Glossary

To temporarily or permanently cease production from a well or to cease further drilling operations. (Source: OSHA 2001)

The Clean Air Act Amendments of 1990 specify a list of 187 hazardous air pollutants that are known or suspected to cause cancer or other serious health or environmental effects. These hazardous air pollutants are referred to throughout this report as air toxics. The list includes volatile organic compounds (VOCs), such as benzene and formaldehyde, as well as other pollutants, such as diesel particulate matter (PM). (Source: U.S. Environmental Protection Agency 2016)

The study of why and how a health problem occurs. Analytical epidemiology uses comparison groups to provide baseline or expected values so that associations between exposures and outcomes can be quantified and hypotheses about the cause of the problem can be tested. (Adapted from: Centers for Disease Control and Prevention 2014)

An elongate depression in the crystalline basement complex which contains a great volume of predominantly sedimentary stratified rocks. The Appalachian Basin extends from the Adirondack Mountains in New York to central Alabama, encompassing an area of about 207,000 square miles, including all of West Virginia and parts of New York, New Jersey, Pennsylvania, Ohio, Maryland, Virginia, Kentucky, Tennessee, North Carolina, Georgia, and Alabama. (Source: Colton 1961)

The application or practice of epidemiology to control and prevent health problems. (Adapted from: Centers for Disease Control and Prevention 2014)

The systematic distortion of the assessment of outcome measures by researchers or study participants. (Adapted from: Sedgwick 2014)

The statistical relation between two or more events, characteristics, or other variables. (Adapted from: Centers for Disease Control and Prevention 2014)

Hydrocarbon-producing geological formation consisting of sedimentary rocks and stretching from the Dallas–Fort Worth metroplex west and south, covering 5,000 square miles. (Source: Texas Commission on Environmental Quality 2018)

A synclinal structure in the subsurface, formerly the bed of an ancient sea. Because it is composed of sedimentary rock and its contours provide traps for petroleum, a basin is a good prospect for exploration. (Source: Schlumberger 2019)

A systematic deviation of results or inferences from the truth or processes leading to such systematic deviation; any systematic tendency in the collection, analysis, interpretation, publication, or review of data that can lead to conclusions that are systematically different from the truth. In epidemiology, bias does not imply intentional deviation. (Adapted from: Centers for Disease Control and Prevention 2014)

An uncontrolled flow of gas, oil, or other well fluids from the well. (Source: Schlumberger 2019)

Water containing more dissolved inorganic salt than typical seawater. (Source: Schlumberger 2019)

An instance of a particular disease, injury, or other health condition that meets selected criteria. (Adapted from: Centers for Disease Control and Prevention 2014)

An observational analytic study that enrolls one group of persons with a certain disease, chronic condition, or type of injury (case-patients) and a group of persons without the health problem (control subjects) and compares differences in exposures, behaviors, and other characteristics to identify and quantify associations, test hypotheses, and identify causes. (Adapted from: Centers for Disease Control and Prevention 2014)

Heavy steel pipe placed in an open hole and cemented into place. Casing is designed to withstand high pressures, large tensile loads, and resist chemical reaction and corrosion. A casing string refers to a series of connected segments of casing or pipe that serves to prevent the hole from caving, keep the fluids inside the casing string from migrating to porous formations, prevent unwanted fluids from entering the hole, and protect freshwater aquifers. (Source: Michigan Department of Environmental Quality 2013)

The geographic locations served by an institution, such as a hospital. (Adapted from: Centers for Disease Control and Prevention 2014) 

To prepare and pump cement into place in a wellbore. Cementing operations may be undertaken to seal the annulus after a casing string has been run, to seal a lost circulation zone, to set a plug in an existing well from which to push off with directional tools or to plug a well so that it may be abandoned. (Source: Schlumberger 2019)

An experimental study that uses data from individual persons. The investigator specifies the type of exposure for each study participant and then follows each person's health status to determine the effects of the exposure. (Adapted from: Centers for Disease Control and Prevention 2014)

A well-defined group of persons who have had a common experience or exposure and are then followed up, as in a cohort study or prospective study, to determine the incidence of new diseases or health events. (Adapted from: Centers for Disease Control and Prevention 2014)

