well-logging lab no. 6 - porosity logs

play a part in the variation, especially since, in thiscase, the data were used directly from the logs with noattempt at environmental corrections. Formation thick-ness and, hence, the effect of adjacent beds on themeasurement in the target bed also play a part.

The variability of the formations themselves proba-bly has a more significant effect. In a geologic envi-ronment like this one (Figure 4.21), it can be expectedthat the lithologies encountered are not always pure.The response of the individual measurements to thatmixture vary between measurements and contribute tothe variation seen here.

The results of the crossplots, while not wildly vari-able, underscores the need for other data, in this case,samples of the formations themselves. Cores throughthe section would be ideal, but their cost would prob-ably be prohibitive. Cuttings samples would be a goodsource of lithology informaton, even given the poten-tial mixing and contamination problems. Sidewallcores, either in this well (if the log analysis could becompleted quickly) or in subsequent wells in zoneswith questionable predicted lithology, would help toreconcile the results shown here.

REVIEW

1. The three common types of porosity logs are: sonic density neutron

2. The sonic log is a porosity log that measures theinterval transit time (delta t, or DT, or t) of a com-

pressional sound wave through the formation. The unitof measure is microseconds per foot (sec/ft) ormicroseconds per meter (sec/m). Interval transit timeis related to formation porosity.

3. The density log is a porosity log that measuresthe electron density of a formation. The formationselectron density is related to a formations bulk densi-ty (RHOB or b). Bulk density is measured in g/cm3 orKg/m3. Bulk density, in turn, can be related to forma-tion porosity.

4. Density logs made since about 1980 also have aphotoelectric curve (Pe, PE, or PEF) that responds pri-marily to formation lithology and is affected in only aminor way by porosity and the type of fluids in thepore space.

5. The neutron log is a porosity log that measuresthe hydrogen concentration in a formation. In shale-free formations where porosity is filled with water, theneutron log can be related to water-filled porosity(PHIN, or NPHI, or N).

6. In gas reservoirs, the neutron log records a lowerporosity than the formations true porosity because gashas a lower hydrogen concentration than oil or water(gas effect).

7. The neutron-density combination is the mostwidely used porosity measurement combination.Porosity, adjusted for lithology, can be determinedfrom the neutron-density combination either by acrossplot chart or by formula.

8. Additional uses of the neutron-density combina-tion are:

detection of gas bearing zones determination of lithology

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Porosity Logs 59

Figure 4.3. Example density log withgamma ray and caliper.

This example is shown to illustrate acommon presentation format for a densitylog, and to be used in determining aporosity (DPHI) from bulk density (RHOB)in Figure 4.4.

Track 1: This track includes both thegamma ray (GR) and caliper (CALI) curves.Both scales increase from left to right. Thegamma ray values range from 0 to 100API gamma ray units, and the calipermeasures the borehole diameter from 6 to 16 inches.

Track 2: The density log correction curve(DRHO) ranges in value from 0.05g/cm3 to +0.45 g/cm3 in increments of0.05 g/cm3. It is shown as a light, dashedline. The photoelectric effect curve (PEF)ranges from 0 to 10 b/e and is shown asa heavy, dashed line. (The dashes on thePEF curve are longer than those on theDRHO curve.)

Tracks 2 and 3: The bulk-density (RHOB)scale ranges in value from 2.0 g/cm3 to3.0 g/cm3 and the curve is a solid line.

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Table 4.7. Values to be used with the chart in Figure 4.4 to determine the density porosity, DPHI.

DepthRaw Data SPHI (Wyllie) SPHI (RHG) DPHI NPHI

DT RHOB PE NPHI Lime Dolo Lime Dolo Lime Dolo Dolo Sand11,508 51 0.024 0.051 0.053 0.11011,522 47 2.75 0.000 0.025 0.000 0.060

11,545 57 0.067 0.092 0.125 0.17011,560 48 0.005 0.031 0.010 0.073

11,593 50 0.017 0.045 0.040 0.09811,615 51 0.024 0.051 0.053 0.01011,631 67 2.50 0.138 0.163 0.203 0.23511,645 52 0.032 0.059 0.070 0.12211,655 57 2.64 0.067 0.092 0.125 0.17011,665 52 2.68 0.032 0.059 0.070 0.12211,696 50 0.017 0.045 0.040 0.098

Table 4.8. Determination of density porosity.

DepthRaw Data SPHI (Wyllie) SPHI (RHG) DPHI NPHI

DT RHOB PE NPHI Lime Dolo Lime Dolo Lime Dolo Dolo Sand11,508 51 2.73 0.024 0.051 0.053 0.110 -0.013 0.078 11,522 47 2.75 0.000 0.025 0.000 0.060 -0.022 0.068

11,545 57 2.67 0.067 0.092 0.125 0.170 0.022 0.11011,560 48 2.96 0.005 0.031 0.010 0.073 < 0 < 0

11,593 50 2.70 0.017 0.045 0.040 0.098 0.005 0.09511,615 51 2.97 0.024 0.051 0.053 0.010 < 0 < 011,631 67 2.50 0.138 0.163 0.203 0.235 0.125 0.20011,645 52 2.82 0.032 0.059 0.070 0.122 < 0 0.03011,655 57 2.64 0.067 0.092 0.125 0.170 0.042 0.12511,665 52 2.68 0.032 0.059 0.070 0.122 0.020 0.105 11,696 50 2.76 0.017 0.045 0.040 0.098 -0.028 0.063

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Figure 4.4. Chart for converting bulk density (b, RHOB) to density porosity (D, DPHI). Procedure:

1. Find the bulk density (RHOB) taken from the density log in Figure 4.3 on the scale at the bottom of the chart. Note that the scale is displayed from high values on the left to low valueson the right.

2. Follow the RHOB value vertically until it intersects the proper matrix line [calcite (limestone) or dolomite] then move horizontally to intersect the y-axis representing the proper fluiddensity, in this case 1.00 g/cm3 (fresh water, the middle scale) to read the porosity (DPHI).

NOTE: The values in Table 4.8 are decimal fractions, the porosity values on the chart are in percent.

Courtesy Halliburton Energy Services, 1994 Halliburton Energy Services

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well-logging lab no. 6 - porosity logs

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