Abstract:
The present invention is directed to an apparatus and method for coring a borehole in a hard rock sidewall of a well bore in a subterranean formation for testing purposes. The apparatus includes a drive motor for operation down hole, a flexible drive shaft coupled to the drive motor and a coring bit coupled to the flexible drive shaft, such that the coring bit is directly driven by the drive motor. The apparatus also includes a control circuit for controlling advancement of the coring bit into the subterranean formation. The apparatus also includes a rotating carousel for storing multiple core samples. The method includes the steps of activating the drive motor to rotate the output shaft; coupling the output shaft of the drive motor to the flexible drive shaft and rotating the coring bit with the flexible drive shaft.

Description:
BACKGROUND 
   The present invention relates generally to an apparatus and method for hard rock sidewall coring of a borehole, and more particularly to a rotary sidewall coring tool that employs a direct drive mechanism, which operates at an enhanced efficiency, a coring bit control circuit, which provides for precise control of bit advancement, and a carousel core storing device that enables the storage of a large number of core samples. 
   Conventional tools for hard rock sidewall coring of a borehole employ complex drive mechanisms, which are not very efficient. Many of these systems also provide inadequate torque delivery at the coring bit making them incapable of delivering reliable core operation. In one such system, the drive mechanism comprises an electric motor coupled to a hydraulic pump, which in turn is coupled to a hydraulic motor, which drives the bit. There is a significant power loss in the hydraulic pump and hydraulic motor of such systems. This is because the down hole temperatures are very high, which lowers the viscosity of the hydraulic fluid in the hydraulic pump and motor, which in turn causes a significant amount of the hydraulic fluid to seep past the pistons in the hydraulic pump and motor, which results in a loss of power output by the pistons. Up to sixty percent (60%) of the efficiency of the hydraulic pump and motor can be lost through the drop in viscosity of the hydraulic fluid. Additional efficiency of such systems are lost because they employ a second hydraulic pump to drive the auxiliary devices, which is a drain on the power output of the electric motor. Thus, such systems can lose up to seventy percent (70%) of their efficiency. Hydraulic motors, therefore, have losses due to low volumetric efficiency (fluid loss) and mechanical efficiency (losses due to gears and bearings) which make their overall efficient less than ideal. 
   In another conventional system, the drive mechanism comprises an electric motor coupled to a hydraulic pump, which is in turn coupled to a hydraulic motor in turn coupled to a 90° transmission. This system has the same drawbacks of the previously described system, namely that there are significant loses due to the decrease in viscosity of the hydraulic fluid in the hydraulic motor. The drive mechanism in this system outputs a low speed and high torque to the bit. Because of its slow speed, this system takes longer than the other systems to remove each core sample. Thus, it requires more rig operation time, thereby making it more expensive to employ. 
   Furthermore, conventional tools for hard rock sidewall coring of a borehole employ limited feedback of operating conditions. While such devices have the ability to control the advancement of the core bit during coring, they do not have the ability to monitor in real time the torque of the bit. Since torque is a primary factor in determining the rate of penetration of the bit, conventional coring devices lack an important piece of information to prevent stalling of the bit during the coring operation. Rather, such devices infer the torque or RPM from the pressure response or motor current changes during the coring operation. However, because inferential readings are inherently inaccurate, conventional coring devices are susceptible to stalling. 
   Another disadvantage of conventional tools for hard rock sidewall coring of a borehole is that they have limited space in which to store the core samples. Accordingly, only a limited number of samples can be stored in such devices during a single run of the tool. In certain wells, therefore, the tool must be run down hole more than once to collect all of the desired core samples. A tool with larger core sample storage capacity is desirable. 
   SUMMARY 
   The features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the exemplary embodiments, which follows. 
   In one embodiment, the present invention is directed to a rotary sidewall coring tool. The coring tool comprises a drive motor, e.g., an electric motor or hydraulic motor, a flexible drive shaft coupled to the drive motor and a coring bit assembly coupled to the flexible drive shaft, such that the coring bit is directly driven by the drive motor. The coring tool further comprises a clutch, which couples the drive motor to the flexible drive shaft and a gear assembly, which couples the clutch to the flexible drive shaft. As used herein, the terms “couple,” “couples,” “coupled” or the like, are intended to mean either indirect or direct connection. Thus, if a first device “couples” to a second device, that connection may be through a direct connection or through an indirect connection via other devices or connectors. The coring tool according to present invention further comprises a hydraulic pump coupled to the drive motor, which drives auxiliary devices. The coring tool also comprises a bit control circuit and sensor, which controls advancement of the coring bit and measures the rpm of the flexible drive shaft, respectively. 
