On the Qualitative Sampling of Minerals During Well Drilling

S. S. Sulakshin, V. G. Khramenkov

The problem of obtaining qualitative core samples in a quantity sufficient for investigations, which is of great practical importance, includes two fundamental questions:

a) determining the moment at which the beds or deposits of minerals under study are encountered;

b) obtaining and preserving qualitative samples in the required quantity.

Both of these questions have so far been far from fully resolved, especially the determination of the moment at which beds (deposits) of minerals are encountered.

On the other hand, an analysis of certain questions of sampling minerals shows the possibility of solving them with incomplete core recovery. But the necessary minimum quantity of core can be determined only for specific conditions, taking into account a whole series of factors, the principal ones being the physicomechanical properties and the character of the distribution of the mineral (Table 1).

All rocks and minerals, according to the difficulty of obtaining core during rotary core drilling of wells, may be divided into four main groups, taking into account the character of the bond between the particles composing them and their behavior under the influence of various factors in the process of drilling the well and raising the core (Table 2).

For obtaining rock specimens and mineral samples, many drilling methods and means have been developed (Table 3), and a large number of tools have been created (Table 4); however, far from all of them meet the requirements from the standpoint of obtaining representative samples (specimens).

For determining the moment at which beds are encountered, reliable signaling devices are practically absent, although there are many possible ways of solving this important question (Table 5). The possibility of using, for this purpose, developed instruments indicating the mechanical drilling speed does not solve the problem.

factors determining the formation, recovery, and quality of core in rotary core drilling of wells
(after S. S. Sulakshin)

CORE CLASSIFICATION SCHEME

Table 1

Continuation of Table 1

Table 2

CLASSIFICATION SCHEME
of rocks and minerals by the difficulty of obtaining samples (core)
(according to S. S. Sulakshin)

Continuation of Table 2.

Table 3

CLASSIFICATION
of methods and technical means for obtaining samples of useful minerals and cuttings during well drilling
(according to S. S. Sulakshin)

Continuation of Table 3.

Table 4

CLASSIFICATION SCHEME
of core-sampling tools used for obtaining core samples in rotary drilling of geological-exploration wells under complex geological conditions
(after S. S. Sulakshin)

Continuation of Table 4.

Continuation of Table 4.

CLASSIFICATION
of the principal methods and means of monitoring the moment of encounter with contacts of mineral deposits

Table 5

Continuation of Table 5

The Department of Exploration Technology of the Tomsk Polytechnic Institute has been working for a number of years on solving the problem of qualitative sampling of minerals during the drilling of geological-exploration wells.

As a result, the theoretical foundations of certain questions of the problem under consideration were developed, and a set of means was created for taking qualitative core samples when drilling wells for coal: a double core barrel (DKS-TPI) and a bed-encounter indicator (S-TPI).

The universal double core barrel DKS-TPI (Fig. 1) is intended for re-drilling mineral beds of simple and complex structure, from weak to strong. The barrel, of telescopic type, has a shutoff hydraulic device and a core bit made of four steel sectors. The universality of the barrel is determined by fulfillment of the developed requirements, the principal ones being the following.

1. The possibility of flushing the bottom of the well before drilling begins through the inner core barrel and the clearance between the pipes.

2. The use, as the flushing fluid, of both technical water and clay mud.

3. Isolation of the core from the action of the flow of flushing fluid.

4. Free (without back pressure) discharge of fluid from the inner pipe as it is filled with core.

5. Free advance of the core into the inner pipe.

6. Protection of the core from the destructive action of mechanical factors (friction forces, vibrations, shocks, and impacts of the barrel) due to the immobility of the inner pipe during drilling.

7. The possibility of drilling through strong coals and rocks (interbeds of barren rocks occurring in coal seams, in the floor and roof).

8. Drilling at forced regimes.

9. Reliable wedging and breaking off of the core at the bottom in soft and hard rocks by a floating core lifter of sector type.

10. The possibility of monitoring the moment of core wedging before lifting the barrel.

11. Reliable retention of the core in the core barrel during lifting, both in vertical and inclined wells.

12. Protection of the core-lifting device from mechanical damage during lowering and raising of the barrel.

13. Draining of the flushing fluid from the cavity of the drill pipes when raising the barrel from the well.

14. Free extraction of the core from the detachable core-receiving pipe without disturbing its integrity.

15. The possibility of making a run of more than 1.5 m.

16. Simplicity of design and low cost of the barrel.

Tests of the barrels showed that they make it possible to substantially increase core recovery. In most cases at deposits of the Kuzbass, core recovery was obtained within the range of 85–100%.

Technical characteristics of the DKS-TPI double core barrel

Section along I–I

Section along II–II

Section along III–III

View along A

Fig. 1. Double core-barrel assembly DKS-TPI: 1—adapter; 2—cylinder; 3—sealing washer; 4—seal; 5—body; 6—locking plug; 7—locking ball; 8—spindle; 9—spring; 10—thrust nut; 11—ball for shutting off the central flushing channel; 12—adjusting screw; 13—locknut; 14—outer tube; 15—seal; 16—washer; 17—lifter body; 18—lifter ball; 19—lifter; 20—locknut; 21—core-receiver head; 22—core receiver; 23—side cover of the core receiver; 24—receiving cup; 25—bit; 26—gate; 27—spring of the core breaker; 28—sector of the core breaker; 29—body of the core breaker; 30—cutters of form G-1305; 31—ball of the hydraulic seal; 32—plug.

