The Correlation of Neutron and Density Logs and its Application in Tight Gas Reservoirs Identification

Abstract

Abstract It’s a great challenge for petrophysicsts and log analysts in identifying tight gas rservoirs from conventional resistivity logs due to the complicated pore structure, thus leads to high irreducible water saturation. The resistivity contrast between tight gas bearing reservoirs and pure water saturated layers is lower than 2.0. The neutron and density logs, which are used for reservoir porosity estimation, are usable in gas bearing formation identification. In gas bearing formation, the neutron derived porosity is lower than the ture formation porosity due to the effection of excavation effect, and the density derived porosity is higher than the true formation porosity. Thus, porosity estimated from density logs is increased and the neutron derived porosity is decreased, the negative correlation exists between these two derived porosity in tight gas bearing formations. On the contrary, with the increasing of neureon porosity, the density derived porosity also increase, the positive correlation exists between them. Based on these two different correlations between tight gas bearing sands and water bearing formation, a technique, which is used to quantitatively characterize the correlation factor, is proposed and the corresponding correlation function is established. For tight gas resercoirs, the value of the correlation factor is negative, and in water saturated formations, its value is positive. Case studies from two tight gas reservoirs in China illustrate that the proposed technique is available in tight gas sands identification.