   The coring bit is mounted on a platform, which is part of the coring bit assembly. The coring bit assembly includes a gear assembly described below. The coring bit assembly can move from a vertical storage position to a horizontal operable position by a hydraulic piston and lever arms. The hydraulic piston is powered by a hydraulic pump, which is in turn driven by the drive motor. 
   The hydraulic piston also manipulates the coring tool to deposit coring samples into a rotating carousel, which is also powered by the hydraulic pump and ultimately the electric motor. The coring tool further comprises a core separator disposed adjacent to the rotating carousel, which comprises a plurality of labeled discs that identify each core sample collected and a spring loaded plunger that dispenses a labeled disc with each core sample loaded into the rotating carousel. The coring tool also comprises a pair of back-up pistons disposed within the tool, one of which is disposed above the coring bit assembly and the other of which is disposed below the coring bit assembly, which upon activation thrust the tool against one side of the well bore just prior to the coring operation. The coring tool further comprises a potentiometer for measuring the length of the core sample. 
   In another embodiment, the present invention is directed to a method of coring a borehole in a hard rock subterranean formation. The method comprises the steps of activating the drive motor to rotate an output shaft; coupling the output shaft of the drive motor to the flexible drive shaft; and rotating the coring bit with the flexible drive shaft. Other steps of the method include rotating the coring bit from the vertical storage position to the horizontal operable position; advancing the coring bit laterally into the hard rock subterranean formation; reducing the rotational speed being transmitted to the flexible drive shaft by the output shaft of the drive motor. Other steps in accordance with the present invention include driving auxiliary devices with a hydraulic pump driven by the drive motor and providing feedback signals to the bit control circuit, which are indicative of the rpm and torque of the coring bit and lateral advancement of the coring bit. Still further steps include discharging a core sample from the coring bit, measuring the length of the core sample, depositing the core sample into the rotating carousel; dispensing a labeled disc into the rotating carousel; and thrusting the coring bit against one side of a well bore just prior to commencing the coring operation. 
   The features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the embodiment that follows. 

   
     DRAWINGS 
     The following figures form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the description of the embodiments present herein: 
       FIG. 1  is a schematic diagram illustrating a rotary sidewall coring tool in a well bore with a coring bit retracted. 
       FIG. 2  illustrates the coring tool of  FIG. 1  with the tool locked in place and with a coring bit extended. 
       FIGS. 3A-3M  illustrate sections of a longitudinal cross-sectional view of the coring tool in accordance with the present invention illustrating schematically each of the functional sections of the tool. 
       FIGS. 4A-4G  illustrate sections of an enlarged longitudinal cross-sectional views of the coring tool with a focus on the clutch and torque sensor portions of the tool in accordance with the present invention. 
       FIG. 5  is a perspective view of the coring bit and associated coring bit assembly in accordance with the present invention. 
       FIG. 6  is a schematic diagram of a bit control circuit in accordance with another aspect of the present invention. 
       FIGS. 7A-7F  illustrate sections of another enlarged longitudinal cross-section of the coring tool in accordance with the present invention illustrating the details of a core sample storage device and back-up pistons in accordance with another aspect of the present invention. 
       FIG. 8  is a perspective view of a rotary carousel used in storing the core samples in accordance with another aspect of the present invention. 
       FIG. 9  is an enlarged partial lengthwise cross-sectional view of the ratcheting mechanism used to advance the carousel for storing core samples in accordance with the present invention. 
       FIG. 10  is an axial cross-sectional view of the ratcheting mechanism used to advance the carousel for storing core samples in accordance with the present invention taken along line  10 - 10  in  FIG. 9 . 
       FIG. 11  is another axial cross-sectional view of the ratcheting mechanism used to advance the carousel for storing core sample in accordance with the present invention taken along line  11 - 11  in  FIG. 9 . 
