Integration of NMR with other open hole logs for improved porosity, permeability and capillary pressure of gas sand reservoirs

Analysis of tight gas sand reservoirs is one of the most difficult problems. Many tight formations are extremely complex, producing from multiple layers with different permeability that is often enhanced by natural fracturing. Therefore, looking for using new well logging techniques like NMR in individual bases or in combination with conventional open hole logs and building new interpretation methodology is essential to well define and obtain the representative reservoir characterizations. Nuclear magnetic resonance (NMR) logs differ from conventional neutron, density, sonic and resistivity logs because the NMR measurements provide mainly lithology independent detailed porosity and a good evaluation hydrocarbon potential. NMR logs can be used to determine formation permeability and capillary pressure. This paper concentrates on three petrophysical applications of NMR; 1) present a technical method for porosity calculation in which density and NMR porosities are combined and calibrated to core, 2) Use new empirical method of bulk gas volume (BG) in the invaded zone for permeability estimation termed BGMRK and 3) Use T₂ distribution for capillary pressure approximation and correct pore size distribution for gas. These applications of NMR logs have been applied in a gas sand reservoir field of different facies and permeability varies from less than 0.1 md to more than 100md related to facies changes. These applications result in a) The technique of using combined NMR and bulk density data significantly reduces uncertainty in porosity through elimination of the neutron log down to 0.5%. b) The new approach of "BGMRK" resulted in very simple facies independent model to calculate reliable permeability 2. c) Capillary pressure can be approximated from NMR T₂ distribution and then the integration between cap-curves and T₂ distribution can be corrected for partial pores fluid fill.