THE FINE MARGINS OF SUCCESS Cory Langford, Scientific Drilling International, USA, discusses how at-bit gamma ray images and continuous inclination measurements enable pro-active geosteering in tight target windows.
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here are a wide variety of downhole logging tools which are used to effectively steer horizontal wellbores within a desired target window. The selection of the required real-time downhole measurements to be used in the geosteering process is largely based on the target formation’s expected log response across the drilling window. As the development process of an oilfield progresses, new methods for extracting the hydrocarbons are implemented. One of these methods involves extremely precise borehole positioning within the lateral’s target zone to maximise hydrocarbon recovery. Traditionally, operators who wanted to achieve the goal of staying within a geosteering window of <10 ft would utilise a rotary steerable
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system (RSS) for a decreased bit-to-sensor distance and better inclination control. With commodity prices for oil remaining in the US$30 – US$40/bbl range, the development of certain mature oilfields might not be economically feasible when the extra cost of running a RSS is required for geosteering purposes.
Azimuthal LWD data requirements While most horizontal wells can be geosteered with a traditional bulk gamma ray logging while drilling (LWD) sensor, there are instances where the target formation does not provide enough gamma ray contrast to adequately determine where the well is positioned by correlation to offset well logs. Azimuthal functionality of downhole LWD sensors provides an accuracy to well placement decisions made in real-time. Determining if an azimuthal gamma ray sensor or azimuthal resistivity tool is needed to stay ‘in-zone’ is usually decided before the well is drilled by looking at offset well data in a pre-well analysis. With a thin geosteering window, target changes in inclination become a critical component to keeping the well within the desired target zone.
