Patent Publication Number: US-7594434-B2

Title: Downhole tool system and method for use of same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a continuation application of co-pending application Ser. No. 10/841,780, entitled System and Method for Monitoring Erosion, filed on May 7, 2004. 

   TECHNICAL FIELD OF THE INVENTION 
   This invention relates, in general, to monitoring erosion in a downhole tool system and, in particular, to use of a sensor that is interrogated downhole to determine the erosive effects caused by flowing fluids containing formation sand, gravel, proppants or other erosive agents through downhole tools. 
   BACKGROUND OF THE INVENTION 
   It is well known in the subterranean well drilling and completion art that relatively fine particulate materials may be produced during the production of hydrocarbons from a well that traverses an unconsolidated or loosely consolidated formation. Numerous problems may occur as a result of the production of such particulates. For example, the particulates cause abrasive wear to components within the well, such as joints, chokes, flowlines, tubulars, pumps and valves as well as any components having directional flow changes. In addition, the particulates may partially or fully clog the well creating the need for an expensive workover. Also, if the particulate matter is produced to the surface, it must be removed from the hydrocarbon fluids using surface processing equipment. 
   One method for preventing the production of such particulate material to the surface is gravel packing the well adjacent the unconsolidated or loosely consolidated production interval. In a typical gravel pack completion, a sand control screen is lowered into the wellbore on a workstring to a position proximate the desired production interval. A fluid slurry including a liquid carrier and a relatively coarse particulate material, which is typically sized and graded and which is referred to herein as gravel, is then pumped down the workstring and into the well annulus formed between the sand control screen and the perforated well casing or open hole production zone. 
   The liquid carrier either flows into the formation or returns to the surface by flowing through a wash pipe or both. In either case, the gravel is deposited around the sand control screen to form the gravel pack, which is highly permeable to the flow of hydrocarbon fluids but blocks the flow of the fine particulate materials carried in the hydrocarbon fluids. As such, gravel packs can successfully prevent the problems associated with the production of these particulate materials from the formation. 
   It is sometimes desirable to perform a formation fracturing and propping operation prior to or simultaneously with the gravel packing operation. Hydraulic fracturing of a hydrocarbon formation is sometimes desirable to increase the permeability of the production interval adjacent the wellbore. According to conventional practice, a fracture fluid such as water, oil, oil/water emulsion, gelled water, gelled oil or foam is pumped down the workstring with sufficient pressure to open multiple fractures in the production interval. The fracture fluid may carry a suitable propping agent, such as sand or gravel, which is referred to herein as a proppant, into the fractures for the purpose of holding the fractures open following the fracturing operation. 
   The fracture fluid must be forced into the formation at a flow rate great enough to fracture the formation allowing the entrained proppant to enter the fractures and prop the formation structures apart, producing channels which will create highly conductive paths reaching out into the production interval, and thereby increasing the reservoir permeability in the fracture region. As such, the success of the fracture operation is dependent upon the ability to inject large volumes of hydraulic fracture fluid into the surrounding formation at a high pressure and at a high flow rate. 
   For most hydrocarbon formations, a successful fracture and propping operation will require injection flow rates that are much higher than those required for gravel packing. For example, in typical gravel packing, a single pump capable of delivering one to ten barrels per minute may be sufficient. On the other hand, for a successful fracturing operation, three or four large capacity pumps may be required in order to pump at rates higher than the formation fracture gradient which may range up to 60 barrels per minute or more. 
   It has been found, however, that the high injection flow rates that are associated with fracturing operations and, to a lesser extent, the particulate matter associated with both gravel and fracturing operations cause erosion to the surfaces of downhole components. For example, the surfaces of the cross-over assembly used during these treatment operations are particularly susceptible to erosion. In order to monitor the wear threshold of downhole equipment, erosion detection systems have been utilized that typically include a series of pressure gauges that monitor pressure changes by measuring pressure at a corresponding series of locations. In these existing solutions, a loss in pressure is a possible indication of a failure of an eroded component. 
