PANDIT DEENDAYAL PETROLEUM UNIVERSITY
SCHOOL OF PETROLEUM TECHNOLOGY
B.TECH. SEMESTER - VI
ASSIGNMENT - 4
Course: Petroleum Formation evaluation (PE 308)
Problem 1: Using the following figures, estimate GR corrected under the following
condition
GRlog reads 67 API units
Hole size = 8 inch
Mud weight = 16 lb/gal
Tool is centered
Hints: GR logs are subject to a number of perturbing effects including (a) Sonde
position
(b) Hole size
(c) Mud weight
(d) Casing size and weight
(e) Cement thickness
Since there are innumerable combinations of hole size, mud weight and tool
position, an arbitrary set of standard conditions is defined in the above figures. A
series of charts is available making the appropriate corrections.
Ans: GRcor = 100
Problem 2: On the GR log shown on figure below, choose a value for GR min and
GRmax and compute Vsh in sand C using linear correlation between GR response
and Shaliness.
Ans: Vsh = 30%
Problem 3: In the following example, give most likely cause for the high activity
seen on the total GR intensity curve at the point marked “A”.
Problem 4:
On the log given below, estimate the amount of clay, Vsh, at the following depth:
8,540, 8,549, and 8,560 ft. Do the same using the SP curve and compare. Why
does the SP yield higher values for Vsh in the bottom zone?
Problem 5:
Using the total GR curve of the following figure, compute the value of Vsh at the
top and bottom of the zone indicated as “ sand+mica”.
Problem 6:
From the spectral gamma device described above, where the gamma rays from Th
and U are predominantly in windows 4 and 5, and where the K gamma ray is
contained only in window 3 and below, determine the response equations for U
and Th. This can be done from a single measurement where the window counting
rates are known, as well as the Th and U concentrations. Suppose window 5 and
window 4 have 40 and 100 counts, respectively, in a formation containing 5 ppm
Th and 20 ppm U.
Problem 7:
In the lowest portion (which is water-bearing) of a clean sandstone reservoir
known to be of constant porosity, the density tool reads 2.21 g/cm 3. Further up in
the same reservoir, above the oil-water contact (where the formation is fully
hydrocarbon saturated), the density tool reads 2.04 g/cm 3. What is the density of
the hydrocarbon?
(i)
The equation ρlog = 1.0704ρe – 0.188 relates the electron density ρe to
the tool reading ρlog, which is closely related to the bulk density ρb. This
equation has been defined so that the tool reading corresponds to ρb in
water limestone mixtures. What transform would be used for the log
reading to coincide with the bulk density of mixtures of 120 kppm
saltwater and sand (SiO 2)?
Problem 8:
The following figure is a short section of the LDT log in the carbonate section of
the simulated reservoir model.
(i)
(ii)
(iii)
From the knowledge of the density curve alone, what ranges of porosity
would you ascribe to the seemingly uniform layer about 12,490 – 12,540
ft?
By including the ρe measurement, could you refine your porosity
estimate? What new average value would you estimate it to be?
What proportion of dolomite and limestone does the matrix seem to be?
If not constant, what is the range of mixtures?
Problem 9:
The following figure shows the overlay ρe versus ρb for three different matrices as
a function of porosity for lithology determination.
(i)
Frequently salt plugging occurs in dolomitic formations which
contain very saline formation waters. In this case the preexisting
porosity of the dolomite can be replaced with depositions of NaCl.
Plot the trend line on this cross plot for the case of a 20 p.u. waterfilled dolomite in which the porosity progressively becomes saltfilled. Make use of the mixing law for U.
(ii)
From this cross plot alone, with what might you confuse the fully
plugged case?
Problem 10:
A density log has been run in a sandstone reservoir where core analysis has
determined porosity to be 23%. In this zone the density reading is 2.40 g/cm3.
(i)
(ii)
What do you estimate the grain density of the sandstone to be?
What is the error in porosity if you assume the matrix to be pure
sandstone (SiO2)?
(iii) From the core analysis the formation is known to consist of SiO 2 and
pyrite (FeS2). What volume fraction of the matrix does the apparent
grain density correspond to?
(iv) The actual grain density from core analysis gives a value of 2.76
g/cm3. What does this imply as a value for
ρfl?