Research on adsorption and damage characteristics of slick water in coalbed methane development

Abstract

In the process of coalbed fracturing, the drag reducers absorbed on the surface of the coalbed causes water lock damage and decreases gas production efficiency of coalbed methane. Thus, research on low-damage coalbed drag reducers is of great significance. Aiming at four drag reducers with different molecular weights, the law of drag reducers adsorption was investigated through static adsorption test and contact angle tests. Afterwards, a core displacement experiment was carried out to study the damage rate of slick water to the coabed, and the nanoemulsion CND was used for competitive adsorption to relieve the influence of fracturing fluid adsorption on the core damage. Finally, through the static adsorption test results, combined with core SEM images before and after displacement, the internal relationship between adsorption and damage was revealed, and the optimal principles and control methods for reducing the damage of drag reducers to the coalbed were proposed. The results show that: the drag reducers with larger molecular weight are more likely to adsorb in the coalbed, which caused more severe damage to the coalbed reservoir. In contrast, drag reducers with lower molecular weight show a lower damage rate (less than 20%) to the reservoir, which is suitable for the preparation of low-damage slick water. Mechanistic studies have shown that the drag reducers is adsorbed on the surface of the organic matter in the coalbed, leading to the exposure of the hydrophilic end. Therefore, the hydrophilic area on the surface of the coal powder was enlarged, which forms the water film adhesion in the pore throat, causing the water lock to block the gas and liquid seepage channel, resulting in reduced permeability; Nanoemulsion and coal powder are more closely adsorbed, which can reduce the adsorption of drag reducers through competition with the adsorption matrix, reduce water lock damage, facilitating the subsequent drainage and gas collection process.