Experimental study on water sensitivity and salt sensitivity of lignite reservoir under different pH

摘要

The water sensitivity and salt sensitivity of lignite reservoirs are important factors affecting the productivity of the coalbed methane (CBM) well. This paper took the lignite of Erlian Basin as the research object. The content and occurrence of mineral was analyzed by LTA + XRD combined with scanning electron microscopy (SEM), and distribution of each functional group of lignite was analyzed by infra-red spectrum. Based on the above analysis, standard salt water with different pH and salinity was used as the experimental injection fluid, and water sensitivity (salt sensitivity) experiments were performed under the condition of constant effective stress. The experimental results show that the sample contains a large number of water-sensitive minerals (kaolinite and illite/smectite mixed layer mineral), and the ratio of carbonyl, hydroxybenzene and carboxyl groups is 10:7:1. Hydrogen bonds are mainly OH center dot center dot center dot N hydrogen bonds. The lignite reservoirs with different pH have different degrees of water-sensitive damage, and the damage rates from high to low are partial alkalinity conditions (54.18%), partial acidity conditions (42.6%), and strong alkalinity conditions (23.67%). The dimensionless permeability changes with the decrease of salinity, which can be divided into three stages (no water sensitivity stage, particle migration water sensitivity stage, particle expansion water sensitivity stage) with 1250 mg/L and 10000 mg/L as the boundary. The displacement pressure and pH are the main controlling factors at different stages, and pH controls the wettability of pulverized coal and the hydration expansibility of clay minerals. The dimensionless permeability of lignite reservoir increases with the increase of salinity under the acidic condition. Under alkaline conditions, it is characterized by no-weak salt sensitivity. The greater the alkalinity, the stronger the salt sensitivity, which is mainly controlled by the change of the thickness of the water film under the influence of pH.