Calculation of NMDC Length Required for Various Latitudes Developed From Field Measurements of Drill String Magnetisation

Abstract

Abstract One possible source of error in magnetic surveys is in based charts used for selecting the required length of non-magnetic drill collar (NMDC). Based on theoretical considerations and measurements of pole-strength values in the field, well-defined charts can pole-strength values in the field, well-defined charts can be constructed for the selection of the minimum required NMDC length for a specific bottom hole assembly and acceptable azimuth error. Introduction In recent years there has been an increasing awareness of possible sources of error in directional surveys. Comparison of magnetic and gyroscopic surveys has shown systematic errors of up to 15 in azimath. As previously reported, some of the sources of error for magnetic survey tools result from drill-string magnetization effects when too short a length of non-magnetic drill collar (NMDC) is used. Usually, the NMDC length required is determined by using non-magnetic drill collar selection charts. Since there is little published information on drill-string magnetisation, the derivation, validity and limitations of these charts are, at least questionable; in some cases the charts appear to be based on purely empirical data! To reduce errors in magnetic surveys from this source, Koninklijke/Shell Exploratie en Produktie Laboratorium has made a study of drill-string magnetization which included measurements of magnetisation of drill-string components both at the surface and downhole. Furthermore, a theory has been developed to extend the area of application of the observed data from the North Sea area to other latitudes. Finally, using these data, a general procedure has been derived for determining the required length of NMDC and the optimum spacing of the compass within that length, for a specified acceptable maximum compass error. Erroneous Magnetic fields in NMDC Sections A drill string in use is in essentially static orientation relative to the earth's magnetic field and will become magnetized. Since the drill string is long and slender and is rotated, this magnetization is orientated along the drill-string axis. At the bit an above and below the NMDC section the magnetic flux is forced to leave the steel, i.e. magnetic poles occur. The strength of a magnetic pole is defined as equal to the magnetic flux that leaves or enters the steal at the pole position and is expressed in weber (Wb). (In practice, the microweber (mu Wb) is a more convenient practice, the microweber (mu Wb) is a more convenient submit.) At Northern latitudes, for instance in the North Sea area, the upper pole above the NMDC section is normally positive, whereas the lower pole is negative (Fig. 1).A compass placed near a magnetic body aligns itself according to the total magnetic field, which is the vectorial sum of the earth's magnetic field and the field of the magnetic body. The magnetic field strength (officially flux density) at a distance r from a magnetic monopole is equal to the quotient of the flux, or pole strength, P, and the area of a sphere of radius r, around the pole, i.e. (1) In the SI-system, B is expressed in Tesla (T), although in practice the microtesia (mu T) is used. In a non-magnetic section of a drill string the erroneous magnetic field is caused by the magnetic bodies above and below the section, and is orientated in axial direction (Fig. 1). Taking account of the three poles at the bottom of the drill string, the erroneous field (delta B) is given by (2) P. 217