Research and application of the cooling system for water-based drilling fluid in hot dry rock drilling

Abstract Hot dry rock (HDR) is a new source of environmentally friendly energy. However, during the former HDR drilling process without the cooling system, the high temperature resulted in the breakdown of drilling fluid and downhole drilling tools. The drilling fluid cooling technology has been studied in our country and abroad and has been successfully utilized in global drilling projects in high-temperature conditions. Nevertheless, traditional cooling system has a complex structure and a high power cost. In order to carry out the HDR drilling in Gonghe basin successfully, the water-based drilling fluid cooling system was designed, optimized and manufactured. The system adopted a combination of air cooling and spraying, which is equipped with cooling packing layer, cooling fan, water collector and spray system, etc. It showed benefits such as a simple structure, low total power, anti-corrosion, blocking prevention, high efficiency, convenient operation and maintenance.The indoor test showed the drilling fluid cooling system could reduce the temperature more than 25°C. The system has been applied in the HDR directional drilling project in Gonghe basin, which temperature is above 200°C at the depth of 4000m.The drilling fluid temperature was reduced from 166.3°C to 132.8°C after the cooling system running for 1h. As a result, it ensured the stability and working life of the measurement while drilling (MWD) and positive downhole motor (PDM) and the track was precisely controlled. This technique can accumulate valuable experience for similar high temperature drilling projects and improve the exploration and development of HDR.