SYNOPSIS
S . 1 General
Drilling is a complex phenomenon, which is influenced by many factors. Among them the most important ones are :
(a) Bit types and their geometry, (b) Drilling thrust, its rotational speed and flushing rate and (c) Geo-technical properties of the rock formations. Out of these while factors, a and b are some of the controllable drill parameters, factor c is the uncontrollable parameter.
Economic and efficient drilling demands the accurate prediction of drilling rates achievable with a given system, by the optimisation of the controllable drilling parameters with the geo-technical properties of the rock formations,
irrespective of the mode of drilling namely (i) percussive and (ii) rotary.
S. 2 Experimental Investigations
The earlier works, available in published form, report inadequate quantum of investigation because of the limited purpose with which the investigations have been carried out.
Thus, results for one rock formation with limited level of thrust and rotational speed are only available in each of them. Thus it was thought appropriate to conduct comprehensive experimental investigations for a wide range of the variables of drilling by impregnated diamond core bits. The details of the parametric variations investigated
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Table 1 : Details of Parametric Variations Investigated.
PARAMETERS VARIABLES
I . LABORATORY INVESTIGATIONS A. Drill Machine Parameters
(i) Type
(ii) Geometry
(iii) Thrust
(iv) Rotational Speed
(v) Flushing rate
Thin walled impregnated diamond core bits.
Six sizes of diameters 15/11 mm;
22/18 mm; 30/26 mm; 3 5/31 mm; 4 5/41 mm and 55/51 mm.
Six magnitudes (120N; 195N; 275N;
3 50N; 424N and 5 0 0 N ) .
Four magnitudes (1180 rpm; 750 rpm;
475 rpm and 300 r p m ) .
At a constant flushing rate of 300 1/hr.
B. Rock Parameters (i) Type
(ii) Rock properties considered
Seven varieties, namely; Quartz, Granite, Dolerite Sandstones, Limestone (Grade I ) , Limestone
(Grade II) and Carbonaceous shale.
Physical properties
(Specific gravity, Density and Porosity).
Strength properties
(Compressive, Tensile, Punch shear, of internal
and Poissons
Shear wave Cohesion, Angle
friction.
Elastic properties (Young's modulus r a t i o ) .
Dynamic properties (Longitudinal and velocities).
Index properties
(a. Hardness, based on Shore, Vickers and Micro-bit drilling;
b. Abrasivity based on : Quartz percentage, Micro-bit drilling;
c. Cerchari and d. Protodyakonov).
Contd.
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(iii) Measured parameters
a. Penetration rate
b. Torque generated at bit-rock interface.
II. FIELD INVESTIGATIONS A. Drill Machine Parameters
(i) Type
(ii) Geometry (iii) Thrust
(iv) Rotational speed
(v) Flushing Rate
'NQ" size industrial diamond core bit.
Diameter 76/46 mm.
Rock Type Thrust Range RPM Range Quartzite
Granite BQA
Sandstone Phyllite
5000-6000 N 4000-5000 N 3250-4750 N
500-650 N 350-700 N
450-500 350-500 350-450 400-750 400-600 At a constant flushing rate of 350 1 /hr.
B. Rock Parameters (i) Type
(ii) Rock properties
Quartz, Granite, Biotite Quartzite Amphibolite (BQA), Sandstone and Phyllite.
Strength properties (Compressive and Tensile st r e n g t h ) ,
Modulus of elasticity.
C. Measured Parameters Penetration rate
S . 3 Highlights of the Experimental Investigations
The entire results of the present laboratory investigations as indicated above in Table 1, have been suitably grouped and presented spread over 3 06 illustrations. Only a few of them have been included, in this, selecting one from each group, as a sample (Figs. 5.2, 5.8, 5.13, 5.15, 5.21, 5.31, 5.39 and 5.48). The detailed
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investigations have been included in the original thesis, out of which, only the most conspicuous once are mentioned here. They are :
I. For each thrust level, rotational speed and bit diameters, the magnitudes of the torque developed at the bit-rock interface, as well as the rate of penetration in different rock types, increased in the following order; Quartz, Granite, Dolerite, Sandstone, Limestone (Grade I) and Limestone (Grade II) (Figs. 5.1 through 5.6, 5.7 through 5.9, 5.11 through 5.16, 5.14 through 5.22 and 5.2 3 through 5.25).
II. With the increase in the diameter of the drill bit, the magnitude of both the torque developed at the bit-rock interface and the rate of penetration decrease sharply when the bit diameter is greater than 45 mm. This happens because of the combined action of the two distinct and simultaneous effects resulting from the increase of the diameter of the drill bit, namely (i) decrease of the pressure at the bit-rock contact and
(ii) increase of the cutting speed.
III. Both the torque developed at the bit-rock interface and rate of penetration decreases non-linearly w i t h the increase of each of the sixteen rock properties, as indicated in Table l, in all the cases, the variation is asymptotic to both the axes (Figs. 5.4 0 through 5.57).
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S. 4 Theoretical Investigation
Considering drilling with impregnated diamond bit as a multipoint scratching operation, analytical zone of fracture, under a single diamond grit, has been determined for both ideal rocks, having no discontinuities, as well as rocks having different quantum of discontinuities. For this, both Von-Mises elasto-plastic and extension strain criteria of fracture analysis have been considered. The quantum of discontinuities in a rock medium have been effectively represented by the ratio of the Young's modulus (E) to the shear modulus (G) . The results of the analytical analysis have been presented in a concise form in (Fig. 6.1 ). The analysis with the parameter E/G clearly indicates the expected phenomena of achieving greater quantum of fracturing with the increase in the quantum of discontinuities present in the rock mass.