WELL LOGSCOMMON MISCONCEPTIONS ABOUT WELL LOGS

BY OGUNROMBI TIJESUNI (Intern,PED)

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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

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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.

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HISTORY

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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.

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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- FORMATION TRAVEL TIME• Gamma Ray- AMOUNT OF RADIOACTIVITY

Shaliness• Resistivity- RESISTIVITY Water Saturation• Caliper-HOLE SIZE etc

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HUGE AMOUNT OF DOLLARS??

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.

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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 profit while a bad well does not. Logs make this discrimination.

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IMPORTANCE OF WELL LOGS TO SEVERAL DISCIPLINES

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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.

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DIP TREND

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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 formation

• Vertical Resolution- This is the thickness of the thinnest bed the tool can resolve.

• There is always a contrast between these two parameters in tools.

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MYTH 2 CONTINUED….

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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 Magnetic 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.

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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 many 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.

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MYTH 3 CONTINUED..ASHAKA 4 WELL

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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. - Δtlog = ɸΔtfl + (1-ɸ). Δtma

ɸ = ACOUSTIC- =ɸ. + (1-ɸ).

ɸ = DENSITY

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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.

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EXAMPLES

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

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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.

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TORNADO PLOT 

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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.

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PRECISION OF THE TOOLS

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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

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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

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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.

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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.