METHOD AND APPARATUS FOR DETERMINING CHANGES IN LENGTH OF A WELL LOGGING CABLE

Abstract: A method and apparatus for determining the amount of stretch in a well logging cable is disclosed. An optical fibre (10) is run into a well with a logging cable (11) and looped back to the surface. A second optical fibre (14) of nominal length is maintained on the surface. A beam of monochromatic light from a source (17) is split and passed through both fibres and recombined. As the well logging tool (1) is raised and lowered into the wellbore, the number of interference fringes passing a fixed point is determined. The change in length of the optical fibre and, therefore, the cable will be equal to 1/2 the number of interference fringes passing a fixed point times the wavelength of light is used. A precise determination of vertical location of the well logging tool (1) can then be made when this information is combined with information from a surface mounted length measurement device, such as an odometer. This information further allows a precise determination of interval bulk density or other formation parameters in which it is important to know the depth of a tool.

Description: This invention relates to a method and apparatus for determining a change in the length of a cable when subjected to a vertical stress. Thus the invention makes it possible to accurately determine the vertical position of a load attached to the lower end of the cable, for example a borehole gravity meter. In the exploration for and production of oil and gas reservoirs, various types of well logging tools are lowered into the well for the purpose of determining the physical characteristics of rock surrounding the well. Often, it is important to know with a great deal of accuracy the depth of the logging tool. It may also be important to accurately know the distance between locations which have been occupied by the tool. One such tool is a borehole gravity meter. A borehole gravity meter is used to estimate the interval bulk density of rock surrounding the well. When the borehole gravity meter is raised or lowered in the well, the change in the relative value of the vertical component of the intensity of the gravitational field can be accurately determined. If the distance between the vertical locations is also accurately known, the interval bulk density can be accurately determined. The present technology for determining tool location utilized a mechanical length measurement device at the surface. Odometers, radioactive tape, inking, painting, and mechanical wheels have been described as being useful in determining the length of wire which has entered the hole (e.g., see "Cable Marker/Detector is a Magnetic-Type System", Oil and Gas Journal , February 21, 1982, page 148). These devices do not account for downhole cable stretch due to friction, tool and cable weight, or thermal expansion or shrinkage. These variations in cable length can introduce an error of several feet in absolute tool location or several inches in the relative distance between two locations in a borehole. The use of laser light in determining the pressure and temperature conditions within a wellbore has been described (see U.S. Patent No. 4,375,164) wherein the index of refraction of each of two optical fibres are compared by examining the interference pattern of laser light in a photomultiplier. In none of the above technology is an accurate means of determining the vertical location of a tool or logging sonde within a wellbore described in which the interference pattern of a laser light is utilized. Consequently, it is an object of this invention to provide a method and means for accurately determining the location of a tool in a borehole or casing. It is another object of the invention to provide a method and means for accurately determining the change in location of a tool in a borehole or casing. It is a further object of the invention to provide a method and means of accurately determining the interval bulk density of rocks which occupy the space between two locations. It is a further object to accurately measure the depth at which data samples are taken such as resistivity, sonic travel time, etc. In accordance with one aspect of the invention, there is provided a method of determining a change in the length of a cable located in a substantially vertical borehole when subjected in use to a stress by a load connected to the lower end portion of the cable, which method comprises: (a) incorporating in the cable prior to use thereof a longitudinally extending optical fibre, the optical fibre extending over the entire length of the cable in a manner such as to provide a light path from the upper end portion of the cable to the lower end portion thereof and vice versa; (b) splitting a monochromatic light beam between the free end of the optical fibre incorporated in the stressed cable at one end of an unstressed optical fibre of fixed length; (c) combining the light beams emitted from the other ends of the stressed and unstressed optical fibres respectively to form a combined beam; (d) detecting an interference fringe pattern produced by said combined beam; and (e) monitoring the interference pattern and determining from changes thereof any change in length of