1. Field of the Invention
This invention relates to downhole electrical logging methods and apparatus for more accurately determining resistivity and conductivity measurements of the formation, and more particularly the utilization of a differential array of measuring electrodes that are selectively coupled to a single source electrode. The voltage distribution in an earth formation penetrated by a borehole resulting from imposed current flow in the formation permits a measurement of resistivity and/or conductivity. From the resistivity measurement, the hydrocarbon saturation of the formation can be determined.
The formation being logged is penetrated by a borehole containing drilling fluid and formation fluids have a resistivity (Rm) different than the resistivity of the adjacent formation. In an invaded zone in which the drilling fluid has penetrated has a resistivity (Rxo) different than the true resistivity (Rt) of the formation. The measurement of Rt for the formation is dependent on the effects of Rm and Rxo, coupled with the variations in the diameter (Dbh) of the borehole and the radius of the invaded (Rinv) zone.
Certain electrical logging instruments have been developed to obtain better measurements of Rt, such as the non-focused electrical logs (conventional resistivity logs). Another is the focused electrical logs (laterologs for measuring resistivity) which use focusing currents to control the paths taken by the measuring current. However, the focused logs have been successful in measuring the uncontaminated formation resistivities, not affected by mud filtration, while the unfocused logs have been successful in measuring near formation resistivities in the invaded zone, but no single tool has been able to provide accurate resistivity measurements over a significant portion of the radial depth of the formation surrounding the borehole including the invaded zone and the uncontaminated zones.
One primary objectives of the differential array instrument is to provide a better determination of the resistivity of the invaded (R.sub.xo) and uncontaminated (Rt) zones in a wider set of well conditions, which in turn will produce a more predictable pore content and saturation value. Another primary objective of the differential array instrument is to provide a more complete visualization of the resistivity profile in the invaded zone and the uncontaminated zones in order to provide additional petrophysical data to the well owner.
Accordingly, one primary feature of the present invention is to provide a differential resistivity instrument that will more accurately determine resistivity derived parameters, such as flushed zone resistivity, uncontaminated zone resistivity, invasion zone radius and fluid saturation over a wider set of invasion conditions.
Another feature of the present invention is to provide a better determination of movable and residual oil saturation.
Still another feature of the present inventions to provide a more accurate definition of hydraulic units.
Yet another feature of the present invention is to provide a complete visualization of the resistivity profile in the invaded zone.