Well logs provide subsurface data by measuring physical rock properties against depth. They justify the cost by determining if a well will be productive. There are common misconceptions about what logs can and cannot do. Logs have uncertainty due to tool limitations and environmental factors. Proper calibration and quality control procedures are important to improve log measurement precision and accuracy.

1. WELL LOGS
COMMON MISCONCEPTIONS ABOUTWELL LOGS
BY OGUNROMBITIJESUNI (Intern,PED)

2. OUTLINE
• History
• What are well logs?
• What do they measure?
• Do logs justify the $$$ and time spent acquiring them?
• Myths about well logs
• Uncertainty analysis in well logs
• Precision of the tools
• Quality Control
2

3. HISTORY
• Around the 1800s,the demand for whale oil went crashing down because of the fierce
competition from high surge of oil and kerosene from the oil industry. Most sailors
were out of job, forcing them to move into the drilling industry.
• It turned out that the sailors were naturals at working with heights, rigging, tall
derricks and drilling rigs.They brought along most of their nautical expressions with
them. E.g. captain's log, where the captain recorded what happened to the ship in his
logbook.The term was then adopted in drilling.
• The drillers wrote what they saw as they drilled deeper into the rocks, which was
recorded against depth.This is referred to as a driller's log. It was the only subsurface
data they had back then. It contains the information about types of rock, how fast
the bit was drilling, any oil or gas inflows, equipment malfunctions and any other
significant occurrences during drilling.
• Today, a "log" means any information recorded against depth (or time) in graph form-
a term we owe much to the sailors.
3

4. HISTORY
4

5. WHAT ARE WELL LOGS?
• Logs are used to get a continuous data of a formation’s rock
properties recorded against depth or time.
• The rock properties measured are then used to infer
information like Hydrocarbon saturation and formation
pressure so we can describe our reservoir quantitatively.
5

6. WHAT DO LOGS MEASURE?
• Logs do not measure the reservoir properties which we
seek in-situ. What logs measure are physical properties
which have a form of relationship with the interested
reservoir parameters.
• Bulk Density- ELECTRON DENSITY
• Neutron- HYDROGEN INDEX Porosity
• Acoustic- FORMATIONTRAVELTIME
• Gamma Ray- AMOUNTOF RADIOACTIVITY Shaliness
• Resistivity- RESISTIVITY Water Saturation
• Caliper-HOLE SIZE etc
6

7. HUGE AMOUNT OF DOLLARS??
7
Yes! logs justify the huge amount of
money spent obtaining them and could
even save us extra cost which might be
incurred drilling the hole blindly i.e.
without a well log.

8. HUGE AMOUNT OF DOLLARS ????
• The reason for running logs is to determine whether a
well is good or bad. A good well is commercially
productive-it produces enough oil or gas to pay back
its investors for the cost of drilling and leaves a
while a bad well does not. Logs make this
discrimination.
8

9. IMPORTANCE OF WELL LOGS TO SEVERAL DISCIPLINES
9

10. MYTH 1-YOU CAN SEE BEYOND YOUR WELL
WITH A WELL LOG
• A well log can only measure a few feet (even inches) around the
wellbore.This doesn’t mean that one will get exactly the same
well log properties beyond the well especially when reservoir
properties vary horizontally and vertically.
• If we detect fractures from say acoustic log or image log, this
doesn’t necessarily mean there are fractures far beyond the
reservoir.
• But, if there is more than one well, probably fractures/dip trends
and hole breakouts trends can be established for that reservoir.
• Trends are what we are interested in. Once we have established well
trends, then only we can see beyond our well.
10

11. DIP TREND
11

12. MYTH 2: EACH LOG SEES THE SAME THING
• Each logging sensor sees different things. A specific
logging tool measures unique depth of investigation
and vertical resolution.
• Depth of investigation-How far the tool can see into the
into the formation
• Vertical Resolution- This is the thickness of the thinnest
thinnest bed the tool can resolve.
• There is always a contrast between these two
parameters in tools.
12

13. MYTH 2 CONTINUED….
13

14. MYTH 2 CONTINUED..
• Assuming that we want to measure porosity. Although different
porosity logging tools can produce similar porosity outputs, they
are measuring different properties at distinct volumes of
investigations. Porosity can be measured from Nuclear
Resonance (NMR), Density, Neutron or Sonic logs.
• A NMR log measures only a few inches from the sensors, while
Neutron logs sees deeper than the NMR.The NMR log may give
porosity values that are around the invaded zones. In contrast,
the Neutron log may see porosity from the invaded zone and
even further than the NMR would.
• However, the resulting processed porosity values should be close
to each other after taking into account the borehole and
environmental conditions.
14