said stressed cable. According to another aspect of the invention, there is provided an apparatus for use in determining a change in the length of a cable located in a substantially vertical borehole when subjected in use to a stress by a load connected to the lower end portion of the cable, said apparatus comprising: an optical fibre having a length such as to provide a light path from the upper end portion of the cable to the lower end portion thereof and vice versa when the optical fibre is incorporated longitudinally in the cable prior to use; an unstressed optical fibre of fixed length; a monochromatic light source; means for splitting a beam of light generated by said light source into separate beams and introducing these into respectively the free end of the optical fibre incorporated in the cable and one end of the fixed length optical fibre; means for combining the light beams emitted from the other ends of two optical fibres respectively; and means for detecting any change in the interference fringe pattern produced by the combined beam, whereby a change in length of the cable can be determined. In putting the invention into practice, a monochromatic laser light beam is divided into two optical paths into each of two fibres. One fibre is of fixed and nominal length, and the second is generally several thousand feet long and is integrated into a cable. The cable/fibre is lowered into a wellbore or casing with a logging tool suspended from the cable. When the light from the two fibres is recombined, an interference pattern of light and dark bands (interference fringes) is generated. Any change in length of the second optical fibre will cause the fringes to shift. The number of fringes which pass a fixed reference point is a direct measure of the length change in the second optical fibre. If the cable length (and, thus, the second optical fibre) remains the same, there will be no shift in the interference pattern. If the cable contracts, the fringe pattern shifts in one direction, while it shifts in the other direction if the cable stretches. The number of fringes passing a fixed location can be determined. The absolute location of the tool can be determined by combining the information from an odometer or encoder of the type well known in the art with that of the interferometer. The odometer provides the depth for a non-stretching cable and the interferometer provides information concerning actual stretch or shrinkage. The combined information establishes the absolute depth. Likewise, the distance traveled between two locations can accurately be determined. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Fig. 1 schematically shows the overall placement of optical equipment with respect to a wellbore; and Fig. 2 schematically shows the optical cables, laser, wellbore, and the manner in which fringe pattern shift is detected. Referring to FIG. 1, the invention is intended to measure the vertical location (depicted by arrow 2) or change in location of a logging tool 1. The logging tool 1 is held in place by a cable 3 attached to a wire line vehicle 4 at the surface. An optical fibre is built into the cable and looped back to the surface through the cable. The cable/fibre may be routed over one or more pulleys 5 and stored on a reel 12. As the well logging tool 1 is lowered into the well 7, the cable will stretch due to both tension and thermodynamic factors. An odometer or optical encoder 8 is used to measure the length of the cable 3 as if it were unstretched. Logging measurements are generally performed while the logging tool is being raised. Friction on both the tool and the cable, as well as the weight of the cable and tool itself, tends to change the length of the cable. The interferometer/laser combination 9 is used to measure the change in length of the optical fibres which have been lowered into the well with the cable 3. Since the cable and optical fibre are both rigidly connected to the well logging tool, the change in length of the optical fibre will be exactly the same as the change in length of the cable. The precise vertical location of the logging tool can, therefore, be determined. Referring now to FIG. 2, the long optical fibre 10 and the cable 11 are depicted separately for clarity. The optical fibre would be contained within the cable and enter the interferometer 15 mounted outside the cable reel 12. As the cable 11 and well logging tool 1 are lowered into the well 7, the amount of cable is recorded with an odometer or other cable measuring device 8. The cable containing the long optical fibre is stored on a cable reel 12. It should be noted that additional optical fibres may be lowered into the wellbore along with the long fibre discussed herein and be used for other purposes or the long optical fibre may actually consist of two fibres (one which carries the beam downwards and one which returns the beam to the surface). A source of monochromatic light 17, preferably a laser, is used to generate a beam of light which is divided by a beam splitter 13 of the type well known in the art into the long optical fibre 11 and a second optical fibre of nominal length 14. The logging tool 1 is raised or lowered into the well by revolving the cable reel. After passing through the