ISSUES OF IMPROVING THE EFFICIENCY OF DRILLING LARGE-DIAMETER EXPLORATION AND PRODUCTION WELLS IN HARD ROCKS

V. M. Matrosov, S. S. Sulakshin, A. I. Derevyannykh

(Presented by the scientific seminar of the Department of Exploration Engineering)

In the Soviet Union, thousands of boreholes intended for water-supply purposes are constructed every year. At the same time, the volume of drilling increases from year to year. The development of new and the expansion of old areas of agricultural facilities, and the construction of large industrial centers, require the immediate commissioning of new sources of water supply. A tremendous role in this matter belongs to the extraction of groundwater from drilled wells. In this case, water can be obtained in the required quantities in areas that have no surface reservoirs, with minimal capital investment. It should also be taken into account that water obtained from drilled wells more fully meets sanitary and hygienic requirements.

Exploration and production wells for water are drilled under the most varied mining and technical conditions and have their own specific features. Thus, for example, they differ from core-exploration drilling wells by their much larger diameters, and from deep oil-drilling wells by the fact that they often intersect rocks of high hardness.

One of the urgent issues in drilling wells for the purpose of water supply is the technology of drilling large-diameter wells in hard rocks by the rotary method. This article is devoted to resolving this important issue; it is written on the basis of materials from research work of the Department of Exploration Engineering of TPI, carried out in cooperation with the Vostokburvod trust.

Conditions of Drilling Wells for Water and Technical-and-Economic Indicators

The work areas of the Vostokburvod trust are very extensive and stretch from the Urals to the Pacific Ocean; at the same time, the overwhelming majority of wells are drilled by the rotary method using URB-ZAM, URB-4P, and URB-4Sh rigs, as well as ZIF-650A. Rotary drilling in hard rocks is carried out both with a full-face bottomhole and by the core method, which gives fairly good results. Thus, in the Novosibirsk SU, when drilling with shot bits, the footage per shift in rocks of categories IX–X amounted to 1.0–1.3 m with a well diameter of 168 mm. Owing to the absence of the necessary bottomhole tools and a developed technology, shot drilling has not yet gained wide dissemination.

distribution, and wells in hard rocks are drilled mainly with roller-cone bits of various types.

Well depths vary over a wide range: from several tens of meters to a thousand and more. Table 1 gives data on average well depths for 1963 for individual specialized administrations of the Vostokburvod Trust, drilled by the rotary method.

Table 1

Wells deeper than 200 m, as a rule, are drilled in relatively soft rocks; however, their number is comparatively small. Therefore, despite the small average depths of the wells, the volume of drilling in hard rocks is considerable.

The distribution of the drilled footage by rock categories and well diameters in the trust’s SU for 1963 is given in Fig. 1, a, b. Data on the percentage content of hard rocks (categories VI–X) are given in Table 2.

Table 2

Analysis of the factual material shows that rocks of high categories in terms of drillability account, for the trust as a whole, for 23.4% of the total footage. Although the relative content of high categories is comparatively small, the labor intensity of the drilling operations required for them is significantly greater than for drilling the remaining 76.6% of the footage in rocks of lower categories.

In this respect, the Chita, Khabarovsk, and Novosibirsk SUs are in the most difficult conditions. And if absolute figures are taken, then in the Novosibirsk SU almost twice as much hard rock is drilled as in all the other SUs. Hence follows the necessity

Fig. 1. Distribution of drilled footage by rock categories (a) and well diameters (b). — Novosibirsk Geological Administration; — Krasnoyarsk Geological Administration; — Chita Geological Administration; — Barnaul Geological Administration; — Khabarovsk Geological Administration; — Irkutsk section; — Vostokburvod Trust.

development of technology and technical means primarily for the indicated SUs.

The specificity of water-well drilling technology is especially clearly manifested when considering well diameters (Fig. 1, b), which vary from \(5 \frac{3''}{4}\) to \(23 \frac{3''}{4}\). Characteristic here are large values of well diameters and, accordingly, of bits. Within individual SUs, the range of variation and the specific weight of individual diameters differ, which is due to hydrogeological conditions. Table 3 gives data on the average well diameters and rock categories for individual SUs of the trust, from which it follows that no definite dependence of well diameter on rock category is observed. For example, in the Barnaul SU and the Irkutsk section the average diameters are close, although the rock categories differ sharply.

Table 3

For these same subdivisions of the trust, the largest values of the average well diameters are also characteristic, whereas for the trust as a whole they lie within the range \(9 \frac{3''}{4}\)—\(15 \frac{3''}{4}\).

