2D inversion-based interpretation of logging-while-drilling thermal-neutron and gamma-ray time decays

Identification and petrophysical evaluation of thinly bedded formations with borehole resistivity measurements can be problematic due to the large volume of investigation of the measurements. Logging-while-drilling (LWD) Sigma logs can overcome this difficulty because Sigma measurements have a smaller volume of investigation. Sigma is the macroscopic absorption nuclear cross section of a material that quantifies the material's ability to absorb thermal neutrons. The contrast in Sigma between salty formation water and hydrocarbons makes Sigma logs useful for resistivity-independent estimation of water saturation. We assumed LWD Sigma measurements acquired with one near thermal-neutron detector (near), one short-spaced gamma-ray detector (SSn), and one long-spaced gamma-ray detector (LSn). Separation of multidetector Sigma logs may indicate the presence of mud-filtrate invasion in the case of thick formations. However, Sigma measurements acquired in thinly bedded formations are affected not only by invasion, but also by shoulder beds such that the separation between logs with different radial lengths of investigation will not necessarily be due to mud-filtrate invasion. We have developed an inversion-based interpretation method of LWD Sigma measurements to estimate 2D formation properties with reduced invasion and shoulder-bed effects. We tested this method with two synthetic and two field cases. Results indicated that the 2D inversion-based interpretation method improved the calculation of water saturation from Sigma logs sustaining environmental and geometric effects. Conventional interpretation of Sigma logs can lead to overestimation of water saturation by as much as 50% resulting from shoulder-bed effects in uninvaded formations and by as much as 100% in depth zones simultaneously affected by invasion, shoulder beds, and well deviation.