TOWARD SOLVING THE PROBLEM OF ENCOUNTERING COAL SEAMS DURING DRILLING

V. G. KHRAMENKOV

(Presented by the scientific seminar of the Department of Exploration Technology, TPI)

Initial Parameters and Sensors of Signals Indicating the Encounter of Coal Seams

One of the requirements in the exploration of hard-coal deposits, as well as of other types of mineral resources, is high-quality sampling, which consists in obtaining core with an undisturbed structure throughout the entire thickness of the seam. The latter is possible only if the moment at which the drilling bit enters a coal seam during drilling is determined in good time and special means are used for cutting through coal seams. At present, fairly effective double core barrels have been developed for drilling through coal seams, and the primary issue is the creation of signaling devices for detecting the encounter of coal seams during drilling. The use of signaling devices is especially necessary in the exploration of new coal deposits, and still more so of deposits with thin coal seams [1].

Determining the moment at which coal seams are encountered is a particular problem of well logging during drilling; the importance of solving it for further work on automating the drilling process is quite obvious. In doing so, it is necessary to solve a number of problems: to select the initial parameter for the sensor of the signaling device, to develop a conversion circuit, and equipment for reliable transmission and reception of the signal at the surface. Naturally, all changes in the drilling process (changes in the operating mode of the bit, replacement of rock layers) can be recorded only with the installation of bottom-hole sensors.

In this article an attempt is made to solve several particular questions of a large general topic, namely: to consider possible initial parameters and sensors of signaling devices for detecting the encounter of coal seams, and their practical applicability at the present stage.

The choice of the initial parameter determines the type of sensor and the place where it is installed. Table 1 gives a classification of the initial parameters and sensors. All initial parameters may be subdivided into direct and indirect. By direct parameters we shall understand the physicochemical properties of coals that distinguish them from the enclosing rocks, as well as changes in the properties of the flushing fluid of the upward flow when a coal seam is encountered. By indirect parameters we mean changes in the parameters of the operating regime of the drilling rig when rock layers change. According to the place where they are installed, sensors are divided into surface and bottom-hole sensors.

For surface sensors, indirect parameters may be used. An additional corroborating factor for the signal

Table 1

the sensor may be the carrying out of coal particles and the change in the properties of the flushing fluid flowing out of the borehole, but this is practically of little acceptability because of the frequent complete absorption of the fluid. Moreover, depending on the depth of the borehole, the delay in the exit of coal particles to the surface may reach several tens of minutes [2].

Depending on the initial parameter, the following variants are possible for surface sensors:

1. Sensors responding to a change in the mechanical rate of advance. This group of sensors may be constructed on the principle of using:

a) the translational motion of the machine spindle, the driving rod, or the cable; the rotation of the shaft or drum of the winch, or the sheave of the crown block; the amount of advance with reference to time. In this case the sensors may have various connections with the initial impulse (mechanical, hydraulic, electrical, etc.);

b) the flow rate of the stream of oil displaced from the lower cavities of the feed hydraulic cylinders (float-type flowmeter instruments; instruments based on the pressure-drop principle, and other flowmeters).

1. Sensors responding to an automatic change in axial load as the rate of advance changes. Sensors of this group may be built in: into the hydraulic system of the machine or other mechanisms kinematically connected with the traversing unit of the drilling machine; into the “dead” end of the cable (mechanical, hydraulic, electrical, and other dynamometer sensors, weight indicators), and they may be connected with the brake system of the machine—when the tool is fed from the winch drum brake.

2. Sensors responding to a change in torque, built into the kinematic chain of the machine.

3. Sensors responding to a change in the power expended when rock layers change.

As studies [2] have shown, sensors of the third and fourth groups in individual Kuzbass deposits may be unacceptable.

1. Sensors responding to a change in the properties of the flushing fluid as it flows out of the borehole (change in color, radioactive properties, etc.), but, as already noted above, these sensors will be of little acceptability because of the frequent complete absorption of the flushing fluid and because of the large delay of the signals.

2. Sensors recording changes in the amplitude-frequency characteristic of the elastic vibrations of the tool (at a small borehole depth): inductive, piezoelectric, strain-gauge, and others, having a connection with the tool.

3. Sensors responding to a change in the operating parameters of the flushing pump. Owing to the low strength of coal, temporary clogging of the core barrel is observed at the initial moment when the drill bit enters a coal seam, which is indicated by an increase in pressure at the pump discharge. But because of large fluctuations at the pump discharge during drilling, this phenomenon can hardly be used as an independent parameter for monitoring encounters with coal seams.

The probability of an unambiguous solution to the question can be increased by using instruments that operate simultaneously from several sensors, for example from a drilling-rate sensor and a sensor of the torque in the drilling process.

