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Reservoir Surveillance via Reservoir Surveillance via Modern Gravity Methods Modern Gravity Methods

by: Jerry Brady by: Jerry Brady

Other Contributors: Other Contributors: John F. Ferguson John F. Ferguson Carlos L. V. Aiken Carlos L. V. Aiken John E. Seibert John E. Seibert Jennifer L. Hare Jennifer L. Hare Tianyou Tianyou Chen Chen

Fred J. Fred J. Klopping Klopping Tim Tim Niebauer Niebauer John M. Brown John M. Brown Leon Thomsen Leon Thomsen Kurt Kurt Strack Strack

Reservoir Surveillance via Modern Reservoir Surveillance via Modern Gravity Methods Gravity Methods

Topics Topics uIntroduction to gravity u uSurface Gravity Surface Gravity u uBorehole Gravity Borehole Gravity u uPermanent Gravity Sensor Development Permanent Gravity Sensor Development

Gravity Methods Gravity Methods

Introduction: Introduction: u u Not Not API gravity API gravity u u “ “g g” ” = acceleration of gravity = acceleration of gravity u u Deep reading density tool Deep reading density tool u u Monitoring fluid saturation changes Monitoring fluid saturation changes

Definitions Definitions microGal microGal = ? = ?

“ “g g” ” = earth = earth’ ’s acceleration of gravity s acceleration of gravity ≅ ≅ 1000 1000 cm/sec cm/sec 2 2 = 1000 Gals = 1000 Gals

1 Gal = 1 Gal = 1 1 cm/sec cm/sec 2 2

Gravity meter technical limits: Gravity meter technical limits: microGal microGal = = µ µGal = 10 Gal = 10 ­ ­6 6 cm/sec cm/sec 2 2

= "g = "g” ” x 10 x 10 ­ ­9 9 ≅ ≅ “ “ng ng” ” (nano (nano­ ­gravity) gravity)

Surface gravity response due to water injection: Surface gravity response due to water injection: = 10 = 10 ­ ­4 4 cm/sec cm/sec 2 2 = 100s of = 100s of µ µGals Gals

Gravity Example Gravity Example

20 mg house fly lands on a 20 ton whale (1.814 x10 20 mg house fly lands on a 20 ton whale (1.814 x10 11 11 milligrams) = 1 milligrams) = 1 µ µGal Gal L. A. Beyer; Borehole Gravity Surveys, SEG Continuing Education

Gravity Meter Types Gravity Meter Types

u u Relative Relative – – “ “weighs weighs” ” a test mass at two different locations a test mass at two different locations

u u Absolute Absolute – – Free fall interferometric Free fall interferometric

Basic Basic LaCoste LaCoste & Romberg Relative & Romberg Relative Geodetic Gravity Meter Geodetic Gravity Meter

The Micro The Micro­ ­g g’ ’s FG5 Absolute Sensor s FG5 Absolute Sensor

© Copyright KMS Technologies 2000

Relative Gravity & Relative Gravity & Absolute Gravity Meters Absolute Gravity Meters

Absolute gravity meter

Relative gravity meter

Reservoir Surveillance via Modern Reservoir Surveillance via Modern Gravity Methods Gravity Methods

Topics Topics u u Introduction to gravity Introduction to gravity u uSurface Gravity Surface Gravity u uBorehole Gravity Borehole Gravity u uPermanent Gravity Sensor Development Permanent Gravity Sensor Development

Prudhoe Gravity Station Layout Prudhoe Gravity Station Layout

u u Water injection creates a positive gravity anomaly Water injection creates a positive gravity anomaly u u Perform baseline gravity survey Perform baseline gravity survey u u Repeat gravity survey Repeat gravity survey u u 4D gravity signal = gravity after water injection 4D gravity signal = gravity after water injection ­ ­ baseline baseline gravity gravity

u u Perform gravity inversion for reservoir density model Perform gravity inversion for reservoir density model u u Reservoir density = water movement Reservoir density = water movement u u Mass balance Mass balance

Overview of Water Monitoring with Overview of Water Monitoring with 4D Gravity Method 4D Gravity Method

Surface Gravity Survey Requirements Surface Gravity Survey Requirements

Requirements Requirements u u High High accuracy accuracy elevation data elevation data

– – Global Positioning System Global Positioning System u u High High precision precision surface gravity surface gravity

