Page 25 - Primer on Enhanced Oil Recovery

16 Primer on Enhanced Oil Recovery

2.3 Reserve calculation methods

Initial geological reserves of oil/gas in the reservoir refer to complete hydrocarbon
amount in a reservoir and have various names and linked to this abbreviations:
Original Oil In Place (OOIP); Oil In Place (OIP); Stock-Tank Original Oil In Place
(STOOIP); Original Gas In Place (OGIP). As volumetric amount of oil depends on
temperature and pressure one needs to pay attention to the terms meaning.
Methods for calculating reserves (resources) can be grouped into two sets
Deterministic Methods and Probabilistic Methods.
Deterministic Methods the first set, are based on the existing datasets of geo-
logical, geophysical, engineering and economic data. With this approach, for calcu-
lating the stocks, single values of each parameter, such as area, porosity, thickness,
etc. are used. The result of the reserve calculation in this approach is a single value
for the reserve.
Not all parameters in a reservoir are usually known. In this case Probabilistic
Methods are used. The methods take into account the uncertainty of geological,
geophysical, technical and economic parameters. All parameters are taken with the
entire range of possible variations for each initial parameter and might have also
time related variables. Stocks are calculated using continuous distribution curves,
which are usually obtained using Monte Carlo simulation software. This approach
is quite ubiquitous stock calculation.
It is possible, and widely used in practice, to combine two methods as some
parameters can be well known and others need to be estimated. Combined method
leads to more accurate reserve estimates.
Estimation of reserves and resources at the stages preceding the discovery, or at
the stages of the early stage of development, is usually carried out on equivalent, or
similar, fields. After the start of production, as data on changes in sampling rates
and pressures become available, performance assessment methods may be applied.
An increase in amount of information about the reservoir and new data lead to a
narrowing of the range of estimates, e.g. level of certainty increases.

2.3.1 Analogy methods
The method of analogies is widely used as a limited amount of initial data for asses-
sing reserves (resources) at the initial stages of exploration and early development
of deposits is available. The basis of the method is a statement (an assumption)
about the comparability of the properties of reservoir rocks, fluids, thermodynamic
(temperature, pressure, phases) and other parameters affecting the assessment of the
recoverable reserves in the reservoirs under consideration to the formations-
analogues. By selecting an appropriate analogue, the development indicators of
which were obtained through the implementation of a similar project (by type and
number of wells, size of the production grid and flow intensification methods), you
can make a forecast of the dynamics of production of the reservoir in question. The
analogue layers are selected by a number of parameters such as depth, reservoir