Page 284 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Reserve Estimates 251

Natural Gas. This is a mixture of hydrocarbons and varying quantities of
nonhydrocarbons that exist either in the gaseous phase or in solution with crude
oil in natural underground reservoirs. Natural gas may be subclassified
as follows.
Associated gas is natural gas, commonly known as gas-cap gas, that overlies
and is in contact with crude oil in the reservoir.**
Dissolved gas is natural gas that is in solution with crude oil in the reservoir.
Nonassociated gas is natural gas that is in reservoirs that do not contain
significant quantities of crude oil.
Dissolved gas and associated gas may be produced concurrently from the same
wellbore. In such situations, it is not feasible to measure the production of
dissolved gas and associated gas separately; therefore, production is reported
under the heading of associated-dissolved or casinghead gas. Reserves and
productive capacity estimates for associated and dissolved gas also are reported
as totals for associateddissolved gas combined.

Natural Gas Llqulds (NGLs). These are those portions of reservoir gas that
are liquefied at the surface in lease separators, field facilities, or gas processing
plants. Natural gas liquids include but are not limited to ethane, propane,
butanes, pentanes, natural gasoline, and condensate.

Reservoir. A reservoir is a porous and permeable underground formation
containing an individual and separate natural accumulation of producible
hydrocarbons (oil and/or gas) which is confined by impermeable rock and/or
water barriers and is characterized by a single natural pressure system. In most
situations, reservoirs are classified as oil reservoirs or as gas reservoirs by a
regulatory agency. In the absence of a regulatory authority, the classification is
based on the natural occurrence of the hydrocarbon in the reservoir as deter-
mined by the operator.
Improved Recovery. This includes all methods for supplementing natural
reservoir forces and energy, or otherwise increasing ultimate recovery from a
reservoir. Such recovery techniques include (1) pressure maintenance, (2) cycling,
and (3) secondary recovery in its original sense (Le., fluid injection applied
relatively late in the productive history of a reservoir for the purpose of
stimulating production after recovery by primary methods of flow or artificial
lift has approached an economic limit). Improved recovery also includes the
enhanced recovery methods of thermal, chemical flooding, and the use of
miscible and immiscible displacement fluids.

Reserves [a'rl]. These are estimated volumes of crude oil, condensate, natural
gas, natural gas liquids, and associated substances anticipated to be commercially
recoverable and marketable from a given date forward, under existing economic
conditions, by established operating practices, and under current government
regulations. Reserves do not include volumes of crude oil, condensate, or natural
gas liquids being held in inventory.
Reserve estimates are based on interpretation of geologic and/or engineering
data available at the time of the estimate. JCxisting economic conditions are

** Where reservoir conditions are such that the production of associated gas does not substantially
affect the recovery of crude oil in the resemir, such gas may be classified aa nonassociated gas
by a regulatory agency. In this event, reserves and production are reported in accordance with
the classification used by the regulatory agency.