optical fibres of different lengths, the light beams are combined and an interference pattern of light and dark bands is produced. The interference bands 18 are monitored with the interferometer 15. Any change in the length of the long optical fibre will cause the fringes to shift. The number of fringes which pass a fixed reference point is a direct measure of the length change in the long fibre. It should be noted that commercially available devices well known to one skilled in the art are available to generate the light beams, combine the beams and monitor the interference pattern. The absolute location of the logging tool can be determined by combining the information from the odometer 8 and the interferometer 15. The odometer provides the depth for an unstretched cable or a cable with uniform stretch and the interferometer provides information concerning the actual stretch or shrinkage because for each fringe passing a fixed point, the cable length has changed in an amount equal to 1/2 the wavelength of the light being used. The combined information determines absolute depth. Similarly, by establishing the absolute depth at each of two locations, the distances traveled between two locations can be determined. The utility of the invention can be recognized in its application to the determination of the interval bulk density in borehole gravity measurements. The interval bulk density can be estimated using the equation: where: ρb = Interval bulk density. k - Universal gravitational constant. F = Free air gradient. Δg = Change in gravitational force between a first location and a second location. Δz = Change in vertical location between the first location and the second location. Δg can be determined with precision with a borehole gravimeter familiar to one skilled in the art. In fact, Δg can be determined with far greater precision than Δz using previously known methods. The vertical location at each of the two locations can be determined with precision equal to that of Δg using the above-described interferometric method/means and, therefore, Δz. The value of F can be precisely determined by means well known in the art with a knowledge of the latitude, elevation, and rough knowledge of the vertical location of the tool (for example, from the average vertical location between the two locations). "k" is a well-known constant. Therefore, the value of ρb can be accurately and precisely determined with the above-described method of determining Δz.

Claims: A method of determining a change in the length of a cable located in a substantially vertical borehole when subjected in use to a stress by a load connected to the lower end portion of the cable, which method comprises: (a) incorporating in the cable prior to use thereof a longitudinally extending optical fibre, the optical fibre extending over the entire length of the cable in a manner such as to provide a light path from the upper end portion of the cable to the lower end portion thereof and vice versa; (b) splitting a monochromatic light beam between the free end of the optical fibre incorporating in the stressed cable and one end of an unstressed optical fibre of fixed length; (c) combining the light beams emitted from the other ends of the stressed and unstressed optical fibres respectively to form a combined beam; (d) detecting an interference fringe pattern produced by said combined beam; and (e) monitoring the interference pattern and determining from changes thereof any change in length of said stressed cable. A method according to claim 1, wherein the borehole is a well and said load is a well logging tool. A method according to claim 2, wherein the well logging tool is a borehole gravity meter. Apparatus for use in determining a change in the length of a cable located in a substantially vertical borehole when subjected in use to a stress by a load connected to the lower end portion of the cable, said apparatus comprising: an optical fibre having a length such as to provide a light path from the upper end portion of the cable to the lower end portion thereof and vice versa when the optical fibre is incorporated longitudinally in the cable prior to use; an unstressed optical fibre of fixed length; a monochromatic light source; means for splitting a beam of light generated by said light source into separate beams and introducing these into respectively the free end of the optical fibre incorporated in the cable and one end of the fixed length optical fibre; means for combining the light beams emitted from the other ends of two optical fibres respectively; and means for detecting any change in the interference fringe pattern produced by the combined beam, whereby a change in length of the cable can be determined. Apparatus as claimed in claim 4 and further comprising a length measurement device capable in use of determining the length of cable and associated optical fibre which has been fed into the borehole, whereby the actual vertical position of said load can be determined. Apparatus as claimed in claim 4 or 5, wherein the load is a well logging tool.

Inventors: ANGEHRN, JORG A., GRAEBNER, PETER, SIE, SWAN A.
Applicants: CHEVRON RESEARCH AND TECHNOLOGY COMPANY
Application year: 1987
Main classification: G01B11
IPC codes: G01B11, E21B47