Lecture 1
practical part
Nazir mafakheri
Quicklook Log Interpretation outline
Basic Quality Control
Identifying the Reservoir
Identifying the Fluid Type and Contacts
Calculating the Porosity
Calculating Hydrocarbon Saturation
Presenting the Results
Pressure/Sampling
Permeability Determination
Quicklook Log Interpretation
Before starting the log interpretation, the
petrophysicist should have:
1. daily drilling reports:
latest deviation data from the well
last casing depth
Mud Data
Quicklook Log Interpretation
Before starting the log interpretation, the
petrophysicist should have:
2. latest mud-log information:
cuttings description
ROP (rate of penetration)
gas reading
Quicklook Log Interpretation
Before starting the log interpretation, the
petrophysicist should have:
3. Logs and interpretations on hand from nearby wells
4. regional wells penetrating the same formations
5. copy of the contractor’s chart book
BASIC QUALITY CONTROL
Petrophysicist needs to ensure the quality of the log data:
Check list :
1. logger’s TD and last casing shoe depths match
2. derrick floor elevation and ground level
3. log curves are on depth with each other.
4. The tension curve to identify toolstring has become
temporarily stuck
BASIC QUALITY CONTROL
Petrophysicist needs to ensure the quality of the log data:
Check list :
5. caliper is reading correctly inside the casing
(find out the casing ID)
6. Check the density borehole correction curve. It should not
generally exceed 0.02 g/cc
7. Inspect the resistivity curves.
oil-based mud (OBM) water based mud (WBM)
BASIC QUALITY CONTROL
Petrophysicist needs to ensure the quality of the log data:
Check list :
8. observing the transit time in the casing sonic log
should read 47 ms/ft.
9. Look out for any cycling-type behavior on any of the
curves, such as a wave pattern.
may be due to corkscrewing while drilling
BASIC QUALITY CONTROL
Petrophysicist needs to ensure the quality of the log data:
Check list :
10 . presentation scales on the log print should generally accepted
in industry norms.
GR: 0–50 API
Caliper: 8–18≤
Resistivity: 0.2–2000 ohmm on log scale
Density: 1.95–2.95 g/cc (solid line)
Neutron: -0.15 ± 0.45 (porosity fraction) (dashed line)
Sonic: 140–40 ms/ft
IDENTIFYING THE RESERVOIR
most reliable indicator of reservoir rock:
behavior of the density/neutron logs
density moving to the left (lower density) and
touching or crossing the neutron curve
NOTE:
In clastic reservoirs in nearly all cases will correspond to a fall in the
gamma ray (GR) log.
IDENTIFYING THE RESERVOIR
density moving to the
left (lower density) and
touching or crossing
the neutron curve
Shales can be clearly
identified as zones
where the density lies
to the right of the
neutron
IDENTIFYING THE RESERVOIR
The greater the
crossover between the
density and neutron
logs, the better the
quality of the reservoir
However, gas zones will
exhibit a greater
crossover for a given
porosity than oil or water
zones
IDENTIFYING THE RESERVOIR
But it is dangerous
to make a hard rule
that the density curve
must cross the neutron
curve for the formation
to be designated as net
sand
IDENTIFYING THE RESERVOIR
For most reservoirs
following approach is safer:
1- Determine an average GR
reading in clean sands (GRsa) and
a value for shales (GRsh). For
GRsh, do not take the highest
reading observed, but rather the
mode of the values observed.
IDENTIFYING THE RESERVOIR
For most reservoirs
following approach is safer:
Define the shale volume, Vsh, as
(GR - GRsa)/(GRsh - GRsa).
By comparing Vsh with the
density/neutron response,
determine a value of Vsh to use
as a cutoff. Typically 50% is
used.
IDENTIFYING THE RESERVOIR
For most reservoirs
following approach is safer:
If the GR is not usable as a sand
indicator, then for now just treat
the entire gross as being net
sand and apply a porosity cutoff
at a later stage
IDENTIFYING THE FLUID TYPE AND CONTACTS
Why?
Because the porosity calculation will depend
on the formation fluid type
IDENTIFYING THE FLUID TYPE AND CONTACTS
If regional information is available
If the formation pressures have already been
measured:
then any information on possible free water levels
(FWLs) or GOCs can also be marked on the log.
IDENTIFYING THE FLUID TYPE AND CONTACTS
comparing the density and deepest reading resistivity
for any evidence of hydrocarbons.
In the classic response, the resistivity and density (and
also GR) will be seen to “tramline”
(follow each other to the left or right)
IDENTIFYING THE FLUID TYPE AND CONTACTS
GOC:
gas zones will exhibit a
greater density/neutron
crossover than oil zones
But:
Formation-pressure
plots represent a much
more reliable way to
IDENTIFYING THE FLUID TYPE AND CONTACTS
Best way to identify gas
zones is to use the shear
sonic log (if available)
combined with the
compressional sonic.
Vp is much more affected
by gas than Vs, a deviation
will be observed in gas
zones
IDENTIFYING THE FLUID TYPE AND CONTACTS
some hydrocarbon/water
zones will not exhibit such
behavior, the reasons being:
When the formation-water
salinity is very high, the
resistivity may also drop in clean
sands.
In shaly sand zones having a
high proportion of conductive
dispersed shales, the resistivity
may also fail to rise in reservoir
zones.
IDENTIFYING THE FLUID TYPE AND CONTACTS
some hydrocarbon/water
zones will not exhibit such
behavior, the reasons being:
If the sands are thinly laminated
resistivity may remain low.
If the well has been drilled with
very heavy overbalance, invasion
may be such as to completely
mask the hydrocarbon response.
CALCULATING THE POROSITY
Porosity should be calculated from the density log:
rhom = matrix density (in g/cc)
rhof = fluid density (in g/cc).
CALCULATING THE POROSITY
CALCULATING THE POROSITY
For sandstones, rhom
typically lies between
2.65 and 2.67 g/cc.Where
regional core data are
available
CALCULATING THE POROSITY
Note that the porosity calculated from the density log
is a total porosity value:
water bound to clays or held in clay porosity is
included.
This has the advantage:
of being directly comparable to porosities
measured on core plugs
CALCULATING THE POROSITY
Porosity may be made using true resistivity (Rt) and
Archie’s equation, which is:
Rw = formation water resistivity (measured in ohmm)
m = the cementation, or porosity, exponent
Sw = water saturation
n = saturation exponent.
CALCULATING THE POROSITY
Porosity may be made using true resistivity (Rt) and
Archie’s equation, which is:
C is a factor that will depend on the shale porosity and CEC
(cation exchange capacity)