   Hence, the existing solutions are reactive schemes that provide only for a possible detection of failed components. Therefore, a need has arisen for a system and method for monitoring erosion and the structural integrity and health of surfaces subject to erosion and wear. A need has also arisen for such a system and method to monitor the early stages of erosion in downhole components, downhole tubulars, flowlines and surface equipment. Further, a need exists for a proactive approach to monitoring erosion that provides for preventative maintenance of equipment, alterations in treatment or production parameters and minimizes the likelihood of failures caused by erosion. 
   SUMMARY OF THE INVENTION 
   The present invention disclosed herein provides a system and method for monitoring erosion and the structural integrity and health of surfaces subject to erosion and wear. The system and method of the present invention provide detection in the early stages of erosion in downhole components, downhole tubulars, flowlines and surface equipment. The system and method of the present invention achieve these results by monitoring erosion sensors embedded within downhole tools, downhole tubulars, flow lines, surface equipment and the like during completion and production operations such that a proactive approach to monitoring erosion is provided for preventative maintenance of equipment, alterations in treatment or production parameters and minimizing the likelihood of failures caused by erosion. 
   In one aspect, the present invention is directed to a downhole tool system the includes a downhole tool that is operably positionable within a wellbore. A sensor is positioned within the downhole tool. The sensor has a first mode in which the sensor is responsive to RF interrogation and a second mode in which the sensor is not responsive to RF interrogation. The sensor is operable to transition from the first mode to the second mode upon the occurrence of a predetermined level of erosion of the downhole tool proximate the sensor. A detector is operably positionable relative to the downhole tool in communicative proximity to the sensor. The detector interrogates the sensor to determine whether the predetermined level of erosion has occurred. 
   In one embodiment, the system includes a database for recording erosion condition data obtained by the detector. In certain embodiments, the sensor may be a radio frequency identification component. In other embodiments, the sensor may include an antenna. In any of these embodiments, the erosion may be caused by a moving fluid that may contain erosive agents such as formation sand or treatment additives such as gravel or proppants. 
   In another aspect, the present invention is directed to a downhole tool system the includes a downhole tool that is operably positionable within a wellbore. A plurality of sensors are embedded within the downhole tool. Each of the sensors has a first mode in which the sensors are responsive to RF interrogation and a second mode in which the sensors are not responsive. The sensors are operable to transition from the first mode to the second mode upon the occurrence of a predetermined level of erosion of the downhole tool proximate the respective sensors. A detector is operably positionable relative to the downhole tool in communicative proximity to the sensors. The detector interrogates the sensors to determine whether the predetermined level of erosion has occurred and if so, the location of the predetermined level of erosion based upon which of the sensors are not responsive. In one embodiment, each of the sensors is associated with a unique identifier that is utilized in determining the location of the predetermined level of erosion. 
   In a further aspect, the present invention is directed to a downhole method that includes disposing a downhole tool within a wellbore, the downhole tool having a sensor positioned therein, the sensor having a first mode in which the sensor is responsive to RF interrogation and a second mode in which the sensor is not responsive to RF interrogation, the sensor operable to transition from the first mode to the second mode upon the occurrence of a predetermined level of erosion of a surface of the downhole tool. The method also includes flowing a fluid through the downhole tool, running a detector into the wellbore such that the detector is in communicative proximity to the sensor, interrogating the sensor with the detector and determining whether a predetermined level of erosion of the downhole tool has occurred based upon the responsiveness of the sensor. In the method, a plurality of sensors may be embedded along a length of the downhole and substantially equidistant from the surface or such that at least some of the sensors are positioned at different distances from the surface. In either case, the interrogating may involve interrogation of each of the sensors with the detector. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
       FIG. 1  is a schematic illustration of an offshore oil and gas platform following a fracture packing operation wherein a system for monitoring for erosion according to the present invention is being utilized; 
       FIG. 2A  is a half-sectional view of a sand control screen assembly and a cross-over assembly during a fracture packing operation; 
       FIG. 2B  is a half sectional view of the sand control screen assembly and the cross-over assembly following the fracture packing operation wherein the system for monitoring for erosion according to the present invention is being utilized; 
       FIG. 3  is a cross-sectional view of a substrate in the form of a tubular having a transition area wherein an array of sensors is positioned according to the present invention; 
       FIG. 4A  is a cross-sectional view of another substrate in the form of a tubular wherein an array of sensors is positioned according to the present invention; 
       FIG. 4B  is a cross-sectional view of a further substrate wherein an array of sensors is positioned according to the present invention; 
       FIG. 5  is a half sectional view of a system for monitoring erosion at a first time; 
       FIG. 6  is a half sectional view of the system for monitoring erosion at a second time; 
       FIG. 7  is a half sectional view of the system for monitoring erosion at a third time; 
       FIG. 8  is a half sectional view of an alternate embodiment of a system for monitoring erosion; and 
       FIG. 9  is a block diagram of a detector communicating with a sensor according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the present invention. 
