Please be fair to the author. Pay your Shareware fee HERE, and receive the CD-ROM at no extra cost.

	This Page è	Basics	Synthetic Loss	Gas Effect	Spreadsheet	Miscellania
	See Also  è	Density Log	Biot-Gassmann	Mineral Prop	Courses	Site Map

EDITING BASICS
Good quality sonic and density data is required for calculating elastic properties from log data. Rough borehole conditions are the worst problem that will need to be repaired. We call this process log editing, log reconstruction, or log repair.

These problems may be minor and easily repaired, or so severe that the log data is useless and must be reconstructed from the log response equation. To do this, we first calculate a complete petrophysical analysis (based on logs that have no problems), then calculate what the bad (or missing) data should have read. The result is often called a "synthetic log". This term may apply to any curve that has been repaired or reconstructed.

Typically, synthetic log curves would be calibrated in intervals of good data and both the original and  synthetic curves would be plotted versus depth to demonstrate this calibration. In cases where the original curve is missing, the synthetic curve parameters will be developed in offset wells with at least some reasonable data. In the worst case, parameters will be based on rational mineral and fluid assumptions based on experience in an area.

<== Example of synthetic density and sonic logs used to calculate elastic properties for a fracture design study. Track 1 has GR, caliper, and bad hole flag (black bar). Track 2 has density correction (dotted curve), neutron (dashed), original density (red), synthetic density (black). Track 3 shows the original and synthetic shear and compressional sonic log curves. In this well, the sonic did not need much improvement - only small spikes were removed by the modeling process.. 

Log editing and creation of synthetic logs is absolutely necessary in rough boreholes or when log curves are missing.

Fracture design based on bad data guarantees bad design results.

Seismic modeling, synthetic seismograms, and seismic inversion interpretations are worthless if based on bad log data.

FIX THE LOGS OR FORGET 'EM!!!!

The process is very simple:
  1. RECOGNIZE BAD DATA
  2. REPLACE IT WITH BETTER DATA

There are a dozen published methods for generating synthetic logs, many dating back more than 50 years. Most are too simple to do a good job, others are too complicated to be practical. All of them are covered in Chapter 25: Editing Logs, available on this website at www.spec2000.net/25-edit1.htm through to www.spec2000.net/25-edit17.htm.


 

CREATING SYNTHETIC LOGS FROM THE LOG RESPONSE EQUATION
The best and easiest modern method is the method used in the example shown above. It uses the log response equation to reconstruct a log based on a complete log analysis run using good data over the interval of interest.

This is not a new technique. It has been used since the 1960's. It did not receive wide acceptance because it relies on a thorough petrophysical analysis, which were slow, expensive, and thus rare. People unfamiliar with good log analysis methods resisted the urge to become competent in the subject, and looked for simpler techniques. Most of these do a poor job. Others use simplified log analysis methods buried in the editing technique. These often ignore lithology variations, so are useless in complex reservoirs.

This is a new century. All the drawbacks are gone. Here's how to do it right.

The equations needed are:
      1:  DENSsyn = Vsh * DENSSH + DENS1 * Vmin1 + DENS2 * Vmin2 + DENS3 * Vmin3 + PHIe * SW * DENSW
                          + PHIe * (1 - SW) * DENSHY
      2:  DTCsyn = Vsh * DTCSH + DTC1 * Vmin1 + DTC2 * Vmin2 + DTC3 * Vmin3 + PHIe * SW * DTCW
                          + PHIe * (1 - SW) * DTCHY
      3:  DTSsyn = Vsh * DTSSH + DTS1 * Vmin1 + DTS2 * Vmin2 + DTS3 * Vmin3 + PHIe * SW * DTSW
                         + PHIe * (1 - SW) * DTSHY

Where:
  DFNSsyn, DTCsyn, and DTSsyn are synthetic density, compressional sonic, and sher sonic
  DENSx, DTCx, and DTSx are density and sonic parameters for the specific mineral and fluid terms

The SW term varies with what you are trying to model. If you want to model the undisturbed state of the reservoir, SW is the water saturation from a deep resistivity log and an appropriate water saturation equation. If you want to see what a log would actually read in a zone, you need the invaded zone water saturation, because that's what most logs see. Invaded zone saturation, Sxo, can derived using a shallow resistivity curve, or it can be assumed to be SW^(1/5).

If you want to see what a water zone would look like, SW is set to 1.00. To reconstruct a log run through a gas zone to reflect the undisturbed case, you need to do the math twice, once to built a water case (remove residual gas in the invaded zone), then a second time to add the gas for the undisturbed zone. The water case is just used for reference, but it helps you to understand the changes caused by the three environments (water vs invaded gas vs undisturbed gas).

Shale values are chosen by observation of the appropriate log interval. Standard parameters for other minerals  (which may need tuning) are:

DENSITY OF GAS FOR RESPONSE EQUATION
The DENSsyn equation is rigorous and can be used with real hydrocarbon densities based on the temperature, pressure, and phase relationship of the fluid in question. A chart showing approximate gas density versus depth is shown at the right, based on average pressure and temperature data for the western Canadian basin. No correction for vuggy porosity is needed.

