Performance analysis of seal rubber on rotary control head in CBM well underbalanced drilling

Sealing performance of rubber core on rotary control head is the important factor which influences the construction effect of under-balanced drilling for coal bed methane. Based on CBM drilling conditions, the finite element simulation analyses of contact pressures between the rubber core of rotary control head and the drilling tools were conducted under wellhead eccentric condition, trip-out condition and drilling condition, and then the influence of different structural parameters of rubber core on its sealing performance was analyzed, which provides a basis for optimal design and development of low pressure sealing rubber core in coal bed methane drilling. The results show that the contact pressures between the rubber core and drilling tools increase with the growth of eccentricity and the contact pressures near the eccentric side are relatively great. Under drilling condition, the contact pressures of drilling tools and joints were obviously less than that under trip-out condition. Meanwhile, the contact pressures climb with outer taper angle decreasing and decline with inner taper angle decreasing and the variation is very small. The friction increase linearly with the increase of main sealing section length and the friction of drill pipe joint is apparently bigger than that of drilling tools. the analysis results were proved to be true through field application.