US3430712A

US3430712A - Well tool position indicator

Info

Publication number: US3430712A
Application number: US649951A
Authority: US
Inventors: John A Stafford
Original assignee: Schlumberger Technology Corp
Current assignee: Schlumberger Technology Corp
Priority date: 1967-06-29
Filing date: 1967-06-29
Publication date: 1969-03-04
Anticipated expiration: 1986-03-04

Description

March 4, 1969 .3. A. STAFFORD 3,430,112

WELL TOOL POSITION INDICATOR Filed June 29, 1967 Sheet of 2 1 N VEN TOR.

March 4, 1969 J, A. STAFFORD 4 3,430,712 WELJL TOOL POSITION INDICATOR I Z of 2 Sheet Filed June 29, 1967 Aria-W2 United States Patent 8 Claims ABSTRACT OF THE DISCLOSURE The particular embodiment described herein as illustrative of one form of the invention is directed to a position indicator for borehole apparatus used to obtain and collect a sample from earth formations traversed by the borehole. To accomplish this, the disclosed tool includes cutting wheels that are arranged to be extended and make cuts along the face of an adjacent formation as a carrier supporting the wheels is moved longitudinally. An electrical control on the carrier is varied as it moves along a particularly arranged surface on the tool to produce electrical signals characteristic of the relative position of the carrier to the fixed position of the tool.

Accordingly, as will subsequently become more apparent, the present invention pertains to new and improved monitoring means for determining at the surface the positions of relatively movable portions of a well tool; and, more particularly, this invention relates to control means for providing an indication at the surface of the progress of a core-slicing tool as it cuts away samples of earth formations from the Wall of a previously drilled borehole.

Heretofore, formation samples have usually been obtained from previously drilled boreholes by explosively propelling into the adjacent Wall of a borehole one or more tubular bodies or so-called bullets having appropriately arranged forward cutting edges. As these bullets penetrate the borehole wall, a generally cylindrical core of the formation material is driven into each bullet so that, when the bullets are subsequently retrieved, the cores in each will be recovered at the surface for examination. Typical of such core-taking bullets are those shown in Patent Nos. 2,678,804, 2,923,530, 3,072,202 and 3,220,- 490.

It is recognized of course that although such core-taking bullets have been highly successful, the most ideal arrangement would be to obtain a continuous sample of an earth formation from along a substantial vertical interval of a borehole. Heretofore, this has not been commercially feasible at least from boreholes that have been previously drilled.

One tool as shown in Patent No. 3,173,500 has been proposed, however, in which a pair of rotatable outwardly-converging cutting wheels are cooperatively arranged to be extended outwardly to cut their way into an adjacent formation. Then, as they are slowly raised, the cutting wheels will cut an elongated wedge-shaped formation sample out of the borehole wall. This sample is caught by the tool and returned to the surface. This tool is not arranged, however, to provide indications at the surface of the progress of the cutting wheels.

3,430,712 Patented Mar. 4, 1969 ice Accordingly, it is an object of the present invention to provide new and improved position-establishing means for providing a signal at the surface of the progress of a coreslicing tool as it cuts away elongated samples from earth formations.

This and other objects of the present invention are obtained with a core-slicing tool having fixed support means cooperatively carrying cutting means thereon and arranging on one of these means a variable electrical control having a laterally-movable actuator and on the other means a control surface having variable lateral dimensions along its longitudinal length. By positioning the control actuator on the control surface, as the cutting means move relative to the support means, the electrical char-acteristics of the electrical control will be varied in accordance with the relative positions of the actuator as it moves along the control surface. Appropriate circuit means are provided to respond to the resulting changes in electrical characteristics of the control and transmit a signal to the surface that is representative of the corresponding positions of the cutting means in relation to the support. This signal is, therefore, monitored to determine the positions of the cutting means during the course of a sample-taking operation.

The novel features of the present invention are set forth with particularity in the appended claims. The operation together with further objects and advantages thereof, may best be understood by way of illustration and example of certain embodiments when taken in conjunction with the accompanying drawings, in which:

FIGURE 1 depicts a core-slicing tool with control means arranged in accordance with the present invention in a borehole and in position to obtain an elongated sam- P FIGURE 2 is a schematic representation of the intermediate portion of the tool shown in FIGURE 1;

FIGURE 3 is a schematic representation of a groove system that may be employed with the tool shown in FIGURES 1 and 2;

FIGURE 4 is a partial cross-sectional view taken along the lines 44 in FIGURE 3;

FIGURE 5 is a schematic representation of the electronic circuit for use in determining the operating posi tion of the cutting wheels in the tool shown in FIGURES 1 and 2; and

FIGURE 6 depicts one manner of mounting an electrical control for use in the present invention.