The most important factor determining the contradictions in the equipment and technology of drilling water wells is the combination of rocks of high drillability categories and large well diameters. The main bits used in drilling hard rocks (categories VI—X) are: NSU—\(5 \frac{3''}{4}\)—\(9 \frac{3''}{4}\); KSU—\(9 \frac{3''}{4}\)—\(11 \frac{3''}{4}\); ChSU—\(7 \frac{3''}{4}\)—\(11 \frac{3''}{4}\); KhSU—\(7 \frac{3''}{4}\)—\(13 \frac{3''}{4}\); IU—\(11 \frac{3''}{4}\)—\(17 \frac{3''}{4}\); Trust—\(7 \frac{3''}{4}\)—\(11 \frac{3''}{4}\).

For drilling the indicated rocks, three-cone bits of various types are used mainly. At the same time, as a rule, the bits are not loaded to optimal values during drilling. The impossibility of creating optimal bit loads is associated both with the trust’s lack of heavy drill pipes and the difficulty of carrying out tripping operations, and with shortcomings in the design of the URB-3AM rig. At shallow depths, when the length of the BHA is insufficient, drilling is carried out with considerable underloading because of the absence of a special feed mechanism. Conversely, at great well depths and with an optimal BHA length, the weight of the drilling assembly may exceed the lifting capacity of the URB-3AM rig.

Axial load when drilling hard rocks is the most important factor affecting the mechanical drilling rate. This circumstance poses the task of modernizing the URB-3AM rig or of creating an entirely new drilling rig for these conditions of well construction.

Insufficient axial load, and often the mismatch of bits to the rocks being drilled, lead to rapid wear of the rock-breaking tools...

bits. For example, when drilling wells in the settlement of Chul’man (Yakut ASSR), the footage per No. 18 bit was 0.10–0.17 m; in the Novosibirsk SU, when drilling hard rocks, the footage per No. 14 bit is about 2 m. Thus, the wear of existing bits when sinking large-diameter wells in hard rocks is extremely high and substantially increases the cost of drilling operations.

An analysis of the technical-and-economic indicators for wells drilled in the Vostokburvod trust in 1960–64 shows that, as the average rock category increases, the percentage of net drilling time increases, but at the same time the number of accidents and the time required to eliminate them also increase. On the basis of the working-time balance, the organizations of the Vostokburvod trust can be divided into three groups according to time spent on drilling:

Group I — the Krasnoyarsk SU, Chita SU, and Khabarovsk SU, where the percentage of drilling time is comparatively high (42–56%);

Group II — the Novosibirsk SU, in which the percentage of drilling time has an average value (33–36%);

Group III — the Barnaul SU, where a low percentage of drilling time is characteristic (18–28%).

The design perfection of a drilling rig and the correctness of conducting the technological process are most fully characterized by drilling rates, in particular such rates as technical, commercial, and cycle rates. The level of these rates varies over a fairly wide range among the SU of the trust, but the values of the technical rates change regularly in inverse proportion to the average categories of the drilled rocks. Thus, the highest technical drilling rates are in the Barnaul and Novosibirsk SU (6.0–7.0 m/rig-month), and the lowest are in the Chita and Khabarovsk SU (1.1–2.5 m/rig-month). Taking the technical rate in each SU as 100%, it is possible to trace the relative decline in the commercial and cycle drilling rates (Fig. 2).

From consideration of this graph it follows that the greatest decline in the commercial drilling rate relative to the technical rate is observed in the Barnaul SU. Here there is a high percentage of unproductive expenditures (accidents, downtime, etc.). The ratios of cycle drilling rates to commercial rates for individual SU are sharply disturbed, and only in the Barnaul SU does the relative decline in the level of the cycle rate increase even more. The most important factor in this circumstance is the considerable duration of pumping operations.

Finally, the economy of drilling is substantially affected by the accident rate of the work, which is associated mainly with drill pipes. The large number of these accidents is due to the fact that the diameter of the drill pipes does not correspond to the diameter of the wells. The ratio of the well diameter to the diameter of the drill pipes reaches 3.34–4.30, whereas in core drilling and in drilling for oil and gas this ratio is 1.50–2.40.

All the factors listed, to one degree or another, affect the cost of drilling operations. The actual cost of 1 linear meter of wells varies within fairly wide limits among the special departments of the trust, which is connected with differences in the conditions under which the work is carried out. For example, in the Krasnoyarsk SU, when drilling hard rocks, the cost of 1 linear meter is 90–130 rubles; in the Khabarovsk SU, 100–150 rubles; in the Chita SU, 100–140 rubles. In the Novosibirsk SU (1962), the cost of 1 linear meter for various drilling methods was: percussion — 96.3 rubles, rotary — 59.5 rubles, core — 57.2 rubles. Taking into account that wells were sunk by the rotary method both in hard and in soft rocks, and by the core method only in hard rocks, the need for further

the development of technology for drilling water wells by the percussive method is beyond doubt.