– – Relative gravity meters Relative gravity meters – – Absolute gravity meters Absolute gravity meters

u u Economic Economic surveys surveys – – 15 15 – – 20 minute station readings for relative meters 20 minute station readings for relative meters – – 60 minutes station reading for absolute meters 60 minutes station reading for absolute meters

g S = 0.2

S g = 0.4

S g = 0.6

Residual water saturation = 0.32; f = 0.22, r ma = 2.65 g/cm 3 , r o = 0.762 g/cm 3 , r w = 0.986 g/cm 3 , r g = 0.198 g/cm 3 . Dots correspond to gas saturations of 20%, 40% & 60%

Prudhoe: Change in Bulk Density Prudhoe: Change in Bulk Density

Fraction of oil replaced

by water or gas

After Brady, 1998

2.16

2.18

2.20

2.22

2.24

2.26

2.28

2.30

0.0 0.2 0.4 0.6 0.8 1.0

Bulk D

ensity

(g/

cm 3 )

Fraction of Oil Swept

Gas

Water

4D Gravity Modeling 4D Gravity Modeling General Procedure General Procedure u u Reservoir density model Reservoir density model u u Forward modeling of surface gravity Forward modeling of surface gravity u u Survey noise Survey noise u u Inverse modeling Inverse modeling

Reservoir Density Model (6.5 yrs) Reservoir Density Model (6.5 yrs)

Gravity Inversion Vs Density Model Gravity Inversion Vs Density Model Average Flood Front Average Flood Front

Gravity Inversion Vs Density Model Gravity Inversion Vs Density Model Flood Front Leading Edge Flood Front Leading Edge

Gravity Inversion Cross Section Gravity Inversion Cross Section

Gravity Inversion Cross Section Gravity Inversion Cross Section

Mass Balance Mass Balance Percent Water Recoverable by Gravity Inversion Percent Water Recoverable by Gravity Inversion

Years of water injection Years of water injection 5 yrs 5 yrs 15 yrs 15 yrs 30 yrs 30 yrs

Noise Free Noise Free 99% 99% 100% 100% 100% 100% 5 5 µ µGal uncorrelated noise Gal uncorrelated noise 99% 99% 100% 100% 98% 98% 10 10 µ µGal uncorrelated noise Gal uncorrelated noise 95% 95% 99% 99% 99% 99% 5 5 µ µGal correlated noise Gal correlated noise 101% 101% 101% 101% 101% 101% 10 10 µ µGal correlated noise Gal correlated noise 126% 126% 110% 110% 110% 110%

Permanent GPS Base Stations Permanent GPS Base Stations

Relative Gravity Meter with GPS Receiver Relative Gravity Meter with GPS Receiver

Ionosphere Interferes with GPS Ionosphere Interferes with GPS

Preliminary GPS Results Preliminary GPS Results

2002 Gravity Survey 2002 Gravity Survey

Absolute Gravity Meters Absolute Gravity Meters Total Stations Total Stations 205 205 Total Observations Total Observations 224 224 Total Meter Days Total Meter Days 32 32 Gravity STD Gravity STD of repeats of repeats3.5 3.5 microGals microGals

Gravity Differences with Time Gravity Differences with Time 2000, 2001 & 2002 2000, 2001 & 2002

Gravity Tests Line Differences w Time All Lines

­100

0

100

200

300

0 10 20 30 40 50

Distance (1000 ft)

Gravity Difference

(microGals) 2001 ­ 2000

2002 ­ 2001 2002 ­ 2000 2002 ­ 2000 5 yr Grav 20 yr Grav Max Grav

GCWI Surface Gravity Conclusions GCWI Surface Gravity Conclusions

1. 1. General water front shape and movement can be General water front shape and movement can be monitored with the surface gravity technique monitored with the surface gravity technique

2. 2. Mass balance techniques can account for more Mass balance techniques can account for more than 95% of the water added. than 95% of the water added.

3. 3. Inversion modeling can determine the average Inversion modeling can determine the average waterflood front and a reasonable value for the waterflood front and a reasonable value for the leading edge of the waterflood front. leading edge of the waterflood front.