Turning now to FIGURE 1, a core-slicing tool 10 arranged for use with the present invention is shown suspended from a cable 11 in a borehole 12 and in position for cutting means, such as a pair of similar cutting wheels 13, to cut away an elongated prismatic or wedgeshaped sample 14 from the adjacent wall of an earth formation 15. As seen in FIGURE 1, the tool 10 is preferably comprised of a number of tandemly connected housings 16-20 suitably arranged for enclosing the various components of the tool.

It will be appreciated, of course, that to better understand the present invention the general arrangement and operation of the tool 10 as a whole should be first described. Although the following description is believed to be sufficient, copending applications Ser. Nos. 649,929, 649,976 and 649,952 filed simultaneously herewith more fully describe those other portions of the tool 10.

The upper housing 16 preferably encloses suitable circuitry for locating the tool at a desired position in the borehole 12 as well as for controlling the various components in the tool and transmitting information and power through the various conductors in the suspension cable 11. The circuitry preferably employed with the cutting means of the present invention is fully described in a copending application Ser. No. 649,978 filed simultaneously herewith.

The next lower housing 17 preferably include suitable longitudinally spaces, hydraulically actuated pistons 21 for selectively extending a wall-engaging member 22 on the rear of the tool 10 laterally against one side of the borehole 12 to shift the forward face of the core-slicing tool in the opposite direction. To make the wall-engaging member 22 selectively operable from the surface, a hydraulic pump 23 and chamber 24- (shown in dashed lines) are arranged to extend and retract the wall-engaging member by pumping hydraulic fluid into the piston chambers either behind or ahead of the pistons 21. By maintaining an increased hydraulic pressure behind the pistons 21, the wall-engaging member 22 will, of course, urge the forward face of the tool 10 against the opposite wall of the borehole 12 with a corresponding force.

The intermediate housing 18 of the tool 10 supports the cutting wheels 13 that are respectively mounted in converging vertical planes and arranged to rotate about independent, outwardly diverging axes themselves lying generally in the same horizontal plane and intersecting each other at a suitable angle. A longitudinal opening 25 is provided along the forward wall of the housing 18 diametrically opposite from the wall-engaging member 22. The cutting wheels 13 are suitably arranged and sized in relation to one another so that, when extended, their converging peripheral edges will pass through the housing opening 25 and all but come together at about the point of intersection of the three aforementioned planes. Thus, by moving the wheels 13 in unison in a generally vertical direction, the generally wedge-shaped or triangular prismatic sample 14 will be cut from the adjacent formation 15.

To gain entrance for the cutting wheels 13 into the formation 15, means (to be subsequently described) are provided for advancing the wheels outwardly and upwardly through the housing opening 25 to their outermost lateral position. Then, after a longitudinal cut of a predetermined length has been made, the cutting wheels 13 are returned along an upwardly inclined path and back through the housing opening 25 until they are fully retracted. The cutting wheels 13- then return to their original starting position while still fully retracted.

The lower housing of the tool 10 is arranged to receive a plurality of core samples and keep them segregated from one another. Generally speaking, the housing 20 is arranged in such a manner that a plurality of compartments therein (not shown) will be sequentially positioned to each successively receive a formation sample, as at 14, as the tool 10 is operated. In this manner, the tool 10 can be employed on a single trip in the borehole 12 to recover a large number of formation samples that will be separately disposed in the compartments in a predetermined order.

Turning now to FIGURE 2, a schematic representation is shown of the intermediate housing 18 of the tool 10 in which the cutting wheels 13 are confined. In general, the cutting wheels 13 are operatively mounted below an enclosed housing or enclosure 26 that is, in turn, secured to two parallel tubular members 27 (only one shown). These tubular members 27 are each slidably disposed about substantially longer, paralleled longitudinal rods 28 (only one shown) that are secured only at their upper and lower ends to the tool housing 18 and spaced away from the rear wall thereof. The opposite ends of these tubular members 27 are slidably sealed around the elongated rods 28. A piston member 29 (only one shown) is fixed at an intermediate position on each of the elongated rods 28 and slidably sealed relative to the internal bore of its associated tubular member 27 to define therein separate upper and lower fluid-

tight chambers

30 and 31.

