Abstract:
The method of maintaining drill bit advancement in an underground formation that contains shale, including providing an electrical signal from an insulated gap location in a drill string substantially directly behind the bit in the formation, detecting substantial change in a signal as the bit advances, and changing the direction of drilling of the bit as a function of a signal change, to thereby maintain the direction of bit advancement in the formation. 
     A method is disclosed for detecting the existence and direction of adjacent bed boundaries. A short hop transmitter assembly generates a signal that is detected by an associated receiver assembly. The received signal(s) are tied to the azimuthal orientation of the transmitter or receiver and processed to yield the direction and/or the distance of the bed boundary. This information is transmitted to the surface via surface telemetry for real-time control of the drilling assembly to stay within, or to enter, a pay zone.

Description:
This application is a continuation-in-part of pending U.S. Ser. No. 11/584,778, filed Oct. 23, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to sub-surface formation boundary detection and more specifically to method and apparatus for such detection using borehole telemetry apparatus. 
     It is known that shale in natural formations may contain significant amounts of hydrocarbon. It is further well known that significant amounts of hydrocarbon can be recovered by boring holes into the formation and using so-called fracking techniques. Such beds may extend generally horizontally, and be relatively thin in vertical extent. U.S. Pat. Nos. 6,280,000 and 6,425,448 describe examples of such drilling and show particular patterns of holes to drain methane from a coal formation. In the boring of such holes, method and means are needed to steer the drilling progress, so as to remain in the bed and, to the extent possible, bore a straight hole such that up and down variations in the borehole path are minimized. 
     Conventional or current boring, or drilling, operations use some sort of measure-while-drilling (MWD) apparatus. Such an apparatus generally includes inclination and direction sensors, various logging sensors to assist in determining that the borehole trajectory remains in the underground formation and a communication means to transmit data to the surface so that the necessary control operations to control the drill string path can be performed. Typical inclination sensors include accelerometers to sense the earth&#39;s gravity field. The most commonly used direction sensors are magnetometers to sense the earth&#39;s magnetic field although gyroscopic sensors may be used in some circumstances. Logging sensors may include conventional resistivity sensors based in the low-megahertz frequency range, total gamma ray sensors and focused gamma ray sensors. In current practice, the only sensors that can provide reliable information as to whether or not the drilling apparatus is within or out of the selected formations are the various gamma ray sensors. Theses sensors generally have a very short range, perhaps only a few inches, and thus the drill bit may already be out of the selected formation by the time that gamma ray sensors provide an indication of such a condition. Given this limitation, such boreholes may have considerable variation in inclination as the path of the drill bit is steered. Further, conventional resistivity tools would increase the length of the bottom hole assembly at the bottom of the drill string and would increase the cost of drilling. While certain resistivity apparatus and methods are used to steer the drilling apparatus in order to maintain the borehole in a desired geological bed, none of these is similar to or has the advantages of the present invention described below. 
     There is a need for improved sensing method and means that can efficiently detect the boundary of the selected formation, such as shale formation, at a considerably greater depth of investigation around the borehole and most desirably one that can provide some indication of the conditions out ahead of the bit so as to permit correction of the drill path with reduced variation in inclination. 
     In the measure-while drilling (MWD) process for drilling, the borehole telemetry technique of choice is the electric field technique that involves direct injection of electric current into the surrounding formation at a point below an insulating gap in the generally conducting steel drill string. This injected current flows out into the formation and develops a detectable electric voltage between a remote contact to the earth and the drill string at the surface of the earth. Examples of such apparatus are disclosed in U.S. Pat. Nos. 5,130,706, 5,883,516, 6,188,223 and 6,396,276. It has been observed experimentally, and confirmed analytically, that when the drill bit is employed in a coal seam the apparent driving-point impedance, defined as the ratio of the output voltage to the output current, seen at the output stage of an electric field borehole telemetry apparatus decreases as the drill bit below an insulating gap approaches a coal seam boundary and penetrates into an adjacent rock layer. Further, it has been observed experimentally and confirmed analytically that the received signal strength at the surface of the earth increases for the same approach to and penetration into an adjacent rock layer. 