   Referring initially to  FIG. 1 , a system for monitoring erosion of a downhole tool operating from an offshore oil and gas platform is schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over a submerged oil and gas formation  14  located below sea floor  16 . A subsea conduit  18  extends from deck  20  of platform  12  to wellhead installation  22  including blowout preventers  24 . Platform  12  has a hoisting apparatus  26  and a derrick  28  for raising and lowering pipe strings such as workstring  30 . 
   A wellbore  32  extends through the various earth strata including formation  14 . A casing  34  is cemented within wellbore  32  by cement  36 . Workstring  30  includes various tools for completing the well. On the lower end of workstring  30  is a fracture packing assembly  38  that includes sand control screens  40  and cross-over assembly  42  that are positioned adjacent to formation  14  between packers  44 ,  46  in annular region or interval  50  that includes perforations  52 . When it is desired to fracture pack formation  14 , a fluid slurry including a liquid carrier and proppants is pumped down workstring  30 . The fracture fluid exits workstring  30  though cross-over assembly  42  into annular interval  50  and is forced at a high flow rate through perforations  52  into formation  14 . The fracture fluid tends to fracture or part the rock to form fissures extending deep into formation  14 . As more rock is fractured, the void space surface area increases in formation  14 . The fracture operation continues until an equilibrium is reached where the amount of fluid introduced into formation  14  approximates the amount of fluid leaking off into the rock, whereby the fractures stop propagating. The proppant material in the fracture fluid fills the voids and maintains the voids in an open position for production. 
   Once the fracture treatment is complete, the gravel packing portion of the fracture operation may commence. The fluid slurry is injected into annular interval  50  between screen assembly  38  and wellbore  32  through cross-over assembly  42  as before. During the gravel packing operation, a surface valve is operated from the closed to the open position allowing the gravel portion of the fluid slurry to be deposited in annular interval  50  while the fluid carrier enters sand control screens  40 . More specifically, sand control screens  40  disallow further migration of the gravel in the fluid slurry but allow the liquid carrier to travel therethrough and up to the surface in a known manner, such as through a wash pipe and into the annular region above packer  44 . 
   As illustrated, a detector  54  is coupled to a conveyance  56  such as a wireline, a slickline, an electric line or the like and run downhole from a control unit  58  located on platform  12  to a position proximate cross-over assembly  42 . Detector  54  may be utilized in wellbore  32  before, after or during the treatment operation. As will be described in further detail hereinbelow, an array of sensors is embedded within components of the downhole tools, such as cross-over assembly  42 , to monitor erosion. Each sensor of the array of sensors has a first mode in which the sensor is responsive to RF interrogation generated by detector  54  and a second mode in which the sensor is non-responsive to RF interrogation. Each sensor of the array of sensors transitions from the first mode to the second mode to indicate that a predetermined level of erosion is present. Erosion may be caused by an erosive agent such as fluids containing particulate matter including sand, gravel, proppants or the like present in treatment fluids, production fluids and the like. As used herein, an erosive agent is any material that wears away at the surface of a substrate by continuous abrasive action typically accompanied by high fluid velocity. Moreover, as previously discussed, the high injection flow rates associated with fracturing operations accelerate the erosion of the surfaces of the components of downhole tools and, in particular, at regions where the direction of the fluid flow is altered such as at cross-over assembly  42 . In order to monitor the erosion, detector  54  is positioned in communicative proximity to each sensor in order to interrogate each sensor. If the sensor responds, a predetermined level of erosion has not occurred. On the other hand, if the sensor is non-responsive, a predetermined level of erosion has occurred rendering the sensor disabled and thereby non-responsive. It should be appreciated that the erosive agent not only wears away the substrate but the sensor too. Specifically, once the surface behind which the sensor is positioned is eroded, the sensor is subjected to erosion and eventually disabled by the abrasive action of the erosive agent. 
