The Relationship Between Specific Capacity and Porosity

Porosity is one of many parameters that determine the flow of groundwater. However, it is generally difficult to measure or calculate porosity of sediments with much accuracy without expensive downhole equipment and sampling disturbs the sediment. When a well is drilled, a drillers log is recorded with some general details, usually including the depth of the well, screened sections, and water levels under static and pumping conditions, etc.. From these data, we can easily calculate the Specific Capacity (Q/S). Since groundwater flow is dependent on the porosity of the sediments, there should be a relationship between Q/S and porosity that can be used to calculate the effective or apparent porosity at the well, considering the construction and measurement details.

A 200 liter tank was used to create a simulated well with observation wells, and simulate groundwater conditions under a controlled environment. There were five different sediments sizes representing medium sand to fine gravel. Each sediment size was tested at various fixed pumping rates, and the drawdown in each well was recorded. Each sediment size was also tested to determine the total porosity. These data were used to develop a relationship between pumping rate, drawdown, and porosity under laboratory conditions, by combining and manipulating several existing formulas. Seven formulas were created to represent this relationship.

A spreadsheet was programmed with the seven formulas. First the laboratory experiment data were entered and compared. A published database of well logs was used to select real wells under real conditions. These data were entered into the spreadsheet and the formulas were applied to these data. Four of the formulas did not produce credible results and were eliminated. The remaining three formulas are being analyzed with the well data to determine which formula most accurately represents the relationship.