An observational analytic study in which enrollment is based on the status of exposure to a certain factor or membership in a certain group. Populations are followed, and disease, death, or other health-related outcomes are documented and compared. Cohort studies can be either prospective or retrospective. (Adapted from: Centers for Disease Control and Prevention 2014)

A group in an analytical study (e.g., a cohort or case-control study) with whom the primary group of interest (exposed group in a cohort study or case-patients in a case-control study) is compared. The comparison group provides an estimate of the background or expected incidence of disease (in a cohort study) or exposure (in a case-control study). (Adapted from: Centers for Disease Control and Prevention 2014)

A generic term used to describe the events and equipment necessary to bring a wellbore into production once drilling operations have been concluded, including but not limited to the assembly of downhole tubulars and equipment required to enable safe and efficient production from an oil or gas well. (Source: Schlumberger 2019)

Any combination of facilities that supply the energy to move gas in transmission or distribution lines or into storage by increasing the pressure. Compressor stations might include equipment to remove liquids, particles, and other impurities from the natural gas, which are disposed of or sold as desired. (Source: U.S. Energy Information Administration 2019)

Identifying sources of exposure, how the contaminant behaves in the environment, and the context in which people come into contact with the contaminant that results in an adverse health effect. (Adapted from: Boston University School of Public Health 2016)

A range of values for a measure (e.g., rate or odds ratio) constructed so that the range has a specified probability (often, but not necessarily, 95%) of including the true value of the measure. (Adapted from: Centers for Disease Control and Prevention 2014)

A factor that distorts the association between an exposure and a health outcome that is related to both. (Adapted from: Centers for Disease Control and Prevention 2014)

The distortion of the association between an exposure and a health outcome by a third variable that is related to both. (Adapted from: Centers for Disease Control and Prevention 2014)

In a case-control study, a member of the group of persons without the health problem under study (see also comparison group and case-control study). (Adapted from: Centers for Disease Control and Prevention 2014)

Crude oil and natural gas that is produced by a well drilled into a geologic formation in which the reservoir and fluid characteristics permit the oil and natural gas to readily flow to the wellbore. (Source: U.S. Energy Information Administration 2019)

A study in which a sample of persons from a population are enrolled and their exposures and health outcomes are measured simultaneously; a survey. (Adapted from: Centers for Disease Control and Prevention 2014)

When referring to a rate, an overall or summary rate for a population, without adjustment. (Adapted from: Centers for Disease Control and Prevention 2014)

Personal characteristics of a person or group (e.g., age, sex, race/ethnicity, residence, and occupation) used in descriptive epidemiology to characterize patients or populations. (Adapted from: Centers for Disease Control and Prevention 2014)

The aspect of epidemiology concerned with organizing and summarizing data regarding the persons affected (e.g., the characteristics of those who became ill), time (e.g., when they become ill), and place (e.g., where they might have been exposed to the cause of illness). (Adapted from: Centers for Disease Control and Prevention 2014)

The phase of petroleum operations that occurs after exploration has proven successful, and before full-scale production. For the purposes of this report, development includes exploration, site preparation, vertical and horizontal drilling, hydraulic fracturing, well completion in preparation for production, and associated waste management. (Source: Schlumberger 2019)

A type of information bias in which the proportion of subjects misclassified on exposure depends on disease status (case-control studies) or when the proportion of subjects misclassified on disease depends on exposure (cohort studies). (Adapted from: Rothman and Greenland 1998).

Association between an exposure and health outcome that varies in a consistently increasing or decreasing fashion as the amount of exposure (dose) increases. (Adapted from: Centers for Disease Control and Prevention 2014)

The machine used to drill a wellbore. (Adapted from: Schlumberger 2019)

Fragments of rock dislodged by the drill bit and brought to the surface in the drilling mud. Washed and dried cuttings samples are analyzed by geologists to obtain information about the formations drilled. (Source: U.S. Legal 2019)

A circulating fluid, one function of which is to lift cuttings out of the wellbore and to the surface. It also serves to cool the bit and to counteract downhole formation pressure. (Source: OSHA 2001)

