PROJ00326

• Contract/PO #: C153498
• Contractor Name: U. S. Geological Survey
• Water District: 0
• County: Boulder
• Bill Number: SB 87-15
• Loan Projects - Doc Type: Contract Documents

but cannot always compensate for rugosity due to
washouts, which can cause abrupt changes in diameter
or an irregular shape in the borehole.

The poorly to moderately consolidated forma-
tions in the Denver basin are easily eroded by bento-
nitic drilling fluids, and washouts can be common,
particularly in mudstone or similar fine-grained units.
Forty-six core samples correlated to intervals in
well USGS where logs indicated washouts were
present. About 83 percent of these samples were mud-
stone. The mean specific yield of these mudstone sam-
ples was 0.03. Appropriate drilling techniques and
control of drilling-fluid characteristics can be used to
decrease the frequency of washouts. The erroneous log
bulk density and porosity associated with washout
intervals precludes use of these data in estimating spe-
cific yield. However, the specific yield of fine-grained
units is small and can be assumed to equal zero. By use
of this assumption, washouts in fine-grained materials
generally will not adversely affect the mean specific
yield calculated for a water-yielding interval.

0,5

Porosity Determination

Porosity determined from density porosity logs
from holes CI and CIA is a good measure of porosity
of the formation as indicated by the correlation oflog
and core porosity data in figure 5. The scatter of data
along the solid line, depicting a I to I relation, is due to
differences in log and core values. Data from washout
intervals have been excluded from figure 5, so some
differences are associated with stochastic and grain-
density errors in the logs. Errors associated with the
core porosity include errors in laboratory techniques or
measurements, errors in correlation of core depths to
log depths, and differences in sample volumes. Lab-
oratory porosity was determined from sample volumes
of about 20 cm3; log porosity was determined from
sample volumes of about 10,000 cm3. In nonhomoge-
neous materials, this difference in sample volume can
produce siguificant differences between porosity
determined from core and logs.

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:t ONE STANDARD ERROR OF ESTIMATE

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DENSITY POROSITY FROM LOGS
Figure 5. Relation of porosity from core samples and density porosity logs for holes Cl and Cl A.

12 Techniques for Estimating Speclllc Ylald and Speclllc Retention from Graln-Slze Data and Geophysical Logs from
Clastic Bedrock Aquifers