ORIGIN AND OCCURRENCE OF OIL 53
properties of its rocks and fluids. Then a recovery factor is assumed,
using assumptions from fields with similar characteristics. The
amount of oil in place is multiplied by the recovery factor to arrive
at a reserve number. Current recovery factors for oil fields around
the world typically range between 10 and 60 percent; some are over
80 percent. The wide variance is due largely to the diversity of fluid
and reservoir characteristics for different deposits. The method is
most useful early in the life of the reservoir, before significant production has occurred.
The materials balance method for an oil field uses an equation
that relates the volume of oil, water, the gas that has been produced
from a reservoir, and the change in reservoir pressure to calculate
the remaining oil. It assumes that as fluids from the reservoir are
produced, there will be a change in the reservoir pressure that
depends on the remaining volume of oil and gas. The method
requires extensive pressure-volume-temperature analysis and an
accurate pressure history of the field. It requires some production
to occur (typically 5% to 10% of ultimate recovery), unless reliable
pressure history can be used from a field with similar rock and fluid
characteristics.
The decline curve method uses production data to fit a decline
curve and estimate future oil production. The three most common
forms of decline curves are exponential, hyperbolic, and harmonic.
It is assumed that the production will decline on a reasonably
smooth curve, and so allowances must be made for shut-in wells
and production restrictions. The curve can be expressed mathematically or plotted on a graph to estimate future production. It has the
advantage of implicitly including all reservoir characteristics, and
it requires a sufficient history to establish a statistically significant
trend, ideally when production is not curtailed by regulatory or
other artificial conditions.
Generally, the initial estimates of the size of newly discovered
oil fields are too low. As years pass, successive estimates of the ultimate recovery of fields tend to increase. The term reserve growth
refers to the typical increases in estimated ultimate recovery that
occurs as oil fields are developed and produced.
Reserve growth has now become an important part of estimating
total potential reserves of an individual province or country. As the
world's known petroleum reserves continue to decline, there will
be more pressure on geologists and engineers in the oil industry to
make the reserve estimates more precise through application of the
properties of its rocks and fluids. Then a recovery factor is assumed,
using assumptions from fields with similar characteristics. The
amount of oil in place is multiplied by the recovery factor to arrive
at a reserve number. Current recovery factors for oil fields around
the world typically range between 10 and 60 percent; some are over
80 percent. The wide variance is due largely to the diversity of fluid
and reservoir characteristics for different deposits. The method is
most useful early in the life of the reservoir, before significant production has occurred.
The materials balance method for an oil field uses an equation
that relates the volume of oil, water, the gas that has been produced
from a reservoir, and the change in reservoir pressure to calculate
the remaining oil. It assumes that as fluids from the reservoir are
produced, there will be a change in the reservoir pressure that
depends on the remaining volume of oil and gas. The method
requires extensive pressure-volume-temperature analysis and an
accurate pressure history of the field. It requires some production
to occur (typically 5% to 10% of ultimate recovery), unless reliable
pressure history can be used from a field with similar rock and fluid
characteristics.
The decline curve method uses production data to fit a decline
curve and estimate future oil production. The three most common
forms of decline curves are exponential, hyperbolic, and harmonic.
It is assumed that the production will decline on a reasonably
smooth curve, and so allowances must be made for shut-in wells
and production restrictions. The curve can be expressed mathematically or plotted on a graph to estimate future production. It has the
advantage of implicitly including all reservoir characteristics, and
it requires a sufficient history to establish a statistically significant
trend, ideally when production is not curtailed by regulatory or
other artificial conditions.
Generally, the initial estimates of the size of newly discovered
oil fields are too low. As years pass, successive estimates of the ultimate recovery of fields tend to increase. The term reserve growth
refers to the typical increases in estimated ultimate recovery that
occurs as oil fields are developed and produced.
Reserve growth has now become an important part of estimating
total potential reserves of an individual province or country. As the
world's known petroleum reserves continue to decline, there will
be more pressure on geologists and engineers in the oil industry to
make the reserve estimates more precise through application of the