     SUMMARY OF THE INVENTION 
     It is a major objective of this invention to provide an improved method to detect a sub-surface formation boundary or boundaries, using an electric field borehole telemetry apparatus. This is useful for example to facilitate subsequent fracking and fracking locating procedure in shale. This method enables use of the telemetry apparatus to transmit inclination, direction and logging parameters to the surface for use in steering the drill string to remain in the formation, i.e. relative to the formation boundary, and particularly both up and down boundaries, in a way that substantially benefits results in terms of better control of the borehole trajectory at a lower cost. The invention also provides a method for assisting in steering a drill bit so as to maintain the drill bit in a sub-surface bed or seam, during drilling, enabling the borehole to be used for reception of fracking equipment. The method of the invention includes detecting the relative position of the drill bit with respect to a formation boundary or boundaries, using an electric-field borehole telemetry apparatus. 
     Another object is to provide a method of maintaining drill bit advancement in an underground in situ formation, that includes 
     a) passing an electrical signal from a location in the vicinity of the bit to a location in the underground formation, above the level of the bit, 
     b) detecting substantial change in that signal as the bit advances, 
     c) and changing the direction of drilling of the bit as a function of said signal change, to thereby maintain or control the direction of bit advancement in the formation or seam. 
     In this regard, the electrical signal is typically electrical current passed from the seam through a seam boundary into the adjacent underground formation. 
     More detailed steps of the method include: 
     1. providing a measure-while-drilling apparatus that includes inclination sensors, directional sensors, logging sensors of choice and an electric-field telemetry borehole telemetry apparatus, 
     2. within the electric-field telemetry borehole telemetry apparatus, in addition to monitoring the inclination, direction and logging parameters, monitoring parameters of the electrical output of the telemetry apparatus such as pulse voltages, pulse currents and/or pulse power, 
     3. transmitting to the earth&#39;s surface the inclination, direction and logging parameters as well as the parameters of the electrical output by means of the telemetry apparatus, 
     4. detecting at the surface the data transmitted, and monitoring the signal strength received at the surface, 
     5. computing the usual drilling parameters needed to guide the drill string along the intended path, 
     6. determining from the transmitted parameters of the electrical output from the downhole apparatus and the signal strength received at the surface, parameters indicative of drill bit approaching or penetrating a formation boundary, and 
     7. making corrections to the direction of drilling to maintain the drill string and bit in the selected seam. 
     A further object is to provide a method of production of hydrocarbon from a selected underground formation, and utilizing an underground coal seam in proximity to the selected formation, that includes 
     a) drilling in the coal seam having a boundary facing the selected formation, 
     b) drilling through the boundary, and into the selected formation and to a preferred location therein, 
     c) producing hydrocarbon fluid from said location, for recovery at the surface. 
     Added objects include fracking the selected formation at said location to enhance hydrocarbon recovery; and wherein producing includes flowing hydrocarbon fluid from said location through a borehole formed by drilling. 
     Yet another object includes providing a method of determining an optimum location in a selected formation, for fracking procedure. 
     A further object includes use of the method as described to determine the location of formation boundaries above and below the zone in which drilling is being effected. 
     The present invention also allows determining the direction and/or distance of sub-surface bed boundaries during drilling, by using short hop telemetry signals. The distance and direction of the bed boundary as determined by the system then can be used to control the direction or inclination at which the well is drilled. 
     In the disclosed and preferred embodiment the LWD tool includes a transmitter/receiver pair for sending short hop telemetry signals into the formations surrounding the wellbore and receiving these signals as at well head. This transmitter/receiver pair is provided on an upper and lower end of the drill-string for communicating information between a sensor located near a drilling bit and the system axially along and behind the drilling motor. One of the antennas is off center with respect to the drill string i.e. is not symmetric with respect to the drill string. The preferred antenna leads to a variation in signal strength of the short hop signal depending on its orientation. 
     The signals detected by the receiver are processed with the respective information of the antenna orientation at the time of their transmission. The variation in signal strength with rotation is used to determine the direction of the adjacent bed boundary. The signal strength for a fixed orientation, or the average of the signal strength covering 360° orientation (antenna facing towards and away from the boundary) can be used to calculate or estimate the distance to the bed boundary. 
