Abstract:
Systems and methods for optically determining casing collar and/or corrosion locations within boreholes, using the diffraction effect of Faraday crystals through which depolarized continuous light is transmitted within optical fibers.

Description:
TECHNICAL FIELD 
   This invention relates to a fully fiber optics based sensor system using a Faraday crystal to detect magnetic permeability anomalies of target elements—such as well string casings, casing collars or casing joints—in well holes from a remote location. The sensing element uses principles of optical diffraction and polarization shift to sense and telemeter permeability changes in the target elements which are subjected to changes in external magnetic fields. The magnitude of the measured permeability within the well hole is a function, among other things, of the mass of the ferromagnetic material present in the target element. A target such as a casing collar presents an additional mass which will appear as permeability anomalies along the length of the casing. Similarly, a corroded section of borehole casing has less mass than a corresponding section of uncorroded casing. The invention permits detection of the permeability anomalies that exist as a result of the differences in mass and, therefore, enables accurate location of downhole collars and casing corroded portions. 
   BACKGROUND OF THE INVENTION 
   In the petroleum drilling and production industry, a string of casing pipe is cemented into a wellbore to provide structural integrity for the bore hole and prevent vertical migration of fluids between formation zones. An additional string of pipe with a smaller chamber, commonly known as production tubing, is usually placed within the casing string as a conduit for the production of fluids out of the well. The downhole string of casing pipe, which may be thousands of feet in length, is made of a plurality of pipe sections which are joined end to end by threaded connections. The pipe joints, also called collars, have increased mass as compared to the pipe sections. After the sections of pipe have been cemented into the well, logging tools are run to determine the location of the casing collars. The logging tools used include a pipe joint locator whereby the depth is recorded of each of the pipe joints through which the logging tools are passed. The logging tools generally also include a gamma ray logging device which records the depths and the levels of naturally occurring gamma rays that are emitted from various well formations. The casing collar and gamma ray logs are correlated with previous open hole logs which results in an accurate record of the depths of the pipe joint across the subterranean zones of interest and is typically referred to a tally log. 
   It is often necessary to accurately determine the location of one or more casing collar joints in a well. This need arises, for example, when it is desired to isolate strata with packers and perforate the well casing between the packers within a producing stratum of the formation, or to identify expansion joints, gas-lift valves, etc. 
   In order to identify casing collar joints, an appropriate well tool is lowered into well casing on a length of tubing, either coiled or jointed. Given the need for precision as to the depth, pipe joint depth information available from previously recorded joint and tally logs taken during well drilling is not sufficiently accurate. Regardless of the care and precision taken during the drilling process, true depth measurements are affected by tubing elasticity, stretch, thermal expansion, non-linearity of the well bore and the casing itself, and other variable regularities. Similarly, the accurate depth of the tubing string lowered into the well is also subject to error from the same causes. In the case of coiled tubing used to lower well tools, there is a tendency to spiral due to forcing the coil down or along a horizontal section of the well. 
   A variety of pipe string joint indicators have been developed including slick line indicators that can produce drag inside the pipe string and wire line indicators that send an electronic signal to the surface by the way of electric cable and others. These devices, however, either cannot be utilized as a component in a coiled tubing system or have disadvantages when so used. Wireline indicators do not work well in highly deviated holes because they depend on the force of gravity to position the tool. In addition, the wire line and slick line indicators take up additional rig time when used with jointed tubing. 
   In recent years, there have been more sophisticated systems and methods devised to improve accuracy of collar locators. These include systems and methods employing magnetic field measurements. While such inventions have advanced the art, there remain problems in the field. For example, certain collar locators operate on the well known principle that an electromotive force (emf) is induced in a coil that is either stationary in a magnetic field that varies in intensity or is moving with respect to a constant magnetic field. Conventional casing collar locators of this type typically rely on the generation of a relatively powerful magnetic field from the locator using either permanent magnet or a coil through which electrical current is passed to induce magnetism. In the latter case, a significant amount of power is required to generate a magnetic field. As the coil passes adjacent to a collar, the flux density of the magnetic field is changed by the additional thickness of the collar. This change is detected by the sensor in the form of a variation in the electromotive force (emf) generated in a coil. The electrical signal is telemetered to the surface and analyzed to determine sensor depth. 
