Page 288 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Reserve Estimates 455

Analogy

The decision to drill a well is based upon the potential reserves that it will
recover. This means that an engineer must be able to predict reserves before a
well is drilled. The lack of information about the reservoir restricts the engineer-
ing methods available.
Analogy is the only method which can be used without specific well infor-
mation such as porosity, reservoir thickness, and water saturation. Because
analogy employs no specific information about a well, it is the least accurate
method of determining reserves. Methods of analogous reserve determination
depend on the proximity of similar reserves. The best analogy can be made by
taking the median ultimate recovery of a number of wells that are closest and
have the same formation and characteristics expected in the proposed well:
When ultimate recovery data are not available, volumetric, decline curve or other
methods of estimating ultimate recovery may be used.
Unless values of ultimate recovery figures for the group are relatively close,
the median ultimate recovery should be calculated by making a normal prob-
ability plot. This plot is made by graphing estimated ultimate recovery against
the cumulative percent of samples. A best-fit line is drawn through the points
and the median is read where the line intersects fifty percent. A straight line
indicates a normal distribution; if the line is not straight the distribution is
skewed. If there are no similar wells in the area, data from those less similar
may be used, but confidence goes down as similarity decreases.

Volumetric
If a well is drilled after reserves are determined by analogy, factual informa-
tion becomes available and reserves can then be determined volumetrically. From
log analysis the porosity, water saturation, and productive formation thickness
are estimated. A reasonable drainage area is assigned and total hydrocarbons
in place are then calculated. When enough wells have been drilled to delineate
the field, a subsurface geological contour map showing the subsea sand top and
bottom depth, oil-water contact, and gas-oil contact can be prepared. From this
map the total areas in acre-feet of each contour are planimetered and graphed
as the abscissa against the subsea depth as the ordinate. Lines are then drawn
to connect the sand-top points and the sand-bottom-points, the area bounded
by the oil-water-contact depth, the sand-top line, the sand-bottom line and the
gas-oil-contact line. This area is the gross oil-bearing sand-volume in acre-feet.
The area, if present, that is bounded by the gas-oil contact depth, the sand-top
line, the sand-bottom-line and the abscissa is the gross gas-bearing sand-volume
in acre-feet. The engineer must determine from core data and/or electric logs
the percentage of the gross sand volume that is productive and must then reduce
the total acre-feet by that percentage. If there is no subsurface contour map
available or if the reservoir is very heterogeneous, an isopach or an isovol map
should be constructed. An isopach map is constructed by contouring net sand
thickness. This kind of map works well when the reservoir is uniform and when
porosity and water saturation are relatively constant. When the water saturation
and porosity vary widely from well to well, an isovol map that indicates hydro-
carbon thickness is useful. This map is constructed by contouring the value of
net pay height multiplied by porosity and by one minus the water saturation.
Care should be taken not to rely on the scale provided on the map especially
when using xerographed copies, as this and other methods of reproduction can
distort one or both axes as much as five percent. A known area such as a section