Gas Drilling Bits Failure Mechanism Studies

In gas drilling, will the drill bit be damaged because thermal conductivity and specific heat capacity of gas is low which the highest of the rubber seal is 170℃, the diamond carbonization is 1200℃and the PDC bit is 750℃in safe?The paper studied cooling theories for three major bits which include the PDC bit, the diamond bit and the tricone rock bit in gas drilling.(1)Establish temperature rising model of PDC bit. The model was divided into three parts which include the tooth, bit and drilling string. First, the heat in friction between cutting tooth and formation was analyzed. Second, the friction heat transfer in tooth, bit and drilling string was analyzed. Third, the heat dissipation by air flow to tooth, bit and drilling string was analyzed. Last, the temperature distribution for each part was gotten. Besides, temperature rising of 81/2" PDC bit after bit balling in gas drilling was analyzed by examples in site that the critical WOB range from 35 KN to 57KN under manageable temperature in normal gas drilling, while the critical WOB range from 10.5 KN to 16.5KN compared to 20 KN to 40KN in site after bit balling that the bit was risk to damage.(2)Establish temperature rising model of diamond bit. The diamond bit is only different from PDC in tooth that the critical WOB of 81/2" diamond bit was 210KN during normal gas during compared to 20KN after bit balling when the revolving speed reach to 100rpm.(3)Establish temperature rising model of tricone rock bit. In the paper, only the bearing was discussed combined with experiment in Jianghan Bit Factory due to its complexity. Then the relationship in temperature, WOB and revolving speed was established. For better prove the benefit for bit cooling in gas drilling, we cooperated with Changqing Oil Field to realize a natural gas drilling in S-242 and S39-14-1. During the cooperation we analyzed the over heat damage of bit in detail, and checked bit with the help of Jianghan Bit Ltd.Co that we got the conclusion that the bit would not be damaged by over heat during normal gas drilling, while it would be damaged seriously by insufficient cooling caused by bit balling in wet well and stabbing in drilling string, etc.(4)Analyse of attrition and life time for tricone rock bit. Attrition equations were established to predict life time of bit during gas drilling with the method of fuzz clustering regression combined with attrition model for tooth and bearing, and the data of 152,81/2" J22 bit in Shengli Oil Field.