   Conventional casing collar locators are subject to operational disadvantages and limitations of their effectiveness. They operate, for example, only in a dynamic mode, because the current induced in the coil requires that the sensor move with respect to the casing. If the device is moved too slowly, the changes in emf become subject to signal-to-noise ratio problems, effectively degrading their accuracy. On the other hand, in a rapidly moving sensor, signal strength may be problematic. In any event, the precise location of the sensor is itself lowered by the necessity of pinning down the position of a moving object at any given moment. Other problems are associated with the generation of strong magnetic fields in the wellhole, such as interfering with other instrumentation. 
   Although methods and apparatus has been known in the past to identify downhole casing collars and problem points of casing corrosion, a need exists for enhanced joint locator and/or pipe corrosions locator information. The foregoing is not intended to identify all of the problems and limitations of previously known systems but should be sufficient to demonstrate that collar and corrosion location systems existing in the past will admit to worthwhile improvement. 
   SUMMARY OF THE INVENTION 
   By use of Faraday crystals through which de-polarized light is passed in the presence of an imposed magnetic field, diffraction effects inherent in such crystals permit sensing of magnetic field anomalies which are a function of the mass of nearby casings, tubes and pipes of material with high magnetic permeability. As the casing collars are associated with changes of mass, the system and methods of the present invention are useful as a casing collar locator. Since corrosion and other defects in well casings, tubes and pipes present similar magnetic anomalies, the present invention has further utility in locating zones of corrosion in in-situ well pipe. 

   
     THE DRAWINGS 
     Other aspects of the present invention will become apparent from the following detailed description of embodiments taken in conjunction with the accompanying drawings wherein: 
       FIG. 1  is a representation of the system of the invention in the context of its application at an oil well site; 
       FIG. 2  is a schematic diagram of the components of a preferred embodiment of the invention; 
       FIG. 3  represents the schematic details of a sensor employed in the invention using a reflected light source; 
       FIG. 4  depicts a typical plot of Faraday crystal losses (in dB) as a function of an applied magnetic field intensity (in Oersteads) wherein a bias point may be located within the regions of linearity on the curve, thus permitting more convenient determinations of magnetic field anomalies; 
       FIG. 5  is a representative signal output from a sensor which uses polarized light; 
       FIG. 6  is a representative signal output from a sensor which uses light that has been depolarized; 
       FIG. 7   a  depicts illustrative positions of a sensor which has been lowered in a well casing in the vicinity of a casing collar; 
       FIG. 7   b  is a signal output corresponding to such positions, shown as an oscilloscopic trace of voltage (v) as a function of time (t); 
       FIG. 7   c  depicts illustrative positions of the sensor indicated in  FIG. 7   a  as it is retrieved by raising it within the casing and thus passing it again through the region of a casing collar; 
       FIG. 7   d  is a sensor signal output corresponding to the indicated positions, shown as an oscilloscopic trace of voltage (v) as a function of time (t); 
       FIG. 8  depicts a differentiation (dv/dt) of the data presented in  FIGS. 7   c  and  7   d , as a result of which the permeability anomalies present in the vicinity of a casing collar are reflected as sharply defined optical output variances when the sensor traverses either a downward pass ( FIG. 7   a ) or an upward pass ( FIG. 7   c ) in the wellhole; and 
       FIG. 9  depicts another embodiment of the sensor in which the light beam from a light source is not reflected backwards after passing through the Faraday crystal but is instead guided through an optical fiber loop coupled to the descending optical fiber from the surface. 