Accordingly, it will be appreciated that developing a higher fluid pressure in the upper hydraulic chambers 30 than that in the lower hydraulic chambers 31, the tubular members 27 and enclosure 26 secured thereto will be moved upwardly along the elongated rods 28 relative to the tool housing 18. Similarly, by imposing a higher pressure in the lower hydraulic chambers 31 than that in the upper hydraulic chambers 30, the enclosure 26 will travel downwardly along the rods 28.

To develop such higher pressures in the

chambers

30 and 31, a suitable reversible hydraulic pump 32 is mounted within the enclosure 26.

Fluid lines

33 and 34 are respec tively connected between the

hydraulic chambers

30 and 31 and the pump 32. By selecting a motor-driven pump 32 and filling the chambers 30- and 31 with a suitable hydraulic fluid, the pump can be selectively operated from the surface to transfer fluid between the hydraulic chambers to accomplish the desired travel of the enclosure 26 along the elongated rods 28.

By arranging a typical bellows or piston (neither shown) at a convenient point in a Wall of the enclosure 26, the hydraulic fluid in the enclosure and

chambers

30 and 31 will be maintained at a pressure at least equal to the hydrostatic pressure of fluids or so-called mud in the borehole 12. In this manner, by pressure-balancing the hydraulic system relative to the borehole hydrostatic pressure, the hydraulic pump 32 needs only to develop a pressure suflicient to overcome the weight of the enclosure and whatever friction that may be encountered in moving the cutting wheels 13 and the enclosure 26.

To power the cutting Wheels 13, an electric motor 33 is also fitted into the enclosure 26 and its shaft 34 connected to the cutting wheels by suitable power-transmission means, such as a universal joint 35 which is connected by way of another shaft 36 to a right-angle gear drive 37 having outwardly diverging Wheel shafts 38 at an angle to one another. By locating the cutting wheel motor 33 in the enclosure 26, it will also be pressure-balanced in the same manner as the motor for the pump 32. Similarly, by enclosing the

shafts

34 and 36 and universal joint 35 in an oil-filled tube (not shown) that is fluidly sealed at its opposite ends to the enclosure 26 and gear drive 37 and in fluid communication with each, the power-transmission means will also be completely pressure-balanced.

A pair of depending arms 40 disposed on opposite sides of the protective tube are connected at their lower ends to the gear drive 37 and pivotally connected at their upper ends to the enclosure 26 so as to pivot about an axis lying generally in the same horizontal plane as the pivotal axis of the universal joint 35. outwardly-biased pins 41 (only one seen in FIGURE 2) near the free ends of the pivoted arms 40 are slidably disposed in a labyrinthlike system of grooves 42 (only one system seen in FIG- URE 2) formed in the interior side Walls of the intermediate housing 18 on opposite sides of the longitudinal opening 25 therein. As will subsequently become apparent, these groove systems 42 are so arranged that upward longitudinal travel of the enclosure 26 from its full-line position to its dashed-line position shown in FIGURE 2 will be effective (through the coaction of the guides 41 in their respective groove system) to direct the cutting Wheels 13 along the path A-B-C-D depicted in FIG- URE 2. Then, upon downward travel of the enclosure 26 back to its full-line position shown in FIGURE 2, the groove systems 42 and guides 41 will direct the cutting wheels 13 along the path D-A toward their initial position.

As seen in FIGURE 3, the groove systems 42 are each arranged in a closed loop having two parallel

longitudinal portions

43 and 44 of unequal length and spaced apart from one another. The shorter grooves 44 are connected at their opposite ends to the longer grooves 43 by oppositely directed

inclined grooves

45 and 46 which re spectively intersect the longer grooves at longitudinally spaced intermediate points.

Accordingly, as the cutting wheels 13 move along the path A-B, they will be moving upwardly and outwardly as they cut their way into the formation 15. Then, as the cutting wheels 13 move upwardly from their position at B to their position at C, they will be cutting along a straight path of a length determined by the vertical height of the shorter grooves 44. Upon reaching their position at C, the cutting wheels 13 will be retracted as they move further upwardly and cut their way toward their position at D. Thus, once the cutting wheels 13 have reached the position at D, a prismatic sample, as at 14, with tapered ends will have been cut out of the formation 15 and dropped into the core-receiving housing therebelow.

