Damage Mechanism and Optimization of Coalbed Methane Fracturing Fluid Reservoir in Guizhou Province

The commonly used fracturing fluid for coalbed methane wells in Guizhou Province is active water fracturing fluid, which is composed of water and anti-swelling agent. The field construction formula is water + 2 wt.% KCl, and the application effect shows that it causes greater damage to the reservoir. In view of the problems of high clay content and great damage of fracturing fluid in Guizhou coal seam, water sensitivity test experiments were carried out on the field coal t samples to analyze the damage of the existing fracturing fluid system to the reservoir. The damage mechanism of fracturing fluid to coalbed methane reservoir was clarified by scanning electron microscope and contact angle experiment. A new active water fracturing fluid system was optimized and evaluated based on anti-swelling experiment, core damage experiment and nuclear magnetic resonance experiment. The results show that: (1) The water sensitivity of Guizhou coal seam is moderate to strong water sensitivity, and the existing fracturing fluid system has weak anti-swelling performance, and the damage rate to the coal seam is more than 46%. (2) The surface wettability of coal body changes after adsorption of fracturing fluid, the contact angle becomes smaller, and the hydrophilicity of coal body increases. The fracturing fluid is adsorbed on the surface of coal seam, blocking the pores and cracks of coal seam, resulting in the width of pores and cracks being smaller, and the seepage capacity of coal seam being reduced. (3) A new active water fracturing fluid system was obtained by using the organic and inorganic compound anti-swelling method: water + 2 wt.% KCl + 0.2 wt.% AS-55 + 0.05 wt.% bactericide. The damage rate of the optimized fracturing fluid formula to the coal seam is reduced by 18% ~ 20% compared with that before optimization. (4) The main stability mechanism of organic-inorganic composite: the hydrophilic head of K+ ions and cationic small molecules in AS-55 aggregates on the surface of inorganic substances, further reducing the hydration of clay mineral particles and increasing the attraction between clay mineral particles. In addition, the hydrophobic tail chain of cationic small molecules in AS-55 can be adsorbed on the surface of hydrophobic coal rock organic matter, agglomerating coal powder particles, and better achieving the purpose of controlling particle migration and preventing coal seam swelling. (5) The nuclear magnetic resonance shows that the main damage mechanism of on-site fracturing fluid is the blocking effect in the pore throat. The pore throat space in the matrix is reduced by 70.4% due to clay swelling, and the optimized fracturing fluid can effectively alleviate the blocking effect in the pore throat. The research results of this paper have reference and guiding role for the efficient development of coalbed methane in Guizhou.