The invasion of drilling fluid into coal under fluctuating wellbore pressure

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.