounts of water were also produced. Sensitivity studies indicated that the methane release from the hydrate accumulations increased with the decomposition surface area, the initial hydrate stability field (P-T conditions), and the thermal conductivity of the formation. Methane production appears to be less sensitive to the specific heat of the rock and of the gas hydrate. 

In a confined aquifer, the quantity corresponding to specific yield is called storativity, 5, and is typically much smaller than Sy. Storativity is a function of the compressibility of water and aquifer material and is proportional to aquifer thickness. A related term is specific storage, Ss, which is storativity divided by aquifer thickness. Storativity increases as thickness of the aquifer increases, whereas specific yield is not a function of aquifer thickness. Removal of large volumes of water from confined aquifers can result in measurable ground subsidence. 

A well in a phreatic aquifer 100 meter thick supplies a small manufacturing plant, and is pumped continuously at a rate of 1200 liter/min. The difference in water levels between observation wells 01 and 02, located 10 and 20 m away, respectively, is seen to stabilize at 3 cm, whereas before pumping the level in Ox was 1 cm higher than in 02. The specific yield of the aquifer material is estimated as 0.2. 

Material Total porosity % Specific yield (galljcu. ft.) Rate of water movement (ft./day) Aquifer class... 

The Middle Jurassic Sandstone Fractured Confined Aquifer. It is above the coal seam with distance of 4.0-54.0 m. The layer consists of pack-sand, sandy mudstone and conglomerate with the average thickness of 180 m. The gravels mainly contain quartzite, quartz sandstone and limestone. The water yield property is relatively weak and this aquifer is the direct water bearing strata. The water quality type is HCOj Na Ca. The permeability coefficient is 0.000405-0.0322 m/d and the specific capacity is about 0.00007-0.00622 L/(s m). This aquifer doesn t have the identical water level with poor connection. In the shallow parts, it is supplied by precipitation and river. Currently, its discharge form is drainage. 

The commonest way of recovering groundwater is to sink a well and lift water from it (Fig. 4.12). The most efficient well is developed so as to yield the greatest quantity of groundwater with the least drawdown and the lowest velocity in the vicinity of the well. The specific capacity of a well is expressed in litres of yield per metre of drawdown when the well is being pumped. It is indicative of the relative permeability of the aquifer. Location of a well obviously is important... 

---