1. State Key Laboratory of Petroleum Resources and Prospecting,China University of Petroleum (Beijing),Beijing　 102249,China; 2. China United Coalbed Methane Co. ,Ltd. ,Beijing　 100016,China; 3. Chinese Academy of Geological Sciences,Beijing　 100037,China

Coal reservoirs have the characteristics of low pressure,low permeability and micro-crack development. In the process of coalbed methane exploitation,it is easy to cause damage to the coal reservoir and reduce the production of coalbed methane wells. In terms of these problems,the molecular structure of a new type of high-efficiency gemini foaming agent was designed and the foaming agent was synthesized in the laboratory. At the same time,based on the gemini foaming agent,a micro-foam drilling fluid system suitable for coalbed methane drilling was developed through some experiments. The coal rock composition,micro-structure and properties of coal seams roof and floor rock samples were analyzed,and the applicability of micro-foam drilling fluid to coal seams and coal seams roof and floor rock sam- ples were evaluated by laboratory experiments. The research results indicate that the new foaming agent has good per- formance property,thus,it can significantly reduce the surface tension of water. When the foaming agent concentration is 0. 1% ,the drop rate of water can reach 71. 82% . The half-life period of the new gemini foaming agent is 3 times more than that of the conventional foaming agent,and the performance of the stable foaming film is good. Furthermore, the effect of new foaming agent on the wettability of coal rock samples were studied by contact angle measuring instru- ment. The results show that the new foaming agent has little change to the wettability of coal rock,and the recovery val- ue of coal rock contact angle is 86% ,which is beneficial to improve the protection effect of coal seam. The micro-foam drilling fluid system has the characteristics of moderate viscosity,good rheological property,high dynamic plastic ratio and stable foam quality. These characteristics can improve the cuttings-carried ability of micro-foam drilling fluid. The results of foam micro-morphology test show that the hydrated film of micro-foam drilling fluid system is thicker and has strong stability,the micro-foam particle size exhibits a normal distribution with an average bubble size of 144 μm and the half-life period is more than 82 h. The linear expansion rate of the coal seam roof and floor rock sample in the mi- cro-foam drilling fluid is less than 3% ,and the rolling recovery rate is greater than 92% ,indicating that the micro- foam drilling fluid has a good inhibition ability and can maintain the wellbore stability of the coal seam roof and floor formation. The recovery rate of coal core gas permeability is over 90% ,which has good reservoir protection effect and can meet the drilling requirements of low pressure and low permeability coal seam. The adsorption and desorption abili- ty of the treatment agents in the micro-foam drilling fluid were evaluated by establishing the experimental device and method. Adsorption and desorption experiments show that the micro-foam drilling fluid has small adsorption capacity and easy desorption,and the desorption rate in coal rock sample is 82. 4% . The micro-foam drilling fluid based on the new gemini foaming agent can effectively maintain the wellbore stability of the coal seams roof and floor rock samples, desorb easily on the surface of the coal seam sample,improve the protection effect on coal reservoirs. Therefore,it is suitable for coalbed methane drilling operations.

国家科技重大专项基金资助项目(2016ZX05044003-003)

LI Zhiyong,LI Qiang,SUN Hansen,et al. Research on micro-foam drilling fluid suitable for coalbed methane drilling [J]. Journal of China Coal Society,2020,45(2):703-711.