Page 277 - Pipeline Rules of Thumb Handbook

264 Pipeline Rules of Thumb Handbook

Categories of natural gas and reserves terminology

Associated and non-associated natural gas Recovery from Existing Fields” and the “Expectation from
Future Discoveries.”
Natural gas is found in underground structures similar to Ultimate Recovery from Existing Fields is the sum of
those containing crude oil. There are three types of natural the “Cumulative Production” from such fields and the
gas reservoirs: “Reserves from Existing Fields.”
Reserves from Existing Fields are the sum of “Proven
1. Structures from which only gas can be produced eco- Reserves,” “Discounted (i.e., 50% of) Probable Reserves” and
nomically—called non-associated gas (or unassociated “Discounted (i.e., 25% of) Possible Reserves.”
gas). Proven Reserves represent the quantities of crude oil
2. Condensate reservoirs which yield relatively large and/or natural gas and natural gas liquids which geological
amounts of gas per barrel of light liquid hydrocarbons. and engineering data demonstrate with reasonable certainty
Although many condensate reservoirs are produced pri- to be recoverable in the future from known oil and/or gas
marily for gas, there are cases where gas is re-injected reservoirs. They represent strictly technical judgments, and
or “re-cycled” to improve liquid recovery, particularly if are not knowingly influenced by attitudes of conservatism or
no gas market is yet available. This gas also is termed optimism.
non-associated. Discounted (i.e., 50% of) Probable Reserves are those
3. Reservoirs where gas is found dissolved in crude oil quantities of crude oil and/or natural gas and natural gas
(solution gas) and in some cases also in contact with liquids for which there exists a probability of 50% that they
underlying gas-saturated crude (gas-cap gas). Both are will materialize. Such reserves are usually allocated to some
called associated gas. (Gas-cap gas is almost never pro- conjectural part of a field or reservoir as yet undrilled or
duced until most of the economically recoverable oil has incompletely evaluated where, while characteristics of the
been yielded.) productive zone and fluid content are reasonably favorable,
other uncertain features may have equal chances of being
In such fields, gas production rates will depend on oil output, favorable or unfavorable for commercial production.
with the oil usually representing the major part in terms of Discounted (i.e., 25% of) Possible Reserves are those
energy equivalents. quantities of crude oil and/or natural gas and natural gas
liquids thought to be potentially producible but where the
chance of their being realized is thought to be on the order
Natural gas reserves of 25%. Reserves in this category are usually allocated to pos-
sible extensions of existing fields where, although geological
There are no generally accepted definitions of natural gas data may be favorable, uncertainties as to the characteristics
reserves, and wide variations both in terms and principles may of the productive zone and/or fluid content are such as to
be met. The following definitions are the most commonly used: preclude a probability greater than 25%.
Expected Ultimate Recovery is the total volume of It should be noted that the terms “gas-in-place” and
hydrocarbons, gas and/or oil, which may be expected to be “oil-in-place” represent estimates of the total hydrocarbons
recoverable commercially from a given area at current or present which, in conjunction with various engineering and
anticipated prices and costs, under existing or anticipated economic criteria, provide the basis for the “reserve” esti-
regulatory practices and with known methods and equipment. mates. They should never be confused with “reserves,” which
This total can be expressed as the sum of the “Ultimate word implies that physical recovery is possible.

Terminology of Reserves
EXPECTED ULTIMATE RECOVERY

ULTIMATE RECOVERY FROM EXISTING FIELDS
CUMULATIVE PRODUCTION RESERVES FROM EXISTING FIELDS EXPECTATION FROM
FUTURE DISCOVERIES
Proven Discounted Discounted
Reserves (i.e. 50% of) (i.e. 25% of)
Probable Possible
Reserves Reserves