While geophysical prospecting is the initial method used to evaluate potential hydrocarbon sources, the only definitive way to indicate oil and gas presence is by drilling an exploratory well. At first, one exploratory well is typically drilled on a particular pad (although a number of exploratory wells may be drilled in a geographic area to estimate the extent of the resource), but once the company hones in on the best spots for the resource, then multiple wells may be drilled on the pad during the subsequent development phase. It is important to note, however, that many exploratory wells are not successful and thus are never fully developed. The process for well construction and drilling is described below.
The operator often first works with the landowner to select a well pad site and determine the need for access roads. After upgrading local roads or constructing new ones, the operator clears and levels the land and builds the pad. Construction of a well pad can take several weeks to months, depending on the characteristics of the site, the target formation, and the company’s exploration and development approach. 1 As part of the well pad infrastructure, the operator installs facilities for storing drilling fluids and disposing of wastewater in either pits or tanks. Pipelines may also be built for the transport of water to and from the site (for more information on pipelines, see Appendix E). Companies may also construct temporary residences for their workers. In doing so, they are required to follow local and state health department regulations for housing and waste disposal. 2
Over the construction period, heavy trucks move earth and transport equipment and supplies to and from the site, including the drilling rig, storage containers, temporary worker housing, and office trailers. The amount of traffic can vary substantially depending on the activity at the site, peaking in the days before and after the drilling and completion of each well. According to one estimate, there can be a total of 1,148 one-way heavy truck trips and 831 one-way light truck trips during the early phase of well development. 3 Throughout the exploratory drilling phase, trucks will continue to provision the site with water, food, and fuel. The operator must follow the U.S. Department of Transportation’s regulations for planning and permitting the transport of heavy loads.
At this exploratory stage, companies typically bring in experienced contract workers to work on the drilling rigs. As an example of the workforce that can be required, the consultancy IHS Energy estimates that the drilling and fracturing of a typical oil well in the Bakken Shale requires 50 full-time employees. 4 During the drilling phase, employees might work 12-hour shifts on a rig that operates 24 hours a day. At other times, the crew working on a rig might be much smaller. Some workers might be present on a site for only a matter of hours to perform a specific task, or could rotate among multiple wells on the same day. 5
Once the equipment, infrastructure, and drilling rig are in place, the operator prepares to drill the well. A blowout prevention device is installed for safety purposes, in case a high-pressure zone is encountered. Then the operator begins drilling a hole in the earth called a wellbore. Drilling fluid, also known as drilling mud, is pumped into the wellbore to lubricate the drill and maintain the proper balance of pressure in the uncased wellbore. At selected depths in the underlying geology, the bit is removed from the wellbore and layers of steel casing and cement are installed to seal the well off from the surrounding rock, both to stabilize the wellbore and protect underground water sources (see Figure 2). Each casing is pressure-tested after the cement is installed and has set. Depending on the well location and geology of the site, the well is then drilled vertically to a median depth of 8,100 feet, 6 typically thousands of feet below groundwater resources, and gradually angled to a horizontal drilling position as it reaches the shale formation.
Source: The Geological Society of America.
During drilling, measurements will be taken in the well to characterize the subsurface thickness and depth of formations, the mineralogy, and the types of fluids present. Drill cuttings - rock fragments generated by the drill bit – are examined to help determine if oil and gas are present and if so, in what quantity. Models based on this combined data can help establish a reliable prediction for hydrocarbon presence on a basin-wide scale. If the resource appears promising, the operator will proceed with completing and flow testing the well, and will likely drill, complete, and flow test additional exploratory wells to evaluate a particular geographic area.
After casing the wellbore, the operator begins the completion process, preparing it to produce oil or gas by removing the drilling rig and replacing it with a workover or completion rig. The well is first tested for integrity. Then the process of hydraulic fracturing begins. To fracture the shale, the operator inserts a perforating tool into the wellbore at the depth of the shale formation, which creates holes in the well casing. This is done in stages. High volumes of fracturing fluid - a mixture typically composed predominantly of water, along with sand and chemicals – are injected into the well at high pressure so that the fluid can flow through new or existing fractures in the shale rock. The sand holds these fractures open, allowing the oil or gas to flow back towards the wellbore. During the exploration phase, the natural gas produced by the well (or co-produced, in the case of a shale oil well) might be released into the atmosphere (vented), burned off (flared), or captured and sent to market. For more information on venting and flaring, see the Air Quality section below.
According to the American Petroleum Institute, oil and gas exploration and production generated 149 million barrels of drilling waste in 1995 (the last time an analysis was conducted), which is primarily composed of drill cuttings and mud. 7 Most of the waste is buried onsite or temporarily stored and then transported to landfills. 8 As with produced water, solid wastes may also be disposed of in Underground Injection Control (UIC) wells, which are regulated under the Safe Drinking Water Act (see Appendix C for more information on UIC wells). In some cases, the drill cuttings may be reused – for example, applied to roads for dust suppression purposes. Although such waste applications are regulated in many states, concerns have been raised about potential soil and water contamination from re-purposing the waste in this way (see Box 8). 9
After the exploratory wells are completed and flow tested, the company studies the data collected to determine whether operations in the area are financially viable, a calculation that includes production potential, the acreage under the company’s control, and the current value of the resource. A site could remain dormant for several years while the company weighs the costs and benefits and waits for the right economic conditions to materialize. The time for deliberation is limited by states, however, which require the operator to either put a well into production within a certain timeframe – which varies by state – or temporarily or permanently abandon the site.
- George Blankenship, Blankenship Consulting LLC, personal communication on August 2, 2014. ↩
- For an example of state guidelines for temporary housing, see Ohio Environmental Protection Agency, Guidance for Temporary Housing Associated with Oil and Natural Gas Drilling Operations (May 2012) ↩
- New York State Department of Environmental Conservation, High-Volume Hydraulic Fracturing in NYS: 2015 Final Supplemental Generic Environmental Impact Statement (SGEIS) Documents (April 2015), 6-305. ↩
- Ken Cohen, “What Does It Mean to Frack a Well? Part 1,” ExxonMobil Perspectives (June 15, 2015). ↩
- George Blankenship, personal communication on August 2, 2014. ↩
- Based on an EPA analysis of operator disclosures to FracFocus. Well depths ranged from 2,900 – 13,000 feet (5th to 95th percentile). U.S. Environmental Protection Agency (EPA) Office of Research and Development (ORD), “Analysis of Hydraulic Fracturing Fluid Data from the FracFocus Chemical Disclosure Registry 1.0” (Washington, DC: U.S. EPA, March 2015), 63. ↩
- U.S. Environmental Protection Agency (EPA), “Exemption of Oil and Gas Exploration and Production Wastes from Federal Hazardous Waste Regulations,” (Washington, DC: October 2002), 2. ↩
- Ground Water Protection Council, “State Oil and Gas Regulations Designed to Protect Water Resources” (2014), 12. ↩
- Maryland Institute for Applied Environmental Health (School of Public Health, University of Maryland), “Potential Public Health Impacts of Natural Gas Development and Production in the Marcellus Shale in Western Maryland” Prepared for the Maryland Department of the Environment and the Maryland Department of Health and Mental Hygiene (July 2014), 47, 93. ↩