The failure of inferences based on group-level (aggregated) analyses to reflect the effect at the individual level. (Adapted from: Rothman and Greenland 1998)

Arises when the level of association between an exposure and an outcome varies by a third factor. (Adapted from: Centers for Disease Control and Prevention 2014)

An extrinsic factor (e.g., geology, climate, insects, sanitation, or health services) that affects an agent and the opportunity for exposure. (Adapted from: Centers for Disease Control and Prevention 2014)

The study of the distribution and determinants of health conditions or events among populations. (Adapted from: Centers for Disease Control and Prevention 2014)

A group whose members have had contact with a suspected cause of, or possess a characteristic that is a suspected determinant of, a particular health problem. (Adapted from: Centers for Disease Control and Prevention 2014)

Having come into contact with a cause of, or possessing a characteristic that is a determinant of, a particular health problem. (Adapted from: Centers for Disease Control and Prevention 2014)

A type of information bias in which a study participant’s exposure status is erroneously assigned. (Adapted from: Rothman and Greenland 1998)

An accumulation, pool, or group of pools of hydrocarbons or other mineral resources in the subsurface. A hydrocarbon field consists of a reservoir with trapped hydrocarbons covered by an impermeable sealing rock or trapped by hydrostatic pressure. (Source: Schlumberger 2019)

A tall stack equipped with burners used as a safety device at wellheads, refining facilities, gas processing plants, and chemical plants. Flares are used for the combustion and disposal of combustible gases. The gases are piped to a remote, usually elevated, location and burned in an open flame in the open air using a specially designed burner tip, auxiliary fuel, and steam or air. Combustible gases are flared most often due to emergency relief, overpressure, process upsets, startups, shutdowns, and other operational safety reasons. Natural gas that is uneconomical for sale is also flared. Often natural gas is flared as a result of the unavailability of a method for transporting such gas to markets. (Source: U.S. Energy Information Administration 2019)

The fracturing fluid that returns to the surface through the wellbore during and after a hydraulic treatment. (Source: The Geological Society of America 2019)

A body of rock strata, of intermediate rank in the hierarchy of lithostratigraphic units, which is unified with respect to adjacent strata by consisting dominantly of a certain lithologic type, or by possessing other unifying lithologic features. (Adapted from: Schlumberger 2019)

The water and chemical additives used to hydraulically fracture the reservoir rock, and proppant (typically sand or ceramic beads) pumped into the fractures to keep them from closing once the pumping pressure is released. (Source: The Geological Society of America 2019)

The amount or number of occurrences of an attribute or health outcome among a population. (Adapted from: Centers for Disease Control and Prevention 2014)

Intentional or unintentional release of volatile chemicals during extraction, processing, and delivery of fossil fuels to the point of final use. (Source: Carras et al. 2006)

Hydrocarbon liquid dissolved in saturated natural gas that comes out of solution when the pressure drops below the dewpoint. (Source: Schlumberger 2019)

A well completed for the production of natural gas from one or more gas zones or reservoirs. Such wells contain no completion for the production of crude oil. (Available: U.S. Energy Information Administration 2019)

The pipes used to transport oil and gas from a field to the main pipeline in the area. (Source: Schlumberger 2019)

Validity of inferences as they pertain to people outside of the source population. (Adapted from: Rothman and Greenland 1998)

Techniques and equipment used to capture methane, volatile organic compound, and hazardous air pollutant emissions from flowback operations (Adapted from: U.S. Environmental Protection Agency 2011).

Information that falls outside the mainstream of published journal and monograph literature, not controlled by commercial publishers. (Adapted from: National Institutes of Health Office of Management 2019).

