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Glossary 231

Appendix B: Glossary

This glossary contains simplified definitions of geo- Arch. A regional geologic structure in which units logic and technical terms and acronyms used in this are folded upward, such that the elevation of a rock report. Most words are defined relative to their use in unit along the axis of the arch is greater than the carbon storage research. A good source for definitions elevation of the rock unit on either flank. Arches for geological terms is J.A. Jackson’s “Glossary of Ge- are similar to, but are typically larger scale than, ology” (1997). Good online resources include the Uni- . Arches may separate basins. versity of California at Berkeley’s glossary Argillaceous. Adjective used to describe rock or sedi- (www.ucmp.berkeley.edu/glossary/gloss2geol.html) ment that contains significant amounts of -size and Cengage Learning’s Geolink glossary at (college. particles (shaly). Very fine-grained particles can cengage.com/geology/resources/geologylink/glossary. clog pore spaces and decrease permeability in res- html). The source for most of the oil and gas field defi- ervoirs. nitions is the Schlumberger oil field glossary (www. glossary.oilfield.slb.com). Arkose. A type of containing at least 25 per- cent . It is typically pink or red and usually Acidize. A common method of treatment or well derived from rapid of granitic rocks. In stimulation where is pumped fact, sample cuttings from deep Cambrian arkoses under into a specific rock reservoir com- have been misidentified as in some . posed of carbonate rocks or having carbonate ce- Arkoses are typical of and lowest ment, in order to increase the permeability of the Cambrian in Kentucky. The relatively reservoir near the wellbore and increase the flow high content in arkoses causes higher of fluids or gases out of the reservoir into the well. than normal gamma readings on subsurface geo- The acidizing treatment is sometimes referred to physical logs, which can be misinterpreted as shaly as an acid job. zones. Adj. Arkosic Adsorption. The adhesion of liquid or gas . A dark-colored () igneous (intrusive or onto a surface. For example, CO molecules 2 extrusive) rock formed from . are a will adsorb onto carbon-rich surfaces, such as are possible carbon storage reservoir where they have found in coal or carbon-rich black . Adj. Ad- porosity. Carbon could theoretically be stored sorptive. through trapping in basalts. Basalts occur Anion. An with a negative charge. at depth in the Precambrian Middle Run Formation in Kentucky. . A geologic structure in which rock units are folded in a convex-upward configuration, such that Basement. General term for Precambrian rocks in the the elevation of a rock unit along the axis of the subsurface. The term is informally used to differ- fold is greater than the elevation of the rock unit on entiate dominantly igneous and metamorphic rocks either flank. These structures are smaller scale than of the Precambrian from the layered sedimentary regional arches. The opposite type of structure is a rocks above. In some parts of Kentucky, the base- . ment also contains thick Precambrian sedimentary rocks. The term “crystalline basement” may be Aquifer (general). An underground layer of permeable used for Precambrian igneous and or that can yield significant quanti- rocks to aid in differentiating them from Precam- ties of water to a well or . Water generally brian sedimentary rocks. is held in the pore spaces between mineral grains of the rock or sediment. Freshwater aquifers occur Basement . A fault that extends into the Precam- near the surface at relatively shallow depths and brian (basement) rocks. Sometimes referred to as must, by law, be protected from contamination. faults “rooted in basement.” Saline reservoirs are saltwater (brine)-bearing rock Basin. A structurally low or depressed area in the units that occur at depth and are potential carbon ’s in which thick sequences of sedimen- storage reservoirs. 232 Appendix B

tary rock accumulated. Typically, they are large re- es. Man-made sources include combustion of hy- gional areas that had or have persistent drocarbons for electricity and transportation fuel, for periods of geologic time. cement manufacturing, and ethanol production. Below drainage. Term used to define depth below Carbon sequestration, carbon storage. A technology the lowest level of a stream in an area. In hilly or or process that captures carbon dioxide from man- mountainous terrain it is often important to desig- made emissions or the atmosphere and stores them nate depth below the lowest drainage, rather than in the biosphere (plants and animals), lithosphere depth below the surface, since wells drilled at the (earth), or hydrosphere (ocean sequestration). Ter- top of hills or mountains may be well above the restrial carbon sequestration uses properties of the lowest surface drainage to the side of the hill or and plants to remove or store carbon dioxide. mountain. Geologic carbon sequestration involves the injec- tion of carbon dioxide into subsurface rock units . A clay layer composed chiefly of the clay such as unmineable coals, depleted oil and gas res- mineral (smectite), which is usu- ervoirs, carbon-rich shales, or saline reservoirs. ally derived from altered volcanic ash. Because smectites swell in water, bentonite layers can form Carbonate. General term for rocks composed of 2– confining beds that prevent vertical migration of rocks rich in carbonate (CO3 ) such as

liquids. Bentonite is commercially used as a drill- (CaCO3) and (CaMg(CO3)2), and some- ing . times rocks cemented by . Carbonate rocks are generally more reactive to CO -saturated brine Bioturbation. Features in rocks or sediment that indi- 2 than noncarbonate rocks. cate the sediment was disturbed or inhabited by or- ganisms, including churning, burrows, tracks, and Carbonic acid. A weak acid formed by the dissolution

trails. Some types of burrows are indicative of the of CO2 in water. Carbonic acid is formed by the

original depositional environment of the rock. This reaction CO2 + H2O = H2CO3. feature is more common in marine than nonmarine Casing (case the well). Pipe that is lowered into a rocks. Burrows commonly have different fills than wellbore and cemented in place. Casing is placed surrounding rock, which can influence porosity in wells in order to isolate parts of the wellbore and permeability. Adj. Bioturbated. from surrounding rocks and fluids. In most wells, Brecciated. In geology, a rock fabric consisting of an- it is required through at least the intervals of fresh gular or broken rock fragments in a or ce- water in order to protect any freshwater aquifers ment. The rock is called a . from drilling fluids or gases. Casing may also be used deeper downhole to isolate other rock units as Brine. Formation water that has salinity significantly needed, stabilize the wellbore, or to control pres- greater than seawater (35,000 mg/L TDS). sure and fluid flow. Putting casing in the ground is Calcite. A common rock-forming mineral composed called “running pipe” or “casing the well.” of (CaCO ). Calcite is common 3 Cation. An ion with a positive charge. in carbonate rocks but can occur in other rocks as well. CBM. See coalbed methane.

