This study aims to analyze the impact of the sub-working layer width on the drilling performance of

Sandwich-type Impregnated Diamond Bit (SIDB) in extra-hard, compact and weakly abrasive rock formation

called the slipping formation and determine the appropriate sub-working layer width for improving the drilling

performance.

For the field drilling test, SIDB of Φ59/41 mm with the sub-working layer width of 1.25mm was manufactured

and applied to the quartzite formation of Ongjin mining area. The result of field drilling test shows that the rate of

penetration (ROP) and the drilling footage are 1.05m/h and 10.8m, respectively, and are 3-5 times bigger than

those of the conventional bit.

The particle-size of the cuttings produced during the drilling ranges from 34.5-108.6 μm, which is much larger

than that of the conventional bit. This indicates that the rock-breaking mode is clearly transferred from

micro-cutting mode to the combination mode of micro-cutting and volumetric-breaking.

This study has proposed a new design approach to effectively overcome the slipping formation.

compressive strength of 150MPa or higher) due to composition of very small mineral grains (0.01-0.2mm) and

local siliceous cementation; (3) weak abrasiveness [3, 4]: The low abrasiveness of rock formation results in low

rate of penetration (ROP) and small rock cuttings. Therefore, the abrasive action of cuttings to the bit matrix is

limited, and diamond grains don’t easily protrude from the bit matrix [5, 6]. When the conventional diamond bit

(generally ranging from 0.2 to 0.3m/h) [7].

In order to improve the ROP of ID bit in slipping formation, many researches are conducted. A researcher

proposed a method putting down abrasive material (quartz sand) into the borehole [8]. This method promotes the

wear of matrix and protrusion of diamond grains by putting down abrasive material into the borehole, thereby the

drilling performance is improved. Other researchers have proposed the grinding of bit bottom surface and acid

matrix and low retention of the matrix on diamond. Another study for improving the drilling performance of

diamond bit in slipping formation is to change the bottom shape of bit [11]. In other words, they increased the

thrust per unit area acting on the bottom of the bit by changing the bottom shape of the bit to the inner protrusion

shape, the outer protrusion shape, the central protrusion shape, the cross-section shape and concentric circular

shape and improved the drilling efficiency. However, this method also has disadvantage that the bottom surface

of bit gradually changes the flat shape as the bit matrix is worn during drilling.

To overcome this shortcoming, a researcher has proposed the SIDB maintaining the concentric circular shape of

the bottom surface of bit during drilling and remarkably improved the drilling performance in the slipping

formation [12]. The matrix part of SIDB consists of the diamond-containing layers and diamond-free layers

toward the radial direction. These layers may be worn at different speeds during drilling due to different wear

resistance. That is, the working layers with high wear resistance (diamond-containing layer) are slowly worn and

the sub-working layers with low wear resistance (diamond-free layer) are rapidly worn. As a result, a zigzag

is relatively large, it promotes the wear action on the bit matrix and is also favorable for diamond protrusion. The

excellent rock-breaking effect of SIDB is greatly influenced by sub-working layer width.

However, further studies are needed to analyze the impact of sub-working layer width on the drilling

performance and rock-breaking characteristics of SIDB in slipping formation and to determine the appropriate

sub-working layer width for improving the drilling performance.

This study aims to investigate the impact of sub-working layer width on the drilling performance and rock failure

characteristics of SIDB in slipping formation and determine the appropriate of the sub-working layer width by

using PFC3D-based rock failure simulation, field drilling test and particle-size analysis of cuttings produced.

This study has the certain significance in effectively overcoming the slipping formation and improving the

drilling performance.

small particle-size of the cuttings and wear abrasiveness of the rock in the drilling of the slipping formation, the

abrasive ability of the cuttings to the matrix should be enhanced by increasing the particle-size of the cuttings

and the abrasive resistance of the matrix should be weakened.

To achieve the design purposes, the sandwich-type impregnated diamond (SID) cutter with two sub-working

layers and three main working layers was designed. Due to the main working layer and sub-working layer are

different in the wear resistance, the flat bottom of SIDB at the beginning gradually changes to a zigzag structure

during the drilling process (Fig. 1).

Fig. 1. Structure of SID cutter: before wear (a), after wear (b).

