Iraj A. Salehi1, Sherif I. Gowelly1, and Samih I. Batarseh1
Search and Discovery Article #80004 (2006)
Posted October 16, 2006
*Oral presentation at AAPG Annual Convention, Houston, Texas, April 9-12, 2006.
1Gas Technology Institute, Des Plaines, IL ([email protected])
In this paper, we present results of a series of field and laboratory experiments aimed at determination of seismic resolution relative to thin Illinois coal seams and study of viability of time-lapsed seismic imaging (4-D seismic) for monitoring the position of the injected carbon dioxide front during CO2 sequestration process, or desorbed methane in the case of coalbed methane production. The project was a cooperative effort between Illinois Clean Coal Institute (ICCI) and Gas Technology Institute (GTI) with valuable technical contribution and logistical support from Illinois State Geological Survey (ISGS).
Illinois coal seams are shallow and thin with the thickness rarely exceeding 10 feet. To investigate viability of seismic technology relative to imaging of Illinois coal seams a series of seismic data acquisitions including surface seismic, vertical seismic profiling, and crosswell seismic imaging carried out at the ISGS pilot site in Southern Illinois. Results were encouraging in that the data proved that thin coal seams can be reliably mapped by properly designed seismic surveys.
Our second objective was to verify viability of 4-D seismic as a monitoring tool for the potential CO2 sequestration projects in coal seams of Illinois. In pursuing this objective, a number of elaborate laboratory measurements of acoustic velocity in gas and water saturated coal samples were carried out. Results of these measurements showed that the magnitude of velocity change resulting from addition of a gas phase into water saturated coal samples is large enough to render the time-lapsed seismic technique useful for monitoring the position of injected or evolved gas fronts.
Location map of four seismic lines and wells used in crosswell survey, White County, Illinois.
Crosswell seismic imaging: final crosswell reflection image.
Summary of results of seismic surveys. Coal seams can be clearly identified on well logs from wells Hon 3 and Hon 9. Each vertical band is for a range of frequencies that increases to the right; i.e., low frequency band on the left (10-50 Hz) and high frequency band (100-1200 Hz) on the right. The upper part of the figure exhibits the seismic response of individual seams before velocity reduction, and the lower part shows the response after 20% velocity reduction. Note that changes in the seismic response resulting from injection of carbon dioxide or evolution of coalbed methane can only be observed at frequencies in the 100-800 Hz and 100-1200 Hz bandwidths (area within the blue rectangles).