Well test
Encyclopedia
This article discusses water well testing; the testing of other wells, eg. petroleum wells
, is a separate field.
A well test is conducted to evaluate the amount of water that can be pumped from a particular water well. More specifically, a well test will allow prediction of the maximum rate at which water can be pumped from a well, and the distance that the water level in the well will fall for a given pumping rate and duration of pumping.
Well testing differs from aquifer test
ing in that the behaviour of the well is primarily of concern in the former, while the characteristics of the aquifer
(the geological formation or unit that supplies water to the well) are quantified in the latter.
When water is pumped from a well the water level in the well falls. This fall is called drawdown
. The amount of water that can be pumped is limited by the drawdown produced. Typically, drawdown also increases with the length of time that the pumping continues.
in a pumping well
were first described by Jacob (1947), and the test was refined independently by Hantush
(1964) and Bierschenk (1963) as consisting of two related components,
,
where s is drawdown (units of length e.g., m),
is the pumping rate (units of volume flowrate e.g., m³/day), 
is the aquifer loss coefficient (which increases with time — as predicted by the Theis solution) and 
is the well loss coefficient (which is constant for a given flow rate).
The first term of the equation (
) describes the linear component of the drawdown; i.e., the part in which doubling the pumping rate doubles the drawdown.
The second term (
) describes what is often called the 'well losses'; the non-linear component of the drawdown. To quantify this it is necessary to pump the well at several different flow rates (commonly called steps). Rorabaugh (1953) added to this analysis by making the exponent an arbitrary power (usually between 1.5 and 3.5).
To analyze this equation, both sides are divided by the discharge rate (
), leaving 
on the left side, which is commonly referred to as specific drawdown. The right hand side of the equation becomes that of a straight line. Plotting the specific drawdown after a set amount of time (
) since the beginning of each step of the test (since drawdown will continue to increase with time) versus pumping rate should produce a straight line.
Fitting a straight line through the observed data, the slope of the best fit line will be
(well losses) and the intercept of this line with 
will be 
(aquifer losses). This process is fitting an idealized model to real world data, and seeing what parameters in the model make it fit reality best. The assumption is then made that these fitted parameters best represent reality (given the assumptions that went into the model are true).
The relationship above is for fully penetrating wells in confined aquifers (the same assumptions used in the Theis solution for determining aquifer characteristics in an aquifer test
).
can produce per unit of drawdown. It is normally obtained from a step drawdown test. Specific capacity is expressed as:
where
is the specific capacity ([L2T−1]; m²/day or USgal/day/ft)
is the pumping rate ([L3T−1]; m³/day or USgal/day), and
is the drawdown ([L]; m or ft)
The specific capacity of a well is also a function of the pumping rate it is determined at. Due to non-linear well losses the specific capacity will decrease with higher pumping rates. This complication makes the absolute value of specific capacity of little use; though it is useful for comparing the efficiency of the same well through time (e.g., to see if the well requires rehabilitation).
Oil well
An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...
, is a separate field.
A well test is conducted to evaluate the amount of water that can be pumped from a particular water well. More specifically, a well test will allow prediction of the maximum rate at which water can be pumped from a well, and the distance that the water level in the well will fall for a given pumping rate and duration of pumping.
Well testing differs from aquifer test
Aquifer test
An aquifer test is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" in observation wells...
ing in that the behaviour of the well is primarily of concern in the former, while the characteristics of the aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
(the geological formation or unit that supplies water to the well) are quantified in the latter.
When water is pumped from a well the water level in the well falls. This fall is called drawdown
Drawdown (hydrology)
In water-related science and engineering there are two similar but distinct definitions in use for drawdown.*In subsurface hydrogeology, drawdown is the change in hydraulic head observed at a well in an aquifer, typically due to pumping a well as part of an aquifer test or well test.*In surface...
. The amount of water that can be pumped is limited by the drawdown produced. Typically, drawdown also increases with the length of time that the pumping continues.
Well losses vs. aquifer losses
The components of observed drawdownDrawdown (hydrology)
In water-related science and engineering there are two similar but distinct definitions in use for drawdown.*In subsurface hydrogeology, drawdown is the change in hydraulic head observed at a well in an aquifer, typically due to pumping a well as part of an aquifer test or well test.*In surface...
in a pumping well
Water well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
were first described by Jacob (1947), and the test was refined independently by Hantush
Mahdi S. Hantush
Mahdi S. Hantush was a prominent Hydrologist known for his analytical work on leaky aquifers and well hydraulics. He is the founder of the Hydrology Program at New Mexico Tech.-Academic:...
(1964) and Bierschenk (1963) as consisting of two related components,
,where s is drawdown (units of length e.g., m),
is the pumping rate (units of volume flowrate e.g., m³/day), is the aquifer loss coefficient (which increases with time — as predicted by the Theis solution) and is the well loss coefficient (which is constant for a given flow rate).The first term of the equation (
) describes the linear component of the drawdown; i.e., the part in which doubling the pumping rate doubles the drawdown.The second term (
) describes what is often called the 'well losses'; the non-linear component of the drawdown. To quantify this it is necessary to pump the well at several different flow rates (commonly called steps). Rorabaugh (1953) added to this analysis by making the exponent an arbitrary power (usually between 1.5 and 3.5).To analyze this equation, both sides are divided by the discharge rate (
), leaving on the left side, which is commonly referred to as specific drawdown. The right hand side of the equation becomes that of a straight line. Plotting the specific drawdown after a set amount of time () since the beginning of each step of the test (since drawdown will continue to increase with time) versus pumping rate should produce a straight line.Fitting a straight line through the observed data, the slope of the best fit line will be
(well losses) and the intercept of this line with will be (aquifer losses). This process is fitting an idealized model to real world data, and seeing what parameters in the model make it fit reality best. The assumption is then made that these fitted parameters best represent reality (given the assumptions that went into the model are true).The relationship above is for fully penetrating wells in confined aquifers (the same assumptions used in the Theis solution for determining aquifer characteristics in an aquifer test
Aquifer test
An aquifer test is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" in observation wells...
).
Well efficiency
Often the well efficiency is determined from this sort of test, this is a percentage indicating the fraction of total observed drawdown in a pumping well which is due to aquifer losses (as opposed to being due to flow through the well screen and inside the borehole). A perfectly efficient well, with perfect well screen and where the water flows inside the well in a frictionless manner would have 100% efficiency. Unfortunately well efficiency is hard to compare between wells because it depends on the characteristics of the aquifer too (the same amount of well losses compared to a more transmissive aquifer would give a lower efficiency).Specific capacity
Specific capacity is a quantity that which a water wellWater well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
can produce per unit of drawdown. It is normally obtained from a step drawdown test. Specific capacity is expressed as:
where
is the specific capacity ([L2T−1]; m²/day or USgal/day/ft) is the pumping rate ([L3T−1]; m³/day or USgal/day), and is the drawdown ([L]; m or ft)The specific capacity of a well is also a function of the pumping rate it is determined at. Due to non-linear well losses the specific capacity will decrease with higher pumping rates. This complication makes the absolute value of specific capacity of little use; though it is useful for comparing the efficiency of the same well through time (e.g., to see if the well requires rehabilitation).