Thus, the analysis conducted of the conditions for sinking water wells in hard rocks by the rotary method shows: 1) the difficulties and specific nature of this type of work; 2) shortcomings in the equipment and technology of well drilling; 3) the causes of certain negative phenomena that reduce the efficiency of well construction.

Fig. 2. Relative decrease in drilling rates

On the basis of the foregoing, certain ways are determined for increasing productivity in the sinking of exploration-and-production wells in hard rocks.

Principal Ways of Increasing the Productivity of Water-Well Drilling in Hard Rocks

1. Hydraulic feed on URB-3AM rigs. On URB-3AM rigs, at shallow depths, and also in the absence of a heavy bottom assembly, it is impossible to create the required axial pressure on the bottom of the borehole. In this connection, the task arises of creating a forced-feed mechanism for these rigs.

The first drawings of such a mechanism were developed at Kazgiproproekt; an experimental prototype was manufactured from them, but the test results are unknown to us. Analysis of the developed design shows that it has two significant shortcomings: 1) the presence of a single hydraulic cylinder, with the adopted connection of its rod to the driving drill rod, will necessarily cause the crosshead to skew, which will not allow feed regulation to be carried out. This is evidenced by experience in operating ZIF-type machines when one cylinder fails; 2) one cylinder does not permit load regulation over a wide range.

In view of this, a hydraulic-feed scheme for the URB-3AM rig with two cylinders and the corresponding modification of individual units is proposed for development (Fig. 3).

Hydraulic-feed cylinders 9 from the ZIF-300 machine are mounted on the working platform 13 of the rig. Uprights 7, which serve as rod guides, are also fastened to the working platform and, in addition, are attached at the top to the mast belt by means of clamps 3. The connection of the hydraulic-cylinder rods with the driving drill rod 5 is made by means of crosshead 6 and two chucks 4—the upper and the lower. These chucks are connected to the rod through ball bearings so that, when it rotates, they remain stationary, but can be fixed in a position that prevents their displacement along the rod in the axial direction. When making a connection, the crosshead is thrown back, rotating on the rod of one of the cylinders.

The hydraulic cylinders of the rig are controlled from distributor 1 through a system of high-pressure hoses 11. When an additional load is created on the bottom, the hydraulic cylinders will operate as jacks and can create a force of 10 t. Given the weight of the entire URB-3AM rig, 13.6 t, the axial load will probably have to be somewhat limited so that the rear part of the rig does not lift off the ground.

Most of the units and parts of the proposed hydraulic-feed scheme are standardized. It is only necessary to manufacture the crosshead and both chucks.

At present there are no specialized rigs for drilling large-diameter wells in hard rocks. Therefore, the creation of a hydraulic attachment for the URB-3AM directly in production organizations is a realistic way to increase the efficiency of water-well drilling.

2. Shot drilling. In the Vostokburvod trust, shot drilling for water has a small relative share, but, as was shown earlier, the technical and economic indicators of this method are fairly high. The wide spread of shot drilling is hindered by a number of factors, such as the lack of developments in the technology of drilling large-diameter wells, the imperfection of bit designs, issues of core breaking, and others.

The primary task is to create a rational type of large-diameter bit. In order to eliminate the need to break off the core, it is advisable to have a bit for drilling with a solid bottom, which will also lead to an increase in the duration of a run. In exploration—

Fig. 3. Diagram of the hydraulic feed system for the URB-3AM rig:
1 — distributor; 2 — pressure gauge; 3 — clamp; 4 — chuck; 5 — driving rod; 6 — crosshead; 7 — post; 8 — mast; 9 — hydraulic cylinder; 10 — base of the post; 11 — high-pressure pipe; 12 — channel beam; 13 — rig platform; 14 — rotary table; 15 — stiffening ribs.

Fig. 4. Shot bit for drilling with a full-face bottomhole: 1—upper part of the body; 2—hydraulic nozzle; 3—lower part of the body; 4—bit; 5 and 6—bushings.

many designs of full-face drilling bits have been proposed. The most successful should be considered the design proposed by V. N. Pavlovskii and R. I. Ostroushko. On its basis, a bit for large-diameter boreholes is proposed (Fig. 4). The bit is intended for drilling boreholes with a diameter of up to 166 mm (allowing for reaming of the borehole walls to 180 mm). A distinctive feature of the bit is that, because of its considerable dimensions, it is manufactured from separate parts of the drilling tool: the upper 1 and lower 3 parts of the body are made from tool joints, the bit 4 from a coupling blank, and the sleeve 5 from weighted drill pipes.

