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

November 19, 2001 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.

November 19, 2001 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 CO 2

November 19, 2001 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

November 19, 2001 Study Area

November 19, 2001 Stratigraphic Column

November 19, 2001 Geologic Framework

November 19, 2001 Geologic Framework

November 19, 2001 Geologic Framework

November 19, 2001 Geologic Framework

November 19, 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

November 19, W4, 30 Hz Coal X Coal Y

November 19, W4, 90 Hz Coal X Coal Y

November 19, 2001 Resolution ParametersCoal X (2.23 m)Coal Y (1.56 m) Min.Phase/Sonic 250 Hz210 Hz Min.Phase/Sonic + Density 210 Hz190 Hz Min.Phase/Density 190 Hz170 Hz Zero Phase/Sonic 230 Hz Zero Phase/Sonic + Density 230 Hz250 Hz Zero Phase/Density 190 Hz210 Hz Theoretical Calculated Hz Hz

November 19, W4, 30 Hz

November 19, W4, 90 Hz

November 19, 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

November 19, 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

November 19, 2001 Model used for Raytracing

November 19, 2001 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

November 19, m overburden, NI tracing, 100 Hz

November 19, 2001 Model used for Raytracing

November 19, m overburden, NI tracing, 150 Hz

November 19, 2001 Model used for Raytracing

November 19, m overburden, CMP tracing, 150 Hz

November 19, 2001 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

November 19, W4 P-P 1-D Synthetic

November 19, W4 P-S 1-D Synthetic

November 19, 2001 P-P vs. P-S Modelling

November 19, 2001 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

November 19, 2001 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

November 19, 2001 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

November 19, 2001 Acknowledgements Fran Hein, Alberta Geological Survey Alberta Energy Utilities Board CREWES sponsors