Facilities that have temporary abnormally high emissions that are caused by an equipment malfunction, accident, operator error, or some other unintended failure of a process to operate in a normal or usual manner. Sometimes referred to as super-emitters. (Source: Zavala-Araiza et al. 2015)

Deviation of a wellbore from vertical toward a horizontal inclination in order to intersect targeted fractures or maximize contact with a productive formation. (Source: Michigan Department of Environmental Quality 2013)

A stimulation treatment routinely performed on oil and gas wells in low-permeability reservoirs. Specially engineered fluids are pumped at high pressure and rate into the reservoir interval to be treated, causing a vertical fracture to open. The wings of the fracture extend away from the wellbore in opposing directions according to the natural stresses within the formation. Proppant, such as grains of sand of a particular size, is mixed with the treatment fluid to keep the fracture open when the treatment is complete. Hydraulic fracturing creates high-conductivity communication with a large area of formation and bypasses any damage that may exist in the near-wellbore area. (Source: Schlumberger 2019)

Organic compounds of hydrogen and carbon, whose densities, boiling points, and freezing points increase as their molecular weights increase. Although composed of only two elements, hydrocarbons exist in a variety of compounds because of the strong affinity of the carbon atom for other atoms and for itself. The smallest molecules of hydrocarbons are gaseous; the largest are solid. (Source: OSHA 2001)

A supposition, arrived at from observation or reflection, that leads to refutable predictions; any conjecture cast in a form that will allow it to be tested and refuted. (Adapted from: Centers for Disease Control and Prevention 2014)

A man-made excavation or diked area for the retention of waste fluids.  (Source: West Virginia Department of Environmental Protection 2012)

A measure of the frequency with which new cases of illness, injury, or other health condition occurs among a population during a specified period. (Adapted from: Centers for Disease Control and Prevention 2014)

Earthquakes related to human activities. Events are typically small in magnitude and intensity of shaking. (Source: National Research Council 2013)

Systematic differences in the collection data regarding the participants in a study (e.g., about exposures in a case-control study or about health outcomes in a cohort study) that lead to an incorrect result or inference. (Adapted from: Centers for Disease Control and Prevention 2014)

The time from exposure to a causal agent to onset of symptoms of a disease. (Adapted from: Centers for Disease Control and Prevention 2014)

Removal of accumulated fluids from mature gas wells to maintain production. (Source: U.S. Environmental Protection Agency 2014)

A quantified relationship between exposure and a particular health problem (e.g., risk ratio, rate ratio, and odds ratio). (Adapted from: Centers for Disease Control and Prevention 2014)

A naturally occurring mixture of hydrocarbon gases that is highly compressible and expansible. Methane is the chief constituent of most natural gas (constituting as much as 85% of some natural gases), with lesser amounts of ethane, propane, butane, and pentane. Impurities can also be present in large proportions, including carbon dioxide, helium, nitrogen and hydrogen sulfide. (Source: Schlumberger 2019)

Variables used to detect sources of bias or confounding in a study that are expected to be associated with the same unmeasured factors as the exposure of interest but not the outcome of interest. (Adapted from: Centers for Disease Control and Prevention 2014)

Variables used detect sources of bias or confounding in a study that are expected to be associated with the same unmeasured factors as the outcomes of interest but not the exposure of interest. (Adapted from: Lipsitch et al. 2010)

A type of information bias in which the proportion of subjects misclassified on exposure does not depend on disease status (case-control studies) or when the proportion of subjects misclassified on disease does not depend on exposure (cohort studies). (Adapted from: Rothman and Greenland 1998).

NORM is any terrestrial material (rock, soil, or water) that contains elements that emit radiation. TENORM is produced when activities such as those associated with oil and gas development concentrate or expose radioactive materials that occur naturally in ores, soils, water, or other natural materials. (Source: Babcock et al. 2015)

The supposition that two (or more) groups do not differ in the measure of interest (e.g., incidence or proportion exposed); the supposition that an exposure is not associated with the health condition under study, so that the risk ratio or odds ratio equals 1. The null hypothesis is used in conjunction with statistical testing. (Adapted from: Centers for Disease Control and Prevention 2014)

A study in which the investigator observes rather than influences exposure and disease among participants. Case-control and cohort studies are observational studies. (Adapted from: Centers for Disease Control and Prevention 2014)

A measure of association used in comparative studies, particularly case-control studies, that quantifies the association between an exposure and a health outcome; also called the cross-product ratio. (Adapted from: Centers for Disease Control and Prevention 2014)

A mixture of hydrocarbons usually existing in the liquid state in natural underground pools or reservoirs. Gas is often found in association with oil. (Source: U.S. Energy Information Administration 2019)

