Changes

| Charts tend to overcorrect in elliptical boreholes

| Charts tend to overcorrect in elliptical boreholes

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The logging service company chart books contain correction charts for most logging tools and instructions for their use. Users of well log data should become familiar with these so that they can either apply the corrections or recognize the environmental conditions where corrections are not significant. Table 1 gives a summary of environmental corrections for common logging tools emphasizing the conditions for which corrections can be large. Table 1 also contains a partial list of recent service company charts used for typical corrections<ref name=pt04r19>Welex, 1985, Welex log interpretation charts: Houston, TX.</ref><ref name=pt04r4>Dyos, C. J., 1987, Inversion of induction log data by the method of maximum entropy: SPWLA 28th Annual Logging Symposium Transactions, Paper T.</ref>a, b; <ref name=pt04r20>Western Atlas International, 1985, Atlas log interpretation charts: Houston, TX.</ref>. Older tools may require the use of older chart books. Slim hole models available for some tools may require their own specific charts. The user must become familiar with log headings to identify tool models so that the correct chart can be selected.

The logging service company chart books contain correction charts for most logging tools and instructions for their use. Users of well log data should become familiar with these so that they can either apply the corrections or recognize the environmental conditions where corrections are not significant. Table 1 gives a summary of environmental corrections for common logging tools emphasizing the conditions for which corrections can be large. Table 1 also contains a partial list of recent service company charts used for typical corrections<ref name=pt04r19>Welex, 1985, Welex log interpretation charts: Houston, TX.</ref><ref name=pt04r4>Dyos, C. J., 1987, Inversion of induction log data by the method of maximum entropy: SPWLA 28th Annual Logging Symposium Transactions, Paper T.</ref>a, b; <ref name=pt04r20>Western Atlas International, 1985, Atlas log interpretation charts: Houston, TX.</ref>. Older tools may require the use of older chart books. Slim hole models available for some tools may require their own specific charts. The user must become familiar with log headings to identify tool models so that the correct chart can be selected.

Environmental corrections are often performed serially. For example, Figure 1 is a flowchart illustrating the steps used to determine true resistivity from a dual induction log. It should be noted that shoulder bed effect corrections are difficult and require complex algorithms<ref name=pt04r4 />. Also, it is possible that not performing all corrections (e.g., invasion correction without borehole corrections) may be worse than making no corrections at all.

Environmental corrections are often performed serially. For example, [[:file:preprocessing-of-logging-data_fig1.png|Figure 1]] is a flowchart illustrating the steps used to determine true resistivity from a dual induction log. It should be noted that shoulder bed effect corrections are difficult and require complex algorithms<ref name=pt04r4 />. Also, it is possible that not performing all corrections (e.g., invasion correction without borehole corrections) may be worse than making no corrections at all.

 

The SP curve presents special environmental problems that are not addressed by charts. Since the SP responds in part to the mud filtrate resistivity, no two SP curves will be recorded under exactly the same environment (Bateman, 1985). (For more information on SP logs, see the chapters on “[[Basic open hole tools]]” and “Determination of Water Resistivity” in Part 4.)

The SP curve presents special environmental problems that are not addressed by charts. Since the SP responds in part to the mud filtrate resistivity, no two SP curves will be recorded under exactly the same environment (Bateman, 1985). (For more information on SP logs, see the chapters on “[[Basic open hole tools]]” and “Determination of Water Resistivity” in Part 4.)