Research on borehole forming characteristics of two-wing polycrystalline diamond compact bit in coal mines

ABSTRACT

Numerical simulation and field experiments were carried out to study the characteristics of reaming law, bottom-hole topography, and borehole volume in this paper that is a great significance to further understand the rock-breaking mechanism of the drill bit, modification of the calculation model of anchorage length and mechanical specific energy. Firstly, the numerical simulation was employed to analyze the characteristics of boreholes during drilling in different strength rock formations. Secondly, a series of two-wing Polycrystalline Diamond Compact (PDC) bits of different sizes were used for drilling in limestone, sandstone, sandy mudstone, and mudstone that was used to study the bottom-hole topography and reaming law. Finally, the collection of cuttings and recording displacements were carried out when drilling in sandy mudstone that was used to verify the proposed borehole volume calculation model. The results have shown that the reaming range is 0.273 ~ 2.056 mm when the uniaxial compressive strength of rock is between 25 and 98 MPa, and the bottom of the borehole is “bump-like.” The borehole volume calculation model considering the reaming law and the bottom shape of the borehole has improved flexibility. Furthermore, a new model for calculating the anchorage length is obtained in this paper.

KEYWORDS:

• Anchorage length
• borehole bottom shape
• borehole volume
• reaming law
• two-wing PDC bit

Nomenclature

PDC	=	Polycrystalline Diamond Compact
UCS	=	uniaxial compressive strength
MSE	=	mechanical specific energy
V	=	borehole volume, cm3
V1	=	borehole tail volume, cm3
V2	=	borehole bottom volume, cm3
L	=	borehole depth, cm
L1	=	borehole depth according to the traditional borehole volume calculation model, cm
L2	=	borehole depth according to the newly established borehole volume calculation model, cm
l	=	the top length of bump-like at the borehole bottom, cm
l1	=	anchoring agent length, cm
l2	=	anchoring length, cm
D	=	borehole diameter, cm
d	=	drill bit diameter, cm
d1	=	anchoring agent diameter, cm
d2	=	anchor (cable) diameter, cm
r	=	the radius of the semicircle at the borehole bottom, cm
M	=	mass of the drill cuttings, g

Acknowledgments

Thanks to all co-authors for their valuable efforts in preparation of this paper. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Additional information

Funding

This research was funded by the 2017 Special Project of Subject Frontiers Scientific Research in China University of Mining and Technology [2017XKQY047].

Notes on contributors

Cancan Liu

Cancan Liu He is a PhD student and majoring in mining engineering at China University of Mining and Technology.

Xigui Zheng

Xigui Zheng He is a professor at China University of Mining and Technology and mainly studies mine pressure and rock formation control.

Niaz Muhammad Shahani

Niaz Muhammad Shahani He is a PhD student and majoring in mining engineering at China University of Mining and Technology.

Zhishang Li

Zhishang Li He is a master and majoring in mining engineering at China University of Mining and Technology.