       FIG. 12  is an isometric view of the coring bit depositing a sample in one of the storage tubes of the carousel and the core separator device depositing core separator in another one of the storage tubes. 
       FIG. 13A  is a partial cross-sectional view of the coring tool illustrating the gear assembly in the coring bit assembly that translates rotation of the flexible drive shaft into rotation of the coring bit. 
       FIG. 13B  is a partial cross-sectional view of the coring tool which illustrates the activation of the push rod that forces the coring sample out of the coring bit against a trap door which covers the opening of a sample storage tube. 
   

   DETAILED DESCRIPTION 
   The details of the present invention will now be described with reference to the accompanying drawings. Turning to  FIG. 1 , a rotary coring tool in accordance with the present invention is shown generally by reference numeral  10 . The coring tool  10  is suspended by wire line  12  in a well bore  14  defined by sidewall  16 . Wire line  12  engages a sheave  18  associated with a surface control unit  20 . The surface control unit  20  includes processing means for programming and controlling various functions of the coring tool  10 . The electronic signals are transmitted through wire line  12 , which can serve both as a conductor and a stress member. This process of communicating, programming and controlling data via a wire line is well known in the art. 
   The coring tool  10  includes a coring bit  22 , which is shown in  FIG. 1  in a retracted position. Coring tool  10  further comprises a pair of back-up pistons  128  and  130 , which in the extended position, as shown in  FIG. 2 , lock the tool against the sidewall  16  of well bore  14 . The coring tool  10  may also include a dedicated section  30  in which the tool&#39;s on-board electronics and telemetry system is housed. Section  32  houses a pressure compensator, whose function is to equalize the internal pressure to the external well bore pressure. Section  32  also houses an accumulator  36  (shown in  FIG. 3C ). The accumulator  36  accumulates hydraulic fluid for uses requiring an immediate hydraulic boost to close all the extended functions of the tool while coring, e.g., bit retract, bit tilting and back-up retract in case of electronic power loss. Section  40  houses a hydraulic control circuit, shown in more detail in  FIG. 3E . The hydraulic control circuit is a complex network of hydraulic flow lines and valves used to control the operation of all of the control mechanisms in the tool  10  operating off of hydraulic power. These devices are described in somewhat more detail below but are well known to those of ordinary skill in the art. Section  42  houses the power unit  46 , which powers the auxiliary devices, such as the pistons, which control movement of the coring bit  22 , described below, and other control devices. The power unit  46  is shown in greater detail in  FIGS. 3G  and described more fully below. Section  48  contains the coring bit assembly  50  (shown in  FIG. 5 ) and section  52  houses the core storage device, namely rotatable carousel  54 , as described in further detail below. 
     FIG. 2  shows the coring tool  10  locked in position opposite the subterranean formation of interest. The coring bit  22  is shown extended laterally through a bit opening in the side of the coring tool  10  cutting a core sample in the formation of interest. 
   Additional details of the coring tool  10  in accordance with the present invention will now be described in connection with  FIGS. 3 through 8 . Referring to  FIGS. 3G ,  3 H,  4 C and  4 D, the details of the power unit  46  will now be described. The power unit  46  comprises a drive motor  60 , which in this example is a one horsepower (1 hp) AC electric motor, which receives voltage through wire line  12 . Drive motor  60  has an output shaft  62  which rotates at 3,000 rpm during operation. Output shaft  62  of the drive motor  60  is coupled to an input shaft  64  of clutch  66 . Input shaft  64  of clutch  66  is coupled to main shaft  68  of clutch  66  via a plurality of clutch plates  70 . As those of ordinary skill in the art will appreciate, when the clutch plates  70  are engaged the input shaft  64  of clutch  66  drives main shaft  68 , as shown in  FIGS. 3H and 4D . When the clutch plates  70  are disengaged, the main shaft  68  is disengaged from input shaft  64 , and is thereby stationary. The clutch plates  70  are brought into engagement and disengagement by a piston-energized electrically controlled valve controlled by the tool&#39;s electronic control system. It should be noted and appreciated that some of the tool&#39;s electronic control will occur at the surface control unit  20  another and other electronic control may occur in the tool&#39;s on-board electronics control device in section  30 . The pressure is provided to the valve from the hydraulic pump  106 , ultimately driven by the drive motor  60 . Those of ordinary skill in the art will understand how the engaging device works and therefore the details of such device will not be described in any greater detail herein. 