Downhole sensors, depending on the initial parameter, may be divided into the following groups:

1. Sensors responding to changes in the physicochemical properties of rocks.

2. Sensors responding to changes in the operating regime of the drill bit (torque, axial load, rate of penetration, bit temperature, change in the frequency–amplitude characteristic of vibrations of the drilling assembly).

3. Sensors responding to changes in the properties of the ascending flow of drilling fluid.

For downhole sensors, variants involving the combination of several initial parameters are also possible.

In core drilling of boreholes, downhole sensors (except for a temperature sensor indicating the degree of heating of the drill bit) will be located above the core-receiving tube. In this case, logging signals may be taken either from the bottom of the borehole or at the place where the sensor is installed. On this basis, downhole sensors should be subdivided into remote and contact sensors. Logging signals from contact sensors will arrive with a depth lag equal to the interval at which they are installed above the bottomhole, which does not solve the problem of timely recording of the moment when coal seams are encountered. In this case, the discussion may concern remote sensors of changes in the properties of the rocks being penetrated or of the drilling fluid; in the latter case there will be an insignificant delay (2–3 sec) in recording changes in rock properties during the time required for cuttings particles to pass from the bottomhole to the point where the sensor is installed.

Coreless drilling increases the possibilities for installing more effective sensors and, by bringing them closer to the bottomhole, increases their resolving capacity.

Having considered the possible initial parameters and sensors, it is necessary to emphasize the prospects for installing downhole apparatus. In our opinion, the most acceptable initial parameters for downhole apparatus are the following:

a) the density of rocks and coal (gamma–gamma logging);

b) the radioactive properties of coals (gamma logging);

c) the different degree of absorption of ultrasonic waves by rock and coal;

d) the dielectric and magnetic permeability of rocks and coal;

e) a combination of several parameters, for example, the mechanical rate of penetration and the torque required to break the rocks at the bottomhole, with the torque sensor possibly being downhole and the mechanical-rate-of-penetration sensor being surface-mounted. In this case only one parameter need be transmitted, which will simplify the apparatus. However, when installing downhole sensors it is necessary to solve a number of problems. The principal issue here is the creation of downhole apparatus and of a communication channel between the bottomhole and the wellhead, operating reliably under specifically difficult conditions. A certain difficulty is presented by placing downhole apparatus in a small-diameter core barrel (for example, 89 and 75 mm); in core drilling it is practically impossible to place a signal sensor directly at the bottomhole. Logging of core-drilled boreholes during their drilling, using geophysical methods and with signals lagging behind the bottomhole by the length of the drilling assembly (the height at which the signal sensor is installed above the bottomhole), will certainly be of great positive value, but, as already noted earlier, will not solve the question of a timely encounter with coal seams (5–10 cm of drilling through coal is permissible).

Thus, for the purposes of monitoring the encounter with a coal seam during the drilling process, at present the simplest and most acceptable are surface instruments using already known paramet—

rocks: the drillability of rocks (the mechanical rate of penetration and, as a consequence, the automatic change in axial load when the rate of penetration changes), the torque, or a combination of these.

The S1-TPI Coal-Seam Encounter Signaling Device

To determine contacts between rock beds and a useful mineral (coal in particular), at present the most acceptable and most often used in practice is the difference in drillability of the rocks of the interval being penetrated [1, 2].

Because the mechanical drilling rate depends on the operating parameters, and because the mechanical drilling rate in rock and in coal may be the same, recording the contacts of coal seams during drilling (the encounter of seams) is, in the general case, not solved unambiguously. However, it can be solved for most specific deposits, where a significant difference is observed between the drillability of rock and coal. Most often the drillability of coals is higher than that of rocks, and a sharp increase in the mechanical drilling rate (without a change in the operating regime) gives grounds to suppose that a coal seam has been encountered. An additional confirmation of this is the bringing of coal particles to the surface during circulation of the flushing fluid.

In practice, because of the absence of instruments, an increase in the mechanical drilling rate is most often assessed visually, and therefore not always objectively; cases of missed seams are not uncommon. The practice of exploring coal deposits requires the development of simple and reliable instruments for determining the mechanical drilling rate and signaling devices for the encounter of coal seams. Tests of the most up-to-date apparatus of the PKM and IRB types in Kuzbass did not give positive results.

We propose the S1-TPI coal-seam encounter signaling device, developed at the Department of Exploration Technology of the Tomsk Polytechnic Institute.

The S1-TPI signaling device is an indicating-type instrument for the mechanical drilling rate and is intended to signal the attainment of a certain drilling rate at which coal seams are expected to be encountered; it can be used at deposits with differing drillability of rock and coal. The signaling device is intended for drilling rigs with hydraulic feed of the ZIF type.

The signaling device is based on the principle of measuring the flow rate of the oil displaced from the lower cavities of the feed hydraulic cylinders when the piston moves downward. The mechanical drilling rate is determined from the oil flow rate. When a certain drilling rate is reached, at which coal seams are expected to be encountered, the electrical circuit of the instrument is actuated and light and sound signals are switched on.