15. MYTH 3: WHAT YOU SEE IS THE TRUTH
• Apparent log readings will be different from the final log values. This means that the log
values obtained directly from the well log maybe very different from the actual log
values you want to measure.The log measurements are affected by the tool geometry,
tool physics, borehole environments including well trajectory and invasions and
other factors.
• Take neutron log readings in a gas bearing zone and in a shale interval for example.
Let's say from our knowledge we know that the actual porosity is around 20 p.u.
• In the gas bearing zone, the neutron reading will be super low since the neutron can't
see a lot of hydrogen in gas. But, in the shale zone, a higher neutron porosity is
observed due to clay bound water effect.This implies that both the apparent neutron
values (one from the gas interval and once from shale interval), are misleading.
• The real porosity value is obtained by combining porosity logs e.g. Density-Neutron and
correcting for presence of Hydrocarbons.
15

16. MYTH 3 CONTINUED..
16
ASHAKA 4 WELL

17. MYTH 4: WHAT YOU MEASURE IS WHAT IS REQUIRED
• The intended properties cannot be obtained directly from well logs.. In fact,
well log measurements are indirect measurements that you solve to get the
measurement that you want. We have to make certain assumptions when we
derive the log properties. If the assumption is true, we will get reasonable
estimates of hydrocarbon or mineral resources.
• If I say I am measuring porosity, I am actually using the measured properties
either from electron density, acoustic travel time of the rock, hydrogen index
or mean T2values to derive porosity values.
17
- Δtlog = ɸΔtfl + (1-ɸ). Δtma
ɸ =
Δ𝑡𝑙𝑜𝑔
−Δ𝑡 𝑚𝑎
Δ𝑡 𝑓𝑙
−Δ𝑡 𝑚𝑎
ACOUSTIC
- ρ 𝑏 =ɸ. ρ 𝑓𝑙 + (1-ɸ). ρ 𝑚𝑎
ɸ =
ρ 𝑚𝑎
− ρ 𝑏
ρ 𝑚𝑎
− ρ 𝑓𝑙
DENSITY

18. UNCERTAINTIES
• According to Ballin (1993), uncertainty is defined
as a lack of assurance about the truth of a
statement or about the exact magnitude of an
unknown measurement or number.
• Well logs suffer from several uncertainties including
the geological environment, data acquisition
technique and laboratory measurements and must
be corrected before usage.
18

19. EXAMPLES
• Monte Carlo Simulation
• Tornado plot (a form of sensitivity analysis)
19

20. UNCERTAINTIES ANALYSIS-EXAMPLES
• Monte- Carlo simulations enable us model phenomena
with significant uncertainty in input. Using Monte-
Carlo simulation the uncertainty in the inputs is
determined by randomly selecting input values from
their uncertainty distributions.
• Tornado plot is a bar chart that compares the impact
of a variable in a process, a workflow or a
computation. It is designed such that the largest bars
appear at the top of the chart indicating the highest
uncertainty.
20

21. TORNADO PLOT
21

22. PRECISION OF THE TOOLS
• The accuracy of a set of observations is how close a measured
value is to the true/accepted value of the observed quantity.
The precision of a set of measurements is a measure of the
range of values found, i.e., of
the reproducibility/repeatability of the measurements.
22

23. PRECISION OF THE TOOLS
23

24. CAUSES OF POOR LOGS
• At manufacture; inadequate design, inadequate testing
• At contractor base; inadequate maintenance, incorrect
shop calibrations
• At well-site; incorrect calibration, poor procedures, bad
hole, abnormal muds(Nacl, Kcl), abnormal lithologies
(e.g. Groningen effect)
• After logging: poor editing may cause erroneous
responses on the logs stored on the database
24

25. CALIBRATION AND QUALITY CONTROL
• The Correct working of a logging tool will
require it being previously calibrated. Several
methods exist
–Primary standards
–Secondary standards
–Tertiary standards
–In-situ calibrations
• Repeat Sections
25

26. QUALITY CONTROL BY INTERPRETATION
• Check reservoir thickness on GR, density/neutron, SP
and microlog
• Compare lithologies obtained from mudlog, sidewall
samples and density/neutron (and/or sonic) logs
• Calculate Porosity from a combination of density,
neutron and sonic
• Ensure the consistency of fluid types obtained from
resistivity, density/neutron, cuttings, sidewall samples,
gas chromatograph, WFT samples and pressure tests.
26

27. CONCLUSION
• Logs are interpreted based on a model the interpreter
has chosen and an idea of what the true value of the
input parameters should be.
• Logs undergo several processes to correct for the
uncertainties associated with its acquisition so as to
improve the values obtained for later reservoir
modelling.
27