Reservoir Estimates
One of the important functions of the reservoir engineer is
the periodic calculation of reservoir oil and gas in place and
the recovery anticipated under the prevailing reservoir drive
Reverse estimation methods are usually categorized into three types:
1. Analogy
2. Volumetric methods
3.performance based techniques which are:
Material balance calculations
Decline curve analysis
Pressure transient analysis
Numerical simulation techniques.
• 1.Analogy:
During this period , before any wells are drilled on the
property , any estimates will be of a very general nature based
on experience from similar pools or wells in the same area. i.e
by analogy.
• 2. Volumetric methods
The volumetric methods involve a determination of the bulk
reservoir rock volume , average porosity , fluid saturations ,
formation volume factors from which the total reservoir
hydrocarbon volume is calculated.
• Recoverable reserves are then estimated by
application of a suitable recovery factor and the
formation /surface volume factor for the produced
Recoverable oil =
Vb x Ø x (1-Sw) x R.F.
Where : Vb is the bulk reservoir volume
Ø is the fractional porosity
(1-Sw) is the hydrocarbon saturation
R.F. is the recovery factor
Bo is the oil formation volume factor
A recovery factor is approximated considering :
• laboratory measurement of oil displacement
in cores
So –Sor
• type of displacement mechanism involved
• correlation of sweep efficiency based on a
similar reservoir
Reservoir estimates are needed at
various stages of a project
1)Geophysical exploration stages
• The first estimate is based on the volume of the
structure determined from seismic maps
supplemented by information on local geological
trends which may indicate the thickness of porous
beds which may be encountered.
• By applying the common range of rock parameters ,
porosity (7 to 30%) , water saturation (8 to 40%) and
recovery factor (10 to 50%) a possible range of
reserves that the structure might contain is
2)Exploration stage
With the drilling of a discovery well the uncertainty
of encountering hydrocarbons is removed , and
measured values for porosity , and water saturation
became available for the section of pay traversed.
Assuming that well log data corroborate the prior
seismic data , now only the contour of the
hydrocarbon /water contact (O.W.C.) is required to
make reasonable estimate for this stage.
3)Field development stage
• As new wells are drilled the volume and geometrical
distribution of the reservoir become even more
accurately defined as well as the average reservoir
porosity and saturation values .
On the other hand , fluid withdrawals and injections
into the reservoir and the corresponding changes in
fluid interfaces must be accounted for as the
inventory of reserves is continuously upgraded .
• Obviously , the trend in reservoir studies is toward
numerical simulation on which not only the static
inventory of reserves is kept , but which can predict
the future behavior of a field.
Calculation of the reserve
• The gross reservoir rock volume enclosed by the
structure above the hydrocarbon /water contact is
calculated in the following steps:
• 1-A net isopach map , giving the contour of equal
thickness of pay with the water contact assigned zero
elevation contour is the most convenient basis for
rock volume calculations.
• 2-The area within each contour is determined by
planimetering , and a plot prepared of area
contained in each contour versus depth:
3) The gross rock volume is A dh which may be found
by planimetering again or by application of a
numerical integration rule . In the Schlumberger
field studies approach the volumetric reservoir
distribution is calculated numerically and plotted by
machine as isopach and isovolume maps.
- Pyramidal formula  V= h [An + A n+1 + ( An * A n+1)0.5]
- Trapezoidal formula  V = h
An +An+1
3.Performance based techniques
Material balance calculations
• In many cases , porosity , saturations , and reservoir
bulk volume are not known with any reasonable
accuracy , and emphasis on volumetric calculations
for reserve estimates is not advisable.
• The material balance is a useful auxiliary tool for
confirming reservoir estimates.
• The material balance equation allow dependable
estimates of the initial hydrocarbons in place as well
as prediction of the future reservoir performance.
• The material balance equation (MBE) relates the
volumes of fluids withdrawn and encroached to the
resulting reservoir pressures.
• Its principal utility , however , lies in predicting
reservoir behavior and not in the estimation of
initial hydrocarbon in place.
There are basically six driving mechanisms
that provide the natural energy necessary
for oil recovery:
• Rock and liquid expansion drive
• Depletion drive
• Gas cap drive
• Water drive
• Gravity drainage drive
• Combination drive
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