Carbon dioxide (CO2). A consisting of one Cement (rock). Natural mineral binding material that carbon covalently bonded to two at- welds framework grains together in a rock. There oms. At earth surface and it may be several types of cement in a sedimentary exists as a gas and it comprises about 0.04 percent rock. of atmospheric gas. It is one of the main green- Cement (well). Material used during drilling to bind house gases, and its atmospheric concentration casing to the wellbore. has risen coincident with the industrial revolution. Carbon dioxide is nontoxic and an important part Cement bond log. A type of geophysical well log that of the global carbon cycle, in which it is produced uses acoustic measurements to graphically image by a large number of natural and man-made sourc- the cement used to hold the wellbore casing in Glossary 233

place in a drillhole. The purpose of the log is to or milliSiemens, conductivity is the reciprocal of analyze the cement for holes that would diminish resistivity. the ability of the cement to prevent fluid movement Confining interval (zone).An impermeable rock inter- between the casing and borehole wall. The annulus val or zone that does not allow vertical migration between the rock wall of the borehole and the cas- of fluids or gas from underlying reservoirs. Ade- ing installed in a well. quate confining intervals must be demonstrated to Chert. A hard, microcrystalline -rick rock. Chert regulatory agencies for permitting an underground is harder to drill through than many other rock injection well. There may be multiple confining in- types, so rocks with abundant chert may require tervals above injection reservoirs. Also known as a longer to drill through or cause well deviations. caprock or seal. Chlorite. A (phyllosilicate) mineral group as- Core. A cylindrical section (sample) of rock removed sociated with low- to medium- meta- through drilling with a special bit and drill rig. Full morphic rocks or hydrothermal deposits. An - (whole or conventional) core is cut at near the di- aluminum--silicate mineral. ameter of the wellbore (generally 3 to 5 in.) dur- ing drilling and is recovered in vertical sections, Clean (sandstone). Adjective used by drillers to de- usually in 30-ft increments. Sidewall cores are scribe a relatively pure or homogeneous sandstone, much smaller cores (generally 1-in. diameter and typically quartz-rich or quartzose in contrast to a several inches in length) drilled horizontally from sandstone composed of many different kinds of the wellbore after a well has been drilled. Cores grains, which appears speckled or “dirty.” provide actual samples of rock strata for reservoir Closure. See structural closure. testing and analysis. Coalbed methane (CBM). Natural gas (methane) held Crop out. Exposed at the surface. within and produced from coal beds. Carbon di- Crystalline rock, crystalline basement. Metamorphic oxide might be used to enhance coalbed-methane or igneous rocks, generally in deep Precambrian recovery. Because the coals are carbon-rich, the strata. In much of the Midwest, crystalline rocks carbon dioxide should be adsorbed (stick) onto the form the bottom (basement) on which younger coal matrix. sedimentary rocks were deposited. The overlying Completion (well completion, complete a well). Fin- sedimentary rocks contain reservoirs for oil, gas, ishing a well so it is ready to produce oil or gas. water, and possibly carbon sequestration. This usually involves installing production casing Cuttings. Fragments and chips of rocks that are cut and cement, perforating the casing into the produc- from the wellbore by drilling. The chips are brought ing interval, treating the producing interval, run- up the hole to the surface as part of normal drill- ning production tubing, and installing pumps, etc., ing operations, are screened from the drilling mud, for production. When examining well records, a and then described by drilling personnel. Descrip- completion indicates that there were sufficient hy- tions may include information about their apparent drocarbons (and presumably porosity and perme- composition, color, texture, hydrocarbon content ability in the host reservoir) to go to the expense of (if any), and depth. Cuttings are bagged and saved installing tubing, etc. for some wells. The Kentucky Geological Survey Condensate. Volatile hydrocarbons often co-produced Well Sample and Core Library has a large invento- with natural gas that are liquid at surface tempera- ry of cutting samples from many of the deep wells ture and pressure conditions: “wet gas.” These in Kentucky. generally are high-gravity oils. Darcy. Standard measure of permeability that is equal Conductivity. A measurement used to approximate the to the passage of 1 cm3 of fluid of 1 centipoise vis- salinity of a formation water sample. In general, as cosity flowing in 1 sec under a pressure differential the number of in a formation water increases, of 1 atmosphere through a porous medium having the conductivity increases. Measured in Siemens 234 Appendix B

a cross-section area of 1 cm2 and length of 1 cm to displace residual natural gas in a reservoir so it (see millidarcy). can be more easily extracted. Density log. A type of geophysical well log that shows Enhanced oil recovery (EOR). The third stage of oil a graphic representation of the bulk density of recovery, in which a variety of methods can be used rocks and their contained fluids in close proximity to alter the chemical or physical properties of the to the wellbore. Density measurements are taken remaining oil in a reservoir so it can be more easily by a wireline tool that is lowered down the well. extracted. EOR typically involves injecting fluids Porosity can be calculated based on a mass-balance or gases into a reservoir or heating the reservoir in relationship between bulk density and porosity. order to lower the (stickiness) of the re- maining trapped oil. Differs from secondary recov- Detrital. Grains that have been eroded from other ery, where the goal is repressurization or pressure rocks or organic material. stabilization in the reservoir with the simple intent

