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The UBC Geophysical Inversion Facility. EM geophysics for hydrocarbons: Inversion applications and current research at UBC-GIF. Scott Napier Doug Oldenburg Jamin Cristall. May 2005. http://www.eos.ubc.ca/ubcgif. Acknowledgments. This research was sponsored by NSERC and:. AGIP

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the ubc geophysical inversion facility

The UBCGeophysical Inversion Facility

EM geophysics for hydrocarbons:

Inversion applications and current research at

UBC-GIF

Scott Napier

Doug Oldenburg

Jamin Cristall

May 2005

http://www.eos.ubc.ca/ubcgif

slide2

Acknowledgments

This research was sponsored by NSERC and:

    • AGIP
    • Anglo American
    • BHP Billiton
    • EMI
    • Falconbridge
  • INCO
  • Kennecott
  • MIM
  • Muskox Minerals
  • Newmont
  • Placer Dome
  • Teck Cominco

Thanks to UBC-GIF personnel:

  • Colin Farquharson
  • Eldad Haber
  • Roman Shekhtman

http://www.eos.ubc.ca/ubcgif

outline
Outline
  • Electrical conductivity and hydrocarbons
  • Introduction to forward modelling
  • Introduction to inversion methodology
  • Case Studies
      • Shallow gas
      • Oil sands
  • Current research
      • Marine CSEM
  • Conclusions
electrical conductivity and hydrocarbons

DC Resistivity

Electrical conductivity and hydrocarbons

GR/SP

Resistivity

FDEM

Med

Deep

SP

TEM

GR

  • Can we detect hydrocarbons with EM measurements at the surface?

Marine CSEM

  • Hydrocarbons are resistive
    • Measurements of ρ are common in the borehole

0.2 Ωm

2000 Ωm

outline5
Outline
  • Electrical conductivity and hydrocarbons
  • Introduction to forward modelling
  • Introduction to inversion methodology
  • Case Studies
      • Shallow gas
      • Oil sands
  • Current research
      • Marine CSEM
  • Conclusions
slide6

Forward Modelling: The airborne FDEM survey

Waveform: Frequency domain

I

Time

Recorded Data: Amplitude and Phase

or Real and Imaginary parts

Boundary conditions

at z = 

forward modelling em data in 1d
Forward Modelling EM data in 1D:
  • The EM skin depth
  • Divide the earth into stack of layers
    • fixed thickness
    • constant internal conductivity

F[m]

Tx

Rx

z1

z2

z3

.

.

Basement half-space

zn

5 frequencies from 385Hz to 102kHz with real

and imaginary parts

outline8
Outline
  • Electrical conductivity and hydrocarbons
  • Introduction to EM methods
  • Introduction to inversion methodology
  • Case Studies
      • Shallow gas
      • Oil sands
  • Current research
      • Marine CSEM
  • Conclusions
slide9

The inverse problem: Unconstrained Optimization

F-1

?

  • Geophysical data are: F[m] +  = d
        • m: model --- unknown
        • F: forward mapping operator
        • : errors
        • d: observations (data)
  • Given:
    • data, errors, a forward modelling method
  • Find:
    • the model that generated measurements.
  • Major Difficulty: Non-uniqueness

d

slide10

Inversion as an optimization problem

  • Define
    • Model objective function.
    • Misfit function.
  • Minimize

 = d +  m Subject to d < Tol.

 : Regularization parameter

: Observed data

: Model and Reference model

Wd,W : Data error, model weighting

slide11

Minimizing the Model Objective

Objective function 

Differentiate

where sensitivity matrix

slide12

Gauss-Newton method

  • Iterate

Linearize F[m+m] = F[m] + J m

  • Update the model

mk+1=mk + α(δm)

  • Repeat until convergence
outline13
Outline
  • Electrical conductivity and hydrocarbons
  • Introduction to EM methods
  • Introduction to inversion methodology
  • Case Studies
      • Shallow gas
      • Oil sands
  • Current research
      • Marine CSEM
  • Conclusions
slide15

gas well: 07-25-110-4W6

-Clay till with recent

narrow gravel channels

-Quaternary Lacustrine clays

-Oligocene/Miocene braided river

channels,

gas or water charged

-Cretaceous shale with

possible deeply incised

gravel/sand channels

-Cretaceous shale

Sand / Gravel/

Conglomerate

Glacial Till

Clays

Shallow Gas: Geologic Background

~ 10 m

< 5 m

~ 100-200 

< 5 m

100-1000 m

water or gas

charged

proof of concept surveys over existing shallow gas field
Proof of Concept: Surveys over existing shallow gas field
  • Airborne frequency domain EM (FEM)
  • 2D DC resistivity
slide17
FDEM:
  • Advantages
    • Covers large areas at low cost
      • low ecological impact
    • 3d images from densely sampled data
  • Disadvantages
    • Inductive method
      • Not particularly sensitive to resistors
    • Shallow depth of investigation (maximum 100-150m)
fdem result
FDEM: Result
  • Gas saturated areas are detectable with FDEM data
    • Forward modelling indicates resistivity will be underestimated
    • gas field could have benefited from this survey

depth = 46 m

dc resistivity
DC Resistivity:
  • Advantages
    • galvanic method
      • sensitivity to resistors
    • good depth of investigation
      • Wenner Array
      • a spacing maximum 400m
  • Disadvantages
    • Ground based
      • slower more expensive acquisition
        • 2D interpretation
    • Data quality based on good electrical contact with ground
      • Suffers in swampy terrain
      • Difficult to penetrate conductive layers
dc resistivity result
DC resistivity: Result

