day 1 borehole geophysics w

7/21/2019 Day 1 Borehole Geophysics W

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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

day 1 borehole geophysics w

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