Disclosed herein are methods and system for determining an elemental composition of a downhole formation using neutron-induced gamma ray spectroscopy and doping the borehole fluid with a neutron absorber to remove at least part of the borehole thermal neutron flux before it can capture borehole nuclei and emit gamma rays within a spectral range of interest. For example, a method for determining the elemental composition includes adding a dopant to a borehole fluid, wherein the dopant absorbs thermal neutrons in the borehole, lowering a logging tool comprising a gamma ray spectrometer, emitting neutrons into a downhole environment, generating neutron-induced gamma rays comprising borehole gamma rays and formation gamma rays, detecting gamma rays from the downhole formation, and extracting the elemental composition of the downhole formation. The dopant may not emit gamma rays above 0.5 MeV or their peaks background should be predictable and/or consistent to be removed during data processing.

The techniques as described herein enhance the accuracy and precision of mineralogy analysis in elemental spectroscopy logging by utilizing oxide standards and/or rock-forming mineral compounds as reference materials that provide more representative and realistic signatures of geological formations. A method comprises logging a wellbore with a tool and fitting reference spectra from oxide compounds and/or rock-forming mineral compounds against measured gamma spectra obtained with the tool, to characterize a subterranean formation that the wellbore extends through.

A time lapse water saturation model for a naturally fractured subsurface reservoir. A fracture model may be generated using a deformation and geomechanical model, and a fracture density index (FDI) is determined from the fracture model using a critical stress analysis. Additionally, a water saturation vs time is determined using from pulsed neutron lifetime (PNL) logs and a corresponding water saturation log. A time lapse water saturation model is determined using a cross-correlation of the fracture density index (FDI) and water saturation.

The present disclosure is directed to methods for evaluating a gravel pack, a frac-pack, or cement in a wellbore. In at least one embodiment, a method for evaluating a gravel pack, frac-pack or cement in a wellbore, includes pumping a first material into the wellbore, wherein the first material comprises a first tracer that is not radioactive. The method includes pumping a second material into the wellbore, wherein the second material comprises a second tracer that is not radioactive. The method includes obtaining a set of data using the downhole tool in the wellbore after the first and second materials are pumped into the wellbore. The method includes obtaining a baseline using the downhole tool in the wellbore in a depth interval without the first or second material. The method includes comparing the set of data with the baseline.