| In the process of drilling a borehole, geosteering is the act of adjusting the borehole position (inclination and azimuth angles) on the fly to reach one or more geological targets. These changes are based on geological information gathered while drilling. From 2D and 3D models of underground substructures, deviated wells (2D and 3D) are planned in advance to achieve specific goals: exploration, fluids production, fluids injection, etc,. A well plan is a continuous succession of straight and curved lines representing the geometrical figure of the expected well path. A well plan is always projected on vertical and horizontal maps. While the borehole is being drilled according to the well plan, new geological information is gathered from mud logging, Measurement While Drilling and Logging While Drilling. These usually show some differences from what is expected from the model. As the model is continuously updated with the new geological information (formation evaluation) and the borehole position (well deviation survey), changes start to appear in the geological substructures and can lead to the well plan being updated to reach the corrected geological targets. Technical Geosteering Value By observing a cross-section of TVD versus MD that displays the entire well path and the payzone as defined from the 3DSBs, the best “big picture” can be seen and drill-up/hold-steady/drill-down planned well path revisions can intelligently be made. See Figure 3. In practice, the number of target changes communicated to the directional driller can range from few to dozens. Updating the target entails specifying inclination and TVD at vertical section of zero. Post-drilling application of technical geosteering provides value by training personnel on how to geosteer/interpret, and it produces a most-complete understanding of the geologic structure and actual well path / reservoir completion. Such “in/out” understanding is often critical for example for reservoir simulation of wells drilled horizontally. It can also affect completion procedures that use fracture stimulation. The best possible geologic interpretation can be attained after drilling because there are no data depth-lag issues or general human fatigue conditions that inherently accompany live operations. Technical geosteering is a numerical tool that augments other data sources—akin to another “dimension”—to assist the operator to interpret where the wellbore is stratigraphically located. Other data that may help with geosteering may include multiple fluid-return-line-derived measures, such as sample drill cuttings analysis, gas chromatograph measurements, oil shows, gas flare height and casing pressure in underbalanced drilling operations, and general rate of penetration characteristics. Most fluid-derived measures suffer from bottoms-up lag-time issues and relatively significant source-depth uncertainty compared to LWDFE data. Technical geosteering defines locally and helps to refine globally the geologic model of the marker bed along and nearby the actual drilled wellbore. Small-scale geologic features—often ignored with legacy geosteering methods that rely only on plain drafting tools—like faulting and zone undulation become better communicated via the TVD versus MD cross-section displaying calibrated 3DSBs and may help explain subsequent production behaviors related to hydrocarbon and or water flows, and issues related to water sumps in wellbore low-spots. |
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Geosteering
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