Post on 04-Feb-2018
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Introduction to
Borehole geophysics
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Borehole geophysics is the science of
recordingand analyzingmeasurements of
physical properties made in wells or testholes
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Borehole geophysics
MEASUREMENTS
ro!es" measure different properties are
lowered into the !orehole to collect
continuous or point data that is graphically
displayed as a geophysical log
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Borehole geophysics
INTER#ATE$ #E%&'SIA( (%##IN#Multiple logs are collected
A suite of logs as a group than !y the analysis ofthe same logs indi)idually
Borehole geophysics is used in ground*water and
en)ironmental in)estigationsto o!tain information
on well construction+ roc, lithology and fractures+permea!ility and porosity+ and water -uality
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Borehole geophysics
Probes+ Cable Drawworks+ Powerand processing
modules Data recordingunits. State*of*the*art logging
systems are controlled !ya computer and cancollect multiple logs withone pass of the pro!e.
#E%&'SI/A( (%##IN# S'STEM /onsists of "
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Some objectives of geophysical
well logging
012 Identification of lithology and stratigraphic
correlation032 Measuring porosity+ permea!ility+ !ul, density+
and elastic properties
042 /haracterizing fractures and secondary porosity
052 $etermining water -uality062 Identifying contaminant plumes
072 8erifying well construction
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Benefits of logging
The main o!9ecti)e of !orehole geophysics is to o!tainmore information a!out the
su!surface than can !e o!tained from drilling+ sampling+
and testing Although drilling a test hole or well is an
e:pensi)e procedure+ it pro)ides access to thesu!surface
where )ertical profiles or records of many different ,inds of data can !e ac-uired.
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continuous analog or digital records of
in situ properties of soils and rocks, their
contained fluids, and well construction.
(ogs may !e interpreted in terms of" lithology, thickness, and continuity of
aquifers and confining beds, permeability,
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porosity+ !ul, density+ resisti)ity+ moisture content+ and specific yield;
and the source+ mo)ement+ chemical and physical characteristics ofgroundwater and the integrity
of well construction
(og data are repeata!le o)er a long period of time+ andcompara!le+ e)en when measured with different e-uipment
Repeata!ility and compara!ility pro)ide the !asis for measuringchanges in a groundwater system with time. /hanges in ana-uifer matri:+ such as in porosity !y plugging+ or changes inwater -uality+ such as in salinity or temperature+ may !e recorded
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Some pro!es record data from roc, !eyond that
distur!ed !y the drilling process
Samples provide
oint data from la!oratory analysis
Borehole logs provide Usually continuous data+ and can !e analyzed in
real time at the well site to guide completion or
testing procedures
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It may pro)ide information on some characteristic not recognized
at the time of geophysical logging $ata from geophysical logs are useful in the de)elopment of digital
models of a-uifers and in the design of groundwater+ supply+recharge+ or disposal systems
A log analyst with the proper !ac,ground data on the area !eingstudied can pro)ide reasona!le estimates of hydraulic propertiesneeded for these purposes
Stratigraphic correlation is a common use of geophysical logs; logsalso permit the lateral e:trapolation of -uantitati)e data from test orcore holes. Using logs+ a data point in a well can !e e:tended inthree dimensions to increase its )alue greatly
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Many techniquesused in surface geophysicsare related closely to techni-ues in borehole
geophysics+ and the two are considered together
when setting up comprehensi)e groundwater+
en)ironmental+ or engineering in)estigations #eophysical logs+ such as acoustic*)elocity and
resisti)ity+ can pro)ide detailed profiles of data that
are useful in interpreting surface sur)eys+ such as
seismic and resisti)ity sur)eys
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Limitations of logging
#eophysical logging cannot replace sampling
completely+ !ecause some information isneeded on each new area to aid log analysis
A log analyst cannot e)aluate a suite of logs
properly without information on the localgeology
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ogs do not have a unique response! for e:ample+ high gamma radiation from
shale is indistinguisha!le from that produced!y granite.
"o ma#imi$e results from logs,
at least one core hole should !e drilled in each depositional !asin or uni-ue a-uifer
system.
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Calibration of logscarried out in one
roc, type may not !e )alid in other roc,
types !ecause of the effect of chemicalcomposition of the roc, matri:.
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Planning a logging program A logging program must !e properly planned to !e of
ma:imum !enefit. Borehole geophysics is fre-uently
applied to en)ironmental in)estigations+ such ashydrogeology to aid site selection+ monitoring+ determiningwell construction+ and planning remediation. In planning alogging program for en)ironmental applications
one of the most difficult -uestions to answer is what
geophysical logs will pro)ide the most information for thfunds a)aila!le. There are se)eral important steps in the
decision*ma,ing process.
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Log analysis
In recent years+ computer techniquesha)e dominatedlog analysis; howe)er+ this de)elopment has not changedthe !asic re-uirements for getting the most information
from logs.
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=uantitati)e analysis
Bed "hicknesses Density Porosity
&olume estimations 'ater Saturation (esistivity etc
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)nother important technique
for log analysis
Cross Plot
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Summary
Basic Units of (ogging rogram
lanning
$ata Ac-uisition
rocessing
Interpretation $esired Results