In order to understand concerns associated with historical oil/gas exploration and production, it is important to understand the actual processes undertaken during the exploration and production of oil/gas.  Oil and gas drilling in the U.S. began in Pennsylvania in 1859, and 1893 in Texas. This early type of drilling was straightforward, and the production of the wells was not optimal. Hydraulic fracturing experiments began in the late 1940s in the US and the practice was common in the 1960s/70s.  Hydraulic fracturing, or fracking, is used to enhance the flow from a well after drilling is complete. This process is carried out by sending a mixture of water, sand and certain additives into a deep-rock formation at high pressure. Horizontal oil or gas wells were unusual until the late 1980s and have since become common practice.

Oil/Gas Exploration and Production Procedures

If geophysical evidence suggests the possibility of finding oil in a specific location is good, operators secure the required surface and mineral rights to the claim and prepare for drilling. If drilling is approved an exploratory, or “wildcat” well is drilled. Despite the high level of effort dedicated to locating potential oil reserves, only 1 in 7 wildcats finds hydrocarbons, and even less find enough oil under the right conditions to make production economically feasible.

Typically, oil and gas are found commingled in the same reservoirs and are produced together (“oil/gas well”). In addition, gas occurs in unique areas not associated with economic oil production (“gas well”).

As the well begins to produce less over time, additional energy may be supplied by the controlled injection of water from the surface into the formation through injection wells. The injected water acts to push the oil toward the well bores. Much of the water used for the injection is pumped out of the producing well along with the oil.  The water is then separated from the oil and reinjected back into the formation through a disposal well. Eventually all wells are either temporarily shut down or abandoned/plugged due to increasing effort/cost of production over time.

Plugging Wells

For wildcat wells that are not successful, the procedure used to plug a drilled hole varies depending on the conditions of the hole and regulatory requirements. The objective in plugging a hole is to prevent cross flow between major geological formations. A cement slurry is often used to plug dry holes. In some cases, rather than fill the entire hole with cement, operators may plug only those particular formations that regulatory agencies specify must be isolated. The remainder of the borehole space between the cement plugs is then filled with muds chemically treated to degrade more slowly than typical drilling mud.

When plugging a well, the well head assembly is removed and the tubing and liners are pulled out of the well. Cement plugs may be installed above and below the fresh-water. A cement mixture or sometimes an upgraded mud mixture is circulated downhole to balance the back pressure or formation pressure. The well casing is cut and pulled from about 100 to 200 feet from the surface or ground level depending on local requirements. A final cement plug is set all the way to the surface and sometimes a concrete slab is placed on top of the cement plug at ground level.

Common Sources of Contamination

Crude oil tanks are used to store crude oil at a well pad or central tank battery prior to transfer to a refinery. Some oil fields pipe oil directly downstream and do not have tanks in the field. Staining surrounding the tank piping, and valves, or around the tanks themselves is commonly observed when they show signs of degradation/corrosion. According to the US Energy Information Administration, in 2018, the average oil well produced 24.4 barrels per day, and the average natural gas well produced about 156,000 cubic feet per day. A barrel of oil equals 42 gallons, so an average oil well produces roughly 1,000 gallons of oil per day.

Another common source of contamination is reserve pits.  Reserve pits are usually plastic-lined earthen pits commonly used to store waste mud, base oil or brine prior to final disposal. Reserve pits can contaminate soil, groundwater, and surface water with metals and hydrocarbons if not managed and closed properly. As reserve pit fluids evaporate, water-soluble metals, salts, and other chemicals become concentrated. Precipitation, changes in shallow groundwater levels, and flooding can mobilize these contaminants into adjacent soils and groundwater. Reserve pits are often left in place after the drilling equipment is removed from the site. Reserve pit fluids are allowed to dry, and the remaining solids are encapsulated with the reserve pit synthetic liner (if one exists) and buried in place. 

Well Abandonment

Abandoned wells are wells with no recent production that are not plugged.  Common terms for abandoned wells are inactive, temporarily abandoned, shut-in, dormant, and idle.  Wells with no recent production and no responsible operator are often called orphaned, deserted, long-term idle, or abandoned. 

According to the US EPA, two-thirds of abandoned wells were unplugged as of 2016. For decades, regulations regarding plugging did not exist. Over time, states began instituting guidance and regulations regarding plugging; but, historically, wells were drilled with very limited documentation of their locations and status. Regulations grew more stringent in the 1950s, requiring cement to be used for sealing the producing intervals and the top of the wellbore. Prior to the 1950s, thousands of wells were left unplugged or ineffectively plugged (e.g., using very little cement).  In the 1970s, regulations developed further to focus on environmental protection. Most wells are still plugged with cement using methods and materials developed in the 1970s.

Some plugged wells in the southern and southwest U.S. are converted for water production. Typically the oil well is plugged up to the base of the usable water and then converted to a water production well.  Current landowners may not be aware that their water well was a historical oil/gas well and may not be testing the well for petroleum constituents in addition to the standard bacteria analysis.  

The National Pipeline Mapping System (NPMS)

The National Pipeline Mapping System (NPMS) viewer depicts active and abandoned pipelines along with basic information about any spills/incidents reported from the pipelines.  Owners/ operators of pipelines that have been physically removed/ deconstructed must apply to have the pipeline removed from the NPMS viewer.  

Summary

As explained above; one must be familiar with the common activates associated with the exploration and production at oil/gas wells to be able to identify the likely sources of contamination left behind.  It is important to consider that some infrastructure used in exploration and production of oil/gas can be repurposed; such as abandoned oil/gas wells used for water production, and reserve pits utilized as livestock ponds.  The process of well abandonment, and documentation was lax in the past.  Contaminants left behind when encapsulated in reserve pits can mobilize under various conditions and impact fairly large areas; thorough documentation of reserve pit locations may not exist and could require creative ways to identify their locations on todays landscape.