Page 127 - Introduction to Petroleum Engineering

VOLUMETRIC ANALYSIS, RECOVERY FACTOR, AND EUR 113

FIGURE 6.10 Examples of reservoir rocks.

Two important characteristics that control the economic viability of the reservoir
are porosity and permeability. Porosity helps quantify the volume of fluid that can
be stored in the rock, and permeability helps quantify the rate that fluids can be
produced from the rock.
Overriding all of these factors is the fourth key factor: timing. A trap must exist
when hydrocarbon migrates from source rock to reservoir rock. If a trap forms at a
location after the hydrocarbon has passed, there will be no hydrocarbon to capture.
Reservoir rocks are usually sandstones, carbonates, and shales. Sandstone is
compacted sediment and includes consolidated rock with cemented grains, uncon-
solidated rock with uncemented grains, and conglomerates. A conglomerate con-
sists of larger grains of sediment, such as pebbles or boulders, embedded in a matrix
of smaller grains of sediment. Carbonate rock is produced by chemical and
biochemical sources. Examples of carbonate rock are limestone (calcium carbonate)
and dolomite (calcium magnesium carbonate). Shale is laminated sediment that is
formed from consolidated mud or clay. Examples of reservoir rocks are shown in
Figure 6.10.

6.5 VOLUMETRIC ANALYSIS, RECOVERY FACTOR, AND EUR

Different calculation procedures and data sources can be used to estimate reservoir
fluid volumes. Geologists determine volume using static information in a procedure
called volumetric analysis. Static information is information that does not change
significantly between the time the reservoir is discovered and the time production
begins in the reservoir. Static information includes reservoir volume and the original
saturation and pressure distributions. By contrast, engineers use dynamic information
to estimate reservoir fluid volumes. Dynamic information is information that changes
with respect to time such as pressure changes and fluid production. Material balance
is an engineering procedure for estimating original fluid volumes from dynamic data.
Reservoir fluid volume estimates obtained from different procedures and sources of
data provide a means of assessing the quality of information used by different disci-
plines. Calculated original fluid volumes can be combined with a recovery factor
(RF) to calculate estimated ultimate recovery (EUR) for a given economic limit.
In this section, we present the equations for volumetric estimates of original oil
and gas in place and then define RF and EUR.