Abstract Relative to the mud system used in a well drilling operation, a monitoring system is provided from which a log of the magnetic susceptibility of earth formations traversed by a borehole can be obtained. The system employs a mud sampling channel where the flow of mud is temperature stabilized and passed through a sampling coil so that the entrained formation particles can influence the value of the magnetic field induced by a signal generator in the sampling coil by virtue of their magnetic susceptibility. The sampling coil is a part of a bridge detecting system which is driven with a standard excitation signal so that a change in the permeability (or susceptibility) of the material on the interior of the coil produces an output voltage signal. The amplitude of the output voltage signal which is in phase quadrature with the excitation signal is detected by a phase sensitive detector. A corresponding deviation signal is generated by amplifying this voltage signal to drive a clock operated chart recorder. The chart recorder is connected to the traveling block of the drilling rig for producing a depth recording also. Thus, the recorded deviation signal which is indicative of magnetic susceptibility is correlatable in a well-known manner to the drilling depth. Identical systems may be used to monitor the mud input and output to the well, so that a differential value of the magnetic susceptibility signal can be obtained. Reference liquids can also be supplied to the system for obtaining calibration values having known magnetic susceptibilities.
A method of logging the magnetic susceptibility of earth formations traversed by a well borehole during a well drilling operation of the borehole by measuring the magnetic susceptibility changes of the drilling fluid being circulated in the drilling process caused by entrained particulate matter from the earth formations being penetrated by the drill bit, comprising the steps of: circulating drilling fluid through a drill string, a drill bit and the annulus between the drill string and the borehole wall while drilling a well bore; diverting a continuously flowing sample of the return circulation of the drilling fluid into a measurement apparatus; stabilizing the temperature of the diverted portion of said return circulation of said drilling fluid to form a temperature stabilized measurement sample flow; passing said temperature stabilized measurement sample flow through a coil forming one leg of a balanced bridge electrical circuit; driving said balanced bridge electrical circuit With an alternating current reference excitation signal; and detecting the amplitude of any component of an alternating current output signal from said bridge circuit which is in phase quadrature with said reference excitation signal due to changes in the permeability of said coil caused by entrained particulate matter in said temperature stabilized measurement sample flow to derive a signal representative of the magnetic susceptibility of such entrained particulate matter in said temperature stabilized measurement sample flow.
Claim 2
Independent Claim
225 words
A method of logging the magnetic susceptibility of earth formations traversed by a well borehole during a well drilling operation of the borehole by measuring the magnetic susceptibility changes of the drilling fluid being circulated in the drilling process caused by entrained particulate matter from the earth formations being penetrated by the drill bit, comprising the steps of: circulating drilling fluid through a drill string, a drill bit and the annulus between the drill string and the borehole wall while drilling a well bore; diverting a continuously flowing sample of the return circulation of the drilling fluid into a measurement apparatus; stabilizing the temperature of the diverted portion of said return circulation of said drilling fluid to form a temperature stabilized measurement sample flow; passing said temperature stabilized measurement sample flow through a coil forming one leg of a balanced bridge electrical circuit; driving said balanced bridge electrical circuit With an alternating current reference excitation signal; and detecting the amplitude of any component of an alternating current output signal from said bridge circuit which is in phase quadrature with said reference excitation signal due to changes in the permeability of said coil caused by entrained particulate matter in said temperature stabilized measurement sample flow to derive a signal representative of the magnetic susceptibility of such entrained particulate matter in said temperature stabilized measurement sample flow.
Claim 3
Dependent Claim
28 words
The method of claim 1 and further including the step of recording said representative magnetic susceptibility signal as a function of the depth of the borehole being drilled.
Claim 4
Dependent Claim
22 words
The method of claim 1 and further including the step of recording said representative magnetic susceptibility signal as a function of time.
Claim 5
Dependent Claim
79 words
The method of claim 1 and further including the step of, prior to making a magnetic susceptibility measurement of said temperature stabilized measurement sample flow of said drilling fluid: passing a temperature stabilized flow of a control fluid of known magnetic susceptibility properties through said coil for a period of time sufficient to alter the permeability characteristic of said coil in a manner functionally related to the magnetic susceptibility of said control fluid, to thereby calibrate said measurement apparatus.