   Even though  FIG. 1  depicts a vertical well, it should be noted by one skilled in the art that the system for monitoring erosion of the present invention is equally well-suited for use in deviated wells, inclined wells or horizontal wells. In addition, it should be apparent to those skilled in the art that the use of directional terms such as above, below, upper, lower, upward, downward and the like are used in relation to the illustrative embodiments as they are depicted in the figures, the upward direction being toward the top of the corresponding figure and the downward direction being toward the bottom of the corresponding figure. Also, even though  FIG. 1  depicts an offshore operation, it should be noted by one skilled in the art that the systems and methods described herein may be utilized in onshore operations. 
     FIG. 2A  depicts a fracture packing operation  70  wherein the system for monitoring erosion according to the present invention may be utilized. In the illustrated embodiment, a sand control screen assembly  72  including a plurality of sand control screens is placed in a wellbore  74  proximate formation  76 . Wellbore  74  includes a casing  78  that is secured therein by cement  80 . Sand control screen assembly  72  and a wash pipe  82  are connected to a tool string  84  that includes a gravel packer  86 , a sump packer  88  and a cross-over assembly  90 . 
   To begin the completion, an interval  92  adjacent formation  76  is isolated by operating gravel packer  86  and sump packer  88  into sealing engagement with casing  78 . Cross-over assembly  90  is located at the top of sand control screen assembly  72  and traverses gravel packer  86 . During the fracture treatment, a frac fluid is first pumped into tool string  84  and through cross-over assembly  90  along the path illustrated by arrows  94 . The frac fluid passes through cross-over ports  96  below gravel packer  86  into the annular area  98  between sand control screen assembly  72  and casing  78  as depicted by arrows  100 . 
   Initially, the fracture operation takes place in a closed system where no fluid returns are taken to the surface. Although fluid from the frac pack flows through sand control screen assembly  72  and toward the surface via washpipe  82 , as depicted by arrows  102 , a valve positioned at or near the surface prevents fluids from flowing to the surface. As illustrated by arrows  104 , the frac fluid, typically viscous gel mixed with proppants, is forced through the perforations that extend through casing  78  and cement  80  and into formation  76 . The frac fluid tends to fracture or part the rock to form open void spaces in formation  76  depicted as fissures  106 . As more rock is fractured, the void space surface area increases in formation  76 . The larger the void space surface area, the more the carrier liquid in the frac fluid leaks off into formation  76  until an equilibrium is reached where the amount of fluid introduced into formation  76  approximates the amount of fluid leaking off into the rock, whereby the fractures stop propagating. If equilibrium is not reached, fracture propagation can also be stopped as proppant reaches the tips of fissures  106 . 
   As previously discussed, the high flow rates associated with the fracture operation can cause erosion to the surfaces through which the fracture fluids flow. Sensors  108  are positioned at transition zones  110  of cross-over assembly  90  to monitor erosion at these particular erosion vulnerable locations. As will be described in further detail hereinbelow, a valve  112  may be opened to permit a detector to be lowered into the cross-over assembly  90  so that sensors  108  may be interrogated to monitor for erosion. 
     FIG. 2B  depicts fracture operation  70  at a point in the operation wherein frac fluid is not being pumped. For example, this may be at a time period between portions of the fracture operation or after the fracture operation has been completed. In the illustrated embodiment, a detector  114  is lowered on a conveyance  116  through valve  112 , which is in the open position, into cross-over assembly  90 . In particular, detector  114  is positioned in communicative proximity to sensors  108 . Detector  114  interrogates each of the sensors  108  with a radio frequency or RF signal. If a given sensor responds, a predetermined level of erosion has not occurred in the material surrounding the sensor. On the other hand, if a given sensor is non-responsive, a predetermined level of erosion has occurred in the material surrounding the sensor rendering that sensor disabled and thereby non-responsive. Each sensor  108  returns a unique identifier such as a unique frequency so that detector  114  may discriminate between sensors  108 . Accordingly, the location or locations of any erosion can be precisely and accurately monitored throughout the tools and tubulars of a completion or production string. 