The surface area overlying an oil or gas reservoir or reservoirs. Commonly, the term includes not only the surface area but also the reservoir, wells, and production equipment. (Source: OSHA 2001)

A producing well with oil as its primary commercial product. Oil wells almost always produce some gas and frequently produce water. Most oil wells eventually produce mostly gas or water. (Source: Schlumberger 2019)

Orphaned wells have no known or solvent owner and may or may not be capable of further production. (Source: Interstate Oil and Gas Compact Commission 2009)

Any or all of the possible results that can stem from exposure to a causal factor or from preventive or therapeutic interventions; all identified changes in health status that result from the handling of a health problem. (Adapted from: Centers for Disease Control and Prevention 2014)

The probability of observing an association between two variables or a difference between two or more groups as large or larger than that observed if the null hypothesis were true. Used in statistical testing to evaluate the plausibility of the null hypothesis (i.e., whether the observed association or difference plausibly might have occurred by chance). (Adapted from: Centers for Disease Control and Prevention 2014)

A small, discrete mass of solid or liquid matter that remains individually dispersed in gas or liquid emissions. Particulates take the form of aerosol, dust, fume, mist, smoke, or spray. Each of these forms has different properties. (Source: U.S. Energy Information Administration 2019)

To pierce the casing wall and cement to provide holes through which formation fluids may enter or to provide holes in the casing so that material may be introduced into the annulus between the casing and the wall of the borehole. Perforating is accomplished by lowering into the well a perforating gun, or perforator, that fires bullet-shaped charges that are electrically detonated from the surface. (Source: OSHA 2001)

A measure of the ease with which fluids can flow through a porous rock. (Source: OSHA 2001)

An area in which hydrocarbon accumulations or prospects of a given type occur. The shale gas plays in North America, for example, include the Barnett, Eagle Ford, Fayetteville, Haynesville, Marcellus, and Woodford, among many others. (Source: Schlumberger 2019)

The total number of inhabitants of a geographic area or the total number of persons in a particular group (e.g., the number of persons engaged in a certain occupation). (Adapted from: Centers for Disease Control and Prevention 2014)

Water suitable for drinking. (Source: Water Education Foundation 2019)

The reduction of random error in measurement and estimation. (Adapted from: Rothman and Greenland 1998)

The number or proportion of cases or events or attributes among a given population. (Adapted from: Centers for Disease Control and Prevention 2014)

The proportion of a population that has a particular disease, injury, other health condition, or attribute at a specified point in time (point prevalence) or during a specified period (period prevalence). (Adapted from: Centers for Disease Control and Prevention 2014)

Study participants in which the outcome occurs before selection for or enrollment into the study. (Adapted from: Rothman and Greenland 1998)

Water produced from a wellbore that is not a treatment fluid. The characteristics of produced water vary and use of the term often implies an inexact or unknown composition. It is generally accepted that water within the pores of shale reservoirs is not produced due to its low relative permeability and its mobility being lower than that of gas. (Source: Schlumberger 2019)

The phase of the petroleum industry that deals with bringing the well fluids to the surface and separating them as well as with storing, gauging, and otherwise preparing the product for distribution. For the purposes of HEI-Energy, production includes extraction, gathering, processing, and compression of gas; extraction and processing of oil and natural gas condensates; management of produced water and other wastes; and operation of gathering pipelines. (Source: OSHA 2001)

The last string of casing or liner that is set in a well, inside of which is usually suspended the tubing string. (Source: OSHA 2001)

A granular substance (silica sand, aluminum pellets, or other material) that is carried in suspension by the fracturing fluid and serves to keep the cracks open when fracturing fluid is withdrawn after a fracture treatment. (Source: International Association of Drilling Contractors 2019)

An analytical study in which participants are enrolled before the health outcome of interest has occurred. (Adapted from: Centers for Disease Control and Prevention 2014)

A sample of persons chosen in such a way that each one has the same (and known) probability of being selected. (Adapted from: Centers for Disease Control and Prevention 2014)

An expression of the relative frequency with which an event occurs among a defined population per unit of time, calculated as the number of new cases or deaths during a specified period divided by either person-time or the average (midinterval) population. In epidemiology, it is often used more casually to refer to proportions that are not truly rates (e.g., attack rate or case-fatality rate). (Adapted from: Centers for Disease Control and Prevention 2014)

