9 IMPROVED POROSITY ESTIMATION IN TIGHT GAS RESERVOIRS FROM NMR AND DENSITY LOGS

Nuclear magnetic resonance (NMR) logs differ from conventional neutron, density, sonic and resistivity logs because the NMR measurements provide information only on formation fluids and are generally insensitive to matrix properties. The main task of formation evaluation is to calculate accurate formation porosities, permeabilities and identify properly the nature and amount of saturating fluids (water, oil, gas). The focus of this paper is to present an empirical method in which density and NMR porosities are combined and calibrated to core. The new porosity is termed DMR (density-magnetic resonance) porosity. Historically, neutron/density measurements have been used to derive formation porosity. While this yields acceptable results in clean liquid-filled lithologies (sands), the effects of lithology, hydrocarbon fill (gas), and formation heterogeneity in shaly sands increases the uncertainty in computed porosities. The technique of using combined NMR relaxation times and bulk density data described in this paper significantly reduces uncertainty in derived logging parameters through elimination of the neutron log. This removes the need to characterize, and correct for flushed zone fluid densities. This paper describes the application of DMR technique in cored wells in the Obaiyed field