N3LIP: Lithology Independent Porosity Calculations Using Three Neutron Measurements

This paper describes a lithology independent porosity computation using three nuclear well logging measurements (N3LIP): thermal neutron porosity, epithermal neutron porosity and formation thermal absorption cross section. These three nuclear measurements are used to calculate four neutron parameters for the formation. These parameters are the slowing down length, migration length, diffusion length and diffusion coefficient. N3LIP treats the formation regardless of its lithological composition as two regions: the pore space and the matrix. The matrix is considered as a black box where its four nuclear parameters are also calculated using the tool responses. The thermal and the epithermal tool responses, and the capture cross section for the pore fluid should be identified. These values can be obtained using a forward model once the hydrocarbon type is identified. The N3LIP method determines porosity by calculating the diffusion length of the matrix using two different solution techniques while iterating on porosity. The final solution is determined by characterizing the difference of the two techniques. The N3LIP algorithm is expected to have application to complex lithologies when a neutron thermal, epithermal and absorption cross section logs have been run in open holes, anticipating the new generation logs (e. g. NPLT). The physics of N3LIP method is demonstrated using simple combinations of different formations with fresh water in the pore space. The algorithm has been tested using artificial logs representing different real formation lithologies. X-ray diffraction data (XRD) for shaly sands (North Slope reservoir) and carbonates (Permian Basin reservoir) formations have been used to generate the thermal neutron, epithermal neutron and formation thermal absorption cross sections using SNUPAR as a forward model. These artificial logs have been used to examine this model.