Page 28 - Primer on Enhanced Oil Recovery

Hydrocarbon and oil reserves classification 19

Table 2.2 Main equations for exponential decline production analysis.
Production rate q f 5 q i xe 2Dn(t)

Accumulated (produced) volume N p 5 (q i q f )/D n
Production decline D n 52ln(1 2 D e ) и D e 5 (q i 2 q f )/q i
Effective decline D e 5 1 e 2Dn(t)
Production time t 5 ln(q i q f )/D n

In the half-logarithmic coordinates can be expressed as:

qf 5 qi e 2 D n t, where: q production (flow rate) of the well at time t, bar/day; qi
initial production (flow rate), bar/day; D n nominal exponential rate of production
decline, 1/day; t time, day.
In the Table 2.2 the relevant equations are shown.
For deposits at a later stage of operation, production forecast can be defined with
an accuracy sufficient to substantially narrow the range of uncertainty on technical
risk factors. In such cases, the 2P “best estimate” scenario can also be used to make
production forecasts for the 1P and 3P assessment scenarios. Nevertheless, the eco-
nomic risk factors can remain at a fairly high level, significantly affecting the dura-
tion of the project, which should be considered when categorizing reserves and
resources.
As oil more and more oil fields mature and there is an increase in the number of
previously judged unproductive wells being put into operation, there is currently a
tendency towards analysis of the fall rate as proportional to production (hyperbolic
and harmonic dependencies). Although some wells have such trends, extrapolation
of the dip curve by a hyperbolic or harmonic law should be used with care as exces-
sive use of such dependence in calculation methods can lead to an overestimation
of recoverable reserves.
In order to obtain reliable results, when using performance analysis, one, most
definitely, requires a stable operating conditions for a sufficient period of time after
the start of the production in the established drainage zones. When assessing recov-
erable reserves and resources, such complicating factors affecting the dynamics of
production change as reservoir rock properties, fluid properties, unsteady and
steady-state flow ratio, operating conditions, well interaction effects, and depletion
mechanisms should be taken into account. In the early stages of production, there
may be significant uncertainty in relation to both final operational indicators and
industrial factors that affect the duration of a reservoir development project.

2.3.4 Material balance
The material balance method is designed to assess the initial and recoverable
reserves (resources) by analyzing the dynamics of pressure changes in the reservoir
as fluid is withdrawn from it. The best application of this method is achieved with
the availability of the required data points (places where data are taken) of reservoir