Diagenesis. The sum of all of the biological, chemical, to displace additional oil. Miscible CO2 could be and physical alteration of sediment after its burial, injected into old oil reservoirs in order to lower and as it turns to stone (lithification), but not in- the viscosity of stubborn trapped oil, allowing it to cluding weathering and when the rock is flow to a producing well. reexposed at the surface. EPA. U.S. Environmental Protection Agency. The EPA Dissolution. A type of chemical weathering in which has primacy over underground injection in Ken- water or acidic waters dissolve parts of tucky. Any underground injection control permit or rocks from the surrounding . Most com- (typically referred to as a UIC permit) must be ap- mon in carbonate rocks. proved by EPA Region IV offices in Atlanta, Ga. DOE. U.S. Department of Energy. This federal orga- Equation of state. Functions that describe the values nization oversees carbon capture and sequestration of pressure (P), temperature (T), and volume (V) research in the . for phases (gas, liquid, solid, or some combina- tion) at equilibrium; that is, no net change in the Dolomite. A dominated by the min- properties or composition of the system without an eral dolomite, which is a magnesium-rich, rather external influence. than calcium-rich, . In some cases, the term dolostone is used to differentiate the sedi- . The recurrent and persistent assemblage of rock mentary rock referred to as dolomite from the min- type, fossils, and thickness that characterize strata eral dolomite. Adj. Dolomitic. of a specific origin. Facies are typically used to describe parts of sedimentary rock bodies. A rock . The process by which a carbonate formation will generally consist of several differ- rock is altered to dolomite. This can occur dur- ent facies. Each facies may have its own porosity ing or shortly after burial, as in arid and permeability characteristics. shoreline (sabkha) environments, or can occur later in burial through the migration of hot, mineral-rich Fault. A crack in the earth’s crust across which move- fluids through carbonate rocks. ment has occurred. The fault is a geometric plane between two rock masses. See normal, reverse, Drillstem test (DST). A standard formation or reser- strike-slip, and thrust fault for different types of voir test in which a subsurface interval of rock is faults defined by the relative movement of rock isolated (usually by packers down the wellbore) masses on either side of the fault. Sealing and and fluid is allowed to flow from the formation transmissive faults refer to their ability to transmit into the drillstem and pressures are recorded over fluids or gases. In carbon storage research it is very time. A variety of pressure responses can be tested important to identify any fault within the area of by opening and closing valves that allow fluids to potential injection. flow into the drillstem. Feldspar. A group of common rock-forming minerals Enhanced gas recovery (EGR). The stage of gas re- composed of aluminum and silica (aluminosili- covery in which a variety of methods can be used Glossary 235

cates) with potassium, sodium, and calcium. Adj. grains — the pore space — can be entirely or partly Feldspathic. filled with cement or matrix. Feldspathic. Adjective used to describe a rock con- Friable (sandstone). Adjective used to describe a rock taining abundant feldspar grains. that is poorly cemented and easily broken or crum- bled. . Adjective used to describe the light-colored, silica and aluminum minerals in igneous rocks. Fugacity. Describes the effective concentration of gases under nonideal conditions (that is, high pres- FDC (compensated formation density) log. A type sure), in which molecules react more strongly with of geophysical well log that shows a graphic repre- other molecules. sentation of the bulk density of rocks and their con- tained fluids along the sides of the wellbore, which FutureGen. A federal- and industry-sponsored project has been corrected (compensated) for fluctuations to construct the first near-zero-emissions power in the diameter of the borehole and the influence plant. The plant would use geologic carbon se- of drilling fluids and mud cake downhole. Density questration as part of a strategy to mitigate carbon measurements are taken by a wireline tool that is emissions. lowered down the well. Porosity can be calculated Gamma-ray log. A type of geophysical well log that based on a mass-balance relationship between bulk shows a graphical representation of natural gam- density and porosity. ma-ray emissions from subsurface rock units. Formation (rock unit). The basic rock unit used for Measurements are taken by a wireline tool lowered mapping in geology. Formations have distinctive down a wellbore. Useful for identifying shales be- upper and lower boundaries and must be mappable cause shales emit more gamma rays than other for large distances. Formations may be composed common rock units. of smaller units termed members and beds. Mul- Gas-drive reservoirs. Reservoirs in which the primary tiple formations may be combined into larger units recovery mechanism is dissolved and frees natural called groups. gas in the reservoir. Expansion of the gas is used to Formation water. Water present in the porosity of drive the oil from the reservoir into the wellbore. subsurface reservoirs or other types of buried Geophysics. The study of the physics of the earth, pri- rocks — typically sedimentary. marily through seismic, gravitational, magnetic, Frac, fracing, fracking. See hydraulic . radioactive, or electrical means. In geology, geo- physics refers to a wide array of techniques used to Fracture (frac) pressure. The pressure at which a unit directly or passively gather information from be- of rock fractures. This pressure must be calculated neath the surface of the earth. Adj. Geophysical. and tested for any injection well in order to deter- mine the limits of injection pressure that will be Geophysical well log (well log). General term for a allowed so that overlying confining intervals are recording or measurement of subsurface rock not fractured. and fluid properties gathered from a wireline tool lowered down a wellbore using geophysical tech- Fracture gradient. The pressure at which a rock for- niques, such as measurements of spontaneous po- mation will fracture at different depths in the sub- tential and resistivity (electric logs), or gamma ray surface, typically noted as pressure per unit depth and density (radioactivity), etc. (for example, psi/ft = pounds per square inch per foot). Geothermal gradient. The rate of increase of temper- ature in the earth with depth. The gradient varies Framework grains. The grains that are the principal by region, but the overall average gradient for the supporting structure in sedimentary rocks. Frame- crust is 25°C per kilometer of depth. work grains may consist of a single mineral (e.g., quartz grains), rock fragments, or fossil grains. Gigatonne. Metric unit equal to 1 billion metric tonnes The intervening space between the framework or 1.103 billion U.S. tons (standard short tons). 236 Appendix B

