Experimental evaluation of coalbed reservoir damage induced by sequential contacts with working fluids

With high capillary pressure and large specific surface area, tight coalbed may be damaged by fluid invasion during drilling, completion and stimulation, which may reduce engineering technical efficiency significantly. Previously, only single working fluid damage to coal reservoir has been reported, seldom taking superimposed effect of damage induced by sequential contact of working fluids with the coal reservoir into account. Collecting coal samples from coal seams of upper Permian Longtan Formation in Guxu mine, southern Sichuan coal field, we conducted compatibility experiment between fluids and the permeability damage experiments of sequential contact of drilling and completion fluids and fracturing fluids with coal samples. According to the result of the reservoir damage evaluation experiment, the damage of fracturing fluids to the reservoir is the most serious, followed by drilling and completion fluids. As shown in the result of compatibility of working fluids and sequential contact damage evaluation experiment, Guar gum fracturing fluid comes first in the terms of extent of damage, followed by ion balance fracturing fluid, active water fracturing fluid and the clean fracturing fluid in sequence. Sequential contact damage experiment can simulate the engineering practice process and provide theoretical basis for evaluation and optimization of drill-in-fluids which is beneficial to promote the protection of coal seam and efficient exploitation of coalbed methane.