Observed Data

Predicted Data

Recovered Model

0

5400

comparing results
Comparing results:
  • Challenging environment for DC resistivity surveying

0

5400

western canada oil sands regions
Western Canada Oil Sands Regions

Athabasca

Peace River

Fort

Peace River

McMurray

Wabasca

Cold

Lake

Edmonton

Calgary

  • Source: Mark Savage, “Oil Sands Characteristics - Geology,” 9 April 2002
the mcmurray formation
The McMurray Formation

Source: David R. Taylor, “McMurray Fm. Geological Model,” 28 May 2003

airborne time domain em surveying the geotem system
Airborne Time Domain EM Surveying: The GEOTEM system

Waveform: Time Domain

Inverted Data: Time Domain

dB/dt

I

Time

Time

time domain em the geotem system
Time Domain EM: The GEOTEM system
  • Advantages
    • No primary field during recording stage
      • secondary fields only
    • Depth of investigation
      • (maximum 150-250 m)
    • Large areas at low cost
      • low environmental impact
  • Disadvantages
    • Inductive method
      • not particularly sensitive to resistors
outline29
Outline
  • Electrical conductivity and hydrocarbons
  • Introduction to EM methods
  • Introduction to inversion methodology
  • Case Studies
      • Shallow gas
      • Oil sands
  • Current research
      • Marine CSEM
  • Conclusions
slide30

Introduction: The marine CSEM survey

  • Towed Transmitter
    • horizontal electric dipole
  • Seafloor Receivers
    • record Ex , Ey
    • possible to record Ez , Hx and Hy
why marine csem
Why Marine CSEM?
  • Reduce risk for expensive deep water wells
    • Recover reservoir resistivity
    • Recover reservoir geometry
  • Why might it work?
    • Galvanic source
      • sensitive to resistors
    • Seawater provides shielding from EM noise sources
      • can detect signals of extremely low amplitude
slide32

Forward Modelling: Theory

  • FD Maxwell’s equations (e-it )
  • Boundary condition
slide33

3D Forward Modelling: Introduction

  • A Helmholtz decomposition with Coulomb gauge
  • System equations for A and 

where

  • Discretize on a staggered grid
forward modelling response of a large reservoir
Forward Modelling: Response of a large reservoir

Tx

0.3 Ωm

1150 m

6 km

1 Ωm

850 m

100 m

50 Ωm

Amplitude of E-field (1 Hz)

Reservoir

-11

10

No Reservoir

-12

10

-13

10

|E| [V/m]

-14

10

-15

10

-16

10

0

1

2

3

4

5

6

7

8

Offset [km]

forward modelling the reservoir model
Forward Modelling: The Reservoir Model

σ (S/m)

profile view

plan view

1650m depth

  • 1000 m
  • 600 m
  • 100 m
  • 2000 m
  • Key model parameters
    • water depth
    • depth of burial
    • thickness
    • horizontal extent
  • Transmitter parameters
    • orientated in x direction
    • 100 m long
    • 300m east of center of the reservoir
slide36

Forward Modelling: Reciprocity

I

V

V

I

  • Reciprocity solves this problem

A

B

M

N

A

B

M

N

  • Practical surveys consist of few Rx and many Tx
  • Each Tx requires a separate forward model
    • time consuming processing

20000

20000

13000

13000

20000

20000

forward modelling ex and ey
Forward Modelling: Ex and Ey

Real Ex

Real Ey

Imag Ex

Imag Ey

inversion results 2 frequencies 1hz 5hz
Inversion: Results - 2 frequencies (1Hz & 5Hz)

σ (S/m)

z=-1600 m

Z= -1500 m

Isosurface at 0.2 S/m

Recovered

z=-1600 m

Isosurface at 0.2 S/m

True

conclusion
Conclusion:
  • Frequency Domain EM, DC resistivity inversions could be very important
    • in exploration
    • in production

0

5400

conclusion41
Conclusion:

m

Source: David R. Taylor, “McMurray Fm. Geological Model,” 28 May 2003

  • Airborne TEM in conjunction with an inversion code can clearly locate oil sand channels
  • Oil sands are a growing proportion of Canada’s hydrocarbon production
conclusion42
Conclusion:
  • Marine CSEM can significantly reduce risk in expensive offshore exploration
  • Potential to help define reservoir geometry