Standard international unit used for measuring car- Hematite. A red or silver iron-oxide mineral. Adj. bon dioxide emissions. ­Hematitic. . A green found in sedimen- Heterogeneous. Adjective indicating variable charac- tary rocks and consisting mostly of silica, potassi- teristics. In sedimentary geology, heterogeneous um, and iron. Generally, characteristic of sediments usually denotes lateral variability in thickness, deposited in deeper marine conditions at slow dep- grain size, and bedding. The opposite is homoge- ositional rates. The high potassium content of this neous. Noun. Heterogeneity. can cause false porosity readings on Homogeneous. Adjective indicating relatively uniform neutron logs and high gamma readings. characteristics. The opposite is heterogeneous. Graben. A relatively downdropped block bounded by Noun. Homogeneity. normal faults. Horst. A relatively uplifted (positive) fault block . An intrusive formed from mag- bounded by normal faults. Horsts often bound gra- ma and consisting of quartz (silica), , bens. amphiboles, , and . In Kentucky, Huff-and-puff. In geology and carbon se- granites are known from subsurface Precambrian questration, an enhanced oil recovery method in strata. which CO2 is injected into an oil reservoir through Gravity data/surveys/analyses/anomaly. Measure- a well. The well is then shut-in (closed) to let the

ments of spatial variations in the earth’s gravita- injected CO2 dissipate into the reservoir, potential- tional field. Measurements at different locations ly dissolving into the oil, the “soak” period. Af- can be used to detect different densities of rock ter some of time (determined by the bottom hole strata in the subsurface. Gravity surveys or analy- pressure) the well is opened to allow the oil to be ses are done to map variations in the gravitational produced. Several cycles of injection and shut-in field. When mapped, gravity anomalies (rapid lat- (huff) followed by production (puff) may be used. eral changes in gravity measurements) may indi- Hydraulic fracture. A well-stimulation method in cate changes in geology related to major faults or which fluids are pumped into a reservoir at high igneous intrusions. pressure in order to overcome the natural confining Greenhouse effect. Heating (rising temperature) pressure in the reservoir, causing a vertical fracture caused by greenhouse gases in the atmosphere that in the reservoir. The fracture is then filled (propped) absorb and emit infrared radiation (heat). with (or other material) to keep the fracture open. This method increases the hydraulic connec- Greenhouse gases. Gases in the earth’s atmosphere tivity between the reservoir and the wellbore. that absorb and emit radiation from the thermal infrared range, including water, carbon dioxide, Hydrocarbon. An organic compound consisting of hy- methane, nitrous oxide, and ozone. Although water drogen and carbon. Typically refers to fuels such is the most abundant greenhouse gas in the atmo- as natural gas, oil, and coal. sphere, science indicates that small changes in the Hydrology. The study of water. In petroleum geology amount of carbon dioxide and methane (common and carbon sequestration, hydrology involves the in industrial and fossil-fuel-powered electrical- measurement and physical and chemical character- plant emissions) can cause increases in the earth’s ization of subsurface waters. surface temperature, termed global warming, or global change. Hydrostatic pressure and gradient. The pressure ex- erted on a point overlain by a column of water at Group (rock unit). A rock unit generally composed of rest. The average hydrostatic gradient with depth two or more genetically related formations. This in the earth is 100 bars/km or 0.43 lb/in.2/ft. unit is designated when the formations in the group are more similar to or distinctive from each other Hydrothermal dolomite. Dolomite formed by pre- than formations above or below the group. cipitation from water hotter than the surrounding rocks (hydrothermal fluids) in the subsurface. Glossary 237

Hydrothermal fluids. Fluids, often mineral-rich, number of protons in a nucleus but different num- which are heated within the earth. These fluids can bers of neutrons. The ratio of isotopes for a given move along faults and fractures, altering the rocks element, such as carbon and oxygen, is sensitive to through which they migrate. a wide variety of chemical (e.g., mineral precipi- tation) and physical (e.g., evaporation) processes, IGCC (integrated gasification combined-cycle). A and therefore the isotopes can be used to infer the power-generation process that uses coal gasifica- processes that affected the occurrence and distribu- tion to generate syngas (synthesis gas) to fuel gas tion of the element. Adj. Isotopic. turbines for electricity generation. Waste heat from the gasification and combustion processes are also . An earthy, white to tan clay. captured to make steam that is then used to gener- . The terrain and features associated with disso- ate electricity from steam turbines. IGCC plants are lution of soluble bedrock, usually carbonates, that more efficient than conventional coal-combustion form , sinkholes, springs, and other features. plants, and result in lower levels of emissions. The gasification process produces a nearly pure stream KGS. Kentucky Geological Survey. A research branch

of CO2, which can be easily captured for storage. of the University of Kentucky, which is charged with increasing the knowledge and understanding Immiscible. Phases of fluids or gases that will not mix. of the mineral, energy, and water resources, geo- Relative to carbon storage, immiscible conditions logic hazards, and geology of Kentucky for the occur when pressures and temperatures are too benefit of the commonwealth and nation. low (and depths are too shallow) for CO2 to mix with subsurface fluids (salt water, oil) or natural Kimberlite. Igneous, ultramafic (dark iron-magne-

gas, so that any injected CO2 would remain as a sium minerals), composed of at least separate phase in the reservoir. Shallow, immis- 35 percent (a magnesium iron silicate min- cible enhanced-recovery projects are expected to eral). rely primarily on displacement of hydrocarbons in the reservoir. KYCCS. Kentucky Consortium for Carbon Storage. A State and industry partnership in Kentucky Injection fall-off test. A downhole test used to measure conducting research on carbon sequestration. the injectivity of a formation. During an injection This partnership is administered by the Kentucky fall-off test, gas or fluids are injected into a forma- Geological Survey and is separate from the U.S. tion through tubing at a steady rate and pressure. Department of Energy regional carbon sequestra- The well is then shut-in, and the pressure of the tion partnerships. KYCCS maintains a Web site at reservoir is monitored over time with gauges. The www.kyccs.org. rate at which pressure returns to pre-test conditions is a function of the permeability and volume of the Laminae (laminations). The thinnest unit layer of de- reservoir. position in a sedimentary rock. Laminae are less than 1 cm (0.39 in.) thick. Injectivity. In petroleum geology and carbon seques- tration, a measure of a reservoir’s ability to have Limestone. A sedimentary rock consisting of more gas or liquid injected into it. than 50 percent calcium carbonate formed from physical or chemical processes. Intercrystalline porosity. Microscopic porosity formed between in rock cements. Typi- Litharenite (lithic arenite). A sandstone containing cally formed in carbonate rocks as a result of di- more than 25 percent rock (lithic) fragments and agenesis. having less than 75 percent quartz grains and less than 10 percent feldspar grains. Isopach (map). A thickness map in which lines con- nect points of equal thickness, similarly to eleva- Lithology. The description or physical character of a tion contours on topographic maps. rock. Generally includes rock type, grain size, bed- ding, mineral constituents, and cements. Isotope. One of two or more species of the same chem- ical element, in which the species have the same 238 Appendix B

