A Mechanical Model of Gas Drainage Borehole Clogging under Confining Pressure and Its Application

Abstract

Drilling in a coal seam that has gas and coal outburst activities is closely related to the discharge of drill cuttings into a borehole. Due to the low effectiveness of slagging, there is a risk that the drilling equipment will be lost if the borehole contains too many drill cuttings, especially when drilling in soft coal seams that suffer from borehole deformation and instability problems. In order to investigate the mechanisms underlying clogged boreholes, a mechanical model is established that considers the confining pressure pi that surrounds a borehole. The characteristics of clogged boreholes, which are affected by parameters such as the clogging segment’s length L, the drilling angle θ and confining pressure pi, were analyzed. The results show that the dredging pressure has nearly exponential growth as the clogging segment’s length L increases and the gravity of the clogging segment reduces the demand for dredging pressure, especially in upward drilling. In downward drilling, the blowing-through pressure increases as the absolute value of the drilling angle increases and will reach a maximum value when the drilling angle θD is in the range of −π/2~0. At the same time, the borehole’s confining pressure pi is the dominant factor in borehole clogging. Meanwhile, boreholes with a high confining pressure pi, especially in soft coal seams and coal seams with a coal outburst, constitute a significant risk. Finally, an actual drilling field construction was evaluated and optimized by applying the clogging segment mechanical model. The results show that the drilling depth was improved by 18.5% on average, and the drilling efficiency was improved by 39.7%, in comparison to drilling activities without optimization.