In the framework of a long-term research on Advanced Mud Logging (AML) we have further developed innovative AML methods and techniques to be deployed while drilling, for real time formation characterization on cuttings. AML can deliver integrated, comprehensive Petrophysical Information Logs (PILs) for well evaluation, facilitates improved monitoring and thus optimization of drilling operations, as well as supports formation evaluation. In the following paragraphs we describe an innovative Spectral Gamma Ray (GR) instrument developed with a NaI(Tl) scintillation crystal, geared especially towards well site cuttings measurements. Gamma ray on cuttings complements the well-established AML well site techniques, including direct measurements on cuttings such as grain density, porosity by Nuclear Magnetic Resonance (NMR), X-ray diffraction (XRD), X-ray Fluorescence (XRF), etc. The spectral gamma ray measurements on cuttings allow resolving the ambiguity on cuttings depth shift, by comparison with conventional gamma ray logging tool, reducing the uncertainty and giving significance on the subsequent AML characterization on cuttings.

As part of the development process, extensive laboratory tests were performed to validate and compare results of different types of instruments. A typical spectral gamma ray measurement on cuttings takes about 15 minutes and gives total gamma ray activity in API units, and the normal Uranium (U), Thorium (Th), and Potassium (K) spectral components. Laboratory tests proved a good signal to noise ratio (SNR) and a match with measurements done on large rock pieces with conventional lab tools, in terms of relative value of the GR readings. Next, results from laboratory test done with real cuttings collected from a well in Saudi Arabia and a conventional (non-spectral) GR showed a good correlation. Finally, actual field tests with the spectral gamma ray instrument on cuttings in the AML unit have shown a good match with wireline log results.

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