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Title. Water Saturation from Porosity Logs Alone, Compared with Standard Resistivity Interpretation Gas/Water Systems. Digital Formation, Inc. November 2003. Contents. Benefits Outline Modeling Examples Conclusions. Benefits – Seismic.

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Title l.jpg
Title

Water Saturation from Porosity Logs Alone, Compared with Standard Resistivity Interpretation

Gas/Water Systems

Digital Formation, Inc.

November 2003


Contents l.jpg
Contents

  • Benefits

  • Outline

  • Modeling

  • Examples

  • Conclusions


Benefits seismic l.jpg
Benefits – Seismic

  • Quantifies influence of wellbore environment on log responses, and implication in seismic attributes.

  • Potential application to evaluating reservoir-scale resistivity properties by linking petrophysical measurements with seismic records.


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Benefits – Petrophysics

  • Saturation from porosity logs alone – no resistivity interpretation involved.

  • From pseudo resistivity curves, can verify ‘m’, ‘n’, Rw.

  • Ability to distinguish fresh water from hydrocarbons.

  • Defines influence of invasion on porosity log response.

  • By integrating pseudo resistivity with actual resistivity curves, the invasion profile can be examined.

  • Pseudo resistivity curves give better bed definition, particularly as compared to induction log measurements.


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Benefits – Engineering

  • Quantify the degree of invasion and wellbore damage.

  • Estimate of permeability variation to the invading mud filtrate.


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Outline

  • In hydrocarbon/water systems, total porosity minimizing matrix and fluid effects is available from a density/neutron combination.

  • Using our Fluid Substitution Modeling techniques, pseudo acoustic, density and neutron logs can be constructed for any assumed fluid combination. To date, we have concentrated on gas/water systems.

  • By comparing actual log response with the pseudo logs it is possible to calculate, at each depth level, the water saturation as “seen” by each of the individual porosity logs.


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Modeling

  • For each porosity log, pseudo logs are created:

    • Assuming “wet”, SW=1.0

    • Using “remote” Sg

    • Assuming a uniform Sg of 80%

  • Actual porosity log response is compared with the pseudo logs – if the modeling has been performed correctly, the pseudo log should fall between “wet” and “remote”.

  • The position of the actual curve between “wet” and “remote” is a direct measurement of SW as seen by each porosity log. For the density, SW is a linear relation with respect to position. For both acoustic and neutron, the response is markedly non-linear.

  • From the values of SW, as derived from porosity logs, theoretical resistivity curves, one each for all porosity logs available, are created using any desired values for ‘m’, ‘n’ and RW.

  • Pseudo resistivity curves are compared with actual resistivity, and can be examined for consistency.


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Examples

  • Fast Rocks

    • SW from Porosity Logs – pro forma

  • Fast Rocks

    • Gassmann DTP & DTS

    • SW from Porosity Logs

  • Carbonates

    • Gassmann DTP & DTS

    • SW from Porosity Logs

      On all plots, porosity increases from right to left, 0-40%.


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Fast Rocks – SW from Porosity Logs – pro forma

Invasion profile from porosity log analysis

Pseudo resistivity logs from porosity logs, assuming m=n=2, and Rw=0.1 (input to regular resistivity analysis)


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Fast Rocks – Gassmann DTP/DTS

Density and Neutron do not “see” gas

Density and Neutron both “see” gas

Acoustic affected by gas in wellbore (anomalously high Dt)


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Fast Rocks – Gassmann DTP/DTS

Density irregularly

“sees” gas

Neutron routinely“see” gas


Fast rocks s w from porosity logs l.jpg
Fast Rocks –SW from Porosity Logs

Acoustic response dominated

by gas in wellbore


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Fast Rocks –SW from Porosity Logs

Pseudo resistivity gives

better bed recognition than

original induction log

Variable degree of invasion

from all three porosity logs


Carbonates gassmann dtp dts l.jpg
Carbonates – Gassmann DTP/DTS

Density and Neutron logs“see” gas

Density and

Neutron logs

“see” liquidonly


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Carbonates – SW from Porosity Logs

Density andNeutron logs“see” gas. Acoustic “sees” residual gas

Reconstructed pseudo resistivitiesaids in invasionprofile interpretation

Estimate of wet resistivity

from porosity logs


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Conclusions – Part I

  • Sw as “seen” by each porosity log is available with no requirement for resistivity logs or knowledge of RW, ‘m’, or ‘n’.

  • By comparing with regular log-calculated SW, saturation changes as defined from porosity logs, and as they change with depth, are available.

  • Comparison with invasion as shown by resistivity logs will show if log interpretation is consistent among all logs.


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Conclusions – Part II

  • The technique is particularly appropriate in reservoirs with variable RW, to solve the problem of fresh water.

  • From these calculations of SW, a series of theoretical resistivity curves can be generated, which can be compared with original resistivity logs.

  • Theoretical resistivity curves help in verifying ‘m’, ‘n’ and RW, and also frequently give better bed definition than resistivity measurements.


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