BOREHOLE GEOPHYSICAL METHODS

The simplified version of the lecture by Carole D. Johnson

USGS, Office of Ground WaterBranch of Geophysics

Storrs, Connecticut

BOREHOLE GEOPHYSICAL METHODS

• Characterize:– Borehole conditions – Formation – Fractures – Fluid

• Tools:– Conventional– Borehole-Wall Imaging and deviation– Flowmeter– Radar Methods– Tomography

• Hydrologic Testing

Conventional Borehole Methods

• Caliper• Gamma• Temperature• Fluid resistivity• Formation resistivity• Electric and electromagnetic

Geophysical Logs

• Continuous• Unbiased• High resolution• Synergistic

EM CONDUCTIVITY (mS/m)0 50 100 150

0

8

6

4

2

DEP

TH (

MET

ERS)

Background

Contaminated

Conventional screened interval

CALIPER– Diameter of borehole

– Changes related to drilling and well construction, lithology, fractures, and solution zones

– Mechanical caliper - three arm averaging

– Acoustic caliper – calculated from acoustic transit time and velocity

Mechanical3-arm averaging

caliper

Caliper

Conventional Logs

Well MW-69Norwalk, Connecticut

20

40

60

Depth(Meters)

GAMMA LOGS– Natural emission (cps or API units)

– Uranium, thorium, and potassium 40

– Clay content, alteration, and mineralogy, stratigraphic correlation

– Vertical resolution 1 to 2 ft– Used in:

– Air , water, or mud filled; – Open, PVC or steel cased

Dep

thGamma

Counts per secondLithology

Sand andGravel

Silt andClay

Water Table

Spectral Gamma – Counts gamma emissions (0.1-3 MeV)– Can be used in air , water, or mud filled;

open, PVC or steel cased borheoles– High vertical resolution– Identify uranium, thorium, and potassium 40

sources for: –Structural and stratigraphic correlation –Water-rock interaction studies–Clay content and mineralogy

GAMMA SPECTRAL

Measurement at a fixed depthENERGY

CO

UN

TS P

ER

SE

CO

ND

• Continuous profile in the borehole• Indicates potential inflow or outflow zones• Gradually increases with geothermal

gradient usually 1oF per 100 feet• Sometimes shown as del Temp, which is

the first derivative of the temp. plot –helps identify changes in slope of the line

TEMPERATURE

Deg F

Temperature

Well MW-69Norwalk, Connecticut

20

40

60

Depth(Meters)

• Fluid resistivity • Single-Point Resistance• Long and Short Normal Resistivity• Electromagnetic Induction

RESISTIVITY LOGS

DE

PTH

IN F

EE

T

TEMPERATURE49 (DEG F) 53

GAMMA0 (CPS) 500

ATVN E S W N

OTVN E S W N

SPECIFIC CONDUCTANCE300 (uS/cm) 700

FORMATION CONDUCTIVITY

0 (mS/m) 250MW105R

MECHANICAL CALIPER

5.7 (INCHES) 6.6

FM Fluid Physical characteristics

Hydraulic Tests

Mixing Law

• σtotal = [(1- φ) * σrock] + [φ * σ fluid],

• φ, porosity in fractured rock

• σ rock conductivity, 1 – 2 mS/m

• σ fluid conductivity, 125 mS/m•

• Fluid Resistivity– Electrical resistivity of borehole fluid– Dissolved solids content of borehole water

Specific Conductance, SC

SC uS/cm = 10,000 * 1FL Res (ohm-m)

• Fluid Resistivity– Electrical resistivity of borehole fluid– Dissolved solids content of borehole water

– Compensate to 25 deg C with temperature logSPEC COND uS/cm @ 25 deg C

= 10000/((1 + 0.022 * (((5 * (TEMP deg F - 32))/9) - 25)) * FL RES ohm-m)

Specific Conductance, SC

SC uS/cm = 10,000 * 1FL Res (ohm-m)

• Fluid Resistivity– TDS mg/L =

0.65 * SPEC COND uS/cm @ 25 deg C– Dissolved solids of borehole fluid NOT formation– Interpret with flowmeter and point sample data– Ambient and stressed

