Advances in Acoustic Logs to Predict the Stress Redistribution in Coal Strata as a Result of Degassing/Dewatering

The aim of this project is to study the effect of fluid withdrawal on stress distribution in coal. Recent advances in acoustic log analysis and analytical models as well as field and laboratory data are used to evaluate the stress redistribution caused by fluid drainage from a borehole.

The key objectives of the project are as follows:

• Conducting a parametric study to investigate the Influence of different pore and confining pressures on the P- and S- wave velocities and their variation during the pore fluid depressurization of coal.
• Extracting the poroelastic parameters of coal using the obtained data as well as existing and proposed models. The S- wave anisotropy will be recorded at lab condition.
• Numerical simulation of the stress re-distribution around a borehole in coal strata as a result of degassing-dewatering using obtained poroelastic data.
• Developing a code in Excel to assess the stress redistribution scenarios using proposed analytical solutions and acoustic velocities collected from mines' boreholes.

Main outcomes of this project are as follows:

• In-depth understanding of the stress evolution in coal seam during the process of degassing/dewatering using extensive laboratory experiments including ultrasonic, static and X-ray micro computed tomography (XRCT) of coal specimens with different characteristics (cleat and matrix density).
• A new model based on the percolation theory was developed to estimate the values of effective stress coefficient using acoustic data. Results of the model are in close agreement with that of lab measurements.
• A physics informed machine learning method was developed to predict the coal cleat permeability from a combination of sonic log data obtained from the field and lab measurements and numerical simulation.
• A model to estimate the effective stress evolution in coal accounting for the effect of sorption-induced strain in addition to the conventional poroelastic effect.
• An EXCEL-based tool that evaluates the profile of rock properties and total stresses of coal seams using commonly available borehole log data, lab and field measurements.
• A finite element model coded in FORTRAN to evaluate stress distribution around drainage boreholes. Required import data for this simulation are imported from the above EXCEL-based tool.

A deliverables of this project is an Excel-based tool that can be readily used by site engineers to estimate coal mechanical properties, Biot coefficient, in situ stresses, etc using downhole geophysical data and other field measurements.