Olabode Ijasan

Interpretation of porosity and fluid constituents from well logs using an interactive neutron-density matrix scale

Neutron and density logs are important borehole measurements for estimating reservoir capacity and inferring saturating fluids. The neutron log, measuring the hydrogen index, is commonly expressed in apparent water-filled porosity units assuming a constant matrix lithology whereby it is not always representative of actual pore fluid. By contrast, a lithology-independent porosity calculation from nuclear magnetic resonance (NMR) and/or core measurements provides reliable evaluations of reservoir capacity. In practice, not all wells include core or NMR measurements. We discovered an interpretation workflow wherein formation porosity and hydrocarbon constituents can be estimated from density and neutron logs using an interactive, variable matrix scale specifically suited for the precalculated matrix density. First, we estimated matrix components from combinations of nuclear logs (photoelectric factor, spontaneous gamma ray, neutron, and density) using Schlumberger’s nuclear parameter calculator (SNUPAR) as a matrix compositional solver while assuming freshwater-filled formations. The combined effects of grain density, volumetric concentration of shale, matrix hydrogen, and neutron lithology units define an interactive matrix scale for correction of neutron porosity. Under updated matrix conditions, the resulting neutron-density crossover can only be attributed to pore volume and saturating fluid effects. Second, porosity, connate-water saturation, and hydrocarbon density are calculated from the discrepancy between corrected neutron and density logs using SNUPAR and Archie’s water saturation equation, thereby eliminating the assumption of freshwater saturation. With matrix effects eliminated from the neutron-density overlay, gas- or light-oil-saturated formations exhibiting the characteristic gas neutron-density crossover become representative of saturating hydrocarbons. This behavior gives a clear qualitative distinction between hydrocarbon-saturated and nonviable depth zones.

Rapid, Interactive Assessment of Petrophysical and Geometrical Effects on Density and Neutron Logs Acquired in Vertical and Deviated Wells

Borehole, geometrical, and petrophysical effects can significantly affect density and neutron logs. Tool location around the perimeter of the borehole, tool standoff, and wellbore deviation have a measurable effect on density and neutron logs, hence on the estimation of porosity and fluid density. It is difficult to diagnose and quantify these effects a priori without numerical modeling. The multiple-particle, radiation-transport Monte-Carlo code (MCNP) has traditionally been used by the logging industry to simulate borehole nuclear measurements acquired in complex rock formations. Despite its versatility and accuracy, MCNP is not numerically efficient for rapid simulation of nuclear logs and does not lend itself to interactive testing of multiple petrophysical/fluid hypotheses. We describe the successful application of a new method for nuclear-log simulation based on linear iterative refinement of nuclear sensitivity functions pre-calculated with MCNP. The procedure is fast, accurate, and efficient in most practical logging applications, including the simulation of nuclear logs acquired in invaded formations and in highly-deviated wells. Density and neutron logs acquired over a depth segment of 1000 ft can be accurately simulated within minutes of CPU time, compared to days with MCNP. Simulations are successfully verified against MCNP in a number of extreme cases of borehole, petrophysical, and fluid conditions wherein the error of the simulations does not exceed 2 porosity units. We implement the new simulation method to reproduce several field examples where logs are affected by presence of clay and invasion with water- and oil-base muds. Our rapid simulation procedure enables the interactivel quantification of the relative effect of clay, invasion, fluid density, and rock petrophysical properties on field logs. It also permits efficient integration with induction resistivity measurements for assessment of free and clay-bound water saturation, as well as for the assessment of residual hydrocarbon saturation. .Additional simulations of density and neutron logs are performed to quantify the influence of shale laminations on hydrocarbon-bearing and invaded thinly-bedded formations. Because of their longer radial length of investigation compared to density logs, neutron logs may exhibit false cross-over effects across thin beds.