   A gear assembly  72  is coupled to main shaft  68 , as shown in  FIGS. 3H and 4D . The gear assembly  72  comprises a plurality of intermeshing gears, which decrease the rotational speed of the drive mechanism imparted by main shaft  68  and axially offset the drive output of main shaft  68 . Gear assembly  72  has an output shaft  74  (shown in  FIG. 3I ) which rotates at approximately 2,100 rpm and has a power output of approximately 0.7 horsepower. Thus, the gear assembly  72  imposes a speed reduction of approximately 1.6:1.0. Alternatively, the gear assembly  72  only axially offsets the drive output of the main shaft  68  without reducing its rotational output to the flexible drive shaft  76 . 
   The output shaft  74  of the gear assembly  72  is coupled to a flexible drive shaft  76 , best shown in  FIG. 13A . Flexible draft shaft  76  is any drive shaft, which is capable of transmitting rotational motion and is flexible enough to bend during rotation. In one embodiment, the flexible drive shaft  76  is formed of a metal probe disposed within a Teflon tube. An example of such a flexible drive shaft is the odometer cables that are typically used in automobiles. The flexible drive shaft  76  connects at its other end to a gear assembly  77  (shown in  FIG. 13A ) in the coring bit assembly  50 . The gear assembly  77  comprises a plurality of intermeshing bevel gears  79 , which are configured to rotate the coring bit  22  so long as the clutch plates  70  are engaged. In other words, the coring bit  22  is capable of rotating both in the vertical storage position as well as in the horizontal coring position. The gear assembly  77  imposes a 1.6:1.0 reduction in the rotational speed of the drive assembly, which translates into a rotational speed of the coring bit  22  of approximately 1450 rpm. Furthermore, the connection between the flexible drive shaft  76  and the gear assembly  77  is hinged and as such it allows relative angular movement between the axes of the flexible drive shaft  76  and the axes of the coring bit  22  while still allowing the transmission of rotational power through the hinge point. 
   Referring to  FIG. 4 , the details of a torque sensor  80  in accordance with the present invention will now be described. Torque sensor  80  is defined by a pair of reluctance sensors  82  connected to a fixed member a certain distance apart from one another, as shown in  FIG. 4E . Each reluctance sensor  82  is made from a magnet, a pole piece and a coil. A magnetic field extends from the magnet through the pole piece into the air space at the end of sensor. As the magnetic tooth approaches the pole piece, the magnetic field decreases and then increases as the object moves away from the pole piece. This decrease/increase in the magnetic field induces an AC voltage signal in the coil. The induced AC voltage is in the shape of a sine wave. One sensor determines the rpm of the shaft. The phase difference between the two sensors can be used to determine the twist or torque generated by the output shaft. In other words, the generated frequency signal is directly proportional to the number of ferrous objects passing the pole piece per unit of time. 
   The details of the coring bit assembly  50  in accordance with the present invention will now be described. Coring bit assembly  50  comprises coring bit  22 , which is capable of being rotated from a vertical storage position to a horizontal operable position, as shown generally in  FIG. 2 . As shown in  FIG. 5 , the coring bit  22  is in position for lateral advancement into sidewall  16  of the subterranean formation of interest. The coring bit  22  sits on a platform  86 , which is raised, lowered, and rotated by a pair of linkage assemblies  88  and  90 . 
   Linkage assembly  88  operates to tilt the platform  86  from a vertical storage position to the horizontal operable position. Linkage assembly  88  comprises a generally triangle-shaped lever arm  92 . Lever arm  92  has a slot  94  formed along its base portion. Another lever arm  96  is coupled to lever arm  92 . Lever arm  96  is connected to a positioning piston  97  operated by a hydraulic pump  106  (shown in  FIGS. 3G and 4B ) described later herein. Lever arm  96  through its back and forth movement imparted by the hydraulic pump  106  operates to rotate lever arm  92 . When lever arm  96  is moved in a forward direction, lever arm  92  rotates in a clockwise direction and thereby moves the coring bit  22  from the horizontal operable position to the vertical storage position. When lever arm  96  is moved in a retracted position, it causes lever arm  92  to rotate counterclockwise thereby moving coring bit  22  from the vertical storage position to the horizontal operable position. 
