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The Importance of Groundwater
Groundwater is one of our most important natural resources. It is a major component of our national water supply, and is essential for agriculture, industry, and domestic use.
You might be wondering what groundwater has to do with oil and gas extraction. It turns out that it plays a major role in both environmental and economic aspects of oil and gas extraction.
In this section, we’ll take a look at what groundwater systems are, how they work, and how oil and gas operations may affect or be affected by groundwater.
Storing and Moving Water Underground
Groundwater is held within the pore spaces of porous, permeable rock, just like oil and gas is stored within a reservoir rock. A rock unit containing water in its pore spaces is called an aquifer.
We’ve established that an aquifer is a porous, permeable mass of rock containing fresh water. But where does this water come from? And what controls how it flows? By answering these questions we can begin to explore the implications of Darcy’s Law in the real world.
Water in a aquifer is usually derived from natural precipitation, although aquifers can also be artificially recharged by pumping water into a porous, permeable unit.
When it rains, water percolates into the earth’s surface. It then flows downward through pore spaces under the influence of gravity until it reaches the water table. Below the water table, all of the pore spaces are completely full of liquid. Above the water table, the some of the spaces contain air. The water table is not a flat surface - its depth varies depending on local rainfall and streamflow conditions. Its shape is often a subdued version of the topography on the earth’s surface.
[diagram of the water table]
In some cases, an impermeable layer prevents the downward flow of groundwater. If the water is trapped above the impermeable layer and unable to sink further, it will flow along the top of the layer. If the water is below the impermeable layer, the aquifer is referred to as a confined aquifer.
If the water table intersects the ground surface, water emerges from the ground as a spring.
Movement of Groundwater
The movement of water in an aquifer is controlled by changes in hydraulic head.
Hydraulic head represents the height of a specific packet of liquid above a set datum. For a sloping water table like the one shown here, hydraulic head decreases from the high end of the water table to the low end.
Hydraulic head can be measured by drilling a hole into an aquifer and letting it fill up with water from the surrounding rocks. The height of the water above the chosen datum is the hydraulic head.
[figure showing hydraulic head]
In unconfined aquifers, hydraulic head is the same as the height of the water table at any given point. However, in confined reservoirs and aquifers, pressure exerted by water uphill of a given point cannot escape due to the impermeable layer at the top of the aquifer. When we drill into a confined aquifer, water will rise above the impermeable layer until it is at at a great enough height for its weight to counteract the pressure of water in the aquifer. In this case, this height is the hydraulic head. When this occurs in nature, the result is an artesian spring.
[figure for artesian spring]
When the points representing the hydraulic head at multiple locations are connected, the resulting surface is called the potentiometric surface. The potentiometric surface can be represented using contour lines, which describe the height of the surface above or below a chosen datum level.
In an unconfined aquifer, the potentiometric surface closely matches the shape of the water table. In a confined aquifer, the surface is more complex. In any case, it’s important to remember that the potentiometric surface is an imaginary surface that describes energy content, not a physical surface like the water table.
The best way to think of the potentiometric surface is as a series of hills and valleys. Water will flow in the direction of the greatest slope of the potentiometric surface. This slope is called the hydraulic gradient.
The hydraulic gradient represents change in pressure over a given distance. As the hydraulic gradient increases, the flow rate increases.
Cone of Depression
[figure showing water table and contour lines of water table]
Imagine that a farmer digs a 100’ deep well to bring water to his property. Before the well begins pumping, the water table in his region is horizontal everywhere. Once he begins pumping, however, a cone of depression forms in the water table surrounding his well. What happened?
The removal of water from the well caused a reduction of head in the area of the well. According to what we learned above, water begins to flow from the surrounding rocks down the newly created gradient in water table height. This makes sense - the water is flowing “downhill” on the potentiometric surface. The reason a cone of depression develops is that water is not able to flow towards the well fast enough to replace the water that’s removed.
Don’t worry, though - the cone of depression won’t just get bigger and bigger. As the difference in head between the well and the surrounding area gets bigger and bigger, water flows towards the well faster and faster. Once the amount of water reaching the well is equal to the amount being removed, the cone stabilizes.
Now imagine that our farmer’s rival digs a deeper well nearby - say 200’. The cone of depression for the deeper well will be larger because the head is reduced by a greater amount by pumping. In this case, the cone of depression leaves the first farmer’s well high and dry.
The final size of a cone of depression depends on the pumping rate of the well, the permeability of the aquifer rock, and the depth of the bottom of the well below the potentiometric surface.
Interaction of Surface Water with Groundwater
When considering the potential environmental impacts of oil and gas drilling activity, it’s important to remember that surface water, such as streams, lakes, and runoff, is not isolated from groundwater. Any contaminants that make it into surface water sources could migrate into groundwater, and contaminated groundwater can affect surface waters.