     These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: 
    
    
     
       DRAWING DESCRIPTION 
         FIG. 1  shows a typical drilling process including a drill string, an insulating gap near the bit and various layers of formation material in the region of a selected seam;  FIG. 1  also being representative of drilling in a coal seam; 
         FIG. 2 a    shows a computer simulation of the output current of the electric-field telemetry apparatus when the drill bit and drill string are in the selected seam and not in contact with other layers of the formation; 
         FIG. 2 b    shows a computer simulation of the output current of the electric-field telemetry apparatus when the drill bit is in contact with another layer of the formation above the seam; 
         FIG. 3  is a block diagram showing the borehole telemetry apparatus, the conductive media between the downhole and up-hole regions and the receiving and processing apparatus at the surface; 
         FIG. 4  shows a representative detail log plot from a drilling operation in a coal seam and shows the transmission-parameter variations that are indicative of approaching or penetrating a coal stem boundary; 
         FIG. 5  is a schematic view of a drilled-into preferred location to enable hydrocarbon production after fracking; 
         FIG. 6  is a schematic view of a metallic strip antenna, in a drill string, and offset from a string axis, for enabling upper and lower shale boundary determination; 
         FIG. 7  is a schematic view of drilling through a boundary between a coal seam and a selected seam; 
         FIGS. 8-12  are schematic views; 
         FIGS. 13-17  are graphs; 
         FIGS. 18-21  are current density plots; and 
         FIG. 22  is a block diagram. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a typical drilling process including a drill string, an insulating gap in the string near the bit, and various layers of underground material in the region of a selected seam or formation layer. A drill rig  1  at the surface of the earth is connected to a drill string  2  penetrating down into the earth. The upper portion of the borehole is shown with casing  4 , and the open hole  3  continues below the casing. An insulating gap  7  in the string is at or proximate the lower end of the drill string. Below the insulating gap a non-magnetic collar  8  in the string contains a measure-while-drilling (MWD) apparatus indicated at  8   a . A mud motor  9 , below  8 , is or may be used to rotatably operate a rotary drill bit  10   a . A future projection of the location of the drill bit indicated at  10   b  shows where the drill bit is projected to be at some future time. At the surface, an electronics assembly  5  is shown electrically connected to the upper end of the drill string, as by connection  5   a . Connection  5   b  provides an electrical connection from a remote contact  5   c  with the earth to the electronics assembly  5 . Information is communicated from the measure-while-drilling apparatus to the electronics assembly  5  by applying output voltage or current signals across the insulating gap  7 , as by means  7   a  (see plus and minus voltage zones +v and −v. Current then flows from the lower region below the insulating gap  7  through the earth to the surface. This current then causes a voltage difference between the upper end of the drill string connected to lead  5   a  and the remote connection to the earth connected to lead  5   b . The drill string between the insulating gap  7  and the upper end of the drill string connected to lead  5   a  is generally of steel and therefore has much greater conductivity than the path through the earth. 
     The earth formation going downward from the surface is indicated typically by layer boundaries  6   a ,  6   b ,  6   c ,  6   d  and  6   e . These boundaries will, in general, represent different kinds of rock, and the region between the boundaries  6   d  and  6   e  are the upper and lower boundaries of a shale seam or layer  6   f  that is to be drilled. The location of this seam is generally known as by prior work before drilling is begun. By well-known techniques, such as using a mud motor and a bent sub in the string above the bit, the borehole  3  is drilled downward from the surface and then caused to turn toward a horizontal condition as shown when the depth of the shale seam is reached. That may be nominally horizontal, but there may be a known or approximately known small inclination angle to the seam. The object of the drilling process is to drill for an extended distance while maintaining such drilling within the shale seam to provide a path for enhancement of fracking equipment, and/or the recovery of hydrocarbon such as oil or gas from the selected seam. Previously, little information was available to assist in maintaining the drill bit path within the seam, as during horizontal drilling, to different locations within the seam. Gamma ray detectors, either total gamma ray counters or so-called focused gamma ray counters, were frequently used for detecting an out-of-coal seam drilling condition. Such detectors provide very short depth of investigation and are located a considerable distance behind the bit so that the resulting borehole path tended to have considerable up and down bending deviation since the bit had to be out, or nearly out, of the bed or formation layer before deviation from the desired trajectory was sensed, and only then could a correction in drilling direction be made, using known measure-while-drilling techniques to change the inclination of the borehole to return to the desired trajectory. 