A measure of association that quantifies the relation between an exposure and a health outcome from an epidemiological study, calculated as the ratio of incidence rates or mortality rates of two groups. (Adapted from: Centers for Disease Control and Prevention 2014)

Systematic differences in the way subjects remember or report exposures or outcomes (a type of information bias). (Adapted from: Rothman and Greenland 1998)

Process of restoring surface environment to acceptable pre-existing conditions. Includes surface contouring, equipment removal, well plugging, revegetation, and other processes. (Source: U.S. Energy Information Administration 2019)

A general term for measures of association calculated from the data in a two-by-two table, including risk ratio, rate ratio, and odds ratio (see also risk ratio). (Adapted from: Centers for Disease Control and Prevention 2014)

A sample whose characteristics correspond to those of the original or reference population. (Adapted from: Centers for Disease Control and Prevention 2014)

A subsurface, porous, permeable rock body in which oil or gas or both are stored. Most reservoir rocks are limestones, dolomites, sandstones, or a combination of these. The three basic types of hydrocarbon reservoirs are oil, gas, and condensate. (Available: Aspen Environmental Group 2015)

Can occur when factors related to both the exposure and outcome of interest (i.e., confounders) are not controlled for in statistical analyses or are controlled but measured inaccurately. (Adapted from: Centers for Disease Control and Prevention 2014)

An analytical study in which participants are enrolled after the health outcome of interest has occurred. Case-control studies are inherently retrospective. (Adapted from: Centers for Disease Control and Prevention)

The probability that an event will occur (e.g., that a person will be affected by, or die from, an illness, injury, or other health condition within a specified time or age span). (Adapted from: Centers for Disease Control and Prevention 2014)

An aspect of personal behavior or lifestyle, an environmental exposure, or a hereditary characteristic that is associated with an increase in the occurrence of a particular disease, injury, or other health condition.

A measure of association that quantifies the association between an exposure and a health outcome from an epidemiological study, calculated as the ratio of incidence proportions of two groups. (Adapted from: Centers for Disease Control and Prevention 2014)

A selected subset of a population that can be random or nonrandom and representative or nonrepresentative. (Adapted from: Centers for Disease Control and Prevention 2014)

Non-cyclical changes that occur over time. (Adapted from: Centers for Disease Control and Prevention 2014)

The unconsolidated grains of minerals, organic matter, or preexisting rocks, that can be transported by water, ice, or wind and deposited. (Source: Schlumberger 2019)

Systematic difference in the enrollment of participants in a study that leads to an incorrect result (e.g., risk ratio or odds ratio) or inference. (Adapted from: Centers for Disease Control and Prevention 2014)

The ability of a test, case definition, or surveillance system to identify true cases; the proportion of people with a health condition (or the proportion of outbreaks) that are identified by a screening test or case definition (or surveillance system). (Adapted from: Centers for Disease Control and Prevention 2014)

A cylindrical or spherical vessel used to separate oil, gas, and water from the total fluid stream produced by a well. Separators can be either horizontal or vertical. Separators can be classified into two-phase and three-phase separators (commonly called free-water knockout). Gravity segregation is the main force that accomplishes the separation, which means the heaviest fluid settles to the bottom and the lightest fluid rises to the top. (Source: Schlumberger 2019)

A fine-grained, fissile, detrital sedimentary rock formed by consolidation of clay- and silt-sized particles into thin, relatively impermeable layers, it is the most abundant sedimentary rock. Shale can include relatively large amounts of organic material compared with other rock types and thus has potential to become a rich hydrocarbon source rock, even though a typical shale contains just 1% organic matter. (Source: Schlumberger 2019)

Natural gas that can be generated and trapped within shale units. (Adapted from: Schlumberger 2019)

A rock rich in organic matter which, if heated sufficiently, will generate oil or gas. Typical source rocks, usually shales or limestones, contain about 1% organic matter and at least 0.5% total organic carbon, although a rich source rock might have as much as 10% organic matter. (Source: Schlumberger 2019)

To start the well drilling process by removing rock, dirt, and other sedimentary material with the drill bit. (Source: Schlumberger 2019)