   

   DETAILED DESCRIPTION 
   Context of the Invention 
   Turning now to the drawings wherein like numerals indicate like parts,  FIG. 1  is a representation of an oil well drilling system which identifies an operative context of the invention. A conventional drilling derrick  102  is shown positioned above an oil well borehole. A casing  104  has been installed within the borehole and cemented in place as at  106 . The borehole may extend thousands of feet into the earth&#39;s crust, perhaps 25,000 feet or so, into an oil and/or gas bearing formation. Ambient conditions at this depth may be twenty thousand pounds pressure per square inch and 150-175° C. in temperature. 
   Oil well logging managers are able to determine and map, on a real time and historical basis, vast amounts of well and formation data using oil well logging tools. In this a wire line cable  108  is connected to a logging tool  110  which has one or more instrumentation sonde sections  112  and a sensing section  114 . The logging tool is lowered into the wellhole on the wireline  108  using techniques well known to those in the art. The sensing section or sections  114  are positioned within a formation zone  116  where logging is to occur. An optical fiber (not shown) is run along with the wireline to the casing collar locator sensor (not shown) within the sensor section  114 . 
   A source of coherent light  118  is directed through an optical fiber cable  110  containing a first optical fiber (not shown). An optical de-polarizer  122  is connected in line with said first optical fiber. An optical coupler or optical circulator  124  couples the first optical fiber with a second optical fiber (not shown) within a cable  120  which second optical fiber is connected to a signal detection and analyzer stage  126 . 
   Fiber Optic Sensing System Employing Diffraction Effect of Faraday Rotator. 
   One preferred embodiment of the invention is schematically illustrated in  FIG. 2 . A coherent source of light  202  is output into a first optical fiber  204 . The depolarizer  206  is connected in line with the first optical fiber  204 , which in turn is coupled with an optical coupler or optical circulator  208  to a second optical fiber  210 . The light is completely depolarized using any one of a number of commercial depolarizing devices well known in the art. 
   De-polarized light emerging from the depolarizer  206  is guided within the first optical fiber  204  downhole and passed through a magnetooptical sensor (not shown, but described more fully below) within the instrumentation section  212  of the sonde. The sensor is lowered by the wireline (not shown) or other means to a depth in the wellhole within the vicinity of a casing collar  214 . The sensor is placed close to the casing inner wall, approximately 25 mm (one inch). For example, the sensor may be mounted on a caliper arm to maintain a fixed distance relative to the casing inner wall regardless of the casing diameter. Therefore, sensitivity of sensor to the casing inner wall will not change. Once within the vicinity of the casing collar, the sensor is further lowered so that it passes below the casing collar. The sensor is then raised to a position above the casing collar. 
   Light returning from the sensor is guided upward through the optical fiber  204 . At the optical coupler or optical circulator  208 , the second optical fiber  210  branches the light returning from the sensor and directs it to an optical detector  216  where it is transformed into an electrical signal  218  and subjected to analysis. 
   Description of Magnetooptical Sensor. 
   The invention employs a magnetooptical sensor to detect magnetic permeable anomalies caused by the presence of varying masses of ferromagnetic material present in casings, tubing, and pipe in the downhole environment. Such anomalies are sensed by a sensor comprising at least one optical collimator, a Faraday crystal, and magnets which may be used to create a magnetic field in the vicinity of the sensor. One preferred embodiment of such a sensor is shown in  FIG. 3 . 
   De-polarized light  302  is guided into a sensor  304  through a first optical fiber (not shown) and passes through a co-axially mounted magnet  306 . The fiber is connected to a collimator  308 , which assures that light entering a Faraday crystal  310  positioned after the collimator consists of parallel rays. An optical reflective medium  312  is positioned adjacent to and downstream of the Faraday crystal  310  and reflects incident light 180 degrees back through the first optical fiber  302 , as a beam  314  where it is guided back to the surface and through the path described in  FIG. 2 . For example, the optical reflective medium  312  may be a corner cube or a mirror. A second magnet  314  is placed adjacent to and downstream of the corner cube such that lines of magnetic flux exist between the magnets  306  and  318 . 
   Other devices to accomplish the same light guides, such as using reflecting devices other than a corner cube  312 , will readily present themselves to one of skill in the art. One Faraday crystal employed in the embodiment described is an iron garnet. Other magnetooptical crystals are well known in the art. 
