Electric Array Logs
The EAL tool actually records four separate channels of data using the five lowest electrodes, bridle electrode and the mudpit electrode. Data is post-processed to yield a synthetically-focussed resistivity log.'Normal' or two-electrode resistivity logs can suffer from severe artefacts when the bed dimensions are comparable to the electrode spacing. Depth of invasion and proximity of the return (bridle) electrode can also affect the response. The main results of these effects are quite serious. Some of the main problems are that normal resistivity logs read much less than the true resistivity values in resistive beds and normal logs responses will invert in resistive beds whose thickness is less than the electrode spacing (ie the resistive beds less than 16" or 64" will appear conductive).
Several types of 'focussed' resistivity tools have been developed over the years. The Laterolog 3 or Guard Log is the simplest form, with two five foot focusing electrodes placed on either side of a central electrode. Current flowing from the central electrode is focussed into the borehole walls by current flowing from the large electrodes. Focusing currents adjust automatically to keep the central current flowing laterally into the formation. This system has the disadvantages of being physically large, and requiring a large current supply.
The Laterolog 7 utilises seven active electrodes, and focuses current by maintaining a region of
zero potential difference between the central and focusing electrodes. This prohibits the flow of current in any direction except into the formation. Since the electrodes are relativity small the required currents are much less than for the Laterolog 3.However there are practical difficulties with electronic instrumentation of this type of focusing. At bed boundaries where there is a large resistivity contrast, high currents are drawn from the focusing electrodes making the system unstable.
It is possible to imitate the current flow of the Laterolog 7 without the need for complex electronics. The Electric Array Log (EAL) does this by measuring the resistivity with an array of electrodes at different locations, and processing these data to synthesize the Laterolog 7 response.
Advantages of the focussed logs such as the EAL include:
Specifications
Model A607
16"N EALA614
16"N / 64"N EALDetector Sensor Current & measure electrodes Current & 2 measure electrodes Range 1 to 10000 ohm-m 1 to 10000 ohm-m Calibration Resistance Values Resistance Values Units of measure ohm-meters ohm-meters Communication Duplex Digital RS232 Type Duplex Digital RS232 Type Nominal Current 60 100 Power at Sonde (VDC) 50 50 Pressure Rating (KPg/PSI) 21000/3000 21000/3000 Temp Rating (Degrees C) 0-60 0-60 Diameter (mm) 33 45 Length (mm) 1500 2310 Weight (Kg) 11.0 12.0
Auslog