     During employment or use of an electric field borehole telemetry apparatus, and a part of the measure-while-drilling apparatus, that included monitoring and transmitting the value of the output current along with the other data, it was observed when drilling in a coal seam that when the bit was approaching or deviating out of the coal seam, the output current increased. It was further noted that under such conditions, the signal level received at 5 at the surface between connections  5   a  and  5   b  increased. It was also observed that the resistivity of the coal in the coal seam was significantly higher than the resistivity in the adjacent rock layers such resistivity affecting the output current. Typical resistivity for a coal seam may be on the order of 100 ohms-meter while that of adjacent rock layers such as shale may be on the order of 4 ohm-meters. 
       FIG. 2 a    shows a computer simulation of the output current of the electric-field telemetry apparatus when the drill bit and drill string below gap  7  are in a coal seam and not in contact with or penetrating into other layers of the formation. This can be represented by using an electrical finite element model. The region of the formation above the upper boundary  20  of the coal has a resistivity of 4 ohm-meter. The region in the coal below the coal boundary  20  has a resistivity of 100 ohm-meter. The contour lines in the diagram are such that they show electric current density. The current density contours are labeled in terms of amperes per square meter (A/m^2). 
     An insulating gap  22  is provided between the portion of the drill string  23  above (i.e. to the left of) the insulating gap and that portion of the drill string, including the drill bit,  21  below (i.e. to the right of) the insulating gap  22 . Neither the drill bit nor any portion of the drill string as referred to is in contact with the low-resistivity material above the coal boundary  20 . The contour lines going from 1.42e −2  A/m^2 near the drill string section  21  to 3.93e −2  A/m^2 at longer distances from  21  are indicative of low current density resulting from the high resistivity of the coal between the drill string and the layer above the boundary  20 . 
       FIG. 2 b    shows a computer simulation of the output current of the electric-field telemetry apparatus when the drill bit is in contact with another layer of the formation. The same electrical finite element model was used as for  FIG. 2 a   . The resistivities of the layers are the same as for  FIG. 2 a   . In  FIG. 2 b    the drill bit  24  is just in contact with the layer above the edge of the coal  20 . From the much greater distances to the corresponding current density contours of  FIG. 2 a   , in this figure above the seam edge  20 , it is apparent that the current density is much larger in this region than it was for the case of  FIG. 2 a    where there was no contact. The driving voltage applied between the drill string sections  21  and  23 , across the insulating gap  22 , was the same for both computations. The region above the coal boundary and extending to the surface can be considered as an impedance network. Since the current flowing into the network is increased, the so-called driving point impedance seen by the power-output device in the electric-field borehole telemetry apparatus is decreased for  FIG. 2 b    in comparison to  FIG. 2 a   . Driving point impedance for a network is defined as the applied voltage divided by the input current. Such a driving point impedance is generally abbreviated as Z D . This confirms the experimental observation that the driving point impedance seen by the telemetry apparatus decreased when the bit was known to be approaching or out of the coal seam. Further, since the current flowing into the layers above the bit is increased for the conditions of  FIG. 2 b    the voltage received at the surface between the leads identified as  5   a  and  5   b  in  FIG. 1  will be increased. The value of Z D  can be determined from measurements transmitted from the downhole location to the surface and the voltage received at the surface can be measured. Thus there are two measures available from the telemetry apparatus that provide useful information on the positional relation of the drill bit and the boundary of the coal seam. In other drilling situations, not related to coal bed methane recovery, changes in the voltage received at the surface using an electric-field borehole telemetry apparatus have been noted and believed to be related to formation resistivity. 
       FIG. 3  shows a block diagram representative of the borehole telemetry apparatus, the conductive media between the downhole and up-hole regions and the receiving and processing apparatus at the surface. An electric field borehole telemetry apparatus  25  comprises inclination sensors  26   a , direction sensors  26   b , and logging sensors  26   c  connected to a signal conditioning, multiplexing and coding section  27 . The output of the coding section  27  is applied to a power section  28  that is connected to the output line  30  which is connected to the drill bit below the insulating gap  7  of  FIG. 1 . The power section  28  may be of a constant voltage, constant current or other type. Connection  29  transmits information, for example voltage and/or output current, from the outputs line  30  to monitoring elements  26   d . The output of the monitoring elements  26   d  is connected to the coding section  27  so the results of such monitoring are added into the data stream that is transmitted to the surface. Output line  31  is a current return path and represents elements of the conductive drill string above insulating gap  7  of  FIG. 1 . 