   The sensor is preferably housed within a cylindrical capillary  316  to maintain alignment of its components and to protect it from the often harsh ambient temperature and pressure conditions within a bore hole. In one preferred embodiment, the capillary  316  has an outside diameter of 2.7 mm. and a length of about 30 mm total, with the Faraday crystal centered within the length of the capillary. Magnets are positioned in a manner which allows adjustment, as a means of affecting sensitivity of the sensor and of biasing the baseline signal. The sensor unit may be packaged into a pressure-sealed non-magnetic metal housing (not shown) in order to withstand downhole pressure conditions. 
     FIG. 4  is a plot of the attenuation of the light, i.e., insertion loss, (in dB) traversing a Faraday crystal as a function of the applied magnetic field (in Oersteads). Adjustments of the magnetic field strength surrounding the sensor can be made by adjusting the positions of the magnets within the sensor. Such adjustments permit biasing the sensor output so as to establish convenient base line responses from the sensor in regions when the casing, but not casing collars, influence the intensity of the light beam passed through the sensor. Sharper delineation of the effect of the greater mass of a casing collar (or the lesser mass of a zone of corrosion) is thus produced when the sensor is positioned near collars or zones of corrosion. 
   The use of non-polarized light is preferred in the operation of the sensor  304 .  FIG. 5  shows an oscilloscope trace of a baseline  502  of a returned signal in which polarized light was employed.  FIG. 6 , by contrast, shows a baseline  602  of the returned signal in which de-polarized light was used. De-polarized light provides a cleaner, more stable baseline from which magnetic anomalies may be more accurately determined. 
     FIGS. 7   a  and  7   c  illustrate the positions of a sensor as it is first lowered ( FIG. 7   a ) and then retrieved ( FIG. 7   c ) within a borehole casing. The positions  702  through  708  correspond to positions in a region  702  where there is little influence from the lower flux density of the magnetic field in the vicinity of the casing collar; at position  704  where the sensor is in a region of higher flux density due to its location closer to the casing collar; then at position  706  in a region of highest flux density; and finally at position  708  where the sensor has been lowered to a position where the collar has little influence on flux density. The intensity of light passing through the sensor is affected by the magnitude of the field, which is, in turn, a function of the mass of ferromagnetic material in the vicinity. Similarly, the positions  710  through  716  in  FIG. 7   c  correspond to equivalent positions as the sensor is retrieved by raising it in the casing from a position  710  below the casing collar  712  to a position  716  above the casing collar. 
   The data illustrated in  FIGS. 7   b  and  7   d  are oscilloscope traces which plot an electrical analogue of the intensity of light passing through the Faraday crystal in the various positions of the sensor indicated in  FIGS. 7   a  and  7   c , respectively. 
   The reader of ordinary skill in the art will note that the deflections or magnitude of the voltage reflected in the traces of data shown in  FIGS. 7   b  and  7   d  are not necessarily directly linearly relatable to the mass of the ferromagnetic material in the pipe. The actual magnetic flux strengths may be affected by ambient magnetic effects which can increase or decrease the actual flux density at a given point. Notwithstanding, the effectiveness of the invention lies in the relative impact upon light intensity of the magnetic field imparted by the magnets associated with the sensor. 
   Furthermore, it is not to be suggested the utility of the invention requires that the sensor be lowered or raised at constant speeds. Indeed, the sensor may be moved in increments or at varying speeds, the only limitation being that the sensor not be moved so swiftly as to present signal to noise deterioration of data. Indeed, the ultimate aim in the use of the sensor is to provide, in effect, a plot of data which correlates to light intensity as a function of distance along the observed section of well casing, pipe or tube, as the case may be. 