The groove systems 42 must, of course, be arranged to insure that the guide pins 41 are diverted into the lower inclined grooves 45 as the enclosure 26 moves upwardly. Similarly, when the enclosure 26 has reached its uppermost position (as shown in dashed lines in FIG- URE 2), it is necessary that the guide pins 41 be prevented from re-entering the upper grooves 46- so that the cutting wheels 13 can proceed from their position at D and directly back to their initial position at A.

Accordingly, means are provided to direct the guide pins 41 in a predetermined direction around the circuitous groove systems 42 but prevent these pins from moving in the opposite direction. As seen in FIGURES 3 and 4, an abutment 47 is provided in the lower end of each of the longer grooves 43 for preventing the guide pins 41 from entering the longer grooves as they move upwardly. To facilitate the passage of the guide pins 41, the faces of the abutments 47 are extended along the line of the downwardly facing Wall of the lower inclined grooves 45 as shown in FIGURE 3. Similarly, to insure that the guide pins 41 will not re-enter the upper end of the upper inclined grooves 46 as the enclosure 26 is returned downwardly, an abutment 48 (similar to those at 47) is located across the entrance to the upper end of each of the upper inclined grooves 46. Here again, to facilitate the passage of the guide pins 41, the faces of the abutments 48 are made as a continuation of the righthand (as viewed in FIGURE 3) side walls of the longer grooves 43. The height of each abutment, as at 47, is made less than the total depth of its associated groove 43 and an inclined ramp or surface, as at 49 (FIGURE 4), is provided from the bottom of the groove 43 up to the upper surface of the abutment, with this inclined surface rising in the direction from which the guide pin 41 is intended to be coming in that groove. Thus, as the springbiased guide pins 41 approach the inclined surfaces 49, for example, they can retract sufiiciently to move up the inclined surfaces 49 of the abutments 47 as the enclosure 26 is moved downwardly. Once the guides 41 reach the abrupt faces of the abutments, the springs biasing the guides will urge them outwardly to return them to their normal extended position. The inclined surfaces 50 (FIG- URE 3) on the lower ends of the upper abutments 48 in the grooves 46 will, of course, function in the same manner.

Motion-translating means (not shown) are arranged in the housing 19 coupled immediately below the intermediate tool housing 18 for rotating the several compartments of the sample receiver in the housing 20 therebelow sequentially into position to receive successive formation samples, as at 14, as they are freed by the cutting wheels 13 and to segregate these samples from one another. Since a complete description of this sample receiver is not essential to a full understanding of the present invention, suffice it to say that these sample-receiving compartments are basically comprised of a plurality of upright tubes (not shown) that are equally spaced about an axial shaft (not shown) journalled at its opposite ends in the housing 20,

with these tubes being adapted to be successively rotated about the longitudinal axis of the housing into position to receive one of the formation samples 14. The upper ends of these tubes are, of course, open and their lower ends are closed. The motion-translating means in the housing 19 are arranged to rotate the sample-receiving tubes into their respective positions in response to the longitudinal travel of the enclosure 26. For further details, complete descriptions of the motion-translating means in the housing 19 and the sample receiver in the housing 20 are found in the aforementioned copending application Ser. No. 649,976 filed simultaneously herewith.

Turning now to the present invention, as best seen in FIGURE 2, to provide indications at the surface representative of the longitudinal positions of the cutting wheels 13 in relation to the housing 18, a control surface with variable lateral dimensions, such as an elongated tapered ramp 51, is secured to the housing in a convenient position that parallels the elongated rods 28. This tapered ramp 51 is suitably arranged to contact the outer end of a laterally movable actuator 52 that is connected to an electrical control, such as a so-called linear potentiometer 53, in the enclosure 26, with the actuator extending through a suitable fluid seal 54 (FIGURE 6) in the enclosure wall.

