| Application | How NGI Helps | |-------------|----------------| | Shaly sand evaluation | Corrects for non-clay radioactivity (e.g., K-feldspar, mica) | | Source rock identification | High Uranium indicates organic matter | | Clay typing | Th/K ratio distinguishes swelling vs. non-swelling clays | | Unconformity detection | Uranium enrichment below unconformities | | Heavy mineral zones | Thorium peaks (monazite, zircon) | | Borehole environmental correction | Uses near/far ratio to correct for mud weight, standoff |
When drilling through faulted or folded strata, the NGI provides a high-resolution gamma log that can be correlated in real-time with offset wells. By comparing the near-bit gamma to the memory gamma from a tool higher up the BHA (Bottom Hole Assembly), geologists can calculate structural dip. This tells them if they are drilling updip (good for oil) or downdip (risking water). schlumberger ngi tool
| Technology | Advantage vs. NGI | |------------|-------------------| | ECS (Elemental Capture Spectroscopy) | Measures Si, Ca, Fe, S, Ti – full mineralogy, but requires neutron source | | LithoScanner | High-resolution mineralogy with pulsed neutron | | NGI (this tool) | Passive, no source, simpler, cheaper, good for clay typing | When drilling through faulted or folded strata, the
NGI remains preferred for low-cost clay typing, geosteering correlations, and wells where radioactive sources are restricted. No tool is perfect
No tool is perfect. Engineers must understand the limitations of the NGI:
It is important to note that Schlumberger has largely moved toward the NeoScope family of LWD services. The NeoScope integrates the near-bit concept but adds a crucial feature: resistivity at the bit. This allows drillers to see changes in porosity and fluid type instantly.
However, the legacy NGI tool remains in active service in many regions (South America, Middle East mature fields, and North Sea) due to its lower cost and high mechanical reliability. For many operators, the "old" NGI is still the most cost-effective solution for gamma-guided geosteering.