Well logging apparatus and method for determining properties of earth formations that have been invaded by borehole fluid

A method for determining the properties of formations surrounding an earth borehole that have been invaded by borehole fluid to form an invaded zone, comprising the steps of:

• (a) suspending a logging device in the borehole;
• (b) transmitting electromagnetic energy from a transmitter location on the logging device, and receiving the transmitted electromagnetic energy at receiver locations on the logging device for a first transmitter-to-receivers spacing associated with said transmitter and receiver locations;
• (c) deriving, from the received electromagnetic energy, an apparent phase shift PS.sub.-- a and an apparent phase average PA.sub.-- a associated with said first transmitter-to-receivers spacing;
• (d) repeating steps (b) and (c) for a plurality of further transmitter-to-receivers spacings to obtain PS.sub.-- a and PA.sub.-- a for said plurality of further transmitter-to-receivers spacings;
• (e) generating a model of formation of resistivity Rt with an invaded zone of resistivity Rxo and diameter of invasion DI;
• (f) selecting values of Rxo, DI, and Rt that would produce a model phase shift PS and a model phase average PA that substantially correspond, for each of a plurality of said transmitter-to-receiver spacings, with the PS.sub.-- a and PA.sub.-- a for the respective plurality of transmitter-to-receiver spacings;
• and
• (g) outputting the selected model values of at least one of Rxo, DI, and Rt.

Apparatus for determining the properties of formations surrounding an earth borehole that have been invaded by borehole fluid to form an invaded zone, comprising:

• (a) a logging device suspendible in the borehole;
• (b) means for transmitting electromagnetic energy from a transmitter location on the logging device, and for receiving the transmitted electromagnetic energy at receiver locations on the logging device for a first transmitter-to-receivers spacing associated with said transmitter and receiver locations;
• (c) means for determining, from the received electromagnetic energy, measurement characteristics associated with said first transmitter-to-receivers spacing;
• (d) means for repeating operation of elements (b) and (c) for a plurality of further transmitter-to-receivers spacings to obtain measurement characteristics for said plurality of further transmitter-to-receivers spacings;
• (e) means for generating a model of formation of resistivity Rt with an invaded zone of resistivity Rxo and diameter of invasion DI;
• (f) means for selecting values of Rxo, DI, and Rt that would produce model measurement characteristics that substantially correspond, for each of a plurality of said transmitter-to-receiver spacings, with the actual measurement characteristics for the respective plurality of transmitter-to-receiver spacings;
• and
• (g) means for outputting the selected model values of at least one of Rxo, DI, and Rt.