Thus, at all longitudinal positions of the enclosure 26 in relation to the housing 18 and the tapered ramp 51, the actuator 52 will assume corresponding lateral positions that are directly related to the distance between the particular point where the actuator is in contact with the ramp and either end of the ramp. It will be recognized, of course, that the resistance between the moving contact 55 (FIGURE 5) and one end of the potentiometer 53 will vary in accordance with the movement of its actuator 52, with the resistance at any given time being directly related to the longitudinal distance between the present position of the enclosure 26 and its initial position as shown in FIGURE 2. Since the ramp 51 is uniformly tapered in a longitudinal direction, this resistance will, of course, vary at a constant rate as the enclosure 26 moves in either direction in relation to the housing 18. It should be realized, however, that the control surface on the ramp 51 can be shaped to develop other patterns of lateral displacement of the actuator 52. For example, pronounced irregularities such as a depression or a bump could be arranged at one or more predetermined positions on the ramp S1 to provide characteristic variations in the resistance of the potentiometer 53. These interruptions in the otherwise constant rate of change of the resistance of the potentiometer 53 could be employed to designate, for example, that the enclosure 26 has reached its position where the cutting wheels 13 are fully extended as well as where the cutting wheels are about to retract. Similarly, other patterns of the control surface could be devised to signal when the enclosure 26 was at a predetermined longitudinal position.

The varying resistance of the potentiometer 53 as the enclosure 26 moves may be used in various manners to provide a signal at the surface. Although the resistance could just as well be measured directly at the surface, it is preferred, however, to use electronic means 56 such as shown in FIGURE 5 to provide an indication of the present position of the cutting wheels 13 at any given time. Inasmuch as this circuit 56 is more fully explained in a copending application Ser. No. 649,978 filed concurrently with the present application, it is believed necessary only to describe this circuit only so far as to show its general relation to the present invention.

In general, therefore, the circuit 56 seen in FIGURE 5 is arranged to provide repetitive electrical pulses at the surface that have a pulse rate representative of the present longitudinal position of the cutting wheels 13 in relation to the housing 18. As the cutting wheels 13 change position in relation to the housing 18, the rate of these pulses will also change to provide a detectable indication at the surface characteristics of the new position of the cutting wheels.

To accomplish this, the potentiometer 53 is connected across a constant-voltage power supply 57 and its movable contact 55 is connected to the input of a typical socalled constant-current amplifier 58 to provide a voltage-divider circuit with an output voltage directly related to the relative position of the movable contact. Since the input impedance of the amplifier 58 is constant, the current applied to its input will be directly related to the output voltage of the voltage-divider circuit.

The current amplifier 58 desirably has a high output impedance so that the output current of the amplifier will also be constant for any one position of the movable contact 55. Thus, as the input current to the amplifier 58 is varied by the potentiometer 53, the output current from the amplifier will vary accordingly, but will be constant for any single position of the movable contact 55 and, thus, be directly related to the present position of the actuator 52 on the ramp 51. A capacitor 59 connected across the output terminals of the amplifier 58 is charged by this output current, with the rate at which this capacitor is charged being, of course, directly proportional to the magnitude of the output current from the amplifier.

The capacitor 59 and amplifier 58 are connected to the input of a typical voltage comparator 60. A reference voltage for the comparator 60 is derived from the constant-voltage power supply 57 by way of a pair of

seriallyconnected resistors

61 and 62. The comparator 60 will generate a DC output signal whenever the input signal thereto equals the reference voltage which energizes a normally-open gate 63 arranged to shunt the capacitor 59 through a resistor 64. The discharge rate of the capacitor 59 will, therefore, be a function of the values of the resistor 64 and the capacitor.

The output signal from the comparator 60 also energizes a normally-open gate 65 which then connects the junction between the

resistors

61 and 62 through a relatively low-value resistor 66 to ground. Closing of the gate 65 will, therefore, decrease the reference voltage being applied to the comparator 60. Thus, each time the voltage on the capacitor 59 reaches the initial reference voltage of the comparator 60, an output signal is developed by the comparator which continues until the capacitor has been discharged (by the resistor 64) to reach a voltage equal to the now-lower reference voltage of the comparator. At this time, the output signal of the comparator 60 ceases and re-opens the

gates

63 and 65 thus restoring the reference voltage to its initial value.

It can be seen, therefore, that this intermittent operation will produce pulses from the voltage comparator 60. The on time of these pulses is substantially related to the time required for the capacitor 59 to be discharged from a voltage equal to the high reference voltage initially applied to the comparator 60 to a voltage equal to the low reference voltage applied thereto. This time is, of course, a function of the capacitance of the capacitor 59, the resistance of the resistor 64, and the differential between the high and low reference voltages. The off time of the pulses is, however, a function of the magnitude of current charging the capacitor 59. Thus, it can be seen that the frequency or pulse rate of the pulses will be proportional to the output current from the amplifier 58 and, therefore, to the position of the movable contact 55.

