ABSTRACT:

In this study, the water absorbability and acoustic characteristics of coal were measured at downhole temperature and pressure, considering the effects of different kinds of drilling fluids. The inversion was conducted based on these data and revealed the change of internal structure of coal with drilling fluids. Results show that a turning point of head longitudinal wave amplitude was observed in the curve of coal acoustic properties and for different drilling fluids, the turning point occurred at different time. This phenomenon may imply that significant change of coal structure formed at the turning point and it is variant for different drilling fluids. Coal mainly failed along the bedding surface and the failure of coal occurred later and more incompletely when the coal was immersed into biodegradable polymer drilling fluid. Based on the failure characteristics, it can be inferred that drilling fluid would fill the natural fracture of coal before the turning point and then penetrated into the coal bedding plane which may lubricate the bedding plane and reduce the strength. Finally, the coal could fail easily under the effect of fluid pressure.

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