   Linkage assembly  90  comprises a pair of lever arms  98 ,  99  disposed on opposite ends of the coring bit  22 . Lever arms  98 ,  99  are connected by connecting rod  100 , which in turn has a mounting eye hook  102  for connecting to a bit advance piston  101 , also driven by the hydraulic pump  106 . Linkage assembly  90  further comprises guide pin  104 , which attaches to coring bit assembly  50  and slides in slot  94 . As lever arms  98 ,  99  are moved axially by the bit advance piston  101 , they pivot at one end about pivot point  103  and slide at the other end along slot  94  carrying guide pin  104 , which in turn forces the coring bit assembly  50  to move horizontally thereby enabling it to advance into the subterranean formation. In the vertical storage position, coring bit assembly  50  is housed in generally cylindrical recess  105 . The positioning piston  97  and bit advance piston  101  are used to drive linkage assembly  88  and linkage assembly  90 , respectively, are hydraulically connected to the section  40 , which in turn is fed with pressurized hydraulic fluid via hydraulic pump  106 , shown in  FIG. 3 . Hydraulic pump  106  is directly connected to, and driven by, drive motor  60 , also shown in  FIGS. 3G and 4B . 
   Referring to  FIG. 6 , the bit control circuit  600  in accordance with the present invention will now be described. The bit control circuit  600  includes an input fluid line  602 , which is connected to the hydraulic pump  106 . The input fluid line  602  supplies pressurized hydraulic fluid into SV advance  control valve  604 . In one exemplary embodiment, the SV advance  control valve  604  is a three-way, two-position electronically controlled solenoid valve controlled by the tool&#39;s electronic control system. In one position (the powered position), the SV advance  control valve  604  connects the input fluid line  602  to the rod side  610  of the bit advance piston  101 . In the other position (the unpowered position), the SV advance  control valve  604  blocks the fluid from hydraulic pump  106 . The SV advance  control valve  604  operates to supply the bit advance piston  101  with pressurized fluid to advance the coring bit  22  as explained below. As the rod side  610  of bit advance piston  101  fills with pressurized fluid, it forces the bit advance piston  101  to retract, which in turn pivots lever arms  98 ,  99  about pivot point  103  and thereby advances the coring bit assembly  50  horizontally into the subterranean formation. 
   The bit control circuit  600  further comprises a SV dump  control valve  606 , which in one exemplary embodiment is a three-way, two-position electronically controlled solenoid valve also controlled by the tool&#39;s electronic control system. In the first position, the SV dump  control valve  606  connects fluid line  608  and rod side  610  of bit advance piston  101  via fluid line  612 . In the second (unpowered) position, the SV dump  control valve  606  connects fluid line  608  to the hydraulic reservoir tank that supplies the hydraulic pump  106 . The SV dump  control valve  606  thus operates to relieve the pressure of the fluid being supplied to the bit advance piston  101  when the pressure exceeds a desired value. 
   The bit control circuit  600  further includes a SV retract  control valve  614 . In one exemplary embodiment, the SV retract  control valve  614  is a three-way, two-position solenoid valve. In the first (powered) position, the SV retract  control valve  614  connects the input fluid line  602  to the piston side  611  of the bit advance piston  101  via input and output fluid line  616  and fluid control line  618 . In the second (unpowered) position, the SV retract  control valve  614  blocks the pump pressure and connects fluid control line  618  to the tank. The SV retract  control valve  614  operates to retract the coring bit  22  by supplying the piston side  611  of the bit advance piston  101  with pressurized fluid, which in turn advances the piston and correspondingly pivots the lever arms  98 ,  99  about pivot point  103  in a clockwise direction thereby causing the coring bit assembly  50  to retract away from the subterranean formation. The bit control circuit  600  also includes an accumulator  619  which is connected to fluid control line  618 . The accumulator  619  accumulates the fluid during activation of the SV retract  control valve  614  to dampen pressure spikes, which would otherwise occur if the SV retract  control valve  614  connected the piston side  611  of the bit advance piston  101  directly to the hydraulic pump  106 . Accumulator  619  also helps to retract the bit away from the wall if the SV dump  control valve  606  is energized to reduce torque instantly. 