The operating principle of the bit is clear from the diagram and consists in the forced circulation of shot under the action of the flow of flushing fluid. This ensures a steady feed of shot to the bottom of the borehole, which will make it possible to work at high specific pressures and rotational speeds.

3. Shot stepped bit. Of great importance in increasing the efficiency of drilling large-diameter boreholes is the design of the rock-breaking tool. The known designs of roller-cone bits, during operation, form a flat or nearly flat bottom-hole shape. As a result, the teeth of the cones work to crush the rock with one exposed plane, which does not make it possible to use sufficiently some mechanical properties of rocks during destruction. It is known that the shear strength of rocks is 2–10 times lower than their compressive strength, and under bending and tension the strength of rocks decreases still further. In addition, the existing arrangement of cones with a large angle of inclination of their axes to the vertical has a very unfavorable effect on the operation of the bearings and the supporting part of the bit because of large bending loads.

In order to eliminate the indicated shortcomings, a special design of a shot stepped bit has been developed, ensuring the operation of each cone tooth in shearing and breaking off the rock in the presence of two exposed surfaces. This is achieved by reducing the angle of inclination of the cone axis to the vertical and by changing the arrangement of the teeth.

In the diagram (Fig. 5), the shot stepped bit is shown in section (in this case, a three-cone bit, although the number of cones does not affect the design of such a bit). On trunnions 1, arranged at an acute angle to the axis of the bit (20–30°), cones 2 are mounted with the aid of radial-thrust bearings 3 and ball locking bearings 4. The teeth 5 of the cones are arranged on the surface of the latter in such a way that, during rotation, they ensure an optimal angle of engagement with the surfaces of the steps 6 of the bottom hole.

Thus, owing to the adopted arrangement of the cones, during drilling of boreholes a stepped bottom hole is formed, and the teeth interact with the rock along two exposed surfaces, which significantly improves the conditions of rock destruction. The small angle between the axis of the cones and the vertical substantially reduces the bending moment on the trunnions in comparison with existing bits. In addition, the radial forces on the cone bearings are reduced to radial-axial ones, which made it possible to install radial-thrust bearings characterized by a high coefficient of serviceability.

The noted positive aspects of the shot stepped bit allow one to hope for an increase in the productivity and service life of the bottom-hole tool when drilling boreholes of any diameter in hard rocks.

4. Multibarrel drilling. Drilling boreholes having

two or more boreholes, is widely used in core exploratory drilling, as well as in drilling for oil and gas. Such a method yields a significant economic effect.

In the practice of drilling water wells, and first of all in hard rocks, the prospects for multiborehole drilling are obvious. When drilling multiborehole water wells, at least two problems can be solved:

Fig. 5. Roller stepped bit: 1 — trunnion; 2 — roller cone; 3 — radial-thrust bearing; 4 — ball locking bearing; 5 — teeth of the roller cones; 6 — bottom-hole steps

two tasks: 1) to increase the yield of the designed well; 2) to reduce the well diameter while maintaining the designed yield. Both will undoubtedly help improve the quality and productivity of drilling operations and reduce their cost.

Drilling a two-borehole well in hard rocks may be carried out according to the following scheme (Fig. 6). The interval “ab” is drilled with a bit whose diameter is determined by the dimensions of the water-lifting equipment. The depth of the interval must be selected based on the position of the dynamic water level in the well. The next interval “bv” is drilled with a bit of considerably smaller diameter. At point “v,” the position of which will depend on the depth of occurrence of the aquifer, the wellbore is deflected with a transition to the smaller diameter. In this process, both technical means of deflection and natural geological factors of borehole curvature may be used. Drilling of the deflected borehole is completed in the aquifer.

Then, using a rigid assembly with a bit of the same diameter as that used to drill interval “bv,” drilling of the main wellbore is continued to point “d.” Considering that in hard rocks the well may remain uncased, the proposed method is quite feasible and may be used in drilling water wells.

Using a wedge lowered into the borehole on casing pipes, it is possible to drive 5–6 additional boreholes, which will significantly increase the water yield of an exploratory-production well.

Conclusion

The analysis carried out of the conditions and technical-economic indicators of drilling water wells in hard rocks made it possible to identify certain reserves for increasing labor productivity and to outline ways for the further improvement of the equipment and technology for drilling boreholes. Naturally, the recommendations listed do not resolve the entire range of issues associated with drilling and, even more so, with the development of exploratory-production wells. It is sufficient to note the prospects for the use of hydropercussion drilling in hard rocks, and of the newest methods—electrohydraulic, electro-impulse, etc. Nevertheless, taking into account the insufficient attention paid to the technology of drilling water wells in hard rocks, the questions raised and the proposed ways of solving them should contribute to increasing the efficiency of drilling operations.

Fig. 6. Diagram of drilling a two-borehole well