(8)log picking

8/11/2019 (8)Log Picking

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LOG PICKING - "BOXING THE LOG" In order to perform a log analysis, it is necessary to read or pick log values in thevarious zones of interest, and other key locations, such as in shale or water bearingzones. Picking log values and analysis parameters from logs is THE most importantstep in quantitative log analysis. Mathematics cannot compensate for poorselections. Few comments on this subject are found in service company trainingmanuals. As a result, beginners often find it difficult to start with valid data, orassume the task is easy and requires no thought or knowledge.

Selections should be made on a consistent basis from day to day to assistreproducibility of results. When using digital log data, the digits themselves will beused by the computer program, but the analyst must still pick numerous values byobservation of log curves, crossplots, or data listings.

In computer aided log analysis, picks are made continuously with a digitizer or byreading magnetic tapes or discs created when the logs were recorded. Such data tends to be more accurate thanhand picked values. Accuracy can be a hindrance on noisy logs, rounded bed boundaries, or in large or roughholes. Some editing or curve shaping may be required prior to digitizing, hand picking data, or using existingdigitally recorded data.

To select a log value, it is helpful, especially for the novice, to "box the log". Draw horizontal lines at each bedboundary, at the inflection points on each curve. Draw vertical lines on each curve at the peaks and valleys, thus

transforming the log into a series of individual beds with a single specific log reading. For thicker reservoir layers,an average value may be chosen if the log curve does not wiggle too much.

Reading w ell logs is m ade easier by " boxing th e log". Horizontal l ines (bed boundaries) and vert ical l ines(log values or log readings) form boxes that bound the data to al low val id data picks. Be sure to honour the " peaks and val leys" rule in thi n beds.


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With experience, it is possible to simply mark points at the peaks and valleys without drawing horizontal lines, asshown in the lower part of the example. Unless absolutely necessary, values should not be selected on slopes.Slopes indicate transition from one condition, such as porosity or hydrocarbon content, to another. Average values,halfway along the slope may be meaningful, but can also be misleading. Do not select values in thin beds unlessyou are also prepared to make bed thickness corrections.

Be sure to pick the correct curve, its appropriate scale, and edit any noise or bad hole conditions prior to finalizingvalues.

Write the log values picked into a table or preprinted form so that you will have a record of the data you are using.Note that very shaly zones are not usually interpreted. Therefore, this data can be left off the table or marked asshale with no data values entered.

SHALY SAND EXAMPLE Depths in Feet (Logs above were in meters)RAW DATA PICKS S hale Properties:

Gamma Ray clean line (GR0) ___22___ api units Gamma Ray shale line (GR100) ___105___ api units SP clean line (SP0) ____14___ mv SP shale line (SP100) ____78___ mv Density shale line (PHIDSH) __0.03___ Neutron shale line (PHINSH) __0.26___ Sonic shale line (DTCSH) ___NA___ us/m Resistivity shale line (RSH) ___20____ ohm-m

Resistivity of Water Zone (R0) ___2.7___ ohm-m

Log Data Values for Layers A: Sh Sd B: Oil C: Water Layer Top __3364_ _3499_ _3512__ feetLayer Bottom __3406_ _3512_ _3530__ feetDeep Resistivity RESD ___20__ __100_ __2.7___ ohm-mNeutron Porosity PHIN __0.24__ __0.27_ __0.24_Density Porosity PHID __0.12__ __0.22_ __0.19_Sonic Travel Time DTC __NA____ __NA__ __NA__ usec/mGamma Ray GR __75____ __25__ ___30__ api unitsSpontaneous Potential SP __55____ __23__ ___15__ mvPhoto Electric Effect PE __3.4___ __2.0__ ___2.0__ cuCaliper CAL __8.5__ __7.7__ ___7.7__ inches

When using computers, log data is usually digitized at an increment much finer than the tool resolution. Thusanswers are calculated even on slopes and in thin beds. Understanding such results usually requires somethought. Again, be sure to pick the correct curve from the data file.