Requirements for 4D Gravity Success Requirements for 4D Gravity Success 1. 1. Density Density contrast contrast

– – Prudhoe max density contrast = 0.11 g/cc Prudhoe max density contrast = 0.11 g/cc

– – Gas / fluid usually the best Gas / fluid usually the best

2. 2. Depth Depth of burial of burial

– – Prudhoe depth = 8200 Prudhoe depth = 8200’ ’ – – 8800 8800’ ’ tvd tvd

– – Best lateral sensitivity = 1/3 depth of burial Best lateral sensitivity = 1/3 depth of burial

3. 3. Zone Zone thickness thickness

– – Prudhoe thickness = 100 Prudhoe thickness = 100’ ’ – – 500 500’ ’

Reservoir Surveillance via Modern Reservoir Surveillance via Modern Gravity Methods Gravity Methods

Topics Topics u u Introduction to gravity Introduction to gravity u uSurface Gravity Surface Gravity u uBorehole Gravity Borehole Gravity u uPermanent Gravity Sensor Development Permanent Gravity Sensor Development

Conducting a BHGM Survey Conducting a BHGM Survey

∆g= F ∆z ­4πGρ∆z

• gravity increases downward: • by the difference between the free­air gradient

F = which is essentially a constant F∆z = is the increase in gravity downward caused by closer approach to the center of mass of the Earth

• gradient of opposite sign (4πGρ) • varies as the density of the adjacent rocks change..

Fundamental Equation of Borehole Gravity Fundamental Equation of Borehole Gravity

Borehole Gravity Used for Reservoir Borehole Gravity Used for Reservoir Surveillance Surveillance

Problems with Current Surveillance Tools: Problems with Current Surveillance Tools: – – Shallow reading devices Shallow reading devices

• • Straddles Straddles • • Scab liners Scab liners • • Gas channels Gas channels • • Gas cones Gas cones • • Perforations Perforations

BHGM densities for 3 years BHGM densities for 3 years

Repeat BHGM across Rabbi field gas/oil contact in Gabon.

Porosity: 24% Gas ρ : 0.082 g/cc Oil ρ : 0.780 g/cc

After Alixant & Mann, 1995

­10

0

10

20

30

40

50

10000 10500 11000 11500 12000 12500

Small Cone

Differential G

ravity (µ

gal)

Measured Depth (ft)

∆S gas

0.2

0.4

0.6

­10

0

10

20

30

40

50

10000 10500 11000 11500 12000 12500

Small Cone

Differential G

ravity (µ

gal)

Measured Depth (ft)

∆S gas

0.2

0.4

0.6

­20

0

20

40

60

80

10000 10500 11000 11500 12000 12500

Large Cone

Differential G

ravity (µ

gal)

Measured Depth (ft)

∆S gas

0.2

0.4

0.6

­20

0

20

40

60

80

10000 10500 11000 11500 12000 12500

Large Cone

Differential G

ravity (µ

gal)

Measured Depth (ft)

∆S gas

0.2

0.4

0.6

Large Vs Small Gas Cone Large Vs Small Gas Cone

After Brady, 1999

Borehole Gravity Meter Borehole Gravity Meter Summary Summary u uRevolutionize current surveillance techniques Revolutionize current surveillance techniques u uUnaffected by borehole Unaffected by borehole u uVery deep reading density Very deep reading density u uGreater accuracy in gas saturations Greater accuracy in gas saturations

Surface Gravity Monitoring Surface Gravity Monitoring

Topics Topics u u Introduction to gravity Introduction to gravity u uSurface Gravity Surface Gravity u uBorehole Gravity Borehole Gravity u uPermanent Gravity Sensor Development Permanent Gravity Sensor Development

Multiple Absolute Station System Multiple Absolute Station System Surface & Subsurface Surface & Subsurface

• Multiple Stations provide monitoring network • Drift free gravity allows long term monitoring

After Niebauer and Herring,. 1999

Auto Auto­ ­Leveling, Gimbaled Sensor Leveling, Gimbaled Sensor with Tilt with Tilt­ ­meter meter

• Gimbal mount for large angular deviations

• Evacuated chamber • Fiber-optic link • Piezo-launcher • Small size:- (initially 2.5”-

3.5” ultimately 1-11/16”) • Very inexpensive

• Sensors = $1000s

After Niebauer and Herring, 1999 After Niebauer and Herring, 1999

Gravity Summary Gravity Summary u uSurface is a viable reservoir surveillance tool Surface is a viable reservoir surveillance tool

– – Limited applications Limited applications F FDensity contrast Density contrast F FDepth of burial Depth of burial F FThickness or zone Thickness or zone

u uBHGM BHGM – – Huge potential for downhole fluids monitoring Huge potential for downhole fluids monitoring

F FRevolutionize horizontal well monitoring Revolutionize horizontal well monitoring

u uPermanent Gravity Sensors Permanent Gravity Sensors – – Future Future