LEARNINGS FROM SPECTRAL GR MEASUREMENTS FROM LWD AND FROM CUTTINGS IN HIGH AND LOW ANGLE WELLS

Abstract

Spectral gamma-ray (SGR) data were acquired from a new slim logging-while-drilling (LWD) tool and from surface cuttings in a near vertical well and in a horizontal well across clastic deposits. Comparison of the data from both measurements indicates that there are advantages from both methods. X-ray diffraction (XRD) and X-ray fluorescence (XRF) data from cuttings also support the findings. The formation evaluation objective is to quantify the volumes of each mineral and fluid present in the formation. SGR data brings the required additional information to reduce the mineral volume uncertainty, especially for the clays in the formation with complex mineral assemblages. In the studied clastic deposits, several clay types are present (with the dominant contribution from illite and kaolinite) together with feldspars and trace elements like zircon and other heavy minerals. The presence of gas introduces another unknown, since it affects the porosity measurements and fluid volume calculation through bulk density and neutron porosity. The comparison of SGR data from LWD logs and from cuttings brings robustness to our conclusions. Comparison of the thorium, potassium, and uranium concentrations from LWD logs and from cuttings shows good agreement in the measurements for the low-angle well. The high-angle well data also shows good agreement between the two measurements except for the cleaner sand section. The results from the cuttings are affected by the accuracy of sample depth control due to the poor borehole conditions and inefficiency in evacuating cuttings in high-angle wells compared to low-angle wells. The trend of the SGR is maintained. The LWD SGR elemental concentrations are then used to solve the formation mineral fractions, which are compared with the same fractions from the XRD on cuttings. Similar conclusions are drawn for the elemental concentrations. The potassium concentration enables the quantification of illite and potassium feldspar. Uranium brings a significant contribution to the total GR measurement, which could lead to a clay volume overestimation if the uranium contributions weren’t excluded. In conclusion, LWD provides superior quality SGR data compared with SGR from cuttings because of the better depth control and vertical resolution. SGR on cuttings can be an alternative when combined with other LWD measurements and accepting a higher uncertainty, in case LWD SGR cannot be run due to certain borehole conditions. This paper compares the results of a slim tool LWD and cuttings SGR data for the first time and concludes on the applicability of each technique.