Experimental study of rock-breaking with an offset single cone bit
Pet.Sci.
Experimental study of rock-breaking with an offset single cone bit
0 School of Mechanical and Electronic Engineering, China University of Petroleum , Beijing 102249 , China
An experimental study of rock-breaking with an offset single cone bit was completed on the bit bench test equipment. Data such as transmission ratio, weight on bit (WOB), rate of penetration (ROP) and torque on bit were acquired in the experiments. Based on analyzing the experimental results, several conclusions were drawn as follows. The transmission ratio of the offset single-cone bit changed slightly with rotary speed of bit, weight on bit and offset distance. The rate of penetration of the offset singlecone bit increased with increase of WOB and offset distance. The torque on bit increased with increase of offset distance under the same WOB and bit rotary speed, decreased with increase of bit rotary speed under the same WOB. The rock-breaking mechanism of the offset single-cone bit was a scraping action. This indicates that the offset single-cone bit is a chipping type bit.
Offset single-cone bit; transmission ratio; rate of penetration; torque on bit
1 Introduction
In order to accelerate the exploration and development of
China’s oil and gas resources, especially the deep offshore
reservoirs and complex reservoirs, it is necessary to develop
new drilling technology and equipment. Since the 1990s,
the slim hole drilling technology is the focus of research for
new drilling technology, the development of a new type of
bit is the key to slim hole drilling technology
(Fletcher, 2003;
Langford Jr, 1999; Li and Tan, 2000; Qiu, 2000; Wu and Liu,
2000; Yu et al, 1994)
.
Compared with conventional single-cone bit (with
no offset), the offset single-cone bit has new structural
characteristics, providing more selections of bit types for
modern slim hole drilling
(Deng et al, 2001; Ma et al,
2004; Moran and Witman, 2004; Witman et al, 2006; Xia
et al, 2002; Yu and Yu, 2007)
. However, the rock-breaking
mechanism and drilling characteristics of the offset
singlecone bit are still issues to be studied.
The transmission ratio (i.e. the ratio of cone rotary speed
to bit rotary speed) is a key parameter for bit design, which
is essential to analyzing the trajectory of teeth on the cone.
Therefore, it is a vital parameter for the design of cutting
structure
(Yu et al, 1995)
. In general, transmission ratio
for offset single-cone bit under practical conditions cannot
be obtained from theoretical study, so the effect of offset
distance on transmission ratio can be obtained only through
bench tests.
Compared with conventional single-cone bit, the offset
single-cone bit is characterized by an asymmetrical structure,
so the rate of penetration (ROP), torque and rock-breaking
characteristics must be investigated experimentally through
bench tests. At the same time, the bottom hole tracks crushed
or cut by offset single-cone bits can also be analyzed.
2 Experimental apparatus and procedures
The experiment was performed on bit bench test
equipment, developed by Southwest Petroleum University.
Fig. 1 shows its schematic diagram. The offset single-cone bit
used is shown in Fig. 2.
The bit bench test equipment is composed of a
threephase silicon-controlled recti er control box 1, direct current
electromotor 2, gear box 3, lower rotary table 4, sensors 7
and 8, joint 9, measuring joint 10, hydraulic cylinder 11,
centralizer 12, upper rotary table 13, kelly bar 14, balance
beam 15, and a swivel 16.
During testing, the upper rotary table 13 was locked by
pin and was stationary. The lower rotary table 4, on which the
rock 5 was held by a clamping device, was driven by a direct
current electromotor 2 through gearing. The rotational speed
of the electromotor was adjusted by a three-phase
siliconcontrolled rectifier. Then the bit could rotate relative to the
rotary table. The hydraulic cylinder 11 could provide the
given weight on bit (WOB) and control the vertical movement
of bit. Sensors 7 and 8 were installed to monitor rotary speed
of the bit and cone.
WOB and torque were measured by resistance strain
gauges on the measuring joint, and the linear strain was then
converted into voltage signals with dynamic strain gauges.
3 Analysis of the experimental results
Four single-cone bits with offset distances, d, of 0, 3.2, 5.8,
3
4
5
6
7
1-Silicon-controlled rectifier control box; 2-Direct current electromotor;
3-Gear box; 4-Lower rotary table; 5-Rock; 6-Single-cone bit; 7, 8-Sensor;
9-Joint; 10-Measuring joint; 11-Hydraulic cylinder; 12-Centralizer; 13-Upper
rotary table; 14-Kelly bar; 15-Balance beam; 16-Swivel
9.0 mm, were tested respectively. The journal angles of these
bits used were 40o, and their diameters were 152.4 mm. The
rock sample was limestone.
3.1 Measured signals of WOB, torque on bit and bit rotary speed
60 rpm and 2 tonnes respectively. As shown in Fig. 3, the
actual acquired signal of WOB had similar tendency with
that of (...truncated)