The standard deviation of a theoretical distribution of sample means of a variable around the true population mean of that variable. Standard error is computed as the standard deviation of the variable divided by the square root of the sample size. (Adapted from: Centers for Disease Control and Prevention 2014)

Layers of sedimentary rock. (Source: Schlumberger 2019)

Gas that migrates from its usual geological location into aquifers or the vadose zone (the zone between the groundwater and the surface) within the shallow subsurface. (Source: Groundwater Protection Council 2012)

Without apparent symptoms. (Adapted from: Centers for Disease Control and Prevention 2014)

A systematic canvassing of persons to collect information, often from a representative sample of the population. (Adapted from: Centers for Disease Control and Prevention 2014)

A statistical tool used to estimate what would have happened to the exposed group had they not been exposed, while controlling for potential confounders that may change over time. (Adapted from: Bouttell et al. 2018)

Gas produced from a relatively impermeable reservoir rock. Hydrocarbon production from tight reservoirs can be difficult without stimulation operations. Stimulation of tight formations can result in increased production from formations that previously might have been abandoned or been produced uneconomically. The term is generally used for reservoirs other than shales. (Source: Schlumberger 2019)

A relatively impermeable reservoir rock from which hydrocarbon extraction is difficult. Reservoir rock can be tight because it has smaller grains or matrix between larger grains or because it consists mostly of silt- or clay-sized grains, which is the case for shale reservoirs. (Source: Schlumberger 2019)

An umbrella term for oil and natural gas that is produced by means that do not meet the criteria for conventional production (See Conventional oil and gas development). What qualifies as “unconventional” at any particular time is a complex function of resource characteristics, available exploration and production technologies, current economic environment, and the scale, frequency, and duration of production from the resource. Perceptions of these factors inevitably change over time and they often differ among users of the term. (Source: Schlumberger 2019).

For the purposes of HEI-Energy, the phrase is defined as the wave of onshore development and production of oil and natural gas from shale and other unconventional, or low permeability, geologic formations as practiced starting around the beginning of the 21st century through multistage hydraulic fracturing in horizontal wells. UOGD operations include:

• field development: exploration, site preparation, vertical and horizontal drilling, well completion (casing and cementing, perforating, acidizing, hydraulic fracturing, flowback, and well testing) in preparation for production, and management of wastes;
• production operations: extraction, gathering, processing, and field compression of gas; extraction and processing of oil and natural gas condensates; management of produced water and wastes; and construction and operation of field production facilities; and
• post-production: well closure and land reclamation.

A measure of the spread in a set of observations, calculated as the sum of the squares of deviations from the mean, divided by the number of observations minus 1. (Adapted from: Centers for Disease Control and Prevention 2014)

A temporary drilling site. Well pads can contain one well or many wells. (Adapted from: Schlumberger 2019)

A treatment performed to restore or enhance the productivity of a well. Stimulation treatments fall into two main groups, hydraulic fracturing treatments (See Hydraulic fracturing) and matrix treatments (e.g., acid or solvent and chemical treatments to improve the permeability of the near-wellbore formation). Fracturing treatments are performed above the fracture pressure of the reservoir formation and create a highly conductive flow path between the reservoir and the wellbore. Matrix treatments are performed below the reservoir fracture pressure and generally are designed to restore the natural permeability of the reservoir following damage to the near-wellbore area. Stimulation in shale gas reservoirs typically takes the form of hydraulic fracturing treatments. (Source: Schlumberger 2019)

There is no consensus on a single definition of well-being, but there is general agreement that at a minimum, well-being includes the presence of positive emotions and moods (e.g., contentment or happiness), the absence of negative emotions (e.g., depression or anxiety), satisfaction with life, fulfillment, and positive functioning. In simple terms, well-being can be described as judging life positively and feeling good. For public health purposes, physical well-being (e.g., feeling very healthy and full of energy) is also viewed as critical to overall well-being. (Source: Centers for Disease Control and Prevention 2013)

A borehole; the hole drilled by the bit. (Adapted from: Schlumberger 2019)

The system of spools, valves, and assorted adapters that provide pressure control of a production well. (Source: Schlumberger 2019)