Abstract

This paper introduces high-resolution acoustic borehole imaging as a new service available in logging-while-drilling (LWD). Borehole images (BHI) are an important part of the formation evaluation portfolio and contribute significantly to the understanding of subsurface structures and formation characteristics throughout the entire lifecycle of a field; from exploration, where BHIs yield detailed information about the depositional environments and the inferred depositional trends, to late field life where the accuracy of the estimated hydrocarbons in place are determined by the precision of the geological model. This information is crucial for the planning phase of new wells, determining recoverable reserves in place, analysis in production behaviour and a detailed understanding of the target formations. Acoustic images can also provide a detailed picture of the borehole shape that is used to reduce well construction and completions risk on a well and provides valuable geomechanical information.

The new method is based on ultrasonic transducers that scan the borehole wall with acoustic signals, while the tool is rotating in the borehole. The tool sends a high frequency acoustic signal towards the wellbore wall and measures the travel-time and the amplitude of the returning signal. Each measurement is recorded with its azimuthal position and can therefore be used to create a borehole image. Whereas the amplitude image contains information about the scanned formation, the traveltime represents the borehole shape and can be converted into a distance when the borehole environmental conditions are known. Both images can be acquired while drilling or tripping (reaming/washing). Since the selected transducers are highly focused they can reveal small scale features such as fractures, cross-bedding and mud-cracks. The high acquisition frequency enables the tool to acquire high resolution images for almost any ROP/RPM combination and the physical acquisition method results in an independence of the used mud type. High resolution images significantly improve the understanding of reservoir architecture, presence and type of fractures or faults and are now available in WBM and OBM.

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