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Depth & Speed Correction of Logs

NExT TrainingE. Standen

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Logging Operations Tensionmeasurement

Cable is marked every 100ft (50m) with a magnetic mark while under 1000 lbs tension. Correction for cable stretch is made based on depth, tension and temperature or by comparison to the “down-log” with tension on the cable.Tension is recorded at the upper sheave and, with newer systems, at the head of the tool string. Tension is monitored so that the cable breaking strength is never approached (10,000 to 14,000 lbs.).

���� Weak Point

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Tool Measure Points

���� SP, Rm

SP - HALS By combining tools together an additional problem is created with measurements at different depths. The basic solution is to memorize the measurements at the top of the tool string until the lowest-most measurement passes the formation and then play back the upper logs “on-depth” with the last measurement.

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Memorization and Sample Rate

• Some old logs were optically memorized (shifted).• MMP-B was a capacitance switching system. • ILP used solid state electronics to memorize

curves usually with a 6 inch frame. • All older logs were a conversion of analogue

signals, usually with a 6 inch sample rate. • Platform Express samples logs every inch for

speed correction of raw data. • The vertical accelerometer and electrical image

data is sampled every 1/10 inch

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Curve Memorization

SP first reading ����

FR ����

FR ���� FR ����

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Historical Operations

• Speed correction was first used for dipmeter data• Accurate location of electrode response was needed for

determination of bedding dip and azimuth.• Comparison of two electrodes on the same pad a fixed distance

apart determined whether the tool was accelerating or decelerating.

• With introduction of accelerometer measurements speed correction of standard log data could be done in the computing center.

• Platform Express uses a vertical accelerometer to speed correct raw inputs before resolution matching and computation of basic log outputs.

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Why Resolution Matching

• Different measurements investigate different volumes of the formation.

• When we combine the measurements in any interpretation, such as invasion computations from resistivity logs or even saturation of the formation we are mixing measurements of different volumetric resolutions.

• The closer the measurements are to examining the same volume of rock, the more accurate the output data.

• This resolution matching is missing in older log data which we take as true measurements (eg Rhob, NPHI, and induction logs).

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High Resolution

LogsThe high resolution logs from Platform Express are only accurate if there is minimal borehole standoff and minimal corrections to the basic log measurements.

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Speed Computation from Acceleration

�Tool moving

�Tool stationary (or moving at a very constantvelocity)

Acceleration

Deceleration to a stop

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Image Comparisons

����two rows of electrodes show a sawtooth effect when no row depth shift is applied. In this case no speed correction was done as well. Interestingly enough, you don’t need to do speed correction to compute a correct bedding dip as can be seen from the bedding sinusoids on both images.

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Tool StickingAs the tool comes to a stop the tension increases. Flat portions of the logs are stuck zones where data is accumulated at the same depth. Once the tool begins to move the data is recorded over the acceleration period, but, at the same sample rate, producing an under-sampled situation. Stuck intervals for the shallow and deep laterolog curves will be at the same depth as their measure points are the same, however, the MCFL curve will show sticking at a different depth due to the displacement of it’s measure point on the tool string. To speed-correct the log, data must be added to the under-sampled ‘moving’ sections and subtracted from the over-sampled stuck zones.

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Speed Correction of

Log InputsTo properly speed-correct the log data, acceleration of the tool must be measured and then integrated to give velocity and then tool possition. Data is then interpolated in the under-sampled zones of acceleration and removed from the over-sampled stuck zones. The resulting data sets can then be resolution matched and combined for proper interpretation.

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TLD Density Inputs

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Density Outputs

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PEx Speed Correction

Variations in computed saturations are often the result of poor depth-matching of log inputs. This will create discrepancies in thin-bedded formations and lead to inaccurate interpretations of rock types and flow potential.

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Additional Net Pay? – Not Exactly!

���� The fact that the speed corrected data increased the thickness of Zone C is just fortuitous in this case. It could have equally reduced the thickness and in any case this is a tight streak and not a porous reservoir interval.

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Results of Speed Correcting Data

• Like measuring thin beds, speed correction will not guarantee greater net pay.

• It will ensure that the log data is better corrected and is the first step to putting the various measurements on depth.

• Speed correction is a good first step for resolution matching different measurements for interpretation.

• Inherent in the process is a certain amount of loss of precision to gain accuracy.

• It is best done on high sample rate data to minimize the precision loss.