Because the wear resistance of the main working layer and sub-working layer is different, the sandwich-type

structure at the bottom surface of matrix is gradually formed during the drilling process. The zigzag structure

formed at the bottom surface of the matrix reduces the contact area between the bottom surface of SIDB and the

rock, thus creating a favorable condition for drilling. In addition, the zigzag structure at the bottom of SIDB also

forms zigzag rock ridges in the bottom of borehole, and the failure of such rock ridges is affected by the

sub-working layer width). That is, if the width of the rock ridges is too small, the failure effect of the sandwich

structure cannot be fully demonstrated, and if the width of the rock ridges is too large, the rock ridges will not

break and rather will act as a hindrance to the drilling. Therefore, the optimum choice of the sandwich

the drilling performance of diamond bit. In this study, we used MBD

diamond with a particle-size of 60/80 mesh

and designed a diamond concentration of 75% in the matrix [13]. The matrix of diamond bit which targets the

slipping formation with a very hard, compact and weak abrasive should be designed relatively weak [14]. In

particular, the matrix formula design of diamond bit for the slipping formation is one of the most important

design factors for realizing self-sharpening of bit and improving life. In general, various metal powders such as

tungsten carbide, iron, copper, nickel, cobalt and others are used for the matrix of bit [15, 16]. From previous

studies, we designed the hardness of the matrix as HRC 20/25. Table 1 shows the matrix formula design of SIDB

for testing.

Table 1. Matrix formula design

steel. The prepared cold-pressing compacts were sintered using by TLZK2013 vacuum hot-pressure sintering

machine. The sintering parameters are shown in Table 2. Fig. 2 shows the structure of graphite mold for sintering

the cold-pressing compacts.

Table 2. Sintering parameters for SID cutter

Fig. 2. Structure of graphite mold assembly: separator plate (1), presser (2), cold-pressing compact (3), support

(4), side plate(5).

We brazed the SID cutter to bit steel body. The design of brazing steel body and SID cutter is important to

maximize brazing strength and to reduce brazing temperature as much as possible. The increase in brazing

strength is directly related to the problem for preventing drilling accidents and ensuring the working stability of

SIDB. To ensure the stability of SIDB and prevent diamond thermal damage at high temperatures, brazing

temperature while ensuring sufficient brazing strength should be lowered as much as possible. We used the silver

We reduced the brazing temperature to below 700 °C and prevented the strength reduction of diamond grains

generated by thermal shock. A high frequency brazing machine of GP-40 type was used in brazing.

Particle Flow Code (PFC) based on the discrete element method is widely used to solve rock failure and

geotechnical problems [17, 18]. To investigate the impacts of sub-working layer width on the rock-breaking

efficiency and characteristics of the sandwich structure, a rock-cutting simulation using PFC3D software was

carried out.

The hard rock mass of Ongjin mining area is mainly quartzite. Microscopic analysis of rock samples shows that

the main mineral is quartz (silica content is more than 98%) with a small amount of muscovite. This rock was

strength is calculated indirectly by measuring the value of the maximum compressive force and then using the

following equation:

From Table 6, it can be seen that the appropriate distance between the two cutting for achieving the greatest

cutting efficiency is 1.25 mm.

GXY-1 drilling rig was used in the test, and the drilling parameters were set to the weight on bit of 4-5kN, the

revolution speed of 654rpm and the flow rate of 20-30L/min. In the drilling tests, ROP and the drilling footage

were recorded and the cuttings produced during drilling were collected for analysis. Table 7 shows the drilling

test results.

Table 7. Drilling test results

rock cuttings is shown in Fig. 6.

Through the rock cutting simulation using PFC3D, it has been confirmed that the sub-working layer width has

a significant impact on the drilling performance and rock-breaking characteristics of SIDB. From Table 6, it is

shown that the fractured volume and the ratio of fractured volume to number of crack is the greatest as

1.64×10

/unit, respectively when the distance between the two cutters is 1.25 mm. This

indicates that the cracks generated at the two cutters are connected together and cause the failure of the rock in

the central part simultaneously when the distance between the two cutters is 1.25 mm. When the distance

between the two cutters is 1 mm, the cracks generated by the two cutters are connected to each other and the

rock in the central part is broken, but the cutting efficiency is less than 1.25 mm. On the other hand, when the

distance between the two cutters is greater than 1.5 mm, the ratio of the fractured volume to the number of