Schlumberger Ngi Tool -

Schlumberger NGI Tool: Near-bit Gamma Imaging for Geosteering and Formation Evaluation

1. Overview & Purpose

Key Energy Peaks Analyzed: | Element | Primary Gamma Energy (keV) | Geological Indicator | |---------|----------------------------|----------------------| | Potassium (K-40) | 1460 | Feldspar, Mica, Illite clay | | Uranium (U-238 series) | 1760 (Bi-214) | Organic matter, authigenic minerals, seawater influence | | Thorium (Th-232 series) | 2610 (Tl-208) | Heavy minerals, detrital clays (Kaolinite, Chlorite) | schlumberger ngi tool

1. Natural Gamma Ray Detector: Measures the natural radioactivity of the formation. This is crucial for identifying shale beds and correlating with offset wells.
2. Inclination Sensors (Accelerometers): Provide high-resolution deviation and azimuth data, allowing drillers to know exactly where the bit is pointing in three-dimensional space.

⚠️ Limitations

• Shallow investigation → Measures only flushed zone. Requires assumption of deep gas saturation (often ( S_g \approx S_xo ) if invasion is piston-like).
• High salinity → Water’s permittivity is still high, but conductivity losses increase. Works up to ~200 kppm NaCl, but corrections needed.
• Oil vs. Gas distinction – NGI cannot differentiate oil from gas; both have low permittivity. Use density/NMR for fluid typing.
• Clay-bound water – Overestimates ( \phi_w ) if not corrected. Requires clay correction models (e.g., Dual-Water, Waxman-Smits).

Example Deliverables from an NGI Run

• High-resolution annotated images with picks for fractures, bedding, and breakouts.
• Fracture orientation rose diagrams and dip histograms.
• Wellbore stability report with recommended mud-weight range and casing depths.
• Stimulation-target map aligning natural fractures with reservoir zones.
• Geomechanical model inputs and sensitivity analyses.

• The tool includes a fast-response temperature sensor.
• Temperature anomalies (deviations from the geothermal gradient) are key indicators of fluid movement. For example, gas expansion cools the surrounding environment, while water movement can warm it.
• Combining temperature data with noise data helps differentiate between static fluid levels and active flow.

The NGI tool uses a combination of nuclear reactions, including: ⚠️ Limitations

Integrating the NGI Tool into the BHA

A typical Bottom Hole Assembly (BHA) using the NGI tool might look like this: gas expansion cools the surrounding environment