     The block  32  represents the conductive media between the downhole and up-hole regions. As shown it is a typical four-terminal electric network. The terminal connected to lead  31  is the point on the drill string just above the insulating gap  7  of  FIG. 1  and the terminal connected to lead  34  is the point at the top of the drill string connected to lead  5   a  of  FIG. 1 . If the resistivity of the drill string between the insulating gap and the surface is insignificant compared to all other resistivities, the points of connections  31  and  34  may be considered common and the network reduces to a three-terminal network. The lead  33  is equivalent to lead  5   b  of  FIG. 1  and represents the connection from a remote contact with the surface of the earth and the receiving and processing apparatus at the surface  35 . The receiving and processing apparatus  36  provides amplification, de-multiplexing and decoding of the received signal to recover the data transmitted from the downhole location and a measure of the amplitude of the received signal. The block  37  provides any further decoding and data conversion required and provides inclination, direction and logging outputs on lead  39   a  to operators to assist in judging the path of the borehole and planning any needed corrective actions, as for bit steering. Downhole electrical output information, for example voltage and/or output current, as well as a measure of the amplitude of the received signal are transmitted to block  38  as parameters indicative of approaching or penetrating the coal boundary for evaluation of the relationship of the borehole location to the desired in-coal location. Information from this evaluation is transmitted to operators on lead  39   b  for planning any required actions to remain in the coal seam. 
     Some electric-field borehole telemetry apparatus may include a capability to transmit command information downward from the surface to the downhole telemetry apparatus. When such a capability is present and evaluation parameters indicate a possible approach to the coal seam boundary a command may be sent downward from the surface directing the downhole apparatus to increase its output signal power. This may be done by increasing the voltage, current or time duration of the signals being transmitted upward. With such an increase in the transmitted signal uncertainties such as downhole movements, rig noise and surface interference are minimized, thus in effect increasing the signal-to-noise ratio of the boundary detection process. 
     Note that the only apparatus that needs to be added to the electric-field borehole telemetry apparatus as shown in  FIG. 3  to permit the use of the method of this invention includes the block  26   d , the monitoring elements, and block  38 , the block that provides the evaluation of the relationship of the borehole location to the desired in-coal location. 
       FIG. 4  shows a detail log plot from an actual drilling operation in a coal seam, and indicates the transmission-parameter variations that are indicative of approaching or penetrating the coal boundary. A date/time scale  40  is shown at the left of the figure. The major divisions on this scale are one hour, the next level of scale is ten minutes and the finest scale is for two-minute time increments. A trace  41  for the output of a gamma ray detector, a trace  42  for the tool output current, a trace  43  for the tool output voltage, a trace  44  for the received signal at the surface of the earth, a trace  45  for the driving point impedance, Z D , (defined as the ratio of the tool output voltage to the tool output current), and two traces  46  and  47  for a focused gamma ray measurement are provided. Trace  46  is for gamma ray data received from the down direction and trace  47  if for gamma ray data received from the up direction. Other traces are shown for ROP, rate of penetration, TVD, total vertical depth and Bit Depth but these are not used in the discussion below. Note that near point  48  an increase in Pulse Voltage, the received signal at the surface, shown on trace  44  is seen. Further, near point  49  a decrease in the driving point impedance shown on trace  45  is seen. These changes are indicative that the tool bit is approaching the boundary between the coal seam and the adjacent lower-resistivity rock layer. Drilling proceeded for about twenty minutes before an increase in the gamma ray measurement shown on trace  41  is observed. This increase that becomes a maximum near the point  50  in the total gamma ray measurement and indicates that the drilling apparatus is proceeding or deviating out of the coal seam. Further, the focused gamma ray signals, shown on traces  46  and  47  confirm that the tool is out of the coal as shown by points  51  and  52 . Since the amplitude of the gamma-up signal  52  is greater than the gamma-down signal  51 , it is apparent that the tool has gone out of the coal seam at the top of the seam. Corrective action was taken and the tool descended back into the coal, restoring the indicated signal to levels comparable to those seen before the detection of indications that the drill trajectory was going toward an out-of-coal condition. 