   The bit control circuit  600  further includes a pressure transducer  620 , which is disposed in fluid line  608 . The pressure transducer  620  sends a feedback signal to the electronic control system, which in turn monitors the pressure being supplied to the bit advance piston  101 . SV ADVANCE  Control Valve  604 , SV DUMP  Control Valve  606 , and SV RETRACT  Control Valve  614  are all electrically connected to the electronic control system and in turn are controlled by that system. In other words, the each of these valves move between the first and second position in response to electronic control signals received from the electronic control system. 
   The bit control circuit  600  further includes a check valve  622 , which is disposed between the pressure transducer  620  and the SV advance  control valve  604 . The check valve  622  prevents the fluid in fluid line  608  from flowing back to the tank when the SV advance  control valve  604  is in the second (unpowered) position. The bit control circuit  600  also includes an accumulator  624  which is connected to fluid line  612 . The accumulator  624  accumulates the fluid during activation of the SV advance  control valve  604  to dampen pressure spikes, which would otherwise occur if the SV advance  control valve  604  connected the rod side  610  of the bit advance piston  101  directly to the hydraulic pump  106 . In one exemplary embodiment, the fluid pressure being output by the hydraulic pump  106  is approximately 2,500 psi, and the fluid pressure being supplied to the bit advance piston  101  during advancement of the coring bit assembly  50 , is between 1000 psi and 1500 psi. As those of ordinary skill in the art will appreciate, other pressures and pressure ranges may be acceptable depending upon the parameters of the system. 
   The bit control circuit  600  operates as follows. SV ADVANCE  Control Valve  604  and SV RETRACT  Control Valve  614  are initially in the closed (unpowered) position and SV dump  control valve  606  is in the open position (unpowered). In this position, the SV ADVANCE  Control Valve  604  and SV RETRACT  Control Valve  614  block pump flow (normally closed) and SV dump  control valve  606  allows the flow to go to the tank (normally open). When it is desired to advance the coring bit  22 , SV ADVANCE  Control Valve  604  and SV DUMP  Control Valve  606  are powered, i.e., moved to the first position by the electronic control system via electronic control signals. This connects the rod side  610  of the bit advance piston  101  to the hydraulic pump  106 , supplying it with pressurized fluid. Once the fluid pressure reaches the desired range, which in one exemplary embodiment is approximately 1000 to 1500 psi, the SV advance  control valve  604  is removed of power. The SV dump  control valve  606 , however, remains closed (powered). Because the check valve  622  prevents the pressurized fluid from flowing back into the SV advance  control valve  604 , the fluid lines  608  and  612  remain pressurized. Once the pressure drops below the desired minimum pressure, the SV advance  control valve  604  is activated again, i.e., powered, until the pressure is once again back into the desired range. In the event that the fluid pressure exceeds the maximum desired pressure, the SV dump  control valve  606  is opened to connect fluid line  612  to the tank and thereby reduce the pressure in the line with the aid of accumulator  619 . 
   When it is desired to stop the coring operation and retract the coring bit  22 , e.g., once a core sample has been obtained, the electronic control system sends control signals to SV ADVANCE  Control Valve  604  and SV DUMP  Control Valve  606  to connect to the tank. At the same time, the electronic control system sends a control signal to the SV retract  control valve  614  to connect the piston side  611  of the piston to the hydraulic pump  106 . This in turn forces the bit advance piston  101  completely open, thereby retracting the coring bit  22 . 