Mafic. Term used to describe dark, iron-magnesium confining interval because different minerals will

minerals in igneous rocks. react (or not react) with acids, fluids, and CO2 at different pressures and temperatures. Magnetic data/analyses/anomaly. Measurements of

spatial variations in the earth’s magnetic field. Mineral trapping. The process in which CO2 injected Measurements at different surface locations can into a reservoir dissolves into the formation water be used to detect different densities of rock strata ( trapping) and forms ionic species, in the subsurface. Magnetic surveys or analyses such as bicarbonate, that subsequently react with are done to map variations in the magnetic field. cations, such as calcium, magnesium, and iron, When mapped, magnetic anomalies (rapid lateral to form carbonate minerals. Mineral trapping is changes in magnetic measurements) may indicate considered the most permanent form of geologic changes in geology related to mineral-bearing ore storage, but it is also one of the slowest. bodies or large oil fields. Miscible. Phases of fluids or gases that will mix into a Matrix. The ground mass or finest-grained component homogenous mixture. Relative to carbon storage, of a rock in which framework grains or crystals the term is used when pressures and temperatures are embedded. Typically used in carbonate rocks are high enough (and depths are great enough) for

to describe the fine-grained material that surrounds CO2 to mix with subsurface fluids (salt water, oil) and encompasses larger particles or framework or natural gas in a reservoir. grains. Mol or Mole. The mass (in grams) of a substance Matrix porosity. Microscopic porosity of the rock ma- that contains 6.023 X 1023 , ions, or mole- trix, or finer-grained ground mass of a carbonate cules, and which is equal to the atomic or formula rock. weight. Mcf. Thousand cubic feet; a standard measure of vol- Moldic porosity. Pores that result from the removal or ume in gas fields. See unit conversion table for dissolution of a grain or fossil shell that retain the equivalents. Mcfg/d would be thousand standard “mold” or general shape and size of the original cubic feet of gas per day. In this context, MMcf grain or shell. indicates million standard cubic feet. MRCSP. Midwest Regional Carbon Sequestration md. Millidarcy. A standard unit of permeability equal Partnership. One of seven regional partnerships to 1/1,000 of a Darcy. See Darcy. funded by the U.S. Department of Energy for car- bon sequestration research. This partnership is ad- MGSC. Midwest Geological Sequestration Consor- ministered through Battelle Memorial Institute and tium. One of seven regional partnerships funded covers the mid-Atlantic, northern Appalachians, by the U.S. Department of Energy for carbon se- , and Arches Provinces, including questration research. This partnership is adminis- central and eastern Kentucky. The Kentucky Geo- tered through the Illinois State Geological Survey logical Survey is part of the consortium. and covers the , including western Kentucky. The Kentucky Geological Survey is part Mud log. A graphic representation of the rate a drill of the consortium. is penetrating subsurface rock units while drilling, with notes on rock type from drill cuttings, and . A phyllosilicate mineral. Mica is a common con- other parameters noted during drilling. stituent of sedimentary rocks, typically appearing as reflective flakes or sheets.Adj. Micaceous. MW (megawatt). Standard unit in the electrical-power industry equal to 1 million watts. Micrite. Very fine-grained crystalline carbonate rock or matrix of carbonate rocks. Typically formed Neutron log (neutron porosity log). A type of geo- from carbonate . Adj. Micritic. physical well log that shows a graphic representa- tion of the interactions of fast neutrons (or neutron . The study of the mineral components of and gamma rays) emitted from a downhole source a rock. In carbon storage research it is important with rock and pore fluids. A neutron log principal- to know the minerals that compose a reservoir or Glossary 239