     The significant issue that the indications from trace  44 , the surface received signal, and trace  45 , the driving point impedance, showed the existence of the problem about 20 minutes prior to actually going out of the coal. Corrective action based on these indications can prevent going out of the coal and this would result in a smoother borehole trajectory in the seam. 
     It is clear from the discussions above that the indications of approach to and going beyond (i.e. penetrating) the boundary of the coal bed are similar at both the upper and lower boundaries of the bed. Operator experience and the making of minor variations in the inclination of the borehole to observe changes in the indications provide the means to identify which case is most probable. 
       FIG. 5  schematically shows running into hole  3 , of a fracking (hydraulic fracturing) string  80  having a head  81  to which highly pressurized hydraulic fluid such as water is delivered from the well head at  1 . That fluid is delivered to space  82  to pressurize and open up or fracture crevices, to enable oil or gas to leave the bed and enter the hole. A plug  84  may or may not be used or west to localized build-up of hydraulic fracking fluid. String  80  may be withdrawn after fracking, to allow pressurized oil or gas to flow back up the hole, for oil or gas production at the well head. Since accurate drilling is achieved by the invention as for example by centralizing the drilled end of the hole, between up and down boundaries at  6   d  and  6   e , the oil or gas production flow may be maximized. See production flow arrows  85  in the formation at the locus of fracturing, and production flow arrows at  86 . 
       FIG. 6  shows provision of an antenna  90  in the drill string, below the gap  7 . It may take the form of a metallic strip positioned in offset relation to the string axis  89 , as at the bore  91  of the string. Strip  90  may be embedded in insulation  90   a . Thus, as the drill string is rotated as during drilling, the antenna strip  90  is rotated toward and away from the two formation boundaries  6   d ′ and  6   e ′, corresponding to coal boundaries  6   d  and  6   e  in  FIG. 1 . The received signal, or its amplitude, as referred to, varies cyclically as the string is rotated, close to the different electrical resistivities of the shale, and of the non-shale layers  6   f  and  6   g  above and between respective boundaries  6   d ′ and  6   e ′. Such signal amplitude variance is detected, as indicative of the closeness of bit travel to the boundaries as the bit is rotated and advanced in the selected formation. 
       FIG. 7  shows a method of production of hydrocarbon such as oil or gas from an underground formation or layer  200 . A coal seam  201  extends general parallel to and in proximity to  200 , and has a boundary  202 . Steps of the method include: 
     a) drilling at  204  in the coal seam having a boundary  202  facing the adjacent formation, 
     b) drilling through boundary  202 , and into the formation  203  and to a preferred location  203 ′ therein, 
     c) producing hydrocarbon fluid from the location  203 ′, back through the borehole  208 , at  208   a  for recovery at the surface. 
     The formation at location  203 ′ may be subjected to fracking to enhance oil or gas production. 
     A perspective view of a directional drill string assembly  300  with an insulated electrode  301  (lower antenna labelled ‘electrode’) and a gap sub  302  (upper antenna labeled ‘gap’) used for short hop communication is shown in  FIG. 8 . See also drill bit  303  at lower end of string  304 , in hole  305  in an oil producing zone  306 , having upper and lower boundaries  307  and  308 . 
       FIG. 9  shows the insulated gap  302  in the drill string  304  which is used to transmit or receive short hop data from the motor or other sensors closer to the bit. Additionally, the gap can be used to transmit information to the surface.  FIG. 10  shows the insulated electrode  301  used to transmit information from an instrumented motor (Smart Motor) or other near bit device  311  and receives information from the gap. The outer edge  301   a  of the electrode facing the hole bore is shown. The electrode body, other than edge  301   a  is embedded in insulation. Information is communicated from this insulated electrode by applying output voltage or current signals across the insulated electrode and motor housing (or collar)  312 . Part of the current then flows from the lower region of the drill string through the earth formation to the upper section of the drill string. This current then causes a voltage difference between the lower end of the drill string and the upper end of the drill string across the insulating gap. 
