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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

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The ubc geophysical inversion facility l.jpg

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 l.jpg

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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    • Minimizing the Model Objective

    Objective function 

    Differentiate

    where sensitivity matrix


    Slide12 l.jpg

    • Gauss-Newton method

    • Iterate

    Linearize F[m+m] = F[m] + J m

    • Update the model

    mk+1=mk + α(δm)

    • Repeat until convergence


    Outline13 l.jpg

    Outline

    • Electrical conductivity and hydrocarbons

    • Introduction to EM methods

    • Introduction to inversion methodology

    • Case Studies

      • Shallow gas

      • Oil sands

  • Current research

    • Marine CSEM

  • Conclusions


  • Case study em methods for shallow gas exploration in nw alberta l.jpg

    Case Study: EM methods for shallow gas exploration in NW Alberta


    Slide15 l.jpg

    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 l.jpg

    Proof of Concept: Surveys over existing shallow gas field

    • Airborne frequency domain EM (FEM)

    • 2D DC resistivity


    Slide17 l.jpg

    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 inversion results l.jpg

    FDEM Inversion: results

    data

    5

    20

    Ωm

    200


    Fdem result l.jpg

    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 l.jpg

    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 l.jpg

    DC resistivity: Result

    Observed Data

    Predicted Data

    Recovered Model

    0

    5400


    Comparing results l.jpg

    Comparing results:

    • Challenging environment for DC resistivity surveying

    0

    5400


    Western canada oil sands regions l.jpg

    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 l.jpg

    The McMurray Formation

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


    Airborne time domain em surveying the geotem system l.jpg

    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 l.jpg

    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


    Inversion of field data l.jpg

    Inversion of Field Data

    7

    km

    10 km


    Comparison to conductivity depth transform l.jpg

    Comparison to Conductivity-Depth Transform

    m

    Inversion

    m

    CDT


    Outline29 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    Forward Modelling: Theory

    • FD Maxwell’s equations (e-it )

    • Boundary condition


    Slide33 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    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 l.jpg

    Forward Modelling: Ex and Ey

    Real Ex

    Real Ey

    Imag Ex

    Imag Ey


    Inversion results 2 frequencies 1hz 5hz l.jpg

    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


    Inversion observed and predicted data l.jpg

    Inversion: Observed and predicted data

    Ex

    Ey


    Conclusion l.jpg

    Conclusion:

    • Frequency Domain EM, DC resistivity inversions could be very important

      • in exploration

      • in production

    0

    5400


    Conclusion41 l.jpg

    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 l.jpg

    Conclusion:

    • Marine CSEM can significantly reduce risk in expensive offshore exploration

    • Potential to help define reservoir geometry


    Questions l.jpg

    Questions?


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