A new method of gas reservoir evaluation based on neutron cross section logging

Abstract It is a widely used method to evaluate gas reservoirs by measuring inelastic gamma rays. A petrophysical parameter fast neutron cross section (FNXS) was proposed to describe the inelastic gamma for gas reservoir evaluation. FNXS is the 14MeV neutron macroscopic elastic scattering cross section. However, the description of inelastic gamma using FNXS is only a first order approximation. Therefore, in some scenarios, the evaluation of gas saturation is not accurate enough. In this paper, we propose a new parameter total fast neutron cross section (TFNXS) based on the inelastic gamma generation and attenuation theory. TFNXS is the sum of macroscopic elastic and inelastic scattering cross sections of 14MeV neutrons. The physical meaning of TFNXS is the probability of inelastic and elastic scattering between neutrons and formation. TFNXS is a new formation property independent of neutron macroscopic capture cross section (Sigma). TFNXS value of gas is quite different from that of matrix minerals, water, oil and clay, so TFNXS is more sensitive to gas-bearing formation and can identify gas reservoir more effectively. We compared the sensitivity of TFNXS and FNXS to gas. The sensitivity of TFNXS to gas is similar to that of FNXS. The effects of different formation conditions on the measurement accuracy of the TFNXS and the FNXS are analyzed with the Monte Carlo method. Compared with FNXS, TFNXS can be obtained more accurately in water and oil layers. Different clay minerals have different effects on TFNXS and FNXS. TFNXS is less affected by wellbore environment, such as well diameter and wellbore fluid, than that of FNXS. We evaluate the application results of TFNXS and FNXS in various model wells. Both TFNXS and FNXS can effectively identify gas-bearing formations. The accuracy of using TFNXS to obtain gas saturation is higher than that of FNXS. TFNXS can more accurately monitor reservoirs and evaluate low porosity gas reservoirs than FNXS.