November 19, 2001

Seismic modelling of coal bed methane strata, Willow Creek,

Alberta

Sarah E. Richardson, Rudi Meyer, Don C. Lawton, Willem Langenberg*

*Alberta Geological Survey, Energy Utilities Board

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Purpose

Assess the feasibility of high-resolution reflection seismic surveys for coal-bed-methane (CBM) exploration and development.

Numerical reflection seismic techniques are assessed for mapping continuity and coherence of coal zones.

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What is Coal Bed Methane?

• Methane extracted from coal seams• No matrix porosity, gas is stored

within the coal itself• No permeability, production is

dependent on fracturing• Production is either by dewatering

or by injection of CO2

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Why the interest?

• Alberta is coal-rich • Several petroleum companies are

in the early phases of CBM exploration and development

• Lateral extent, coal thickness, and degree of fracturing are three key parameters for development

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Study Area

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Stratigraphic Column

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Geologic Framework

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Geologic Framework

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Geologic Framework

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Geologic Framework

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1-D Synthetic Seismograms

• Created using a frequency series of 30 to 310 Hz wavelets

• Coals seams should appear as a trough at the upper contact and a peak at the basal contact

• Frequency of resolution depends on thickness and contrast with surrounding beds

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9-11-28-19W4, 30 Hz

Coal X

Coal XCoal Y Coal Y

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9-11-28-19W4, 90 Hz

Coal X Coal XCoal Y

Coal Y

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ResolutionParameters Coal X (2.23

m)Coal Y (1.56

m)Min.Phase/Sonic 250 Hz 210 Hz

Min.Phase/Sonic + Density

210 Hz 190 Hz

Min.Phase/Density 190 Hz 170 Hz

Zero Phase/Sonic 230 Hz 230 Hz

Zero Phase/Sonic + Density

230 Hz 250 Hz

Zero Phase/Density

190 Hz 210 Hz

Theoretical Calculated

140-300 Hz 215-430 Hz

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2-13-38-22W4, 30 Hz

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2-13-38-22W4, 90 Hz

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1-D Observations

• Minimum phase wavelets produce a greater number of visible events than zero phase

• Sonic + Density synthetics produce the best results

• Density alone highlights coals, but gives little information about surrounding lithologies

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2-D Raytrace Modelling

• Digital model based on measured cross-section of Willow Creek by AGS

• P-wave velocities based on those of equivalent lithologies in nearby wells

• Three variations examined – 40 m, 200 m, and 400 m of overburden

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Model used for Raytracing

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Modelling Parameters

• Normal incidence (NI) and common mid-point (CMP) ray tracing run on each of the three models

• Frequencies of 50 Hz, 100 Hz, 150 Hz• Base of quaternary removed• CMP parameters for 40 m

overburden: 10 offsets/side, 5 m increments

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40 m overburden, NI tracing, 100 Hz

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Model used for Raytracing

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200 m overburden, NI tracing, 150 Hz

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Model used for Raytracing

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40 m overburden, CMP tracing, 150 Hz

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Preliminary P-S Wave Study

• Coal has very distinctive Vp/Vs ratio of 1.5

• Converted wave seismology shows great potential for CBM work, as it often increases resolution in the shallow section

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9-11-28-19W4 P-P 1-D Synthetic

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9-11-28-19W4 P-S 1-D Synthetic

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P-P vs. P-S Modelling

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Conclusions & Recommendations

• Coal interval is mappable, individual seams of these thicknesses are unlikely to be resolved

• Lateral changes in stratigraphy (e.g. shale plugs) are interpretable

• Optimum depth for seismic imaging of coals is 200 m, with these survey parameters

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Conclusions & Recommendations

• Field test recommended, if primary objective is to determine gross structure of the coal interval

• Based on preliminary work, P-S surveys may improve resolution

• Seismic surveys will likely prove to be a valuable tool in the search for coal-bed methane

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Future Work

• More numerical modelling, varying coal seam thicknesses

• Test converted-wave exploration in shallow section to improve resolution

• Test whether Vp/Vs ratios within coals may be useful for determining gas saturation on a 4-dimensional basis

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Acknowledgements

• Fran Hein, Alberta Geological

Survey

• Alberta Energy Utilities Board

• CREWES sponsors