Page 29 - Primer on Enhanced Oil Recovery

20 Primer on Enhanced Oil Recovery

pressure, the physicochemical properties of fluids at initial reservoir conditions and
the full amount of data on the dynamics of hydrocarbon production and water injec-
tion into the reservoirs.
When developing highly permeable, homogeneous reservoirs of gas deposits and
operating in the natural depletion mode, the reserves assessment carried out by the
material balance method can provide high accuracy in determining the volume of
final recoverable reserves at various depletion pressure values. In more complex
cases, such as water breakthrough into the production wells, complex geological
heterogeneity of the reservoir, carbon multiphase deposits, complex strata structures
with low permeability reliance on only material balance method can lead to inaccu-
rate estimates. Great care should be taken in assessing the uncertainty factors asso-
ciated with a specific reserve development project. It is necessary to take into
account the features of the geological structure of the reservoir, as well as the
dynamics of changes in reservoir pressure during production.
The basis of the material balance methods is to postulate for the oil reserve that
there is a fixed amount (volume) of hydrocarbons. This amount, Q HO , is equal to
the sum of amounts already extracted during the development, Q n , and the amount
still remaining in the reservoir, Q rem . The total value is constant at any time of
development:
Q HO 5 Q n 1 Q rem 5 const (2.4)

Differences in the nature of the indicators, both geological conditions and condi-
tions for the development of each specific deposit, require an specifically developed
approach to the compilation of the material balance equation in definition of initial
reserves and produced hydrocarbons.
The most reliable results in calculating reserves using the material balance meth-
ods can be obtained when the values of the measured and estimated parameters (the
reservoir pressure is very important) are accounted in the values in the equation in
the formula for calculating reserves. The parameters should be representative and
characterise the entire reservoir as a whole.

2.3.5 Reservoir modeling (simulation)
An improved form of the material balance method can be developed to be more
accurate by computer modeling of the reservoir. This modeling will provide highly
accurate estimation of the reservoir response during the application of a specific
technological development schemes. The model’s accuracy is directly dependant on
the reliability of the initial data, such as reservoir rock properties, geometric reser-
voir shape, relative phase permeability functions, fluids properties, etc.
Geological and hydrodynamic models represent the reservoir as a set of three-
dimensional interconnected cells or “reservoirs” and use the initial data based on
the bulk method or the method of production dynamics. Models then can be used to
calculate the initial geological volumes by taking into account the adaptation of the
dynamics of changes in reservoir pressure, hydrocarbon production by wells or