The density-neutron log combination is a common standard for reservoir evaluation, typically used for porosity and lithology estimation. The measurements require the use of radioactive sources that operators are increasingly trying to avoid, particularly in areas where drilling operations are challenging and the lost-in-hole risk is elevated. This paper introduces a workflow that integrates surface-measurements, i.e., advanced cuttings, with nuclear magnetic resonance (NMR) and acoustic log data to estimate formation-porosity and bulk density and to distinguish different lithologies.
We present cases where we integrate the porosity from NMR or acoustic with mineralogical analyses on cuttings to infer matrix density by calculating a weighted average of the mineral densities according to their fractional volume in the rock. Formation fluid density is calculated based on a saturation analysis, with the fluid composition and densities of water and hydrocarbons estimated from downhole fluid sampling. This novel method develops a calculated continuous synthetic bulk density log.
Standalone NMR logging can deliver a very accurate, lithology-independent measurement of porosity in oil and water zones. Even though NMR measurements may face challenges in gas-bearing reservoirs, a very accurate porosity can be obtained by integrating NMR and acoustic logging data together with natural gamma ray and resistivity data. An overall comparison of NMR-acoustic porosity to porosities derived from density-neutron and core data within different lithologies shows that the measurements are consistent within +/- 1 porosity unit (pu).
To deliver the same set of information as the density-neutron log with standalone NMR, acoustic or an NMR-acoustic log combination, bulk density, and lithology information utilizes the presented workflow integrating the data from surface measurements and downhole logging. The calculated synthetic bulk density logs from the integrated analysis of surface and downhole log data are in good agreement with measured bulk density logs.
The presented workflow introduces an alternative to the common density-neutron combination for formation evaluation. It delivers a set of petrophysical properties, such as porosity, bulk density and lithology by integrating NMR with advanced cuttings and fluid analysis without using any radioactive sources in the borehole.