porosity from resistivity logs

8/9/2019 Porosity From Resistivity Logs

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POROSITY FROM RESISTIVITY LOGS

Estimating Porosity from: Microlog Shallow Resistivity Deep ResistivityEstimating SXO and SW for Porosity Calculations

POROSITY FROM MICROLOG

The methods presented below provide a mechanism for analyzing ancient logs bycomputer. Experience has shown them to work well provided some control is

exercised on the mud filtrate and water resistivity values upon which they depend.This is done by comparing results to cores or more modern log suites in the same

formations nearby. When presented by computer, the results will not appeargraphically to be any different or any less accurate than the most sophisticated

multi-log analysis. Therefore, a warning note should be annotated on the results.

These porosity methods also rely on a knowledge of SXOor SW, which cannotusually be derived accurately prior to knowing the correct porosity. Thus, if no

other porosity method is available, these methods could give misleading results,with porosity being too low in hydrocarbon bearing zones.

Many older wells do not have porosity logs, but may have a microlog. Porosity can be derived, but it shouldbe calibrated against core or more modern logs.

The response equation is based on Archie's formation factor and water saturation equations.
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PHIml - Porosity from MicrologCalculate porosity from the microlog if there is positive separation. 1: IF RES2 > RES1 2: THEN PHIml = 0.614 ((RMF@FT * KML) ^ 0.61) / (R2 ^ 0.75) 3: OTHERWISE PHIml = 0

WHERE: KML = correction factor for mud cake effect (fractional)

PHIml = porosity from microlog (fractional) RES1 = shallow microlog (1 inch) reading (ohm-m) RES2 = deep microlog (2in) reading (ohm-m) RMF@FT = mud filtrate resistivity (ohm-m)

COMMENTS:No shale correction can be applied, so use caution. Since there is seldom any positive separation in reallyshaly sands, these will not usually cause any problem, except understate the potential of some shaly sands.

This method works well in good hole conditions, and with medium to high porosity. It should be used only ifno other porosity indicating log is available, which is common in wells drilled before 1957. Morecomplicated programs are available which simulate the microlog butterfly chart, but this simpler formulaworks nearly as well.

The chart and one such program are shown below.
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Chart for Microlog Porosity Method


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FORTRAN Code for Microlog Porosity Method

RECOMMENDED PARAMETERS:

Mud Weight

KML

lb/gal Kg/m3 frac

8 1000 1.000

10 1200 0.847

11 1325 0.708

12 1440 0.584

13 1550 0.488

14 1680 0.412

16 1920 0.380

18 2160 0.350

NUMERICAL EXAMPLE:1. Assume microlog data:RES1 = 3 ohm-mRES2 = 4 ohm-mRMF@FT = 1.0 ohm-mmud weight = 1200 Kg/m3KML = 0.847PHIml = 0.614 * ((1.0 * 0.847) ^ 0.61) / (4 ^ 0.75) = 0.20

POROSITY FROM SHALLOW RESISTIVITY LOGSPorosity from proximity log, microlaterolog, microspherically focused log, spherically focused log, shortnormal, shallow induction, or shallow laterolog can be determined and is often used when no other porositylog is available. It can also be used to check microlog porosity if no other check is available.

The response equation is based on Archie's formation factor and water saturation equations.


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PHIxo - Porosity from Shallow Resistivity 4: PHIxo = (A / ((RXO / RMF@FT) * (SXO ^ N))) ^ (1 / M)

WHERE: A = tortuosity exponent M = cementation exponent N = saturation exponent

PHIxo = porosity derived from shallow resistivity device (fractional)

RMF@FT = mud filtrate resistivity at formation temperature (ohm-m) RXO = resistivity from shallow resistivity device (ohm-m) SXO = water saturation in invaded zone (fractional)

COMMENTS:No shale corrections are applied, so use caution. This method is a last resort, since an assumption aboutSXO must be made. SXO cannot be calculated for this method since it requires knowledge of porosity. Shalecorrected versions of this equation can be created by inverting one of the shale corrected saturationequations.

A nomograph for solving these equations is provided below.