       FIGS. 11 and 12  show cross sectional views of the drill string at the Smart Motor electrode and a nearby boundary  313  which is often encountered in horizontal drilling, where the object of the drilling process is to drill for an extended distance while maintaining such drilling within the desired zone (e.g. a coal or shale or other seam). The metallic electrode  301  is recessed of partially recessed into the outer wall of the drill string, so that the outermost surface of the electrode is exposed to the bore hole  320 . The remainder of the electrode is embedded in insulation  90   d . During the drilling process and during the rotation of the drill string, the electrode will be alternately facing the formation boundary or be looking away from the boundary. A signal strength variation will be seen at the gap if the resistivity contrast between the target bed or zone of drilling and the shoulder bed adjacent the target bed is high enough. This signal variation can be used to infer a proximity to a boundary. See also  FIG. 13 . 
     During employment of a short hop telemetry apparatus, it is expected that the signal strength will vary in an oscillatory way during string rotating.  FIGS. 14 and 15  show these variations as a function of angular position for different cases. It is clear from those plots that it is possible to determine the relative position of the boundary in addition to the fact that there is a boundary due to the shape of the waveform and maximum amplitude variation changing with respect to distance from the boundary. The fact that the insulated electrode antenna is sitting on one side of the string and is therefore lacking the rotational symmetry of an electrical antenna such as a ring or gap type antenna, is the cause for this effect. Given that the resistivities of the formations is known the signal strength or strength variations is useful to determine a distance to the boundary (see  FIGS. 16 and 17 ). 
       FIGS. 18 and 19  illustrate the difference in signal strength at the gap when the insulated electrode is either facing away or towards the boundary. Note the higher current densities at the gap for the case where the insulated electrode is facing the boundary (i.e. closer to the boundary) in  FIG. 19 . When the shoulder bed has a lower resistivity, the currents are drawn more into this shoulder bed (see.  FIGS. 13 and 14 ) especially if the electrode is facing towards the boundary as in  FIG. 14 . This results in a higher signal strength received at the gap. 
     The invention is typically used as a means or method to detect an approaching (relative to bit advancement) boundary between zones of different resistivity, such as a coal bed, shale bed or an oil-water contact, and is useful to determine the direction of the boundary situation with respect to high-side and to give a distance estimate. This information is used for example to maintain drilling in a desired formation, without steering into neighboring zones. The new method provides information as to the direction the boundary is “approaching” from, without having to drill ahead further, by analyzing a few drill string rotations during which there is short hop communication. 
     In  FIG. 10  a possible embodiment of an off-center antenna is shown. This antenna consists of one long, planar electrode strip embedded in an insulating material and is sitting on the side of an instrumented motor (Smart Motor)  311  or near bit sub. This allows driving or sensing currents or voltages between the electrode strip and the drill string. A lengthwise cross-sectional view of this Smart Motor antenna is shown in  FIG. 11  where it is in close proximity of a shoulder bed  342  and facing towards the bed boundary. A transversal cross-section through such a short hop antenna is given in  FIG. 12  again with the electrode strip pointing towards the boundary. 
     The off-center antenna preferably is located close to the drill bit to facilitate the ability to examine the formation as close to the bit as possible. Alternatively the asymmetric antenna could be located further up the BHA (borehole) without departing from the principles of the present invention. 
     Information is communicated from the gap by applying output voltage or current signals across the insulating gap. The current then flows from the upper region of the drill string through the formation to the lower section of the drill string. This current then causes a voltage difference between the lower end of the BHA and to off-center electrode (e.g. the Smart Motor electrode). 
     Referring now to  FIGS. 18 and 19 , the operation of the azimuthal boundary embodiment will now be described for the logging tool shown in  FIG. 8 . The two special orientations with an off-center antenna are when facing the boundary or pointing away from the boundary.  FIG. 18  shows the current densities in the lengthwise cross-section of the drill string for the case where the electrode is pointing away from the boundary (i.e. is furthest away from the interface). The current densities are shown in  FIG. 19  for the case where the electrode is facing towards the boundary. Note the higher current density at the gap compared to the case shown in  FIG. 18 . This higher current density results in a larger signal at the gap for the case when the electrode is facing toward the boundary. 
