Real-Time Formation Dip From a Logging-While-Drilling Tool

Abstract

Formation dip can now be estimated in real time with a logging-while-drilling (LWD) tool. A new method allows the Resistivity-At-the-Bit tool to perform the computation as the well is being drilled. The tool calculates the apparent dip direction, dip magnitude and a quality factor at approximately 3 minute intervals. These data are transmitted to the surface and displayed as apparent and true dip. The information can be used to confirm or modify the drilling plan as the well is drilled since the local structural dip may differ from that predicted from large scale maps such as those made using seismic surveys. The new method is fundamentally different from conventional dipmeter processing and has complementary strengths and weaknesses. Whereas conventional processing is most accurate when the bedding planes are nearly normal to the borehole, this new technique is most accurate when the bedding planes are nearly parallel to the borehole. Thus, it is accurate and useful for determining whether the borehole is moving up or down relative to the bedding planes. An added advantage of this processing is that it uses only data that are acquired at a single instance, making it immune to any errors in the measurement of absolute depth. Because depth errors can cause errors in the computation of dip, the method's immunity to them is especially important in the while-drilling environment, where depth control is more problematic than it is in wireline operations. A field example demonstrates the validity of logging-while-drilling dips by comparing them with dips picked by hand from images recorded by the Resistivity-At-the-Bit tool and with dips from a wireline imaging tool. Introduction The development of a reservoir is usually guided by structural interpretation from seismic surveys. While seismic surveys provide continuous sections, they have poor resolution and rely upon assumptions concerning sonic velocity in the formation. As the wells are drilled, more detailed measurements are acquired including formation dip and acoustic velocity and the reservoir geometry is refined. Dip data in particular can be utilized to recognize structures such as folds and faults and help in the interpretation of the depositional environment. A number of wireline tools have been developed over the years to determine formation dip. Dipmeter tools with pad-mounted electrodes provide high-resolution electrical measurements from which formation dips are computed. In 1986 electrical imaging tools became available and are now commonly used for this purpose. Now it is possible to determine dip from a logging-while-drilling tool, the Resistivity-At-the-Bit or RAB tool. This can be done through conventional processing methods after the resistivity images are downloaded from the tool on the rig floor, but it can also be done using a unique processing method which runs in the tool as the formation is being drilled. While these dips can be used for conventional structural and stratigraphic interpretation, having formation dip in real-time offers a significant advantage over "after-the-fact" dipmeter analysis particularly in geosteering the well. Real time dips make it possible to determine how the well is cutting through the bedding planes. In a high angle or horizontal well it is possible to know very accurately in real-time whether the borehole is traveling "up structure" or "down structure" through the formation and at what angle. This type of information can prevent unwelcome surprises by enabling a well course change to be made in much earlier than would otherwise be possible. P. 41^

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