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Chart for Shallow Resistivity Porosity Method

RECOMMENDED PARAMETERS:Normal values for A, M, N and SXOfor sandstone A = 0.62 M = 2.15 N = 2.00for carbonates A = 1.00 M = 2.00 N = 2.00for water zone SXO = 1.00for hydrocarbon zone with high porosity SXO = 0.60for hydrocarbon zone with medium porosity SXO = 0.70for h drocarbon zone with low orosit SXO = 0.80


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for heavy oil and tar sands, SXO = SW = 0.10 to 0.30

NUMERICAL EXAMPLE:1. Assume shallow resistivity data:RXO = 20 ohm-mRMF@FT = 1.0 ohm-mA = 0.62M = 2.15

N = 2.00SXO = 1.00PHIxo = (0.62 / ((20.0 / 1.0) * (1.0 ^ 2.0))) ^ (1 / 2.15) = 0.20

2. If zone was hydrocarbon bearing, assume:SXO = 0.70PHIxo = (0.62 / ((20.0 / 1.0) * (0.7 ^ 2.0))) ^ (1 / 2.15) = 0.28

POROSITY FROM DEEP OR MEDIUM RESISTIVITY LOGThis method can only be applied in water bearing zones, although correction for hydrocarbon content canbe made if water saturation is reasonably well known from other sources, such as offset wells or capillarypressure data.

The response equation is based on Archie's formation factor and water saturation equations, which aredescribed more fully in Chapter Eight.

PHIrt - Porosity from deep resistivity log5: PHIrt = (A / ((RESD / RW@FT) * (SW ^ N))) ^ (1 / M)

WHERE: A = tortuosity exponent M = cementation exponent N = saturation exponent PHIrt = porosity from deep resistivity (fractional) RESD = deep resistivity log reading (ohm-m) RW@FT = formation water resistivity (ohm-m) SW = water saturation in un-invaded zone fractional
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COMMENTS:No shale corrections are applied, so use caution. This method is not usually used in hydrocarbon zones andis an absolute last resort. The result is often used in a porosity playback log (with SW = 1.00) to look forpossible hydrocarbon zones by observing the separation between PHIrt and the other porosity logs. Shalecorrected methods may be created from saturation equations in Chapter Eight.

A nomograph to solve these equations is provided below.
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Chart for Deep Resistivity Porosity Method

RECOMMENDED PARAMETERS:Normal values for A, M, N and SW:for sandstone A = 0.62 M = 2.15 N = 2.00


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for carbonates A = 1.00 M = 2.00 N = 2.00for water zones SW = 1.00for hydrocarbon zone with high porosity SW = 0.20for hydrocarbon zone with medium porosity SW = 0.40for hydrocarbons zone with low porosity SW = 0.60

NUMERICAL EXAMPLE:1. Assume deep resistivity data:

RESD = 5.0 ohm-mRW@FT = 0.25 ohm-mA = 0.62M = 2.15N = 2.00SW = 1.00PHIrt = (0.62 / ((5.0 / 0.25) * (1.0 ^ 2.0)) ^ (1 / 2.15) = 0.20

If SW = 0.40PHIrt = (0.62 / ((5.0 / 0.25) * (0.4 ^ 2.0))) ^ (1 / 2.15) = 0.46

2. This last result suggests the zone could not be hydrocarbon bearing, otherwise the RESD value wasincorrectly picked. Assume RESD = 50, then;PHIrt = (0.62 / ((50 / 0.25) * (0.4 ^ 2.0))) ^ (1 / 2.15) = 0.16

This is a more reasonable result.

ESTIMATING SXO and SWThe methods presented in this Chapter provide a mechanism for analyzing ancient logs by computer.Experience has shown them to work well provided some control is exercised on the mud filtrate and waterresistivity values upon which they depend. This is done by comparing results to cores or more modern logsuites in the same formations nearby. When presented by computer, the results will not appear graphicallyto be any different or any less accurate than the most sophisticated multi-log analysis. Therefore, a warningnote should be annotated on the results.

These orosit methods also rel on a knowled e of SXO or SW which cannot usuall be derived


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accurately prior to knowing the correct porosity. Thus, if no other porosity method is available, thesemethods could give misleading results, with porosity being too low in hydrocarbon bearing zones.

If approximate porosity is known, water saturation (SW) can be estimated from the Buckle's PHIxSW methodor the resistivity ratio method: 6: SW = KBUCKL / PHIe

PARAMETERS:Sandstones Carbonates KBUCKLVery fine grain Chalky 0.120Fine grain Cryptocrystall ine 0.060Medium grain Intercrystalline 0.040Coarse grain Sucrosic 0.020Conglomerate Fine vuggy 0.010Unconsolidated Coarse vuggy 0.005Fractured Fractured 0.001Use these parameters only if no other source exists.

Invaded zone saturation (SXO) can be estimated from: 7: SXO = (SW) ^ (1 / 5)

WHERE: PHIe = estimated effective porosity (fractional) KBUCKL = porosity saturation product (fractional) SW = water saturation (fractional) SXO = invaded zone water saturation (fractional)

This approach could be more accurate than the guidelines provided earlier for estimating SW and SXO.Shale corrections are not included, so care must be exercised in shaly sands.

Copyright E. R. (Ross) Crain, P.Eng. emailRead the Fine Print
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