     FIG. 3  depicts a substrate  130  in the form of a tubular  132  for transporting fluids. Sensors, such as sensors  134 - 162 , are embedded within tubular  132  at different locations in order to monitor erosion. These sensors  132 - 162  may be arranged in a variety of arrays. Sensors  134 ,  136 ,  138 ,  140  are positioned in a symmetrical and complimentary relationship as highlighted by box  164 . In particular, sensor  134  is positioned across from sensor  136  and sensor  138  is positioned across from sensor  140 . It should be appreciated by those skilled in the art that other types of array arrangements are within the teachings of the present invention. For example, sensors  142 ,  144 ,  146  and  148  are positioned in a staggered relationship as highlighted by box  166 . 
   Regardless of the particular arrangement of sensors  134 - 162 , in a preferred embodiment, the sensors are embedded within substrate  130  at regions which are particularly susceptible to erosion. As illustrated, erosive agents such as particles in the fluid flow through tubular  132  along the path indicated by arrows  168 . As the fluid moves through a transition area  170  of tubular  132 , the flow path becomes nonlinear and the erosive agents contact tubular  132  at erosion zones  172 ,  174 . Sensors  160 ,  162  are embedded within tubular  132  at erosion zones  172 ,  174 , respectively, in order to monitor the erosion at these particularly susceptible locations. In operation, a detector can identify the particular sensors in the array and the particular erosion conditions associated with the sensors. The detector may record each of the erosion conditions in a database to maintain an erosion history, for example, that may be utilized to determine the health of erosion zones  172 ,  174 . 
     FIG. 4A  depicts another embodiment of a substrate  180  in the form of a tubular  182  having an array of sensors  184  therein. In the illustrated embodiment, array of sensors  184  includes a sensor  186  positioned at a first distance from an inner surface  188  of tubular  182  and a second sensor  190  positioned at a second distance from inner surface  188 . As will be appreciated, this arrangement of sensors at different depths is present throughout array of sensors  184 . Positioning sensors at different depths enhances the ability to monitor erosion. For example, sensors  186 ,  190  each have a responsive mode and a non-responsive mode and each of the sensors  186 ,  190  transitions from the responsive mode to the non-responsive mode to indicate respective levels of erosion. Thus, by monitoring sensors  186 ,  190  two predetermined levels of erosion may be monitored. 
   As one skilled in the art will appreciate, the installation of the sensors may be accomplished using a variety of techniques. For example, holes may be drilled into outer surface  192  of substrate  180  such that sensors  184  may be positioned therein. It should be appreciated that due to the small size of the sensors, the holes do not have to be large. Preferably, the holes are formed from outer surface  192  and not from inner surface  180 , which is the surface exposed to the erosion. After installation of the sensors, the holes may be capped with a filler material such as an epoxy, a threaded plug, a weld or the like. 
   The small form factor of the sensors permits the sensors to be employed in a wide variety of downhole and fluid transport related applications. The sensors may be employed in downhole tools, downhole tubulars, flow lines, surface equipment and the like during completion and production operations, for example. In this regard, the substrate may be a pipeline or other fluid transmission line, a riser, a drill bit, an elbow, a joint, a packer, a valve, a piston, a cylinder, a choke, a mandrel, a riser pipe, a liner, a landing nipple, a ported sub, a polished bore receptacle or the like. Moreover, it should be appreciated that the use of the sensors is not limited to downhole applications. As will be explained in further detail hereinbelow, the sensors of the present invention are well suited for flow lines that transport fluids on the surface. Further, the sensors are well suited for use with nonmetallic substrates, such as polymeric and elastomeric materials as well as composite materials. For example, sensors may be integrated into a layer of braided or filament wound material that forms a layered strip within a composite coiled tubing. 
