In our July Newsletter we provided a basic overview of groundwater, hydrogeology, and aquifer systems specific to North Carolina. This month we wanted to expand on those issues by discussing wells, well construction, and some basic parameters that differ between various types of wells. We will focus on general information related to both water wells and oil/petroleum wells.
What is a water well?
According to the American Groundwater Trust, a water well is simply “An engineered device created to access subsurface water. Wells may be bored, or drilled (horizontally or vertically) or constructed as a vertical or horizontal shaft.” Water wells can be used for a variety of applications, including drinking water supply, industrial water supply, and observation/monitoring of groundwater chemistry and contamination. For the purposes of this article, we will be focusing on two general types of water wells: 1) Drinking water supply wells, and 2) Monitor wells
What is the difference between a supply well and a monitor well?
A monitor well is designed to sample groundwater. These samples are typically obtained to analyze the groundwater for contaminants such as petroleum, solvents, and metals. They are usually relatively shallow and have a low yield because they are only used to obtain small quantities of water from the surface of the aquifer. In contrast, supply wells are usually constructed much deeper than monitor wells, and have much larger water intake intervals in order to extract high yields of groundwater for drinking or industrial supply. Additionally, a supply well will have a permanent pump associated with it to bring water to the surface, whereas water is typically sampled from the ground surface by bailers or other temporary extraction methods.
How are these wells constructed?
Water wells can be dug, driven, or drilled. Probably the most common method of installing supply wells in North Carolina is drilling due to the maximum depths required for high yields of groundwater. Regardless of the installation method, there are some basic features that all water wells generally share in terms of their construction (some monitor wells, especially temporary, may not contain all the features listed below):
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Image from the Kansas Geological Survey Borehole: The hole drilled to construct the well
- Casing: Typically metal or plastic pipe inserted into the borehole to keep it from collapsing
- Annular space: The space between the casing and the borehole wall. Depending on geologic conditions, this space can be filled with sand/gravel pack surrounding the well screen (see below).
- Annular seal: Material between the borehole wall and casing typically near the ground surface designed to prevent contamination and surface water from entering the well. Bentonite (clay) and grout are commonly used.
- Screened interval: The section of well casing that is perforated to allow groundwater to enter the well. This is typically within a sand/gravel deposit or fractured bedrock where water flow and porosity are higher.
Supply wells will also generally have an end cap sealing the bottom of the well, and various parts associated with the pumping system. Both supply wells and monitor wells will also generally have a concrete pad at the ground surface to stabilize the well, and some type of cap or seal at the top of the well to prevent surface water and other materials from entering the well casing. Injection wells, also shown in the above figure, are used to inject fluid or wastewater into the deep aquifer, and are not discussed in detail in this article.
If you are interested in viewing the specific North Carolina Administrative Code associated with well construction in our state, it can be found through the Wake County website.
What About Oil Wells?
Oil or petroleum wells are constructed to extract crude oil and natural gas from deep in the subsurface. A well that is specifically designed to extract natural gas may be called a gas well. The very basic principles described above are the same for these wells: they have a borehole that is drilled into the subsurface, a well casing that is inserted into the hole and a screened/perforated zone within the oil/gas deposit that allows the material to enter the well casing and be extracted. However, oil wells have a more complex pumping system associated with extracting the petroleum, and they are generally drilled much deeper that water wells (typically several thousand feet into the subsurface). Additionally, they typically will have multiple layers of casing surrounding the well that extend to different depths in order to obtain the maximum penetrating needed to reach the formation containing the petroleum deposit, as well as to allow both fluids and gas to travel vertically through the well system (conductor, surface, intermediate and production casing). Oil and gas wells can also be drilled horizontally through shale formations to extract natural gas by fracturing the rock unit (hydraulic fracturing).
A Day in the Life of a Certified Well Driller
One of our Senior Geologists and North Carolina Certified Well Drillers, Tim Leatherman, recently spent several months in the field supervising well construction and abandonment. But what does this really mean? We thought it might be interesting to provide you with a “day in the life of a certified well driller” to give you a better idea of what life is like in the field for this type of work:
The day starts early, as it always seems to for field work. Prior to arriving onsite, Tim will need to have some type of scope of work planned for the day, including the approximate number of wells or soil borings to drill, estimated well depths, well diameters, and the specific drilling method to be used. Additionally, ULOCO Public Utility Locating needs to be called approximately 72 hours before drilling starts. This allows for all utilities to be marked to prevent damage to buried cables or pipes.
Once Tim arrives on site, he will gather other subcontractors, the client, and any other parties for a health and safety meeting (tailgate meeting) to discuss personal protective equipment (PPE). Typically for drilling this will involve equipment such as hard hats, steel-toed boots, safety vests, eye protection, and ear protection. At the end of the tailgate meeting Tim will discuss the specifics of the project – walk through drilling locations and discuss any hazards such as overhead utilities. Tim will overview the approximate depth of borings/wells, any soil sampling that is required, and the materials that are planned to be used.
Finally, it’s time for the actual work! The drillers, under Tim’s supervision, will install soil borings, monitoring wells, or supply wells by hollow stem auger, mud-rotary, air-rotary, air hammer, direct push, spit spoon, sonic drilling, or other drilling methods. All work is carefully supervised by the Certified Well Driller, and detailed records are kept of specifics such as well depths, borehole diameters, soil types, materials used and the time taken for the drilling.
At the end of a very long day the crew packs up and prepares to start again the following morning with another safety meeting and the new work plan, and the drilling continues. In brief, this is a day in the life of a certified well driller.
