Page 287 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

254 Reservoir Engineering

within the proved and probable limits, (3) reserves in formations that have
favorable log characteristics but questionable productivity, (4) reserves in
untested fault segments adjacent to proved reservoirs where a reasonable doubt
exists as to whether such fault segment contains recoverable hydrocarbons, (5)
incremental reserves attributable to infill drilling that are subject to technical
or regulatory uncertainty, and (6) reserves from a planned improved recovery
program that is not in operation and that is in a field in which formation, fluid,
or reservoir characteristics are such that a reasonable doubt exists to its success.

Reserve Status Categories. These define the development and producing
status of wells and/or reservoirs. They may be applied to proved or unproved
(probable or possible) reserves.
Developed Reserves. These are expected to be recovered from existing wells
(including reserves behind pipe). Improved recovery reserves are considered
developed only after the necessary equipment has been installed, or when the
costs to do so are relatively minor. Developed reserves may be subcategorized
as producing or nonproducing.

Producing Reserves. These are expected to be recovered from completion
intervals open at the time of the estimate and producing to market. Improved
recovery reserves are considered to be producing only after an improved recovery
project is in operation. Unproved (probable or possible) producing reserves are
in addition to proved prothing reserves, such as (1) reserves that may
be recovered from portions of the reservoir downdip from proved reserves or
(2) reserves that may be recovered if a higher recovery factor is realized than
was used in the estimate of proved reserves.

Nonptvdwhg Reserves. These include shut-in and behind-pipe reserves. Shut-in
reserves are expected to be recovered from completion intervals open at the
time of the estimate, but which have not started producing, or were shut in for
market conditions of pipeline connection, or were not capable of production
for mechanical reasons, and the time when sales will start is uncertain. Behind-
pipe reserves are expected to be recovered from zones behind casing in existing
wells, which will require additional completion work or a future recompletion
prior to the start of production.
Undeveloped Reserves. These are expected to be recovered (1) from new wells
on undrilled acreage, (2) from deepening existing wells to a different reservoir,
or (3) where a relatively large expenditure is required to (a) recomplete an
existing well, or (b) install production or transportation facilities for primary
or improved recovery projects. Undeveloped reserves usually will be distinguished
from developed reserves. The ownership status of reserves may change due to
the expiration of a production license or contract; when relevant to reserve
assignment, such changes should be identified for each reserve classification.

Methods of Estimating Reserves

Methods of determining reserves progress from analogy, before a well is
drilled, to history after it is plugged and abandoned. The accuracy with which
reserves can be estimated progresses along the same path from speculation
to history.