1. Field of the Disclosure
This disclosure generally relates to exploration and production of hydrocarbons involving investigations of regions of an earth formation penetrated by a borehole. More specifically, the disclosure relates to the accurate measurement and imaging of the resistivity of an earth formation using a logging tool in a borehole.
2. Description of the Related Art
Electrical earth borehole logging is well known to persons having an ordinary level of skill in the art, and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of electrical logging apparatus. In the first category, one or more measurement electrodes—current source(s) or sink(s)—are used in conjunction with a return electrode (which may be a diffuse electrode such as a logging tool's body or mandrel). A measurement current flows in a circuit that connects a current source to the measurement electrode(s), through the earth formation to the return electrode, and back to the current source in the tool. In a second category, that of inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present disclosure belongs to the first category.
Electrical logging devices may be operable in one or more modes. In one mode, a constant current is maintained at a measuring electrode while a voltage is measured; in another mode, the voltage of the measuring electrode is held constant and the current is measured. Ideally, if current is varied to maintain a constant voltage, the resultant current is inversely proportional to the resistivity of the earth formation being investigated. Conversely, if current is maintained constant through the circuit, ideally speaking the voltage of the measurement electrode is proportional to the resistivity of the earth formation being investigated. Ohm's law teaches that if both current and voltage vary, the resistivity of the earth formation is proportional to the ratio of the voltage to the current.
Of course, the foregoing is a simplified and idealized description of physical relationships that are more complex in practical application. Those of ordinary skill in the art will appreciate that there are usually other variables to consider, for example the electrical characteristics of the instrumentation itself and the electrical characteristics of the environment under investigation. The prior art suggests innumerable approaches for addressing such non-idealized factors.
It is known that oil-based drilling fluids (“muds”) may be used when drilling through water-soluble formations, and an increasing number of present day exploration prospects are believed to lie beneath water-soluble salt layers. The use of oil-based muds, which have a low conductivity relative to water-based muds, implicates a potential drawback in the use of contact electrodes to inject electrical currents into a wellbore. Any standoff or gap between an electrode and a wellbore wall creates a standoff impedance; and as electrode standoff increases, the corresponding impedance can begin at some point to dominate resistivity measurements taken through the electrode. Increasing borehole rugosity, which tends to increase average electrode standoff, thus becomes especially problematic.
In addition to deteriorating the electrical contact between the logging tool and the formation, invasion of resistive, oil-based mud into porous formations substantially reduces the effectiveness of prior art resistivity imaging devices. This problem is not, generally speaking, fully alleviated by the use of focusing electrodes.
It would therefore be desirable to have an apparatus and method for determining formation resistivity that is relatively insensitive to borehole rugosity and can be used with either water based or with oil-based muds. It is believed that the present disclosure satisfies this need.