Glossary of Terms Hydrate science is rapidly evolving. Both new terms and different terminology for the same gas hydrate related objects and mechanisms are commonly found in the literature and in discussions. Commercial terms may differ somewhat from scientific usage.for descriptions of the same objects or processes. Geophysical and gas hydrate terminology is current usage, mining terms are common usage and taken largely from AGI (1997) and Jackson (1997). Also see the on-line Schumberger glossary <http://www.glossary. oilfield.slb.com/> for more detailed terms relating to hydrocarbon exploration and exploitation. American usage (meter, kilometer) is used. AAPG: American Association of Petroleum Geologists. AGC: Automatic gain control: A system to automatically control the increase in amplitude of an electrical signal from the original input to the amplified output. AGC is commonly used in seismic processing to improve visibility of late-arriving events in which attenuation or wavefront divergence has caused amplitude decay. API: American Petroleum Institute. Aquifer: An underground formation of permeable rock or sediment that can hold water in pore space. Aquifers can be limited in aerial extent or very large, underlying thousands of square kilometers of the earth's surface. With relation to hydrate and related gas deposits, those marine sediments which can either host hydrate and related gas or can provide the hydrate forming gas by flow or diffusion through the pores. A water-bearing layer of rock or sediment capable of yielding water flow. (Confined) Aquifer: An aquifer whose upper, and perhaps lower, boundary is defined by a layer of natural material that does not transmit water readily. Often a gas or petroleum reservoir. (Unconfined) Aquifer: An aquifer which is not bounded by an impermeable layer in at least one, and possibly two directions. See Chapter 7. The terms open or communicating aquifer may also be used. Atm, atm: Pressure measured in atmospheres (1 bar or 14.7 lb/in2) Attribute: A measurable property of seismic data, such as amplitude, dip, frequency, phase and polarity. Attribute analysis includes assessment of various reservoir parameters, including hydrocarbon indicators, using techniques such as amplitude variation with offset (AVO) analysis. Authigenic: Rock or sediment constituents and minerals that have formed place and have not been transported form elsewhere. Applies specifically to minerals that have crystallized locally at the spot where they are 297 298 Glossary of Terms now found. The term often refers to minerals formed during diagenesis following deposition of the original sediment. AVO: Amplitude Variation with Offset is a seismic attribute for the variation in seismic reflection amplitude with change in distance between shotpoint and receiver. It indicates differences in lithology and fluid content in rocks above and below a reflector. Barrel: One petroleum barrel = 159 liters or 42 U.S. Gallons. Bcf: Billion cubic feet (of gas). BGR: Base of Gas Reflector. This is sometimes a strong positive impedance contrast at the base of a gas-rich zone that may be trapped below hydrate. Where a free gas zone occupies pore space, there may be a sharp contact with subjacent water flooded sediments. BHT: Bottom Hole Temperature. Biogenic methane: Methane formed by microbial processes. With respect to oceanic hydrate, the source is deep biosphere acting upon buried organic material within the marine sediments. Blanking: Blanking refers to a decreased reflection strength on reflection seismic records that display true reflection amplitude. It is often located in the lower third of the GHSZ. Blanking is attributed to: hydrate filling of pore spaces replacing water. This reduces the acoustic impedance contrasts found in the original sedimentary bedding. Some apparent blanking may have causes that are unrelated to gas hydrate accumulations. Bm3 Billion cubic meters BGHS: Base of Gas Hydrate Stability. Also: BOH, Base of hydrate. This refers to the actual base of naturally occurring hydrate stability. Normally, this occurs more or less uniformly at some depth in the sediments depending on water depth, seafloor temperature, geothermal gradient and for any particular HFG or mixture of HFGs (which have different stability fields). In abnormal circumstances, for instance where there is a local high heatflow (see 3.2.1; 4.7.2; 4.8), the BGHS may be pushed up to the seafloor. In this instance, cooling of the HFG-enriched vent water by seafloor water has the potential to produce highly HFG-saturated water that will cause rapid growth of seafloor hydrate near the vent. The BGHS may be difficult to identify when there is no BSR, especially where marine sediment bedding is parallel with the seafloor. BOP: Blow out preventer. Usually installed at the wellhead and part of a stack of BOPs. These are optimized to provide maximum pressure control in the event of a well control incident. BSR: Bottom-Simulating Reflector. Seismic reflection from a negative impedance contrast at the contact between higher Vp hydrate-rich sediment zone