Seismic and Strain Relevance of Coal to Its Shrinkage and Swelling Rate during the Sorption Pocess of CH4 and CO

Highly porous Bituminous coals have a high adsorptive capacity for CO₂ and CH₄. This experiment aims to find the adsorption and desorption for coal with CH₄ and CO₂ gases under in situ pressures. Since coal undergoes volumetric deformation during the sorption process, this experiment targets to find the relevance of coal deformation with respect to strain measurements and the change of seismic properties of coal specimen.

METHODOLOGY

Throughout the experiment three major components are integrated. They are: coal sorption setup, strain gauge setup, and seismic transducers with digital phosphor oscilloscope.

Coal sorption setup: About 1 cubic inch of a coal sample is subjected to gas (N₂, CH₄, and CO₂) pressures of up to 1400psi in a reactor cell. A reference cell is used for step-up and step-down procedures. Finally, a mixing cell is used for mixing different gas compositions in the desired ratios. These three cells are interconnected with 1/4” steel pipelines and gas flows are controlled by operating valves. High precision pressure gauges are also attached in each cell for monitoring gas pressure readings.

Strain gauge set up: A Tee Rosette type of 120Ω resistance is glued to the sample. This gauge is wired to a CR10X data logger through a 120Ω module which aids in completing a Wheatstone bridge configuration for accurate strain measurements.

Transducers with digital phosphor oscilloscope: Contact ultrasonic transducers are used to generate and receive longitudinal waves through the sample with a 5077PR pulser/ receiver. The travel time for these pulses through coal is observed in the Oscilloscope and is used to calculate seismic velocity and attenuation.

RESULTS AND DISCUSSION

On a raw basis, powdered Drywood coal having particle size of <20 mesh at 72.0°F has a methane content of 11.4 scc/ gms. This is according to Langmuir’s coefficient under gas pressures of 4.17MPa. In this experiment we attain adsorption and desorption isotherms under temperatures of around 80.0°F and under gas pressures of 9.6MPa. Coal undergoes shrinkage and swelling during adsorption of CO₂ and desorption of CH₄. These volumetric deformations are measured through contact transducers and biaxial strain gauges. Comparing the sorption results with seismic analysis would enhance predictions of coal behavior in the field for CO₂ sequestration.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California