A Method of Creating Pseudo Acoustic (Compressional and Shear) and Density

Logs Involving Fluid Substitution

Title

Prepared for the Canadian Well Logging Society

byMichael Holmes

Digital Formation, Inc.October 8, 2003

Outline

• Introduction

• Data Analysis

• Examples

• Conclusions

Introduction – Part 1

• Gassmann’s model showing the effect of gas saturation on the slowing of rock velocities has been adapted to petrophysical analysis.

• Components of an analogous model of Krief have been integrated into the Gassmann model, to include:– The entire range of shaley formations– Analysis of shear as well as compressional velocities

• Using standard wireline log suites, it is possible to calculate compressional and shear pseudo travel time curves for any input of gas vs. water saturation. Additionally, it is possible to calculate pseudo density curves, again for any input of gas vs. water saturation.

Introduction – Part 2

• Source data required are:– Porosity – as curves– Shale – as curves– Water and Gas Saturation – as curves– Matrix – as component input– Shale – as component input– Fluid – as component input

• An extension of the analysis involves estimating water saturation for each porosity log, independent of resistivity data. The technique consists of solving the porosity log equation for fluid content assuming an accurate knowledge of:

– Porosity– Matrix– Shale

• From the saturations determined from porosity logs, it is possible to create pseudo resistivity logs, using reasonable values of m, n, and RW. These yield a resistivity invasion profile, which can be compared with actual resistivities.

Data Analyzed

• Four wells from Western Canada, plus one US well– Belly River Alberta– Shell et at Rosevear– Amoco et al Hanlan– BP Boundary Lake South– Wamsutter, Wyoming

• All wells have the following log suites:– Compressional DT– Density– Neutron– Gamma Ray– Resistivity

• Two wells (Amoco & BP) also have a Shear DT

Data Analysis

• Corrected porosity logs for bad hole.• Performed standard shaley formation analysis using density/neutron

combination for porosity. This minimizes any gas and matrix effects on porosity calculations.

• Ran fluid substitution to derive pseudo logs assuming:– Wet formation– Residual Gas Saturation– Uninvaded Zone Gas Saturation

• Pseudo compressional and shear acoustic and pseudo density curves were created.

• Ran saturation calculation from porosity logs, and created pseudo-resistivity logs for each porosity log.

• Compared pseudo logs and pseudo saturation curves with raw data and calculated saturations.

Example #1

• Belly River

Belly River: Raw Data

Minor correction required

Belly River: Pseudo Logs

Acoustic log “sees” gas

Pseudo log heavily affected by very small amounts of gas

Example #2

• Shell Rosevear 714

Shell Rosevear 714: Raw Data

Large correction due to hole washout

Shell Rosevear 714: Pseudo Logs

Acoustic log does not “see” gas

Acoustic log “sees” gas

Generally good correlation between actual and pseudo acoustic compressional logs

Shell Rosevear 714: Pseudo Logs

Acoustic “sees” little or no gas

Good correlation between actual and pseudo density curves

Shell Rosevear 714: Composite Log

Probable bad hole

Fair correlation between petrophysics and core data

Example #3

• Amoco et al Hanlan

Amoco et al Hanlan: Raw Data

Significant corrections were required due to bad hole

Amoco et al Hanlan: Pseudo Logs

Gas

Cycle skips

Acoustic compressional log mostly tracks the wet pseudo log except where gas occurs

Good correlation with measured shear curve

Amoco et al Hanlan: Composite Log

Fair correlation between petrophysics and core

Amoco et al Hanlan: Pseudo Logs

Compressional acoustic log does not “see” gas

Good correlation between pseudo and actual shear acoustic

Example #4

• BP Boundary Lake South

BP Boundary Lake South: Raw Data

Extensive curve editing required due to bad hole

BP Boundary Lake South: Pseudo Logs

Acoustic log “sees” gas

Acoustic log does not “see” gas

Anomalous response, possibly due to bad hole

Generally good correspondence between pseudo and measured shear acoustic

BP Boundary Lake South: SW & Resistivity from Porosity Logs

Regular invasion profiles in high porosity rocks

Apparent cycle skipping due to gas in wellbore

BP Boundary Lake South: Composite Log

Good comparison between petrophysics and core

BP Boundary Lake South: Pseudo Logs

Fair to good comparison between actual and pseudo acoustic logs

Example #5

• Wamsutter, Wyoming

Wamsutter, Wyoming: SW & Resistivity from Porosity Logs

Shows variable invasion profile, acoustic log mostly SW=1.0

Conclusions – Part 1

• Pseudo compressional acoustic logs compare well with actual data for the full range of shale content. However, it can be shown that recorded compressional acoustic data “sees” gas saturation in an irregular and unpredictable fashion. This is a consequence of erratic and discontinuous mud invasion.

• Small amounts of gas have large effects on the acoustic log. Unless fluid (particularly gas) saturation is known from other sources, the acoustic log cannot be used reliably to determine porosity.

• For the two wells with shear acoustic there is good to excellent correlation with pseudo logs. This gives confidence in the ability to estimate shear acoustic curves when no measurements have been made.

Conclusions – Part 2

• Saturation calculations from porosity logs show a varied invasion profile. In the general case, the following pattern is observed:

Lowest SW

ResistivityNeutron – reflecting the changing depth of investigationDensity - reflecting the changing depth of investigationSonic – reflecting the changing depth of investigation

Highest SW

• Pseudo resistivities logs frequently show better bed definition than resistivity logs (especially induction) due to better vertical resolution.

• The analytic techniques, in combination, show whether or not the raw measurements are consistent with the deterministic interpretational model. Any inconsistenency in the model, and/or bad/miscalibrated raw data will be reflected in impossible output – such as porosity log saturations less than zero, or unrealistic values of calculated fluid properties.

Increasing

The endHave a nice day!