   Next, positioning piston  97  is operated to rotate coring bit assembly  50  from the horizontal operable position to the vertical storage position. Once coring bit assembly  50  is in the vertical storage position, core sample  131  is ready to be measured and then deposited into the core sample storage device, rotatable carousel  54 . To measure, the core sample  131 , a push rod  121 , which is shown in  FIGS. 7B and 7C , pushes the core sample  131  out of the coring bit  22  up against a trap door  127 , which covers the opening to one of the storage tubes  114 , described in more detail below. The push rod  121  is activated via a pressure control valve (not shown) controlled by the electronic control system, which in turn supplies pressurized fluid from the hydraulic pump  106 . The push rod  121  has a plunger  123 , which pushes the core sample  131  out of the coring bit  22 , as shown in  FIG. 13B . The trap door  127  (shown in  FIG. 13B ) closes over the storage tube, when the back-up pistons  128  and  130  are extended. The same fluid that feeds the back-up pistons  128  and  130  to come out, also feeds another piston (not shown) that pushes on a linkage to close the trap door  127 . Using a linear potentiometer  129  connected to the push rod  121 , the length of the core sample can be determined. The linear potentiometer is a variable resistance device. A precision measurement of position can be made when a moving terminal slides across the resistance element. 
   After the measurement has been taken, the core sample  131  is ready to be deposited into the core sample storage device. This is done by opening the trap door  127  and activating the push rod  121 . The trap door  127  opens when the back-up pistons  128  and  130  are closed. When the back-up pistons  128  and  130  are closed, the same pressure is routed to the back side of the trap door piston to open the door  127 . The push rod  121  is then extended once again and this time the core sample  131  will be pushed into a storage tube  114 . 
   The details of the core sample storage device in connection with the present invention will now be described in connection with  FIGS. 7 and 8 . The core storage device in accordance with the present invention comprises a rotatable carousel  54 , which comprises a pair of support hubs  110  and  112 , best seen in  FIG. 8 . A plurality of removable storage tubes  114  are disposed between support hub  110  and support hub  112 . They fit in recesses  116  formed within the support hubs  110  and  112 . There are six equally-spaced removable storage tubes mounted around the circumference of the rotatable carousel  54 . Removable storage tubes are approximately 20 inches long and capable of storing approximately 10 cores each. Thus, rotatable carousel  54  is capable of storing approximately 60 two-inch long, one-inch diameter core samples. As those of ordinary skill in the art will recognize, however, any number of storage tubes can be used which fit within the design parameters of the tool. Furthermore, the core samples can be of a different size. The rotatable carousel  54  has a larger storage capacity than conventional storage devices and therefore enables the coring tool  10  to store more core samples than conventional devices. Indeed, conventional storage devices typically have one tube arranged lengthwise along the tool and therefore have limited storage capacity. Thus, the coring tool  10  has the benefit of collecting more samples on a single trip and therefore takes less trips into the well bore to collect the desired number of samples than prior art devices. 
   The rotatable carousel  54  is rotated by operation of a ratcheting mechanism shown generally in  FIG. 9  by ratcheting mechanism  900 . The details of the ratcheting mechanism  900  are shown in more detail in  FIGS. 10 and 11 . The ratcheting mechanism  900  includes an indexing wheel  902 , which is rotated by rotating arm  904  via indexing finger  906 , as shown in  FIG. 10 . The indexing wheel  902  has a plurality of generally equally spaced notches  908 , which are engaged by the indexing finger  906  to advance the indexing wheel  902 . The rotating arm  904  is advanced and retracted via a piston  910  and spring  912  (shown in  FIG. 11 ). The hydraulic piston  910  is a single-action piston, which receives pressurized fluid from the hydraulic pump  106  via activation of a fluid control valve (not shown), which is in turn electronically controlled by the surface control unit  20 . When the fluid pressure is removed from the piston, the spring  912  forces it in a retracted position, which in turn moves the indexing finger  906  to engage the next notch  908 , shown clockwise in  FIG. 10 . The next time pressurized fluid is supplied to the piston  910 , the indexing finger  906  operates to rotate the indexing wheel  902  the distance between adjacent notches  908 . 
   Core sample separating device  118  in accordance with the present invention will now be described. Turning to  FIG. 7D , core sample separating device  118  comprises a cylindrical housing  120 , which stores a plurality of stacked discs  122 . Each of the plurality of stacked discs  122  is labeled with a core sample identification number or other similar designation. Each stacked disc  122  is inserted into a removable storage tube  114  disposed opposite to a corresponding storage tube  114  into which a core sample is being ejected by the coring bit  22 . Accordingly, the plurality of stacked discs  122  separate adjacent core samples. The stacked discs  122  are dispensed into the removable storage tubes  114  by a disk dispensing mechanism  124 . This disk dispensing mechanism  124  comprises a preloaded spring and dispensing plate  126 . A plurality of raised portions or lips  125  mounted on a face of support hub  110  between adjacent storage tubes  114  operate to push the discs  122  into the storage tubes as the lips  125  are rotated into engagement with the stacked discs  122  by the ratcheting mechanism  900 . 
