Evaluation of coal reservoir damage caused by fluid of high pH value

The contact between coal and working fluid of high pH value is inevitable in the process of CBM drilling and fracturing operation. The coal seam will be in the environment of alkaline for a long time after contacting with working fluid, which can bring serious damage. To simulate this damage, No.9 coal seam of Ningwu basin is chosen as the research object, the experiment of influence of increased pH value fluid on permeability of coal was conducted for natural columnar coal samples and artificial fracture coal samples; and the experiments of capillary imbibitions and liquid flowback and stress sensitivity were carried out for fluid of different pH values. The results show that coal has strong alkali sensitivity damage, stronger imbibitions ability and lower flowback rate for working fluid of high pH value compared with formation water; the stress sensitivity coefficient of coal increases after contacting with alkaline fluid. Combining the results of wettability determination and scanning electron microscopy (SEM) analysis, it can be got that the higherthe pH value of working liquid is, the stronger the wettability will be, at the same time the contact with high pH value fluid can lead to microstructure instability and cation exchange adsorption; the inorganic scale generated by the contact of formation water and high pH working fluid and the high adsorption ability to all kinds of liquid are the main causes of damage; the mechanical strength is weakened and the stress sensitivity is strengthened after the action of alkaline fluid on coal seam.