GENERAL RULES FOR PICKING LOG VALUES

In order to perform a log analysis, it is necessary to read or pick log values in the various zones of interest, andother key locations, such as in shale or water bearing zones. Selections should be made on a consistent basis fromday to day to assist reproducibility of results.

In computer aided log analysis, picks are made continuously with a digitizer or by reading magnetic tapes createdwhen the logs were recorded. Such data tends to be more accurate than hand picked values. Accuracy can be ahindrance on noisy logs, rounded bed boundaries, or in large or rough holes. Some editing or curve shaping maybe required prior to digitizing, hand picking data, or using tape-recorded data.

To select a log value it is helpful, especially for the novice, to "box the log". Draw horizontal lines at each bedboundary, at the inflection points on each curve. In thinner beds, draw vertical lines on each curve at the peaks andvalleys, thus transforming the log into a series of individual beds with a single specific log reading. Pick peaks orvalleys in thin beds to get the best possible values.

On thicker beds, draw a line through the average value of the curve. It is necessary to create a new bed or layereach time the porosity changes by 2% (porosity units), or when resistivity or gamma ray change by more than 10%(relative units)


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The rule is to draw bed boundaries at the top and bottom of eachclean zone, then draw boundaries at the porosity breaks withineach clean zone. Finally draw any new boundaries needed toaccommodate resistivity changes, which usually represent oil orgas water interfaces. The major interval between the top andbottom boundary of a relatively clean rock is called a ZONE. Eachboundary inside a ZONE defines a LAYER of rock. Each layer willbe analyzed separately by picking log values for each layer, called"Reading the Log"..

This concept is shown on the log at the right.

For thick layers, pick average values. For porosity, gamma ray,laterolog, and array induction logs, "thick" means more than 6 feet(2 meters). For older style induction logs, "thick means greaterthan 15 feet (5 meters).

In thick beds , pick average values(heavy black vert ical l ines) ===>

Old s ty le induct ion log , layer roughly15 feet (5 meters) , pick peaks and valleys;

other log s, pick averages ===>


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be interpreted. If these values are close to the expected matrix value for the known lithology, they may be used withcaution. If lithology is unknown, start with pure mineral values from tables.

4. Shale values for sonic, density, and neutron are determined from the average value of logs in shales, 20 or morefeet thick, below the zone to be interpreted. This applies to clean logs without skips, spikes, and rough or largeboreholes. Caution should be used since shale properties can vary widely within a short interval. Data is used forshale corrections to porosity calculations. Therefore, corrections may be inaccurate if shale properties vary or arepoorly chosen.

5.Gamma ray readings in shale are usually reasonably high (100 - 200 API units and lower in sandstones andcarbonates (10 - 50 API units). SP scales vary widely, but deflections to the left usually represent sandstones anddeflections to the right mean shale, except in shallow fresh water sands. The SP is usually featureless incarbonates.

6. The gamma ray and SP clean sand and shale lines are required to allow us to find the shale volume for use inshale corrections to porosity calculations. To determine the clean line value, find the cleanest or least shaly zone inthe entire well. Lower this value to suit the known shale content in other zones. Caution - never push the clean lineinto more than 5% of the data points. To find the shale line, draw a line through the average data value in thick shalezones. Do not include very radioactive zones which are generally caused by uranium, and not shale minerals. Up to10% of the data points may be above the shale line.


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EXAMPLES OF READING LOGS

Gamm a ray and SP c lean l ines (GR0 and SP0) and shale l ines (GR100 and SP100) picked fo r several zones.Resist ivi ty o f shale (RSH) and resist iv i ty of water zones (R0) are also sh ow n. Note variat ion s with d epth due to

changing geologica l s i tua t ions .


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Sonic tr avel t ime in shale (DELTSH) also v aries with dep th.


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Densi ty, neut ron , and gamm a ray sha le proper t ies vary wi th depth

As do the neutron porosity of shale (PHINSH) and density porosity of shale (PHIDSH).

If base line methods are difficult, certain crossplots may be helpful. Some analysts prefer the crossplot methodalthough it requires an extra computer step and is not appropriate for visual or quick look interpretation. Crossplotsalso mask variations related to depth, so appropriate zoning is required.

(8)log picking

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