Geometrical instruments – Indicator of direction of force traversing natural media – Process
Patent
1994-05-19
1995-03-21
Wirthlin, Alvin
Geometrical instruments
Indicator of direction of force traversing natural media
Process
33304, 33313, E21B 47022
Patent
active
053984218
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The object present invention relates to a method for correcting magnetic measurements performed in order to determine an inclination and an azimuth of a well crossing a subsurface formation, by means of a tool displaced therein, with the method being particularly suitable for correcting measurements achieved by a sonde inserted between a drilling tool and the string linking it to a surface installation so as to enable, for example, to take into account the parasitical magnetic field generated by the drill string, which is superposed on the terrestrial magnetic field.
BACKGROUND OF THE INVENTION
Examples from prior art in the field of well orientation measurement are described in U.S. Pat. Nos. 4,435,454; 4,472,884; 4,559,713; 4,819,336.
While drilling wells and notably more or less deviated deep wells, the angle of inclination of the well and the azimuth thereof are generally to be determined with precision. To that effect, a measuring device included in a drill string inserted above the drilling tool is used.
This device generally comprises three magnetometers for measuring the components of the local magnetic field vector in three orthogonal directions Ox, Oy, Oz. One of the axes Oz is parallel to the axis of the tool and of the drill string. The other two, Ox, Oy are in a plane orthogonal to the axis of the drill string and the orientation thereof with respect to the vertical can be any orientation. Three accelerometers for determining the components Gx, Gy, Gz of the local gravitation vector are also arranged along these three axes. The measurements of the accelerometers enable a calculation of the inclination I of the tool and the orientation thereof, often referred to as TF (for Tool Face), which is the angle between axis Ox and the vertical plane. By combining the measurements Bx, By, Bz of the three magnetometers with the values I and TF obtained, the azimuth of the tool and therefore of the well, which is the angle between the projections in the horizontal plane of the axis of the tool and of the magnetic field, can be calculated.
The drill string, which is made of metal, magnetizes under the influence of the terrestrial magnetic field. Thus, the drill string generates a parasitical magnetic field which is superposed on the terrestrial field and alters the measurements. In order to minimize the parasitical influence, the measuring tool is interposed in a certain length of drill collar made of a nonmagnetic material. The residual disturbance P due to the more distant magnetic parts of the drill string is then assumed to be parallel to the axis of the drill string (FIG. 5).
In fact, the existence of a local magnetization ("hot spots") of pipes reputed to be nonmagnetic is often observed. The field generated by these anomalies is generally not parallel to the axis of the drill string. The case of a magnetic disturbance P (FIG. 6) of any direction provided with an axial component (axial disturbance) along Oz but also a radial component (radial disturbance) orthogonal to the previous one is thus to be considered.
U.S. Pat. No. 4,163,324 describes a method for eliminating errors due to a magnetic disturbance in the case where the latter may be assumed to be purely axial.
In the case, justified in practice, where no hypothesis can be made on the direction of the disturbance field, a method described in U.S. Pat. No. 4,682,421 may be used, which mainly consists in eliminating the influence thereof by rotating the measuring device about the axis thereof which is substantially parallel to the local direction of extension of the well and, for different angular positions distributed over 360.degree., in measuring the components of the magnetic field vector. The transverse component of the magnetic disturbance can be eliminated after comparing the measurements performed in several different orientations.
When the drilling tool is connected to a surface maneuvering installation by a rigid drill string which is progressively extended by fixing pipe sections, the measuring method mentio
REFERENCES:
patent: 4357660 (1982-11-01), Hepp
patent: 4682421 (1987-07-01), van Dongen et al.
patent: 4956921 (1990-09-01), Coles
Gazaniol Didier
Hourcard Michel
Mabile Claude
Nicolle Gilles
Institut Francais du Petrole et Societe
Wirthlin Alvin
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