     FIG. 4B  depicts another embodiment of a substrate  194  in the form of a section of tubular  196  having an array of sensors  198  positioned therein. In the illustrated embodiment, array of sensors  198  includes sensors  198 A- 198 F, which are each placed at consecutively greater distances from surface  199  as expressed by the distance indicators d 1 -d 6  having the following relationship: d 1 &lt;d 2 &lt;d 3 &lt;d 4 &lt;d 5 &lt;d 6 . For example, sensor  198 A is positioned at a distance d 1  from surface  199 , sensor  198 B is positioned at a distance d 2  from surface  199  and sensor  198 C is positioned at a distance d 3  from surface  199 . Positioning sensors at various depths enhances the ability to monitor erosion in a discrete manner. In operation, sensors  198 A- 198 F each have a responsive mode and a non-responsive mode such that each of the sensors  198 A- 198 F transition from the responsive mode to the non-responsive mode to indicate respective specific levels of erosion. Thus, by monitoring the array of sensors  198 , discrete levels of erosion may be monitored. 
     FIG. 5  depicts a system  200  for monitoring erosion. A substrate  202  includes a sensor  204  embedded therein for monitoring erosion. Sensor  204  is discreetly positioned within substrate  202  such that sensor  204  does not affect the structural integrity of substrate  202 . Substrate  202  is defined by an inner surface  206  and an outer surface  208 . Inner surface  206  is subjectable to fluid flow and, although no erosion has occurred, inner surface  206  is a candidate for erosion. A detector  212  is lowered on a wireline  214  and positioned in communicative proximity to the sensor  204  within substrate  202  such that detector  212  is closer to inner surface  206  than outer surface  208 . Detector  212  probes sensor  204  and determines whether the predetermined level of erosion of inner surface  206  of substrate  202  has occurred based upon the responsiveness of sensor  204 . As illustrated, detector  212  transmits RF interrogating signal  216  which is received by sensor  204 . Sensor  204 , in turn, responds with RF response signal  218 , which is received by detector  212 . Based on the responsiveness of sensor  204 , detector  212  determines that the predetermined level of erosion has not occurred to surface  206 . 
     FIG. 6  depicts the system  200  for monitoring erosion at a second time. Inner surface  206  of substrate  202  has been subjected to an erosive agent for some period of time and inner surface  206  has eroded. The erosion, however, has not reached a predetermined level. Specifically, sensor  204  remains operational although a portion of the sensor&#39;s antenna has been eroded along with inner surface  206 . Detector  212  interrogates sensor  204  with RF signal  224  and sensor  204  responds with RF signal  226 . Based on the responsiveness of sensor  204 , detector  212  determines that the predetermined level of erosion has not been reached. 
     FIG. 7  depicts system  200  for monitoring erosion at a third time wherein further fluid flow has eroded inner surface  206  of substrate  202  to the point that the predetermined level of erosion has occurred. Specifically, the erosion has disabled sensor  204  and provided the impetus for the transition from the first mode to the second mode of sensor  204 . In the illustrated embodiment, detector  212  interrogates sensor  204  with RF signal  234 . Since sensor  204  is disabled, however, sensor  204  does not respond to RF signal  234 . Based upon the non-responsiveness of sensor  204 , detector  212  determines that the predetermined level of erosion has occurred to inner surface  206  of substrate  202 . Based on the information that a predetermined level of erosion has occurred, preventative maintenance or other corrective action may be undertaken to ensure the health of substrate  202  before substrate  202  fails. 
   Accordingly, it should be appreciated that the present invention provides a system and method for monitoring erosion and the structural integrity and health of surfaces subject to erosion and wear. In particular, the passive sensors of the present invention, provide an indication of a predetermined level of erosion. Hence, the systems and methods of the present invention provide for the proactive monitoring of erosion which represents an improvement over existing reactive schemes. 
     FIG. 8  depicts an alternate embodiment of a system  240  for monitoring erosion. A substrate  242  includes an inner surface  244  and an outer surface  246 . Substrate  242  may be a flow line positioned on the surface carrying production fluids that requires monitoring for erosion while being used. It should be appreciated that production fluids may carry particulate matter, such as sand, that may cause erosion. 