The main units of the signaling device are the sensor and the monitoring-and-signaling panel. The sensor is mounted in a vertical position in the oil line connecting the lower cavities of the feed hydraulic cylinders with the hydraulic-control unit, and is connected by cable to the monitoring-and-signaling panel, installed in a place visible to the driller.

The schematic diagram of the instrument is shown in Fig. 1.

The sensor of the signaling device, with magnetic transmission of readings, operates on the principle of a float-type flowmeter with a disk float and consists (Fig. 2) of housing 1, float 7 with core 4, and inductive coil 5. Oil passing through the sensor raises the float with the core—

Fig. 1. Schematic circuit diagram of the SG-ShKh signaling device

Visible labels in the diagram:

• Sensor
• To the distributor valve
• From the lower cavities of the hydraulic cylinders
• Green
• Red
• Switch 1
• Switch 2
• Transformer
• 220 V mains
• $\mu A$
• $D_1$, $D_2$
• $R_1^*$, $R_1^{**}$, $R_2$, $R_3$, $R_4$, $R_5$, $R_6$, $R_7$, $R_8$
• $P_1$, $P_2$, $P_3$
• $L_1$, $L_2$
• $C_1$
• $U_1$, $U_2$, $U_3$
• $T_p$
• $I$, $II$, $III$, $IV$
• $0$, $1$, $2$, $3$, $4$, $5$, $6$, $7$, $8$, $9$, $10$, $11$
• $\tau_1$, $\tau_2$, $\tau_3$, $\tau_4$, $\tau_5$, $\tau_6$, $\tau_7$, $\tau_8$, $\tau_9$, $\tau_{10}$

by an amount proportional to the mechanical rate of penetration. The position of the core determines the inductive reactance of the coil, which is included in the bridge circuit of the instrument \(L_1, L_2, R_1^*, R_2, R_3\). The readings of the microammeter \(\mu A\) (current strength in the circuit), whose scale is graduated in cm/min of drilling rate, depend on the degree of change in the inductive reactance of the coil.

When the spindle stops, oil does not pass through the sensor; the float sits in seat 3, the core assumes the extreme lower position, and the instrument reads zero. When the spindle is raised, the oil in the sensor passes through check valve 2; the float with the core assumes the lower position, and the instrument reads zero.

The circuit of the instrument (Fig. 1) includes a sensitive polarized relay of the RP type (\(P_1\)) and a more powerful relay of the RS-13 type (\(P_2\)). When relay \(P_1\) operates, voltage is supplied through its contacts to the winding of relay \(P_2\), which switches off the green lamp and switches on electric bell \(Zb\) and the red lamp—sound and light signals. Activation of the signals means that the rate of penetration is equal to or higher than the encounter control rate preset by means of switch \(P_2\) on the panel. The calculated resistances \(r_i\) and \(r_i'\) of switch \(P_2\) are determined from the condition of constancy of the resistance of the branched circuit and constancy of the current through relay \(P_1\) at any setting of the encounter control rate.

Fig. 2. Diagram of the sensor of the S1-TPI signaling device:
1 — housing; 2 — check valve; 3 — seat; 4 — core; 5 — inductive coil; 6 — core cylinder; 7 — disk float

When installing the instrument on one of the rigs, one of the resistances \(R_6, R_7, R_8\) is switched in by means of switch \(P_1\).

Automatic compensation of the instrument error due to changes in oil viscosity with changes in temperature is achieved by means of thermistor \(R_5\), which is mounted directly in the sensor of the signaling device.

The instrument has two scales: a small one (0–60 cm/min) and a large one (0–120 cm/min). The scale is set by scale switch \(P_{\text{sh}}\), by means of which the shunting resistance \(R_4\) is connected in parallel with the microammeter (large scale) or disconnected (small scale).

The circuit of the instrument is powered from a 220-volt AC mains supply through transformer \(Tr\).

A prototype of the S1-TPI signaling device was manufactured in the institute’s workshops and tested in the drilling laboratory of the Department of Exploration Technology and under field conditions (Leninskaya GRP of the Kuzbassuglegeologiya trust). Laboratory and field tests demonstrated the operability of the instrument and yielded positive results.

Introduction of the signaling device into production will make it possible to carry out exploration of coal deposits with higher quality and more economically.

LITERATURE

1. S. S. Sulakshin. The problem of high-quality sampling of solid minerals. Journal Izv. vuzov—Geology and Exploration, No. 3, 1962.

2. S. S. Sulakshin, V. G. Khramenkov, V. P. Rozhkov. Some experience in solving the problem of encountering coal seams under the conditions of Kuzbass. Izv. TPI, vol. 127, issue 2, 1964.