Gravedad api

SPE 28944
Discussion of New Method To Read and Correct the API Gravity of
Oil From Observed Temperatures to 60°F
S.V. Keaves, V.S.V. Rajan, SPE, Alberta Research Council
JPT – August 1994 –pag. 714
The recent “E&P Exchange” article “New
Method To Read and Correct the API
Gravity of Oil From Observed
Temperatures to 60 ºF” (Jan. 1994 JPT,
Page 7) by S.V. Keaves is interesting anduseful. However, graphical interpolation
could be subject to human error. A simple
formula for quick estimation of the API
gravity at 60 ºF from the measured value
at temperature issuggested below.
131.5
141.5
60
60 = −
g
A ………………...(D1)
and 131.5
141.5
= −
T
T
A
g
.….…..……(D2)
Where:
A = ºAPI
g = Specific gravity
and the subscripts T and 60 refer to
temperaturein ºF and 60 ºF. respectively.
From Eqs. D1 and D2,





 


= + −
T
T
A A
g g
1 1
141.5
60
60 ….........(D3)
From Eq. D1,
131.5
141.5
60
60
+
=
A
g…..…...….…..…..(D4)
Combining Eqs. D3 and D4 yields
( )




 


= + + −
T
T
A A A
g
g 60
60 60
131.5 1 .....(D5)
[1 ( 60)]
1 1
60
− +




 


= T
T
a
g g
………....(D6)
Where a =volumetric thermal expansion
coefficient, which maybe assumed to be
reasonably the same for all API oils and
constant over the practical range of
temperatures. This is consistent with
Keaves’Fig. 1, where the lines showing
the temperature variation of API for all
oils are nearly parallel.
From Eqs. D5 and D6,
( )
1 ( 60)
131.5* 60
60
+ −
− −
=
T
A T
A T
a
a
……….(D7)From an examination of the API data
(45°F and above), the value of a is
estimated to be
0.00045.
( )
1 0.00045( 60)
0.059175 60
60
+ −
− −
=
T
A T
A T ……….(D8)
Eq. D-8 is simpleenough to be
programmed into a hand calculator. The
equation checks out very well for the
examples given in the article.
SI Metric Conversion Factor
(º 32)
9
5
ºC = F −
º 32...