Underground Coal Gasification (UCG)

Overview

Monitoring of the progress of the Underground Coal Gasification (UCG) process as well as the state of the surrounding rockmass provides useful insight into this unconventional mining method. Microseismic monitoring can be used to locate and characterise fracturing occurring in the coal seam and surrounding rock due to the thermal stresses at the burn front. This provides real-time spatio-temporal information of the burn front, as well as information about the state of the gasifier.

Larger seismic events resulting from the stresses imposed by the cavity remaining after combustion or the collapse of the cavity itself, are more typical of mining applications. IMS has decades of experience in monitoring this (more conventional) kind of mining induced seismicity.

IMS has experience of monitoring UCG processes at sites in New Zealand and South Africa.

Operation

Typically an array of tri-axial sensors are installed around the gasifier. Sensors are positioned mostly above the coal seam so as not to sacrifice the sensors as the seam is burned. Calibration of velocities at such a site is difficult as the presence of explosive gasses and valuable infrastructure limit the use of explosives as a seismic source. An air gun borehole source can be used, and IMS has successfully used percussion drilling noise to calibrate the velocities of such a site.

Sensors are typically digitised by IMS seismometers positioned at the collar of each borehole, with communication links to the central seismic monitoring server.

Communications

The seismic stations communicate with a central computer (seismic server) via WiFi digital radios or alternatively seismic data can be written to portable USB drives, using the standalone recording mode. Use of common consumer grade WiFi radios or cellular modems minimises infrastructural set-up costs. The seismometers are linked to GPS modules to provide accurate time synchronization.

Sensors

Typically 14 Hz miniature tri-axial geophones are used to monitor UCG operations, and higher frequency accelerometers may be useful if these can be positioned within a few tens of meters of the burn front, where high frequency signals associated with thermal cracking of the medium will be detectable. Sensors are installed using long (200-500 m) boreholes from surface, grouted into the boreholes at various depths.

Seismic Data Processing & Interpretation

Real-time continuous recording of seismicity is suggested for UCG process monitoring. A recommended pre-ignition monitoring period of a few months is recommended to establish baseline.

IMS can remotely download, process and analyse all the seismic data from our office in South Africa. This continuous, real-time seismological processing service comes with routine (daily, weekly or monthly) analysis and reporting. Monthly reports are recommended for pre-ignition baseline monitoring, and daily/weekly reports are recommended while the burn is in process.