     A transversal cross-section through the drill string and formation is shown in  FIGS. 20 and 21  to illustrate the difference between the two particular orientations and the effect on the different current injection into the formation. 
     During normal drilling operation the BHA will be rotating and with it the off-center electrode. Let us consider the case where the BHA is in proximity to a boundary with enough resistivity contrast. If during rotation a transmission takes place, the signal strength will vary in an oscillatory way.  FIGS. 14 and 15  show these variations normalized to the maximum value for each distance, as a function of angular position for different cases derived from finite element computer simulations.  FIG. 14  shows the signal strength at the Gap with 1 Ωm mud and shoulder bed resistivity and 20 Ωm bed resistivity for two distances to the boundary (1 and 0.5 ft. from the BH (borehole) to the boundary). As a first order estimation of the signal dependence on orientation, a cosine function choice presents itself as applicable. A cosine fit to the simulation data is depicted therefore, for comparison, revealing that there is a pronounced deviation from this fit. The closer the electrode is to the boundary the more focused the sensitivity becomes, i.e. the higher the deviation from the cosine fit (see  FIG. 14  and compare the curve for 1 and 0.5 ft. distance). 
       FIGS. 14 and 15  also show that the extremum (minimum or maximum) of the signal dependence function that has steeper adjacent slopes is marking the orientation where the electrode is facing the boundary. In other words, the signal strength function is more “pointed” where the electrode is closest to the boundary. In  FIGS. 14 and 15  this orientation is at 0° (which is equal to the 360° orientation). Note that two cases are shown in  FIG. 15  with the resistivity contrast being reversed. One case has a 20 Ωm bed and a 1 Ωm shoulder bed and the other case has a 1 Ωm bed and a 20 Ωm shoulder bed. The orientation where the electrode faces the boundary is still identified correctly, nevertheless, in that one is having a signal maximum and the other case is having a signal minimum at those orientations. 
     During employment of a short hop telemetry apparatus it is expected that the signal strength will vary in a periodic way, when rotating. The period of the signal variation depends on the rotation speed. With the available orientation information obtained from directional sensors it is possible to correlate signal strength and position of the off-center electrode at the time of transmission. Such directional measurements can be made by a three axis accelerometer or a three axis magnetometer which then can determine tool face angle of the tool. 
     A signal strength variation will be seen if the resistivity contrast between the target bed and the shoulder bed is high enough.  FIG. 13  shows simulation results that quantify the amount of variation to be expected for different resistivity contrasts for a 1 ft. distance to the boundary. The variation shown in  FIG. 13  is normalized to the mean of the two extreme values (e.g. facing towards and away from the boundary). A certain signal variation means±that value for the extremes (e.g. 130% and 70% for a 30% variation). Note that there is no variation when there is no resistivity contrast between the adjacent zones. The simulation results show a variation of up to ±30% for a distance of 1 ft which will increase when the BHA gets closer. 
       FIG. 16  shows the dependence on distance to the boundary of the simulated signal strength at the Gap with 1 Ωm shoulder bed resistivity and 20 Ωm bed resistivity for the electrode looking towards the boundary. The closer the boundary, the higher the signal. The signal is normalized to the case without boundary, i.e. normalized to the case where the boundary is sufficiently away from the BHA and has no influence on the signal strength. 
     There is signal variation between the extreme orientations (e.g. facing towards and away from the boundary) for a case with 1 Ωm shoulder bed resistivity and 20 Ωm bed resistivity for variable distance to the boundary. The closer the boundary the higher the signal variation at the Gap. 
     Given that the resistivities of the formations are known the signal strength ( FIG. 16 ) or strength variations ( FIG. 17 ) can be utilized to determine the proximity to a boundary. In the absence of resistivity information the signal strength or strength variation can still be useful, e.g. if during the course of the drilling procedure the boundary is approached and detected with an alternative method. In this case the signal levels and/or the signal variation can be used as a gauge for subsequent boundary distance estimations. 
       FIG. 22  is a typical block diagram of circuitry incorporating the antenna electrode  301  and the gap  302  defined between drill string portions above and below the gap.