Optimal design of jet mill bit for jet comminuting cuttings in horizontal gas drilling hard formations

• A new type of bit called jet mill bit (JMB) was presented and optimal designed.
• A model for predicting the optimal cutting's impact velocity is developed.
• A model for designing the optimal length of accelerating tube in JMB is developed.
• The factors affecting the optimal design of JMB are investigated.
• Provide an alternative to eliminate cuttings accumulation in horizontal gas drilling.

Gas drilling is considered as an ideal technique for drilling horizontal wellbore in hard formations. However, due to the low cuttings-carrying energy of gas in horizontal gas drilling, drill cuttings returned to surface are dusk-like, and the large cuttings not only can't be circulated out of down hole in time but also can't be reground by the bit teeth as in vertical gas drilling do. As a result, they are easily accumulated in the horizontal wellbores, which may lead to accidents such as wellbore plugging and pipe sticking. Based on jet comminution technique, a new type of bit called jet mill bit (JMB) was presented and optimal designed in this research, it provides a promising solution to the problem by comminuting cuttings into dust-like scale right after the cuttings are generated. In order to promote the optimal design and execution of the JMB, analytical models were developed for predicting the required cutting's impact velocity and designing the minimum length of accelerating tube in JMB, meanwhile the factors affecting the optimal design of JMB were also investigated. Case study using field data in the published article shows good consistency is observed between the predicted result and field experience. It is also concluded that the required cutting's impact velocity and the minimum length of accelerating tube increase with the initial cuttings size and decrease with the final cuttings size, and they decrease slowly with the final cuttings size when the final cuttings sizes larger than approximately 0.3 mm. This paper provides drilling engineers with a promising technology to eliminate cuttings accumulation in horizontal gas drilling.