Density of gas at reservoir conditions - default approximation ==>

The straight line on the graph is:
For gas, in English units  (gm/cc and feet),
      4. DENSHYgas = Min (0.8, 0.000038 * DEPTH)

For gas, in Metric Units (Kg/m3 and meters).     
      5: DENSHYgas = Min (800, 0.125 * DEPTH)

For oil, in English units (gm/cc):
      6. DENSHYoil =  141.5 / (131.5 + API_GR) 

For oil, in Metric  units (Kg/m3):
      7. DENSHYoil =  141 500 / (131.5 + API_GR) 

Where:
  DENSHYgas = density of gas at DEPTH
  DENSHYoil = density of oil
  DEPTH = depth of reservoir
  API_GR = oil gravity

SONIC TRAVEL TIME OF GAS FOR RESPONSE EQUATION
The DTCsyn equation, an extension of the Wyllie Time Average equation for estimating porosity in water filled rocks, provides the opportunity to compute the sonic travel time (and the seismic velocity) of any hypothetical formation by describing the quantity of rock matrix, shale, water, and hydrocarbon, as well as the acoustic properties of these elements in a given reservoir. The equation works for either compressional or shear waves, as long as the appropriate fluid and rock properties are used.

Laboratory experiments and theory have shown that the time average relationship is usually not true when gas fills the pore space, or is even a small fraction of the pore space. For this reason, we call the hydrocarbon travel time in the Wyllie equation a "pseudo-travel-time" to reaffirm that it represents a velocity which may not be the same as the velocity of the gas at the temperature and pressure of the formation.

The hydrocarbon "pseudo-travel-time" is derived empirically by comparing results from synthetic seismograms and properly processed field data. A very rough approximation of hydrocarbon "pseudo-travel-time" with depth, which has given reasonable results in the western Canadian rock sequences, is shown at left. Travel time for liquids, such as oil and salt water (formation water) are more predictable and may be used in the Wyllie equation without reservation.

<== Sonic travel time in gas at reservoir conditions - default approximation
 

This approach was first introduced by the author and John Boyd and published as "Determination of Seismic Response Using Edited Well Log Data" by E.R. Crain and J.D. Boyd at CSEG Annual Symposium, October 1979.

The straight line portion of this graph is represented by:
      8: DELTHYgas = Max (200, 1000 - 0.08 * DEPTH) for English Units  (us/ft and feet)
      9: DELTHYgas = Max (656, 3280 - 0.2625 * DEPTH) for Metric Units  (us/m and meters)

For oil, we have used:
      10: DELTHYoil =  188 + 1.22 * API_GR       for English units
      11: DELTHYoil =  616 + 4.0 * API_GR       for Metric units

Where:
  DELTHYgas = compressional travel time of gas at DEPTH
  DELTHYoil = compressional travel time of oil
  DEPTH = depth of reservoir
  API_GR = oil gravity

For shear travel time, the porosity can be accounted for by using:
      11. DTSgas = DTSoil = DTSwater (see table above).

"META/MODL" SPREADSHEET -- Modeling Log Response
This spreadsheet models log response based on user supplied assumptions, core data, or log analysis results. It is used to prepare log data for use in Mechanical Properties of rocks or for editing logs prior to Seismic  Modeling or creation of synthetic seismic traces. The program uses the log response equation with appropriate values for fluid and rock matrix replacement.

Model Log Response for fluid and rock replacement. English and Metric Units.

Sample of "META/MODL" spreadsheet for calculating log response based on user supplied assumptions,
core data, or log analysis results.
 

MISCELLANEOUS EDITING ROUTINES
The following algorithms may be useful in creating a shear travel time when none exists, and to quickly see the effect if gas on a sonic and density log.

SHEAR TRAVELTIME FROM STONELEY WAVES DATA
In very slow formations, where shear travel time was impossible to measure on older sonic logs, this formula is used to calculate shear travel time (DTS) from Stoneley travel time {DTDT}:
       5: DTS = (DENS / DENSW * (DTST^2 - DTCW^2)) ^ 0.5

The dipole shear sonic log has reduced the need for this calculation, as it sees shear waves better than older array sonic logs.

SHEAR TRAVELTIME FROM COMPRESSIONAL DATA
A shortcut that cam be used is to determine a multiplier (Vp/Vs) based on the graph at the right:
       6: DTS = KS8 * DTC

Where:
  KS8 = 1.8 -1.9 for limestone and anhydrite
  KS8 = 1.7 to 1.8 for dolomite
  KS8 = 1.6 for sandstone and shale
 

QUICKLOOK METHOD TO REMOVE GAS EFFECT
In gas zones only, the density log and the compressional sonic log data may need to be corrected to a liquid filled state. The sonic reads too high and density too low due to the gas effect. If a full blown log analysis is available, density and sonic can be back-calculated from the porosity and lithology using the response equation method described above, provided that reasonable gas corrections were made in that analysis.

In the absence of a full petrophysical analysis, the following equations will also provide better data than the raw log data. In gas zones only:
      1: DENSsyn = DENS + 0.5 * PHIe * Sgxo * (DENSMA - DENSW)
      2: DTCsyn = DTC - 0.5 * PHIe * Sgxo * (DTCMA - DTCW)
      3: DTSsyn = DTS

WHERE:
  DENSsyn = density corrected for gas effect (gm/cc or Kg/m3)
  DENS = density log reading (gm/cc or Kg/m3)
  PHIe = effective porosity (fractional)
  Sgxo = gas saturation near the well bore (fractional)  default = 0.80 for sonic, 0.70 for density log
  DENSMA = matrix density (gm/cc or Kg/m3)
  DENSW = water density (gm/cc or Kg/m3)
  DTCsyn = compressional sonic corrected for gas effect (usec/ft or usec/m)
  DTC = compressional sonic log reading (usec/ft or usec/m)
  DTCMA = compressional sonic travel time in matrix rock (usec/ft or usec/m)
  DTSsyn = shear sonic corrected for gas effect (usec/ft or usec/m)
  DTS = shear sonic log reading (usec/ft or usec/m)
  DTCW = sonic travel time in water (usec/ft or usec/m)
 

Copyright © E. R. (Ross) Crain, P.Eng.  email
Read the Fine Print