   Sensors  248 ,  250 ,  252  are embedded within substrate  242  in order to monitor erosion of inner surface  244 . Fluid flow contacts inner surface  244  as it flows along the path represented by arrows  254 . As illustrated, detector  256  is positioned within communicative proximity of sensor  250  in order to interrogate sensor  250  and determine if a predetermined level of erosion has occurred. Further, detector  256  is positioned closer to outer surface  246  than inner surface  244 . The receded and jagged inner surface  244  indicates that some level of erosion has occurred, however, the erosion has not disabled sensor  250 . Detector  256  transmits RF signal  258  to sensor  250 , which responds with response  260  as sensor  250  is in a first mode of operation since the predetermined level of erosion has not occurred. It should be appreciated that in operation, detector  256  may move from sensor  248  to sensor  250  to sensor  252  to develop a picture of the health and structural integrity of substrate  242  while substrate is carrying fluid or another erosive agent. 
     FIG. 9  depicts a system  270  wherein detector  272  is communicating with a sensor  274  according to the teachings of the present invention. Specifically, detector  272  and sensor  274  are positioned within communicative proximity of one another. Detector  272  comprises an interrogating signal generator  276  with a sending transducer or antenna  278 . A microprocessor  280  is connected to the interrogating signal generator  276  and an amplifier  282 , which, in turn, is connected to a signal receiving transducer or an antenna  284 . In one embodiment, amplifier  282  includes a demodulator that demodulates the unique RF signal received from the sensor  274 . 
   Microprocessor  280  includes an electronic circuit which performs the necessary arithmetic, logic and control operations with the assistance of internal memory. It should be appreciated, however, that the processing power for detector  272  may be provided by any combination of hardware, firmware and software. Moreover, in an alternate embodiment, detector  272  does not include sophisticated circuitry and memory for storing data, but rather relays the collected data to the surface in real time. 
   As can be seen from  FIG. 9 , power source  290  powers interrogating signal generator  276  to send interrogating signal  286  from antenna  278 . Additionally, power source  290  supplies power to microprocessor  280  and amplifier  282  which receives RF response  288  from sensor  274 . Preferably, power source  290  comprises a battery to enable these operations. Sensor  274  is illustrated as a RFID component that includes a signal receiving and reflecting antenna  292 . In one embodiment, the RFID component includes a reflector modulator for modulating interrogating signal  286  received by antenna  292  as well as for reflecting the modulated signal, response signal  288 , from antenna  292 . In another embodiment, the antenna  292  is integrated with RFID  274  as an embedded antenna. 
   By modulating RF signal  286 , sensor  274  transmits to detector  272  a unique identifier that allows detector  272  to distinguish sensor  274  from other similar sensors. As previously discussed, this feature is particularly useful in the context of an array of sensors that are positioned throughout a substrate. In one implementation, antenna  292  may be constructed of any suitable electrically conductive material such as a suitable nickel-based alloy. As previously discussed, antenna  292  increased the transmission power of sensor  292 . This is particularly useful when sensor  274  is embedded within a metallic substrate. Preferably, antenna  292  erodes at approximately the same rate as the host substrate erodes such that when antenna  292  is completely eroded, sensor  274  is disabled and non-responsive to indicate that a predetermined level of erosion has occurred. 
   Preferably, sensor  274  is a passive device that requires no battery. Passive devices do not require an additional power source as the energy received from the transmission provides sufficient power for the sensor to respond with a weak or periodic reply transmission as along as sensor  274  is receiving the appropriate interrogation signal. It should be appreciated, however, that sensor  274  may be an active device that receives power from a power supply, such as optional power supply  294 . 
   In operation, interrogating signal  286  and response signal  288  are typically RF signals produced by the RF transmitter circuits described hereinabove. Interrogating signal  286  from antenna  278  passes through air or a fluid medium, for example, and is received by antenna  292  at sensor  274 . In one embodiment, the modulator component of sensor  274  modulates the signal to uniquely identify sensor  274  and reflects the amplitude-modulated signal, response signal  288 , from antenna  292  to antenna  284 . Antenna  284  sends the signal to amplifier  282  which processes the signal and forwards the signal to microprocessor  280  for further processing, wherein the system determines that a predetermined level of erosion has not occurred. In the alternative, if sensor  274  has been disabled by erosion then signal  288  is not transmitted. When sensor  274  is in this second operation mode, after a predetermined period of time in which antenna  284  does not receive a signal, microprocessor  280  determines that the predetermined level of erosion is present in the substrate. 
   While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.