ReservoirISTLearn

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From raw well logs to net pay

Turn a set of log curves into zones, porosity, water saturation and a net pay number you can defend.

Field
Reservoir engineering
Level
Beginner
Time
15 min

The picture

240024202440246024802500Depth (m)Gamma ray0150 APIDeep resistivity0.22000 ohm.mDensityand neutron1.952.95 g/ccShaleWater sandGas sandShaleNet pay
Synthetic example. In the gas sand, resistivity jumps and the density and neutron curves cross over (shaded). That combination is what you look for. Neutron porosity (dashed) runs from 0.45 on the left to -0.15 on the right.

The workflow

  1. Check the data

    In: LAS file, header, caliperOut: Curves you trust, on one depth scale

    Open the LAS file and read the header first: units, null value, depth reference. Look for flat lines, spikes and washed-out sections on the caliper. Depth-match curves that came from different runs before you compare them.

  2. Find the reservoir rock

    In: Gamma ray, SP, caliperOut: Clean zones separated from shale

    Low gamma ray usually means clean sand or carbonate, high gamma ray means shale. Mark the top and bottom of each clean zone. Use the caliper to see where the hole is too rough for the other logs to be reliable.

  3. Estimate porosity

    In: Bulk density, neutron, matrix and fluid valuesOut: A porosity curve for each zone

    Density porosity is (matrix density − bulk density) ÷ (matrix density − fluid density). Pick the matrix value for the rock you actually have, for example about 2.65 g/cc for clean sandstone and 2.71 g/cc for limestone. Compare it with the neutron log: where they separate, you may have gas or shale.

  4. Estimate water saturation

    In: Deep resistivity, porosity, Rw, a, m, nOut: Sw curve and a hydrocarbon indicator

    Archie's equation gives Sw = ((a × Rw) ÷ (φ^m × Rt))^(1/n). Rw comes from a clean water zone, water samples or a catalog. High resistivity on top of good porosity points to hydrocarbons. In shaly sands, plain Archie reads too much water, so use a shale-corrected model.

  5. Apply cutoffs and sum the pay

    In: Porosity, Sw and shale volume curves, cutoff valuesOut: Net thickness, net-to-gross, averages

    Choose cutoffs (for example porosity above 8%, Sw below 60%, shale volume below 35%) and keep only the intervals that pass all of them. Report net pay, net-to-gross ratio, and the thickness-weighted average porosity and Sw for each zone.

What you hand over

A zone-by-zone table of net thickness, average porosity and average water saturation, with the cutoffs you used.

Background

What you are trying to answer

A well log is a set of measurements recorded as a tool is pulled up the hole. Your job is to turn those curves into three answers for each reservoir zone: how much rock is worth producing (net pay), how much pore space it has (porosity) and how much of that pore space holds hydrocarbons (1 − Sw).

These numbers feed volumetric reserves, the geological model and the decision to complete a well. That is why every step needs a clear record of the choices behind it.

Read the tracks together

No single curve gives the answer. Look at them side by side:

  • Gamma ray separates shale from clean rock.
  • Density and neutron together give porosity and hint at the fluid.
  • Resistivity tells you whether the pore fluid conducts electricity (salt water) or does not (hydrocarbons).
  • Caliper tells you whether the hole is good enough to trust the others.

Keep a log of your choices

Write down the matrix density, Rw, a, m, n and every cutoff you used. When a colleague asks why your net pay differs from theirs, the answer is almost always in one of those values.

Common mistakes

  • Using the wrong matrix density. A 0.06 g/cc error can move porosity by about 3.5 porosity units.
  • Using Archie in a shaly sand without a shale correction, so pay is missed.
  • Mixing measured depth and true vertical depth when you add up thickness in a deviated well.
  • Taking Rw from a catalog without checking it against a water zone in the same well.
  • Reporting net pay without listing the cutoffs, so nobody can reproduce it.

Check yourself

1. A zone has low gamma ray, density porosity of 22% and deep resistivity ten times higher than the wet sand above it. What is the most likely reading?
2. In Archie's equation, what happens to calculated water saturation if the true Rw is lower than the value you used?
3. Which input does density porosity need besides the bulk density log?
4. Why must net pay always be reported together with its cutoffs?

Try this workflow in LoggiTech

Process raw LAS files through to net pay, correlations and reports.

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