Schlumberger Technology Corporation

Pierre A Moulin

Jean-Michel Hache

Wayne J Phillips

The invention relates to a method of determining the resistivity of a geological formation through which a metal-cased borehole passes, by means of at least three electrodes in contact with the casing and spaced apart in the longitudinal direction of the borehole. In order to eliminate the effect of any differences between the two sections of casing delimited by the electrodes, a circuit connected to the electrodes and forming a Wheatstone bridge is balanced, with said sections of casing constituting two of the resistances in said Wheatstone bridge circuit.

Method and apparatus for determining formation resistivity in a cased

A method and apparatus is disclosed for measuring motion signals of gyroscopes in downhole instruments used to determine the heading of a borehole. An illustrative embodiment of the invention includes a measuring-while-drilling system which may experience motion even while the drill string is suspended in rotary table slips when the heading of the drill string is being determined. Accelerometer and magnetometer data along three orthogonal axes of a measurement sub are used to obtain unit gravitational vectors g at a first time and at a second time and unit magnetic vectors h at the first time and the second time. The difference between the two unit gravitational vectors at the different times, .DELTA.g, and the difference between the two unit magnetic vectors at the different times, .DELTA.h, are used along with the unit vectors g and h and the difference in time .DELTA.t to determine the rotation vector of the probe .OMEGA..sup.p which has occurred during such time difference. The vector representing the rotation of the earth, .OMEGA..sup.e is then determined by subtracting .OMEGA..sup.p from the vector .OMEGA..sup.g from three gyroscope instruments placed along the axes of the measurement sub. The heading of the drill string is determined from the gravitational vector and the earth rotation vector.

Motion compensation apparatus and method of gyroscopic instruments for

The apparatus includes a body member lowered within drill pipes and having an upper part (26) and a lower part (27) arranged for limited movements with respect to each other. Each part of the body member is anchored inside the drill pipes and the movements between said parts are detected when stresses are applied to the drill pipes from the surface. A first radial-coil transformer has a primary winding (35) integral with a part of the body member and a secondary winding (36,37) integral with the other part to detect the angular movements. A second axial-coil transformer has a primary winding (40) integral with a part of the body member and a secondary winding (41,42) integral with the other part to detect the longitudinal movements. The signals delivered by the transformers are linear functions of the respective movements.

Apparatus for conduit free-point detection in boreholes

Downhole cable tension measuring apparatus according to the invention is connected between a cable and well-logging apparatus suspended from the cable in a borehole. A rod is placed between the cable and the well-logging apparatus so as to elongate elastically under the effect of cable tension. A sleeve, fixed at one end of the rod, surrounds the rod. A transformer, the primary of which is wound on the rod, the secondary of which is wound on the sleeve, detects the relative longitudinal displacement of the other end of the rod with respect to the sleeve. Two primary windings displaced on the rod are alternately excited to generate two signals in the secondary coil in order to correct temperature drifts of the circuits. The rod and sleeve are made of the same metal so that each of them will elongate similarly as a function of temperature. The apparatus is placed in a pressure-balanced oil-filled chamber.

Downhole cable tension measuring apparatus

Method and apparatus for automatically inscribing magnetic marks on a moving wireline are disclosed. An alternating magnetic field is generated for application to a zone on the moving wireline. The marks are inscribed on the wireline by interrupting the field in response to a control signal corresponding to the time for a mark. The interruption is synchronized to alternations of the field and movement of the wireline. The interruption begins between alternations of the field and is maintained for a time sufficient for a predetermined length of the wireline to be moved beyond the application zone. This predetermined length is related to the length of the zone on the wireline affected by the alternating magnetic field, which is applied with a U-shaped electromagnet held in proximity to the wireline. Wireline movement pulses are generated by a tangentially coupled measuring wheel driving an encoder. The movement pulses are counted to determine the time for each mark and provide a control signal corresponding to a desired mark. The pulses are also used to determine when the predetermined length of wireline has been moved beyond the application zone and when the interruption of the field ends. Preferably, the field is restored starting with an alternation of a polarity related to the polarity of the field when it was terminated.

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Chaville, France

Pierre A Moulin