Evaluation of reservoir damage caused by drilling fluid

Aiming at reservoir damage caused by drilling fluid invasion during CBM drilling, the paper, by selecting seam No.9 in Ningwu basin and drilling fluid used in site as research object, conducted evaluation of dynamic-static damage of coal samples caused by drilling fluid, capillary imbibition and stress sensitivity. Dynamic-static damage evaluation shows that filtrate damage of drilling fluid is the main cause for reduction of permeability in coal seam, coal imbibitions ability is strong and adsorption retention is serious for drilling fluid when compared with formation water, which lead to the low liquid phase flow-back, after being reacted with the drilling fluid, coal samples has weakened mechanical properties and strengthened stress sensitivity. In combination with the analysis results of water sensitivity, alkali sensitivity, wettability evaluation and scanning electron microscopy (SEM), it is pointed out that fracture blocking, wettability difference and drilling fluid adsorption of coal are the main factors of seam damage caused by drilling fluid.