Trust Your Cuttings – Applications of Spectral Gamma-Ray (GR) Analyses for Drill Cuttings QC

Spectral gamma-ray analyses of washed and sieved cuttings using a Georadis RT-50 scintillation detector are obtained to calculate total GR. The resulting data is compared to well-log derived GR allowing to assess quality and representativeness of cutting samples. Three case studies utilizing legacy and new well-log and cutting data including crystalline basement have been performed to test the outcome. The results show that calculated GR from spectral cutting analyses exhibits the same API ranges than well-log derived GR both for legacy and recent wells. Two main mismatch types in data have been observed. The less significant one caused by small cavings either increasing or decreasing the calculated GR from spectral cutting analyses. The extent of the caving contamination can be identified thus allowing to define cutting samples where quantitative analyses are representative for a certain lithological interval. Strong data-mismatch between both GR curves observed in the case studies are either caused by contamination with lost circulation material (LCM) or caused by drilling activities (e.g. underreaming) that strongly impact cuttings quality. Such cuttings can be used for qualitative analyses only or require more work to be shifted to the respective well-log GR depth for quantitative analyses. Spectral GR from a recent well has been used in case study 2 for comparison with spectral composition obtained from cutting samples. Both methods deliver the same range of data values so spectral composition from cutting samples can be used to explain GR-readings in case no spectral well-log GR is available. Rock-typing schemes for crystalline basement lithologies based on spectral well-log GR have been reproduced with cutting-derived data in case study 3. Here it was possible to distinguish various rock type based on potassium contents. This enables to extent rock-typing schemes into wells without spectral GR.