and sediments containing gas in pores below which have much slower acoustic Glossary of Terms 299 velocities. The BSR is commonly taken as is a remotely sensed reflection that marks the base of the hydrate stability zone and is used as a first-order indicator of the presence of gas hydrate. BSR push-up: Occurs where the BGHSZ is pushed up toward the seafloor by, 1. An anomalous subjacent heatflow, such as over a salt plug, 2. In the vicinity of a usually near vertical fluid-gas flow passage through the HSZ where lateral heat flow results in sub-hydrate stability conditions extending laterally from the passage. Often associated with seafloor vents and seafloor hydrate. Push rather than pull is used because the effect is caused by warmer fluids moving up. btu or (BTU): British thermal unit, A measure of heat energy required to raise the temperature of one pound of water by one degree Fahrenheit. 1 btu = 1055J, 2.973x1-4 kWh. Compound hydrate: A species of gas hydrate with more than one type of HFG occupying guest sites within the same crystal lattice. For example, natural gas hydrate with methane and ethane present. Condensate: Light hydrocarbons that condense when cooled to surface temperature. Also known as natural gas liquids. Connate water: Water that has been entrapped during sedimentation along with the sediments. During compaction of marine sediments, connate water may be expelled into porous strata or secondary porosity from which it may migrate upward into the sea. Darcy: A measurement of the ability of a fluid to pass through a porous material. The permeability that will allow a fluid of 1 centipoise (cP) viscosity to flow at a 1 cm/s velocity for a pressure drop of 1 atm/cm. The millidarcy (md) is the common unit for hydrocarbon reservoirs. Depressurization: The gas hydrate gasification technique whereby pressure in the immediate vicinity of hydrate is lowered, causing ambient conditions of hydrate instability. Diagenesis: The sum of all chemical and physical changes in minerals during and after their initial accumulation. Diagenesis involves addition and removal of material, transformation by dissolution and recrystallization or replacement, or both, and by phase changes. Authigenic refers to minerals formed in place. It is also applied to minerals that are clearly the result of new crystal growth on older crystals. Diagenesis embraces those processes such as compaction, cementation, authigenesis, replacement, crystallization, hydration, etc., that occur under conditions of pressure up to 1 kb and temperature in a range that encompasses gas hydrate stability conditions on Earth. Diffusional pressure: The tendency for a material to be transported through a given medium. A strong tendency, or high diffusional pressure, leads 300 Glossary of Terms to faster diffusion. A weak tendency, or low diffusional pressure, leads to slower diffusion. Dissociation. The breakdown of the hydrate crystal structure or 'melting' of hydrate into its water and gas components owing to rising temperature and/or diminishing pressure (2.6). Dissociation feedback: Gas hydrate contains within it a self- moderating or feedback system which act as a rate-controlling mechanism during dissociation. Whatever the cause of dissociation, heat is absorbed. This cools the system and may cause dissociation to slow or cease. Run-away dissociation feedback may take place when hydrate is brought to near- atmospheric pressures which usually will initiate dissociation; the system may absorb enough heat under this condition to freeze the local water. When a sample becomes coated with ice, the rate of dissociation slows (see Self- preservation). Dissociation point: The temperature, at a constant pressure, or the pressure, at a common temperature, at which a compound or mineral breaks up reversibly to form two or more other substances. For instance, CaCO3 becomes CaO plus CO2 and gas hydrate becomes water and gas. Dissolution: The process of dissolving into a homogeneous solution, as when a gas disperses into its component molecules in a fluid or other gas or where a hydrate structure becomes unstable owing to the diffusion of gas molecules from the hydrate into surrounding gas or fluid. Maximum concentrations of dissolved solids (TDS) are governed by the solubility for each species. Dry gas: Gas with no condensate, generally almost pure thermogenic methane. Effective porosity: That portion of the porosity that is available for storing recoverable gas or fluid. Endothermic: Gas hydrate dissociation is endothermic. It absorbs heat when it dissociates about equal to the heat produced during crystallization. Exothermic: Gas hydrate formation is exothermic. It produces heat when it forms. Fraccing (Frac): Industrial term for the artificial fracturing of rock in an oil or gas reservoir for the purpose of increasing permeability. Fracturing can be caused in a number of ways in the vicinity of a drilled hole but hydraulic fracturing is most common. In this method, high pressure fluid is forced into the reservoir materials so that cracks propagate, often along bedding planes.