ly measures the effect of hydrogen in pore fluids, Paleotopography. Related to an ancient land surface so it indirectly measures relative porosity. Most that was buried. The term implies an uneven sur- neutron-porosity logs are calibrated for fresh water face with buried hills and valleys generally along and rock types, and must be recalibrated (or recal- an unconformity. Adj. Paleotopographic. culated) for different rock types. Typically, neutron Paleotopographic high. A structural high on a paleo­ logs are calibrated to calcite, which means they are topographic surface, such as an unconformity sur- scaled to true porosity in , but have to be face. A buried hill. In some cases, paleotopograph- rescaled for other rock types. ic highs or lows may preferentially have porosity Normal fault. A type of fault in which one side of the or permeability and trap oil, gas, or water. Areas of fault (the hanging wall) has moved down relative preferential permeability development may have to the other side (footwall). The opposite is a re- carbon storage potential. A paleotopographic low verse fault. Most normal faults are high-angle, or would be a buried valley or depression. near-vertical in orientation. Peak ground acceleration. A measure of the maxi- Openhole, open hole. The (usually) lowermost part mum acceleration (change in velocity) of a particle of the wellbore in which no protective casing or during the course of an earthquake motion. Essen- pipe has been. When used to describe production, tially a measure of the maximum amount of shak- it generally means that all intervals below a certain ing that is likely during an earthquake. The U.S. depth to the bottom of the well are not cased. In Geological Survey maps peak ground acceleration an injection well, an openhole completion would in earthquake-prone areas for earthquake hazard mean injection into the entire uncased interval be- analysis. low a certain depth to the bottom of the well. Perforate, perforated interval. After production cas- Oolite. Carbonate rock composed of tiny, rounded, ac- ing or tubing is placed in a wellbore, the casing is cretionary (gradually increasing in size) particles shot with holes that penetrate the casing. The holes or grains. The grains are called ooliths. Adj. Oo- allow oil or gas from the producing formation to litic. enter the casing and be pumped to the surface in a production well. In an injection well, perforations Organic content. A measure of the amount of organic allow fluid or gas to be injected into a specific rock carbon in a material. Organic content is significant reservoir, or specific part of a rock reservoir, from in carbon sequestration because high organic con- the wellbore. tents in some rocks can cause adsorption of carbon dioxide. Permeability. A measure of the degree to which flu- ids can move through a rock; in other words, the Organic rich. Used to describe a rock that contains connectivity of pores and fractures. Permeability large amounts of solid organic carbon components. is generally measured in darcies or millidarcies Organic content is significant in carbon sequestra- (md). tion because high organic contents in some rocks can cause adsorption of carbon dioxide. Petrography. A measure within a rock unit. The study of the mineral and textural relationships of rocks Packer. In drilling, a device that can be expanded using microscopy and other techniques. Adj. Pe- downhole in the wellbore or casing to seal off in- trographic. tervals of the well for testing, cementing, or casing. When two packers are used to isolate a zone, they pH. The measure of hydrogen ion activity in solution are sometimes referred to as straddle packers. and is equal to the negative logarithm (base 10) of the hydrogen ion concentration. It is a measure of Paleokarst. Ancient karst. Surface or interval in a solution’s acidity. rock with evidence of ancient karst, including

dissolution, sinkholes, conduits, formed from Physical trapping. The process in which CO2 as a ancient exposure of a carbonate surface and ancient buoyant free-phase is trapped below low-permea- weathering and erosion of that surface or intervals bility seal rocks in a closed trap — called “struc- beneath that surface. tural and stratigraphic trapping” — or in which no 240 Appendix B

closed trap exists and CO2 slowly migrates in a sa- do not. Part of a standard electric log along with line aquifer over long distances — called “hydro- spontaneous potential. dynamic trapping.” In the latter case, CO is even- 2 Reverse fault. A type of fault in which one side of the tually trapped over time by residual, structural, or fault (the hanging wall) has moved up relative to mineral trapping processes. the other side (footwall). The opposite is a normal Porosity. The ratio of the relative amount of open or fault. void space in a rock to the total volume of the . A silica-rich, extrusive igneous rock; a fel- rock. sic igneous rock. In Kentucky, are known ppm. Acronym for parts per million, which is a standard from subsurface Precambrian strata. unit in which concentration of ions of dissolved . A tensional feature formed when blocks of the constituents are reported in a weight-per-weight earth’s crust pull apart. Typically forms a large gra- basis as a dimensionless ratio. As a practical mat- ben bounded by faults. ter, ppm is equivalent to milligrams per liter. Salinity. Equals the total amount of (milligrams Primary porosity. Porosity remaining from the origi- per liter or parts per million) remaining in a water nal deposition of a sediment. See also porosity and sample that is evaporated to dryness. secondary porosity. Saline aquifer. A subsurface reservoir containing high- Pseudomatrix. Term used to describe very fine-grained ly saline water (typically more than 10,000 ppm of (pasty), interstitial, or intergranular (between grain) dissolved salts). In carbon sequestration research, material, which looks like a matrix but is discon- the term is typically used to denote regionally tinuous or different from the matrix. Pseudomatrix widespread, saltwater-bearing units, compared to typically is formed through the deformation or dis- more local oil and gas reservoirs, which may also solution of weak detrital grains. contain salt water, but only in small areas. psi. Acronym for pounds per square inch, a standard Saline water. Water that contains a significant con- unit of pressure. centration of dissolved salts. Freshwater salinity Quartzarenite. A type of sandstone composed of more equals 1,000 ppm or less of total dissolved solids than 95 percent quartz grains. and seawater salinity averages 35,000 ppm of total dissolved solids. Brines have salinities significant- Quartzose. Quartz-rich rock. For , used as a ly greater than seawater. For purposes of under- general term for a rock that is either a quartzarenite ground injection, it is critical to know the depths or a quartz-rich litharenite. of fresh, potable water and the salinity of deeper Reservoir. A porous and permeable rock body in the waters in potential storage reservoirs. subsurface containing quantities of oil, gas, or wa- Sandstone. A sedimentary rock composed of sand-size ter and generally isolated to a specific interval by grains. Sand grains are often naturally cemented surrounding less-permeable rock, forming confin- together. Porous and permeable sandstones are ing layers or traps. common reservoir rocks. The thickness, lateral extent, mineralogy, porosity, permeability, and ho- Residual trapping. The process in which CO2 fills very small pore spaces between and within grains mogeneity are some of the factors that influence its potential for carbon storage. making up the rock, which renders the CO2 immo- bile. Seal. An impermeable rock layer that does not allow Resistivity log. A type of geophysical well log that vertical or lateral migration of fluids or gas from measures the electrical resistivity of subsurface underlying or adjacent reservoirs. Adequate seals rocks and interstitial pore fluids. It is useful for must be demonstrated to regulatory agencies for delineating hydrocarbons in pore spaces of rock permitting an underground injection well. A con- because water conducts electricity but oil and gas fining layer. Glossary 241

