Electric Indicator for Encountering Coal Seams

V. G. Khramenkov

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

One of the basic requirements in the exploration of coal deposits is to obtain a complete core along the seam of the useful mineral. Full recovery of the core over the entire thickness of the coal seam cannot be achieved without precise registration of its roof during the drilling process.

Determining the moment when the drill bit enters a coal seam during drilling is a problem that requires a solution. At the present stage, the most acceptable way to solve this problem is the use of indicator devices for encountering coal seams, based on the difference in drillability between rocks and coal, the difference in the power expended (torque) on their destruction, or a combination of these two parameters, which can be measured at the surface with a sufficient degree of accuracy. As a rule, the drillability of coals is higher than that of the enclosing rocks, while the power expended (torque) on destruction is lower. A sharp increase in the mechanical drilling rate (without a change in the operating regime), accompanied by a decrease in the torque acting during drilling, gives grounds to assume that a coal seam has been encountered.

The practice of coal-deposit exploration requires the development of simple and reliable instruments for determining the mechanical drilling rate, instruments for determining the torque acting during drilling, and indicator devices for encountering coal seams. Because of the practical absence of the indicated instruments, the moment when the drill bit enters a coal seam is recorded by visual observation of the mechanical rate of deepening; to avoid missing seams during drilling, continuous monitoring of the advance of the tool string is required. Moreover, visual assessment is not always objective.

The proposed indicator circuit is based on summing two parameters that increase the reliability of the instrument data: an increasing mechanical drilling rate when a coal seam is encountered and the simultaneous decrease in the torque acting during drilling.

The schematic diagram of the indicator is shown in Fig. 1. The indicator consists of four main units: I—the sensor of the mechanical drilling rate, II—the sensor of the torque acting during drilling, III—a magnetic amplifier, and IV—a device for controlling the signal feed.

The mechanical drilling rate is measured by the flow rate of oil displaced from the lower cavities of the feed cylinders of the drilling rig when the pistons move downward synchronously with the deepening of the hole. The inductive sensor of the mechanical drilling rate operates on the principle of a float—

of a flowmeter with a disk float; it is a bridge whose arms consist of active ($R_1$ and $R_2$) and inductive ($L_1$ and $L_2$) resistances. The balance of the bridge is disturbed by a change in the variable inductance of the lower section of coil $L_2$. The change in inductance is caused by the displacement in the coil of the core of the sensor float; the magnitude of the displacement is proportional to the oil flow rate, i.e., also to the mechanical drilling speed. The current strength in the circuit of the microammeter $\mu A_1$, whose scale is graduated in cm/min of drilling speed, depends on the degree of bridge unbalance. When the instrument is installed on one of the rigs, the corresponding resistance $R_{1i}$ is switched in.

Fig. 1. Schematic diagram of the signaling device.

The torque sensor (unit II) is a current transformer, Tr. t. The current strength in the circuit of the microammeter $\mu A_2$, graduated in kgm for the drilling process, will depend on the power consumed by the electric motor, i.e., also on the magnitude of the torque on its shaft. The type and power of the electric motor (corresponding to the type of rig) are set by switching in the corresponding resistance $R_{2i}$.

Summation of the signals from the sensor of the mechanical drilling rate and the sensor of torque on the drilling process is performed by means of a magnetic amplifier (unit III), which has two control windings \(OU_1\) and \(OU_2\), the currents in which, from the sensor circuits, create oppositely directed fields.

To set the control rate of encounter, at which the coals in a given deposit are drilled, a switch with a series of calculated resistances \(r_{1i}\) and \(r'_{1i}\) is used. The switch scale is graduated in the control rates of encountering coal seams. The condition for calculating the resistances \(r_{1i}\) and \(r'_{1i}\), connected in parallel and in series with control winding \(OU_1\), is constancy of the resistance (the total resistance of the microammeter \(\mu A_1\) circuit must remain unchanged) of the branched circuit \((r_{1i}, r'_{1i}\), control winding \(OU_1)\) and a definite, equal current \(i_1\) in control winding \(OU_1\) for any settings of the encounter rate, when this drilling rate is reached.

The possibility of using the indicator at any borehole depths and drilling regimes (a wide range of torque on the drilling process) is determined by introducing into the circuit a switch with a series of calculated resistances \(r_{2i}\) and \(r'_{2i}\). The switch scale duplicates the scale of the microammeter \(\mu A_2\) (in \(kGm\)). The conditions for calculating the resistances \(r_{2i}\) and \(r'_{2i}\), connected in parallel and in series with control winding \(OU_2\), are analogous to the conditions for calculating the resistances \(r_{1i}\) and \(r'_{1i}\), namely: the resistance of the branched circuit \((r_{2i}, r'_{2i}\), control winding \(OU_2)\), in order to maintain constancy of the total resistance of the microammeter \(\mu A_2\) circuit, must be constant for any connection of the resistances \(r_{2i}\) and \(r'_{2i}\) (any torque settings); the current \(i_2\) in control winding \(OU_2\), for any torque settings on the switch scale corresponding to the readings of the microammeter \(\mu A_2\) (in \(kGm\)), is a constant value.

In calculating the indicator circuit, the value of the current in control winding \(OU_2\) is taken as

\[ i_2 \ll i'_1, \]

where \(i'_1\) is the current in control winding \(OU_1\) at practical mechanical drilling rates in coal deposits (for example, for the enclosing rocks of the coal-bearing series of the Kuznetsk Basin, a mechanical drilling rate of \(5\text{—}9\ \mathrm{cm/min}\) is characteristic), and \(i'_1 < i_1\).

The operating scheme of the indicator is as follows: on the scale of the switch of resistances \(r_{1i}\) and \(r'_{1i}\), the control encounter rate characteristic of the given deposit is set; the drilling regime is specified and the torque on the drilling process is determined from the scale of microammeter \(\mu A_2\); on the scale of the switch of resistances \(r_{2i}\) and \(r'_{2i}\), the torque on the drilling process is set.

When a coal seam is encountered (an increase in the mechanical drilling rate and a decrease in the torque on the drilling process), the current in control winding \(OU_1\) increases to the value \(i_1\) and higher; the field created by it also increases, while the current in control winding \(OU_2\) (current \(i_2\)) decreases, and the field created by it also decreases; the output voltage of the magnetic amplifier rises to a value sufficient (according to the calculated data) for actuating the control device (unit IV). As the control device, a relay may be used that switches on an encounter signal (audible or visual) or simultaneously switches off the electric drive of the rig. When the mechanical drilling rate increases (an increase of the current in control winding \(OU_1\) and an increase of the field created by it), accompanied by an increase of the torque on the shaft ...

of the electric motor (an increase in the current in the control winding \(OU_2\) and an increase in the field it produces) because, owing to the opposing directions of the fields from the sensor signals, the magnitude of the amplifier output voltage is insufficient to actuate the control device, and the seam-encounter signal is not given.

The indicator is an instrument that indicates the type of mechanical penetration rate and torque during the drilling process. The indicator is installed on rigs with hydraulic feed; mandatory conditions for its use are feeding the tool from a spindle and an electric drive for the rig.

The application of this instrument in practical work will make it possible to carry out exploration of coal deposits with higher quality and greater economy.