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In nowday’s oil exploration field, thorough and effective reservoir evaluation is more important than ever before. The extended Micro-Resistivity Scanning Imaging Logging Tool (MRIT) is a new tool designed to provide a clear and intuitionistic image of stratum around borehole wall. Based on the Six-Arm Dipmeter (SED) technology, the 6 pads of MRIT are each mounted on an independent arm (6 in all), allowing improved electrode- to-formation contact. High quality formation resistivity data are achieved by using 150 pad-mounted button electrodes distributed at 25 per pad. This finally produces tool location data, six micro-resistivity curves and six independent caliper curves with a measurement resolution of 0.2 inch.

Calibration And Verification

MRIT is a non-focus electric logging tool. The tool current loop goes from lower button electrode on pad into formation, then comes back to upper electrode. In fact, the housing of cartridge acts as upper electrode.In the logging process, the pads are forced to contact against the borehole wall by the motor-drive mechanical push-and-pull system. Then the logging software controls emitting current upon measured value, and the current and potential difference measurements on each button electrode meanwhile are recorded, presenting the micro-resistivity variation within the formation around the borehole.

The tool resolution is closely related with the pad-mounted button electrodes arrangement. MRIT takes such a array shown in the following diagram, by which 0.2 in. (5mm) of both the all-around resolution and vertical resolution and 80% coverage of a 6.5 in. hole can be achieved when the tool works at the MAX investigation speed of 1800ft/h.

Pad Buttons Arrangement Diagram

MRIT takes the advanced design concept of digitalized pad so as to greatly enhance the image quality and the measurement range. With the sample signals digitalized in the pad, the button electrode enables serial digital communication with electric circuit. As a result, the coupling jam among signal lines is reduced and the signal to noise ratio is improved, which is beneficial for minor signal acquisition.

MRIT Pad Functional Diagram

The real-time image data produced at the wellsite will go through a series of post-processing and image enhancement. The micro-resistivity image represents the borehole wall formation resistivity hetero-feature changes, in which the change of resistivity reflects variation of the lithology, porosity and mud content. The fluid nature in the alluvion and the irregular borehole wall will have certain impact on the image.

Application of MRIT images in geological analysis mainly includes the following.

Fracture identification and evaluation

Thin bed evaluation

Detailed stratigraphic and sedimentological analysis

Secondary porosity analysis and geological structure interpretation

Core restoring and lithology characterization assistance

The figure below shows MRIT images after post-processing wellsite raw data, in which the fracture bisecting the borehole along a north to south trend and the sand-shale boundary is clearly seen.