Sealing fault. Faults are termed sealing when fluids tings when they come to the surface or increased and gases are confined within a reservoir by the gas readings on gas detection equipment. faults. Sealing faults form structural traps in res- Shut-in well. A well that could produce hydrocarbons ervoirs. When faults are conduits for fluid or gas (oil or natural gas) that is temporarily sealed or migration they are termed transmissive faults. shut off for some economic or technical reason. SECARB. Southeast Regional Carbon Sequestration Sidewall core. A core taken from the side of a well- Partnership. One of seven regional partnerships bore. These cores can be taken after a well has been funded by the U.S. Department of Energy for car- drilled, as opposed to full (whole or convention- bon sequestration research. This partnership is al) core, which is taken during drilling. Sidewall administered through the Southern States Energy cores, however, are much smaller than whole (con- Board and covers the southeastern United States, ventional) core, generally 1 in. or less in diameter including the southeastern part of the Eastern Ken- and 1 to 2 in. long. Cores provide actual samples of tucky Coal Field. rock strata for reservoir testing and analyses. Secondary porosity. Porosity that develops after dep­ Siltstone. A sedimentary rock composed of silt-size osition, typically through fluid migration and dis- grains. solution, fracturing, etc. See also porosity and pri- mary porosity. Seismic data/surveys/analyses. Measurements of elastic waves of energy (typically transmitted by Secondary recovery. The second stage of oil or gas re- P and S waves) used to interpret the composition, covery. The first stage is natural flow due to grav- fluid content, and layering of rock units in the sub- ity or pressure from the reservoir to the wellbore. surface. Typically, a seismic wave is generated at a When that motive force decreases, an external source on the surface and then a series of monitor- fluid or gas is injected into the reservoir through ing devices measure the reflection of the energy as selected wells in order to increase or maintain it reflects off of subsurface rock units. The data are pressure in the reservoir and push or displace hy- then processed to produce a seismic cross section drocarbons to a producing well. Waterfloods are of the subsurface rock layers. Seismic surveys have the most common type of secondary recovery, but been required for all of the DOE-sponsored carbon there are many different methods. In contrast to dioxide injection test wells in order to determine if secondary recovery, where the goal is repressur- there were any faults or structures within the area ization, enhanced oil and gas recovery, or tertiary of influence that could negatively influence- con recovery, involves methods that alter the chemical tainment of the injected carbon dioxide. or physical composition of the residual oil or gas in the reservoir. Seismic risk/hazard analysis. Method for examin- ing the potential consequences or probabilities of . A broad low area in the earth’s earthquakes in an area. Seismic risks are deter- crust in which sediments accumulated and lithified mined based on past occurrences of earthquakes to form sedimentary rocks. in an area and computer modeling of the likely . A lithified, fine-grained sedimentary manner in which bedrock and sediment will propa- composed of clay- and silt-size particles. Shales, gate seismic energy in an earthquake. Seismic- in contrast to , have a finely laminated risk analysis is required for most federal building structure, called fissility, along which the rock and construction projects, and may be needed for breaks readily. Shales typically have low poros- large-scale underground injection projects. The ity and permeability, and, where thick, often form U.S. Geological Survey has published seismic-risk good confining intervals or natural seals. maps for the entire United States. Show of oil or gas. In drilling, an indication of oil or Skeletal grains. Grains in carbonate rocks formed from natural gas in a wellbore. A show is typically de- the calcite or skeletons of marine plants termined at the surface from fluorescence in cut- and organisms. For example, shell fragments are common skeletal grains. 242 Appendix B

Solubility. The extent to which one substance will dis- Standard deviation. A measure of the spread of values solve into another. For carbon storage, the focus from their average. is on the degree to which CO will dissolve into 2 Stimulation. A well treatment used to restore or en- formation waters. hance productivity in a well, or enhance the per-

Solubility trapping. The process in which CO2 dis- meability or hydraulic connectivity between the solves into formation waters. The extent of dis- reservoir and the wellbore. solution generally decreases with increasing tem- Straddle packers. See packers. perature and salinity, and increases with increasing

pressure. The process removes CO2 as a separate . The study and correlation of rock units buoyant phase. to determine their relative stacking, distribution, depositional origins, and ages. Adj. Stratigraphic. Solution features. Any physical feature formed from the solution of soluble rocks, such as limestone, by Stratigraphic trap. A trap or seal on a reservoir in acidic water. Features include any of a number of which changes in rock types, layering, or bedding karst features such as conduits, caves, sinkholes, form the seal or trap. etc. Strike-slip fault. A type of fault in which the rock Sorting (sandstone). Fabric of sedimentary rocks, typ- blocks on either side of the fault slide past each ically applied to sandstones, which describes the other, rather than up or down relative to the other general distribution of grain sizes in the rock based block. Informally termed a tear fault. on standard deviations. Very well sorted means 0.35 to 0.50 standard deviations, which means that Structure. A geologic feature formed from deforma- most of the grains in the rock have similar size. tion of the earth’s crust, such as a fault or fold. See Poorly sorted means 1.4 to 2.0 standard deviations, also structure map. Adj. Structural. which means there is a wide distribution in grain Structural closure. Term used to indicate a closed sizes. Well-sorted sandstones typically have bet- structural contour on a structure map, generally de- ter porosity than poorly sorted sandstones because noting a structural high in a reservoir, which may small grains fill the poor spaces between larger form a structural trap. grains in poorly sorted sandstones (although other factors also influence porosity). Structural relief. The relative difference in depth from the top of a surface to the base of a surface (for Source rock. A rock unit with a high organic content example, the top surface of a rock unit) at depth that if heated could or has generated hydrocarbons. across some structure, paleotopographic feature, Hydrocarbons then migrate to reservoirs. Most or trap, generally referring to an irregular uncon- source rocks are organic-rich shales, typically con- formity surface or a reservoir. taining at least 1 percent organic matter and at least 0.5 percent total organic carbon. In Kentucky, for Structural trap. A trap or seal on a reservoir in which example, the Devonian black shales (Chattanooga the reservoir is sealed along a fault or through the Shale, Ohio Shale, New Albany Shale) are known dip or attitude of beds in a structure such as an an- source rocks. ticline or syncline. Spontaneous potential (SP) log. A type of geophysi- Structure map. A subsurface map of a surface, such cal well log that measures the electrical potential as the top or bottom of a rock unit or reservoir. of subsurface rocks and interstitial pore fluids from Contour lines on the map represent points of equal wireline tools lowered down the wellbore. This log elevation, generally drawn relative to a sea-level is useful for detecting permeability in subsurface datum, although other datums may be used. Hence, rock layers because it detects differences in salin- they are similar to a topographic map of a subsur- ity between drilling muds and formation fluids. It face unit. Structure maps are useful for determining is also useful for determining clay content of beds. the dip of rock strata and the occurrence of struc- It is part of a standard electric log, along with re- tural features such as anticlines and . sistivity. Glossary 243

