Page 26 - Introduction to Petroleum Engineering

10 INTRODUCTION

The ratio of fluid volume remaining in the reservoir after production to the fluid
volume originally in place is recovery efficiency. Recovery efficiency can be
expressed as a fraction or a percentage. An estimate of recovery efficiency is obtained
by considering the factors that contribute to the recovery of a subsurface fluid:
displacement efficiency and volumetric sweep efficiency.
Displacement efficiency E is a measure of the amount of fluid in the system that
D
can be mobilized by a displacement process. For example, water can displace oil in
a core. Displacement efficiency is the difference between oil volume at initial condi-
tions and oil volume at final (abandonment) conditions divided by the oil volume at
initial conditions:
SB / S / B
E D oi oi oa oa (1.7)
/
SB oi
oi
where S is initial oil saturation and S is oil saturation at abandonment. Oil saturation
oi
oa
is the fraction of oil occupying the volume in a pore space. Abandonment refers to
the time when the process is completed. Formation volume factor (FVF) is the
volume occupied by a fluid at reservoir conditions divided by the volume occupied
by the fluid at standard conditions. The terms B and B refer to FVF initially and at
oi
oa
abandonment, respectively.
Example 1.4 Formation Volume Factor
Suppose oil occupies 1 bbl at stock tank (surface) conditions and 1.4 bbl at res-
ervoir conditions. The oil volume at reservoir conditions is larger because gas
is dissolved in the liquid oil. What is the FVF of the oil?

Answer
vol at reservoirconditions
OilFVF
vol at surface conditions
14 RB
.
OilFVF 14 RB/STB
.
.
10 STB
Volumetric sweep efficiency E expresses the efficiency of fluid recovery from a
Vol
reservoir volume. It can be written as the product of areal sweep efficiency and
vertical sweep efficiency:

E Vol E A E V (1.8)

Areal sweep efficiency E and vertical sweep efficiency E represent the efficiencies
V
A
associated with the displacement of one fluid by another in the areal plane and
vertical dimension. They represent the contact between in situ and injected fluids.
Areal sweep efficiency is defined as
sweptarea
E A (1.9)
totalarea