The C-TPI coal-bed encounter indicator is a device that indicates the type of mechanical drilling speed. An increase in drilling speed upon encountering beds of minerals is registered by the instrument—the electrical circuit is actuated, and light and sound signals are switched on.

The indicator is intended for rigs with hydraulic feed, ZIF-300 and ZIF-650A, and in principle may be used on any rig with hydraulic feed.

Measurement of drilling speed is carried out by the consumption of oil displaced from the lower cavities of the feed hydraulic cylinders during the synchronous downward movement of the pistons with the swivel. Therefore, an obligatory condition for use of the indicator is feed of the tool with the spindle.

The indicator consists of a mechanical drilling-speed sensor—a float-type flowmeter—and a control-and-signal panel with an audible signal (bell or siren). The sensor is installed in a break in the oil line running from the lower cavities of the feed hydraulic cylinders to the hydraulic-control device; the control-and-signal panel is installed in a place visible to the drilling foreman.

Fig. 2 shows the sensor diagram and the block diagram of the indicator.

Fig. 2. Coal-bed encounter indicator C-TPI: a—diagram of the mechanical drilling-speed sensor; b—block diagram of the indicator; 1—float; 2—core; 3—two-section induction coil; 4—check valve; D—signal sensor; I—measuring instrument; P1—rig-type switch; P2—encounter control-speed switch; MU—magnetic amplifier; EMR—electromechanical relay; RV—electronic time relay; SP—signaling devices; BP—power-supply unit.

The operation of the sensor is as follows. Oil, passing through the sensor, raises float 1 (Fig. 2, a) with core 2, which enters induction coil 3. The two-section coil constitutes two adjacent arms of a bridge differential-transformer circuit. A measuring instrument I is connected into the diagonal of the bridge (Fig. 2, b). The instrument readings depend on the degree of unbalance of the bridge circuit, i.e., on the magnitude of insertion (height of rise) of the core into the lower section of the coil, which is determined by the oil flow rate, proportional to the rate of deepening. During all stops of the spindle and during its upward motion, the float with the core occupies the lower position, and the instrument indicates zero. When the spindle moves upward, the oil passes through check valve 4.

The signal from sensor D is amplified by magnetic amplifier MU and is fed to the input of electromechanical relay EMR, which operates when the threshold value of the signal is reached and switches on signaling devices SP. The signaling device makes it possible to set in advance (switch P2) the check rate of bed encounter, i.e., the rate at which coal beds are expected to be encountered in the given deposit. When the set rate is reached, the signal at the input of EMR also reaches its threshold value.

When the signaling device is installed on one of the drilling rigs, switch P1, which has markings for rig types, is placed in the corresponding position.

Temporary (15 sec) switching off of the signals during lowering of the tool (high rate of spindle lowering) is achieved by introducing an electronic time relay RV into the circuit. The normally closed relay button is installed on the instrument panel of the rig.

Power for the signaling-device circuit is supplied from a 220-volt AC mains. The power-supply unit BP of the signaling device includes a voltage stabilizer, making it possible to obtain correct readings when the supply voltage varies within ±30%.

Technical characteristics of the signaling device

Tests of the signaling devices were carried out in geological exploration parties of the Kuzbassuglegeologiya Trust: in 1965, in the Uropskaya GRP; in 1966--1967, in the Leninskaya GRP. Five signaling devices had been in operation since August 1967 in the Leninskaya GRP.

The tests were carried out on ZIF-650A rigs. The rocks of the sites are represented by sandy-clayey varieties from coarse-grained sandstones to fine aleurolites and argillites, carbonaceous argillites

and coals of medium hardness. The average drillability category of the rocks is the fifth. The time-study observations of the instruments, carried out in parallel with the tests, were made under practically specified drilling regimes with carbide bits of the CA, CM, and BT types: axial load—700–1200 kg, flushing intensity—100–150 l/min, number of revolutions of the tool with the principal bit diameter of 92 mm—250–480 per minute.

As of December 1967, 11,300 linear m of boreholes had been drilled using the signal device; 1,690 rig-shifts had been worked; 290 seams had been encountered; 9 seams had been missed, with two misses of seams of workable thickness occurring because of a malfunction of the sound signal; the reasons for seven misses of seams of nonworkable thickness have not been established.

The tests showed the effectiveness of using the signal device at specific deposits. The exposure of all seams was clearly recorded by the instrument. As a result of the tests, the fundamental possibility was established of distinguishing easily drillable rocks (usually coarse-grained sandstones) from coals by the amplitude of fluctuations in instantaneous drilling rates: for rocks it is ±10–15 cm/min, whereas for coals it is more often within the range of 2–3 cm/min. The noted smooth increase in drilling rate along a thin (5–10 cm) supercoal interbed makes it possible, when seams are encountered, to reduce the drilling interval to 5–10 cm instead of 20–30 cm.

The use of the signal device also makes it possible to select the most rational drilling regimes according to mechanical speed.

The signal device is simple to handle and reliable in operation. The combined use of the S-TPI signal devices and DKS-TPI double core barrels will make it possible to substantially improve the quality of sampling of coal seams.