Subcrop. Point or line at the surface where a subsur- Tight (rock unit). Adjective used to describe rock units face rock unit comes to the surface of bedrock. The that show little permeability or are well cemented. rock unit may be covered by modern alluvium and TOC. Acronym for total organic carbon. A common soil, and not actually exposed or cropping out. measure of the amount of organic carbon in a rock Sublitharenite. A type of sandstone between lithic unit. It is an important measurement for determin- arenite (litharenite) and quartzarenite. Composed ing original source rocks and for determining ad- of 75 to 95 percent quartz and 5 to 25 percent rock sorption mechanisms relative to the injection of (lithic) fragments. carbon dioxide in the subsurface. Subsea. Below sea level. The term is generally used to Ton. Standard U.S. ton, also called a short ton, equals denote elevation below sea level on structure maps 2,000 U.S. pounds or 0.907 metric tonne. and cross sections. Tonne. Metric ton, equals 1,000 kilograms or 1.103 Supercritical (fluid). Refers to a substance that ex- U.S. short tons. ceeds its critical point (critical temperature and . Igneous, extrusive that is fine- pressure) with near-liquid density and a viscosity grained and dominated by alkali feldspar and mi- similar to the gas phase. For CO , the critical point 2 nor mafic (dark) minerals. is 1,085 psi at 88°F (74.8 bar at 31.1°C), which occurs at depths of approximately 2,500 ft in most Transmissive (fault). Faults are termed transmissive of Kentucky. For industrial-scale geologic seques- when fluids and gases can migrate along the faults

tration, keeping CO2 at supercritical conditions in or fractures associated with faults. Faults can also underground reservoirs will be important, because be sealing and form traps. there is a significant (approximately 250 times) Trap. In oil and gas geology, a rock reservoir capable volume reduction of supercritical CO2 relative to gaseous CO . of holding hydrocarbons and sealed by relatively 2 impermeable rocks through which the hydrocar- Syncline. A local geologic structure in which rock units bons will not migrate. See also structural trap and are folded (downwarped), such that the elevation stratigraphic trap. of a rock unit along the axis of the structure is less than the elevation of the rock unit on either flank. Treatment (well treatment). General term for methods These structures are smaller scale than regional ba- in which fluids are pumped down a well to resolve sins. The opposite type of structure is an anticline. a wellbore or reservoir condition. See stimulation. Synsedimentary. Occurring when the sediment that TVD. Acronym for true vertical depth of an intention- formed the sedimentary rock was deposited. ally deviated or horizontal well from the ground surface to the deepest penetration of the wellbore TD. Acronym for total depth of a well. in a vertical plane. The measured depth of the well is the total length of the wellbore. TVD is used Total dissolved solids, TDS. The total dissolved ma- with wellbore deviation data to correct observed terial in a liquid, measured as the materials small rock unit thicknesses as penetrated at some angle enough to pass through a filter or sieve of 2 mi- to the actual bedding to the true vertical thickness crometers. TDS measurements are used for salinity of the unit. analysis. Deep formation waters are salty and have high TDS values. Unconformity. A rock surface that represents a sub- stantial gap in the geologic record in which rock Tertiary recovery. See enhanced oil or gas recovery. units below the surface are overlain by rock units Thrust fault. A type of reverse fault in which one fault that would not be in depositional succession. Typi- block has moved or been thrust over or across an- cally, it is either an erosive or a surface other fault block. Typically, these are low-angle, of ancient exposure and weathering. A number of sometimes near-horizontal, faults. terms apply to different scales of unconformity (and disconformity) related to the scale (or area) of the surface, or the relative geometry of the units 244 Appendix B

above and below the surface. Oil and gas traps injection wells, and used to push or displace re- (and porosity development) are common beneath sidual oil toward a producing well. unconformity surfaces. Water saturation. The fraction of water that occupies Unconsolidated. In geology, adjective applied to sedi- the pore space, typically expressed as a percent- ments or rocks, to indicate they are uncemented. age. Drillers sometimes use this as an adjective to Wellbore. A hole drilled in the ground. Pertaining to describe poorly cemented or uncemented rocks the drillhole or the rocks that line the drillhole. downhole. Wireline (logging). A method of deploying in a well- University of Kentucky. A large public university in bore retrievable tools that are suspended from an Lexington, Ky., known primarily for the success of electrically conductive cable for the purpose of ac- its basketball team and the Kentucky Geological quiring continuous measurements of rock and fluid Survey, one of its research centers. properties. Nuclear logs measure radioactive prop- Vug. A large pore or cavity in a rock. Typically an ir- erties and provide information on density, poros- regularly shaped pore of nonspecific origin. Adj. ity, fluids, and rock type. Electrical logs measure Vuggy, vugular. natural and induced electrical properties and pro- vide information on rock type, porosity, and fluids. Vuggy porosity. Porosity formed from vugs. It is typi- Geophysical logs provide measurements of physi- cally larger than the grains that comprise the rock, cal properties of rock types (sonic velocities) and or the crystals in the cement holding the grains to- porosity. Mechanical logs provide information on gether. variations in borehole diameter, velocity of fluids Water-drive reservoirs. Reservoirs in which the pri- in the borehole, and other properties. Often the tool mary recovery mechanism is pressure from natural as deployed contains multiple sources and sensors water in the reservoir, generally at a position below of each major type and all are recorded simultane- the oil or gas layer. ously for later analysis. Waterflood. A type of secondary recovery in which water is injected into a reservoir from one or more