CaliperTemperatureFluid

Conductivity

Fluid and Caliper Logs

Well MW-69Norwalk, Connecticut

20

40

60

Depth(Meters)

FLUID LOGGING - DIFFERENCINGCALIPER TEMPERATURE FLUID

RESISTIVITY

Ambient

Pumping

• Single-Point Resistance– Electrical resistance between a surface electrode

and a downhole electrode – Clay content, porosity,

and dissolved solids– Resistance(ohms)

NOT resistivity(ohms-m)– Affected by borehole fluid

and diameter– Water or mud filled

open hole – High vertical resolution

Single-Point Resistance

Depth of Investigation

DUAL INDUCTION

Oil Patch Large-Diameter Tool for Shallow to Deep Induction

DUAL INDUCTION

Slimhole Tool for Medium and Deep EM Conductivity

CALIBRATION MSI DIL45-NG

~ 6.5 ft

~ To

ol le

n gth

2.7

5 m

or

9ft

ILM 1980 mS/m and ILD 582 mS/m

Dep

thGamma

EM Conductivity Lithology

Sand andGravel

Silt andClay

Water Table

EM Conductivity, in mS/m Gamma, in cps

Dep

th, i

n M

eter

s0

0

4

8

16

20

12

40 80 0 40 80 120

Dep

thEM

ConductivityGamma Lithology and Water Quality

Sand andGravel

Silt andClay

Water Table

Zone withelectricallyconductivecontamination

EM Conductivity, in mS/m Gamma, in cpsD

epth

, in

met

ers

30

45

40

50

55

35

0 40 80 0 40 80 120

GAMMA and EM Signatures

• Lithology / stratigraphy• Contamination• Radioactive signatures• Saline intrusions• Well construction

100

20

120

60

40

80

0

EM Resistivity ohm-m1000 200

Limestone Shells

Limestone

Limestone Shells Sand

FW Limestone Limestone Shells Sand

Sand Shells

EM Conductivity mS/m

75000 15000

0

20

40

60

80

100

120

Spec Cond uS/cm

1500 300

DADE COUNTY SALT WATER INTRUSION PROJECT, SOUTH FLORIDA

BOREHOLE GEOPHYSICAL METHODS

• Characterize:– Borehole conditions – Formation – Fractures – Fluid

• Tools:– Conventional–– BoreholeBorehole--Wall Imaging and deviationWall Imaging and deviation– Flowmeter– Radar Methods– Tomography

• Hydrologic Testing

BOREHOLE-WALL IMAGING

• Submersible cameras• Acoustic and optical televiewers• Continuous and oriented 360-degree

image of the borehole wall• Character and orientation of lithologic

and structural features• Borehole diameter, construction, and

deviation

Projected image3-D wrapped image

NW

E

SOUTH

Amplitude

N NS WE

Dipo = tan -1 amplitudediameter

Strike = (175 - 90)o = 85o

N E S W N

BOREHOLE-WALL IMAGING

Strike and DipStrike and Dip

• Dip Azimuth and dip: • N1N1--360360oo 00--9090oo

• Strike in RHR, and dip: • N1N1--360360ooE, 0E, 0--9090oo

• Northern Quadrant:• N0N0--9090oo(E or W), 0(E or W), 0--9090oo (N or S)(N or S)

NW

E

SOUTH

OPTICAL TELEVIEWER

• CCD camera • Conical or hyperbolic reflector• High resolution (RGB)• Uphole or downhole digital• Downhole digital slower speeds but greater

range of cable types and lengths• Water- or air-filled holes• Less affected by water clarity than fisheye,

but more sensitive than ATV’s • Direct viewing of lithology and structure

DE

PTH

BE

LOW

TO

P O

F C

AS

ING

IN M

ETE

RS

N WE NSN WE NS

31

32

33

OPTICALTELEVIEWER

In Color

6-in diameter borehole in sandstone

Uphole digitizedConical mirror

Downhole digitalConical mirror

Uphole digitalHyperbolic mirror

N WE NS

ACOUSTIC TELEVIEWER

• Ultrasound (1+ MHz) pulse-echo system• Acoustic amplitude and transit time• Acoustic caliper• Borehole enlargements related to lithology and structure• Water- or light mud-filled holes• Analog – oscilloscope and Polaroid's• Digitized downhole• Rotating transducer• Fixed transducer bounce acoustic beam off rotating

reflector– Higher resolution– Greater range of diameters (up to ~20”)