The string of output pulses from the comparator 60 are transmitted through the suspension cable 11 to a suitable pulse-rate detector 67 at the surface. The varying DC output of the pulse-rate detector 67 is connected to an indicator, such as a voltmeter 68, which is suitably calibrated to provide an indication of the present position of the cutting wheels 13. Thus, in the operation of the present invention, when the cutting wheels 13 are in one position, the pulse output of the voltage comparator 60 will be at a rate characteristic of this position. As the cutting wheels 13 move up or down, this pulse rate will correspondingly change to provide a different indication on the position indicator 68 at the surface.

Turning now to FIGURE 6, one arrangement is shown for mounting the control or potentiometer 53 within the enclosure 26. The potentiometer 53 is a typical fluid-tight, so-called linear potentiometer which has a rectilinearily movable contact 55. To protect the potentiometer contact member 55 from damage, the potentiometer 53 is completely confined in the enclosure 26 and the actuator 52 movably disposed in a lateral bore 69 spaced from and parallel to the central axis of the potentiometer. As previously mentioned, the actuator 52 is fluidly sealed relative to the wall of the enclosure 26 by the fluid seal 54. The actuator 52 will, of course, be pressure-balanced since the pressure of the hydraulic fluid in the enclosure is maintained equal to that of the fluids in the borehole 11. A spring 70 between the actuator 52 and the rear of the bore 69 maintains the actuator in engagement with the ramp 51.

To operate the potentiometer 53, a rigid connecting member '71 is disposed in an opening 72 between the bore 69 and the forward end of the potentiometer 53. The connecting member 71 is secured at its opposite ends to the actuator 52 and the distal end of the movable contact member 55 so that lateral movement of the actuator as it moves along the tapered ramp 51 will cause a corresponding movement of the contact member.

Accordingly, it will be appreciated that the present invention has provided means for determining at the surface the relative positions of two relatively movable portions of a well tool during the course of their movement in relation to one another. By employing the principles of the present invention, a signal is provided at the surface from which the operation of a well tool such as the disclosed core slicer can be monitored.

While a particular embodiment of the present invention has been shown and described, it is apparent that changes and modifications may be made without departing from this invention in its broader aspects; and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is:

1. Apparatus for obtaining samples of earth formations traversed by a borehole comprising: support means adapted for suspension in a borehole; formation-sampling means on said support means and adapted for travel relative thereto between longitudinally-spaced positions; and means for signaling the longitudinal position of said formation-sampling means relative to said support means including a control surface extending longitudinally along one of said two first-mentioned means and having varying lateral dimensions along its length, an electrical control on the other of said two first-mentioned means, and means operatively arranged between said control surface and said electrical control for lateral movement by said control surface to vary the electrical characteristics of said electrical control in accordance with the relative longitudinal positions of said control surface and said laterallymoving means.

2. The apparatus of claim 1 wherein said electrical control is a variable resistance element.

3. The apparatus of claim 1 wherein said lateral dimensions of said control surface vary uniformly in relation to the longitudinal dimensions of said control surface.

4. The apparatus of claim 3 further including: a distinctive lateral irregularity on said control surface at a predetermined location.

5. The apparatus of claim 1 further including: electrical conductor means connected to said support means and extending to the surface of the earth; means on said apparatus for connecting said electrical control to said electrical conductor means; and indicating means at the surface of the earth connected to said electrical conductor means and responsive to variations in said electrical characteristics of said electrical control to signal the relative longitudinal positions of said formation-sampling means and said support means.

6. The apparatus of claim 5 wherein said lateral dimensions of said control surface vary uniformly in relation to the longitudinal dimensions of said control surface.

7. The apparatus of claim 6 wherein said control surface is on said support means and said electrical control is on said formation-sampling means.

8. The apparatus of claim 7 wherein said electrical control is a variable resistance unit.

References Cited UNITED STATES PATENTS 3/1950 Arutunolf l75-78 6/ 1952 Mennecier 17540 3/1965 Stuart et al 175-78 X 12/1965 Wisenbaker et a1. 166-55 US. Cl. X.R.