Abstract
When drilling a CBM reservoir, the drill string moves up and down in the wellbore, causing fluctuations in the wellbore pressure. Wellbore pressure and pressure fluctuations deform the cracks and pores of coal, damaging the reservoir permeability. Therefore, the invasion of drilling fluid into coal under fluctuating wellbore pressure should be studied. We studied the effect of pressure, applied in various increments, on the permeability of coal, as well as the relationship between the coal’s permeability and the variation of fracture widths during pressure fluctuations. Drilling pressure and pressure fluctuations cause permanent damage to the coal’s permeability. The amount and the rate that different drilling fluids invaded into coal, under conditions of steady wellbore pressure and fluctuating wellbore pressure, were measured by studying core flow. When the wellbore pressure fluctuated, the invasion amount was greater. Large pressure differences and pressure fluctuations should be avoided during drilling for CBM reservoirs. Water invaded into the coal the most, and foam invaded into the least. These results provide technical guidance for planning and protecting a CBM reservoir, and help to improve CBM production.
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Funding
This research was funded by National Natural Science Foundation of China, grant number 51804043, and it was also funded by Yangtze fund for youth teams of science and technology innovation, grant number 2016cqt03.
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S.Z. was in charge of project administration, experiment design and writing-review manuscript. Y.L. and Y.P. were in charge of investigation, experiment validation and writing original manuscript. T.L. and D. D were in charge of data curation and formal analysis.
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Zheng, S., Liu, Y., Pan, Y. et al. The invasion of drilling fluid into coal under fluctuating wellbore pressure. Arab J Geosci 13, 334 (2020). https://doi.org/10.1007/s12517-020-05341-7
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DOI: https://doi.org/10.1007/s12517-020-05341-7