   In the preceding several paragraphs, the embodiment described relates to the use of the invention to detect the location of casing collars, but the invention is not so limited. It is a fundamental feature of the disclosed invention that changes in magnetic flux affect the diffraction of light passing through a Faraday crystal of magnetooptical properties, as discussed by G. B. Scott and D. E. Lacklison, “Magnetooptic Properties and Applications of Bismuth Substituted Iron Garnets,”  IEEE Transactions on Magnetics , Vol. Mag. 12, No. 4, July 1976, and T. R. Johansen et al, “Variation of Stripe Domain Spacing in a Faraday Effect Light Deflector,”  Journal of Applied Physics , Vol. 42, No. 4, Mar. 15, 1971. The disclosures of these publications are hereby incorporated herein by reference. Changes in flux are presented where anomalies occur in the magnetic permeability of well casings, pipes and tubes. Such anomalies are present near casing collars which typically involve greater concentrations of mass over that of the casing tubing they join. They are also presented by the presence of corrosion or other defects in the walls of in-situ casings, pipes and tubes. Therefore, the invention may be used to detect corrosion and such other defects. 
   Returning to the analysis of data obtained by use of the sensor described above, the data presented in  FIGS. 7   c  and  8   d  may be used directly to determine the position of the casing collar. It may also be useful to mathematically differentiate the data so as to present a starker, more sharply defined location of the anomalies.  FIG. 8  reflects such a treatment of the data reflected in  FIGS. 7   a  and  7   c . The figure is a plot of dv/dt of the data in  FIGS. 7   a  and  7   b . The first (left) spike  802  in  FIG. 8  delineates a magnetic anomaly detected as the sensor is moved down the casing through the region where the magnetic flux is affected by the presence of the casing collar; the second (right) spike  804  delineates the same anomaly as the sensor is retrieved upwardly past the same collar. The wellhole depth, and hence the location of the casing collar, is then determined by cross calibrating the position of the sensor with other well logging data, such as a gamma log. 
   In the sensor described above, light traverses the Faraday crystal twice, due to its reflection by the corner cube  312  in  FIG. 3 . This double passage of the light results in larger insertion losses. The loss may be lessened, and sensitivity of the results improved, by the use of a single pass of the light. Such a sensor is depicted in  FIG. 9 . 
   A fiber optic light beam  902  is passed through a collimator  904 , a Faraday crystal  906 , and a second collimator  908 . Instead of being reflected backward, an optical fiber loop  910  is created by means of a mini-bend fiber  912  capable of a bending radius of less than 15 mm. The loop is re-coupled to the first optical fiber  902  through an optical coupler  914 . The sensor is housed in a capillary  916 , which may be composed of glass or other material capable of withstanding ambient temperature and pressure conditions within the well hole, and is provided with magnets  918  and  920  necessary for the operation of the Faraday crystal. As the sensor depicted in  FIG. 9  does not involve dual passage of the sensing beam through the Faraday crystal, the beam experiences reduced insertion losses, and, therefore, the sensor is more sensitive. 
   The foregoing embodiments have each employed a single optical fiber through which light is transmitted downhole to a sensor and simultaneously returned to a photo-detector located at the surface. Other embodiments may be employed in which separate optical fibers are used for insertion of light to a sensor and as a return guide of the light after its passage through a Faraday crystal. Such an embodiment would not require optical couplers which also entail insertion losses and, thus, the embodiment may present more sensitive data results. 
   To the extent that operations described in the embodiments above may be performed by different components, it should be apparent to those of ordinary skill in the art, that different components may be used. For example, a light beam shaping device is not intended to be limited to a collimator, but such shaping may also be performed by a focuser, a lens, or a particular extremity of an optical fiber. Similarly, a light reflection element is not limited to a corner cube but may be performed by other devices known in the art. 
   In this application and claims, the terms casing, pipe and tubing are used in their broadest sense to include all forms of well casing, pipes and tubes and of all compositions, limited only by requirement that they exhibit magnetic permeability sufficient to affect the light intensity of unpolarized coherent light passing through magnetooptical crystals exhibiting a Faraday diffraction effect. 
   The various aspects of the invention were chosen and described in order to best explain principles of the invention and its practical applications. The preceding description is intended to enable those of skill in the art to best utilize the invention in various embodiments and aspects and with modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.