   Turning to  FIG. 12 , it can be seen how the coring bit  22  deposits samples into one of the storage tubes  114  of the rotatable carousel  54 , in this example storage tube no. 4, while the core sample separating device  118 , shown in  FIG. 7D , deposits a disc  122  into storage tube no. 1. Lip  125  pushes the disc  122  into storage tube no. 1 as the ratcheting mechanism  900  rotates the rotatable carousel  54 . 
   Coring tool  10  further comprises a pair of back-up pistons  128  and  130 , which are shown in the retracted position in  FIGS. 1 and 3  and in the extended position in  FIGS. 2 and 7 . The back-up pistons  128  and  130  assume the retracted position during positioning of the coring tool  10  within well bore  14 . Back-up pistons  128  and  130  assume their extended position once the coring bit  22  is positioned in a desired location for obtaining a core sample. Back-up pistons  128  and  130  are hydraulically operated through a hydraulic fluid which is supplied to back-up pistons by a hydraulic fluid control line, which is connected to the section  40 . The section  40  receives a control signal from the electronic control system to open a valve, which in turn connects back-up pistons  128  and  130  to pressurized hydraulic fluid supplied by the hydraulic pump  106 . This in turn extends back-up pistons  128  and  130  into engagement with the sidewall  16 . The section  40  receives a control signal from the electronic control system to close the valve thereby removing the pressurized hydraulic fluid in back-up pistons  128  and  130  and thereby causing them to retract, which in turn disengages the coring tool  10  from the well bore wall. 
   Operation of the coring tool  10  in accordance with the present invention will now be described. The coring tool  10  in accordance with the present invention is positioned within well bore  14  adjacent sidewall  16  in the area of the subterranean formation of interest. Back-up pistons  128  and  130  are activated thereby positioning the coring tool  10  against sidewall  16 . The positioning piston  97  and bit advance piston  101  also controlled by section  40  which receives control signals from surface control unit  20  will operate linkage assemblies  88  and  90  so as to move the coring bit  22  from the vertical storage position to a horizontal operable position and thereafter laterally advance the coring bit  22  into engagement with sidewall  16 . Torque sensor  80  and pressure transducer  620  provide feedback signals to the electronic control system. These control signals supply the electronic control system with the rpm of the bit, a phase shift between the two reluctance sensors from which torque can be derived, and the fluid pressure being supplied to the coring bit, which in turn is indicative of the lateral position of the coring bit  22  relative to the subterranean formation. In the event that the coring bit  22  gets stuck or cannot operate at the desired rpm and/or torque, the electronic control system can reduce the torque on the coring bit  22  or retract the bit completely, if needed. 
   Once the core sample  131  has been cut from sidewall  16  of the subterranean formation, the coring bit  22  is rotated from the horizontal operable position to the vertical storage position. The tool  10  then measures the core sample  131  and deposits it in the removable storage tube  114 . The disk dispensing mechanism  124  then dispenses a labeled disk into the removable storage tube  114  opposite the one into which the core sample  131  is deposited. The back-up pistons  128  and  130  are then retracted and the coring tool  10  is ready to be moved to the next area in the subterranean formation from which a core sample will be obtained. This process is repeated until all of the core samples are collected or the rotatable carousel  54  is full, after which the coring tool  10  is pulled out of the well bore  14 . Once all of the removable storage tubes  114  have been emptied and placed back into the rotatable carousel  54 , the coring tool  10  is ready for use again either in well bore  14  or another well bore in another subterranean formation. 
   Therefore, the present invention is well-adapted to carry out the objects and attain the ends and advantages mentioned as well as those which are inherent therein. While the invention has been depicted, described, and is defined by reference to exemplary embodiments of the invention, such a reference does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts and having the benefit of this disclosure. The depicted and described embodiments of the invention are exemplary only, and are not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.