Focused Acoustic Beam

Focused acoustic signal

Fixed head, piezo-electric element –“shoots” upward and the rotating mirror reflects it towards the borehole wall

RotatingMirror Acoustic

window

Borehole wall

ACOUSTIC TELEVIEWER

Acoustic Caliper

Travel time and

Amplitude

DEP

TH IN

FEE

T B

ELO

W T

OP

OF

CA

SIN

G

28.5

N E S W N N E S W N

28.0

SECOND THIRDPLOTS OF ARRIVAL TIME

DEP

TH B

ELO

W T

OP

OF

CA

SIN

G IN

MET

ERS MULTI-ECHO ATV

Single trace showing first, second, and third arrivals

Tool

Radial plot of arrival times at a single depth

Tool

COMPARISON OF ACOUSTIC AND OPTICAL TELEVIEWER

IMAGES• ATV and OTV - fractures generally

recognizable• OTV – direct viewing of the relation between

lithology and structure• OTV – view redox conditions, free product,

and other indicators of flow and contamination

• ATV – resolution of fractures under a wider range of conditions (dark-colored rocks,

cloudy water, or coated borehole walls)

N WE NSN WE NS

DEP

TH B

ELO

W T

OP

OF

CA

SIN

G, I

N M

ETER

S

Acoustic Optical

61.5

62.0

ACOUSTICand OPTICALTELEVIEWER

3-in diameter borehole in sandstone

COMMON PROBLEMS

• Centralization• Deviation • Magnetic minerals and steel casing

DE

PTH

BE

LOW

TO

P O

F C

AS

ING

IN M

ETE

RS

31

32

33

If the tool is de-centered, the ATV data will have

characteristic banding and a trace laid on top of the

features not be a sinusoid.

OTV

N WE NS N WE NS

ATV

Centralization

BOREHOLE DEVIATION

Because the tool is centralized and slidesparallel to the axis of the borehole, it considers the hole to be plumb.Boreholes are rarely plumb,and you need to correctthe strike and dip ofmapped features.

100

200

100 ft depth

200

0

400

600

800

0 100 200 Distance (feet)

Depth (feet)

N

Borehole Deviation

6.0

6.5

7.0

7.5

8.0

DEP

TH B

ELO

W T

OP

OF

CAS

ING

, IN

MET

ERS

N WE NS N WE NS

Spliced at 7.5 m

Correctedto feature at 8.0 m depth

Bottom of casing at 6 m

Mag. Off and Spliced

Mag. On

ANALYSIS OF ACOUSTIC AND OPTICAL TELEVIEWER IMAGES

• Independent analysis produces data sets with multiple discrepancies

• Combined analysis most efficient and powerful• Integration with other data

– Geophysical logs – flowmeter– Core samples – typically not oriented and missing in

fractured intervals– Hydraulic measurements – packer placement

• Tadpole and stereo plots

Projected image3-D wrapped image

NW

E

SOUTH

Amplitude

N NS WE

Dipo = tan -1 amplitudediameter

Strike = (175 - 90)o = 85o

N E S W N

BOREHOLE-WALL IMAGING

Lower HemisphereStereoplot

N35E, 80North

Tadpole Plot

10

20

30

40

Dip Direction(Strike= + 90o)

Dip (degrees)0 45 90

Dep

th in

ft o

r m

Side by side comparisons, interpretations, and display data

StereographicProjection

Tadpole Plot

ProjectionPlot

N E S W N 0 30 60 90N E S W N

Image Plot

Bedding and Fractures in Sandstone

BeddingBedding

FracturesFractures

NFracturesFractures

Combined interpretation - after hydraulic logging

ALL FEATURES

FOLIATION and LITHOLOGY Shown for different depth rangesOf the borehole

FRACTURES

DE

PTH

IN F

EE

T B

ELO

W T

OP

OF

CA

SIN

G

Optical Televiewer Imaging Of Fractures

Well MW-69Norwalk, Connecticut

SUMMARY

• Multiple logs used together - Synergistic- Integrated- Iterative