In order to improve oil recovery, gelling acid acidizing operation was carried out for Well # 1 and Well # 3. During the fluid discharge, it was found that the returned material contained a certain amount of unknown colloidal substances, which blocked the wellhead channel and affected the flowback construction; At the same time, in the process of lifting the tubing, it was found that unknown gray solid was attached to the tubing. In order to find out the composition of the two unknown substances, study their causes and avoid similar accidents, the author carried out various analysis experiments. It has carried out solubility experiment, roasting experiment, dissolution experiment, infrared spectrum analysis test experiment and compatibility experiment. The comprehensive experimental results show that the gelled substances and unknown gray white solids in Well # 1 and Well # 3 are caused by the incompatibility between the gelling agent in the gelling acid and the solid free kill fluid, resulting in flocculation and cementation, resulting in the formation of gelled substances and gray white solids. Both colloidal substance and gelling acid have a wave peak at 1636, which is a vibration peak of –C=C–. It is speculated that it may be an amide group. The gray-white solid can be dissolved with acid and water. According to the infrared spectrum analysis, the substance is mainly composed of inorganic components, in which K element accounts for 27.4%; Other components include hydroxyl and carbon carbon double bonds, which are mainly polymers. The analysis of gelled matter and gray solid is that the gelling agent in gelled acid is not compatible with solid free kill fluid, resulting in flocculation and cementation, resulting in the formation of gelled matter and gray solid.