Page 31 - Petroleum Production Engineering, A Computer-Assisted Approach

Guo, Boyun / Computer Assited Petroleum Production Engg 0750682701_chap02 Final Proof page 20 22.12.2006 7:08pm

2/20 PETROLEUM PRODUCTION ENGINEERING FUNDAMENTALS
2.1 Introduction r o,st ¼ density of stock tank oil, lb m =ft 3
r w ¼ density of freshwater, 62:4lb m =ft 3
Properties of crude oil and natural gas are fundamental for
designing and analyzing oil and gas production systems in The density of oil at elevated pressures and temperatures
petroleum engineering. This chapter presents definitions of can be estimated on empirical correlations developed by a
these fluid properties and some means of obtaining these number of investigators. Ahmed (1989) gives a summary
property values other than experimental measurements. of correlations. Engineers should select and validate the
Applications of the fluid properties appear in subsequent correlations carefully with measurements before adopting
chapters. any correlations.
Standing (1981) proposed a correlation for estimating
the oil formation volume factor as a function of solution
2.2 Properties of Oil GOR, specific gravity of stock tank oil, specific gravity of
Oil properties include solution gas–oil ratio (GOR), solution gas, and temperature. By coupling the mathemat-
density, formation volume factor, viscosity, and compress- ical definition of the oil formation volume factor with
ibility. The latter four properties are interrelated through Standing’s correlation, Ahmed (1989) presented the fol-
solution GOR. lowing expression for the density of oil:
62:4g o þ 0:0136R s g g
r o ¼ r ffiffiffiffiffi 1:175 , (2:5)
2.2.1 Solution Gas–Oil Ratio g g
‘‘Solution GOR’’ is defined as the amount of gas (in 0:972 þ 0:000147 R s þ 1:25t
g o
standard condition) that will dissolve in unit volume of
oil when both are taken down to the reservoir at the where
prevailing pressure and temperature; that is, t ¼ temperature, 8F
g g ¼ specific gravity of gas, 1 for air.
V gas
R s ¼ , (2:1)
V oil
where 2.2.3 Formation Volume Factor of Oil
‘‘Formation volume factor of oil’’ is defined as the volume
R s ¼ solution GOR (in scf/stb) occupied in the reservoir at the prevailing pressure and
V gas ¼ gas volume in standard condition (scf) temperature by volume of oil in stock tank, plus its dis-
V oil ¼ oil volume in stock tank condition (stb) solved gas; that is,
The ‘‘standard condition’’ is defined as 14.7 psia and B o ¼ V res , (2:6)
60 8F in most states in the United States. At a given reservoir V st
temperature, solution GOR remains constant at pressures where
above bubble-point pressure. It drops as pressure decreases
in the pressure range below the bubble-point pressure. B o ¼ formation volume factor of oil (rb/stb)
Solution GOR is measured in PTV laboratories. V res ¼ oil volume in reservoir condition (rb)
Empirical correlations are also available based on data V st ¼ oil volume in stock tank condition (stb)
from PVT labs. One of the correlations is,

0:0125( API) 1:2048 Formation volume factor of oil is always greater than

p 10 unity because oil dissolves more gas in reservoir condition
R s ¼ g g 0:00091t (2:2)
18 10 than in stock tank condition. At a given reservoir tempera-
ture, oil formation volume factor remains nearly constant
where g g and 8API are defined in the latter sections, and at pressures above bubble-point pressure. It drops as pres-
p and t are pressure and temperature in psia and 8F, sure decreases in the pressure range below the bubble-
respectively. point pressure.
Solution GOR factor is often used for volumetric oil
Formation volume factor of oil is measured in PTV labs.
and gas calculations in reservoir engineering. It is also used Numerous empirical correlations are available based on
as a base parameter for estimating other fluid properties data from PVT labs. One of the correlations is
such as density of oil. r ffiffiffiffiffi 1:2
g g
B o ¼ 0:9759 þ 0:00012 R s þ 1:25t : (2:7)
2.2.2 Density of Oil g o
‘‘Density of oil’’ is defined as the mass of oil per unit Formation volume factorof oil is oftenused for oil volumet-
3
volume, or lb m =ft in U.S. Field unit. It is widely used in riccalculationsandwell-inflowcalculations.Itisalsousedas
hydraulics calculations (e.g., wellbore and pipeline per- a base parameter for estimating other fluid properties.
formance calculations [see Chapters 4 and 11]).
Because of gas content, density of oil is pressure depen-
dent. The density of oil at standard condition (stock tank 2.2.4 Viscosity of Oil
oil) is evaluated by API gravity. The relationship between ‘‘Viscosity’’ is an empirical parameter used for describing
the density of stock tank oil and API gravity is given the resistance to flow of fluid. The viscosity of oil is of
through the following relations: interest in well-inflow and hydraulics calculations in oil
production engineering. While the viscosity of oil can be
141:5 measured in PVT labs, it is often estimated using empirical

API ¼ 131:5 (2:3)
g o correlations developed by a number of investigators
including Beal (1946), Beggs and Robinson (1975), Stand-
and
ing (1981), Glaso (1985), Khan (1987), and Ahmed (1989).
r o,st A summary of these correlations is given by Ahmed
g o ¼ , (2:4)
(1989). Engineers should select and validate a correlation
r w
with measurements before it is used. Standing’s (1981)
where
correlation for dead oil is expressed as

8API ¼ API gravity of stock tank oil 1:8 10 7 360 A
g o ¼ specific gravity of stock tank oil, 1 for freshwater m od ¼ 0:32 þ , (2:8)
API 4:53 t þ 200