Page 33 - Petroleum and Gas Field Processing

1.7.1 Estimation of Reserves
Estimation of oil and gas reserves in a discovered reservoir is one of the
most important factors in evaluating the discovery and deciding on its
viability for commercial development. To determine the volume of oil and/
or gas present in the reservoir, the bulk volume of the reservoir (V b ) is first
determined using the available reservoir description data. The volume of
fluids in the pore spaces of the reservoir rock is then calculated by
multiplying the bulk volume by the rock porosity (’); this is also known as
the pore volume of the rock. Porosity is a property of the rock defined as
the ratio of the volume of the pore spaces within the rock to the bulk
volume of the rock. The pore volume is normally occupied by oil (or gas)
and water. The fraction of the pore volume occupied by water is known as
the water saturation (S w ). The porosity and initial water saturation are
determined from the logs and core samples obtained from the exploratory
wells. Therefore, the initial volume of oil (V 0 ) at the reservoir conditions is
determined by

V 0 ¼ V b ’ð1 S w Þ
This volume of oil is called the initial oil in place (IOIP) or the original oil
in place (OOIP). This method of estimating the OOIP is known as the
volumetric method. Other methods (the material balance method and
decline curve method) also exist for estimating the OOIP.
It is impossible to recover all of the OOIP; certain forces within the
reservoir rock prevent the movement of some oil from the rock to the well.
The fraction of the OOIP that could be recovered is called the recovery
factor (E r ), and the total recoverable volume of oil (E r V 0 ) is called the
proven reserves.
The proven reserve for a reservoir changes with time as a result of
three factors. First, the volume of oil in place decreases as oil is produced
from the reservoir. Second, as more oil is produced, more reservoir data
become available, which could change or modify the initial estimate of the
OOIP. Third, new developments and improvements in recovery and
production methods may increase the possible recovery factor. Therefore,
the proven reserve of any field is continuously updated.

1.7.2 Well Location, Spacing, and Production Rates
Another important function of reservoir engineering is to determine the
optimum locations of the wells to be drilled and the production rate from
each well for the most effective depletion of the reservoir. Where the wells
should be drilled depends largely on the structural shape of the reservoir

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.