1. OIL RECOVERY MECHANISMS AND THE MATERIAL BALANCE EQUATION

2. 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 mechanisms.
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.

3. 1.Analogy: 2. Volumetric methods
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.

4. Recoverable reserves are then estimated by application of a suitable recovery factor and the formation /surface volume factor for the produced fluid.
Recoverable oil = Vb x Ø x (1-Sw) x R.F. Bo
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

5. A recovery factor is approximated considering :
laboratory measurement of oil displacement in cores So –Sor So
type of displacement mechanism involved
correlation of sweep efficiency based on a similar reservoir

6. 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 estimated.

7. 3)Field development stage
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

8. 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.

9. 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:

10. - Pyramidal formula  V= h [An + A n+1 + ( An * A n+1)0.5] 3
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 ( An * A n+1)0.5] 3
- Trapezoidal formula  V = h An +An+1 2

11. 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.

12. 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.

13. 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