Calculation Of Neutron Log Response Of Gas-bearing Formation With Element Scattering Cross Section

The response of neutron logging will be abnormal when gas is contained in rock pore.Quantitative research on this problem can improve the calculation accuracy of neutron logging porosity.Different equations of state are used to describe the state parameters of gas.By comparing the deceleration ability of gas and neutron logging standard formation,the equivalent relationship between fast neutron with the same energy in gas and deceleration length in freshwater limestone is established.The temperature and pressure factors of gas medium are connected with the porosity of standard formation,and the influence of formation temperature and pressure is considered.The calculation method of gas neutron porosity can be directly applied to the calculation of neutron logging response equation.In this paper,the basic principle and implementation steps of neutron logging response calculation based on average scattering cross section of nuclides are described in detail.The correctness of the calculation results is verified by comparing the neutron logging porosity of Schlumberger log interpretation chart,and the neutron porosity relationship of natural gas considering the influence of formation temperature and pressure factors is provided.The specific contents include the following aspects:A method for calculating the average scattering cross section of nuclides based on neutron slowing energy spectrum is presented.The inelastic scattering threshold energy of common nuclides in formation is high.Elastic scattering is the main deceleration effect of fast neutrons in formation medium.Based on this,two kinds of slow energy spectrum distribution are selected to calculate the average scattering cross sections of common nuclides in formation.Because hydrogen in water is the main slowing factor for fast neutrons in formation media,the demarcation energy of water is 0.414 eV.Maxwell-Boltzmann distribution is used in the range of 0.025-0.414 eV neutron energy and Fermi spectrum in the range of 0.414 eV-4.5 MeV neutron energy.Fast neutron deceleration length calculation method based on neutron diffusion theory.In this paper,two kinds of neutron slow-down spectra are used to calculate the average scattering cross sections of nuclides in a given energy range.The neutron groups with different energies are equivalent to the same number of single-energy neutron groups.The deceleration lengths of fast neutrons in different media are calculated by using the theory of single-energy neutron diffusion,which improves the calculation efficiency.Calculating method of gas neutron porosity.By comparing the deceleration ability of gas and neutron logging standard formation,this paper establishes the equivalent relationship between the deceleration length of fast neutrons with the same energy in gas and in freshwater limestone,establishes the relationship between the temperature and pressure factors of gas medium and the porosity of standard formation,and provides the calculation method of gas neutron porosity considering the influence of formation temperature and pressure.Results Verification and application.Comparing the neutron log response of gas-bearing formation substituted by neutron log response equation with Schlumberger log interpretation chart,the coincidence degree is better.The neutron porosity of methane,ethane and natural gas with temperature and pressure is calculated respectively.Finally,the neutron porosity function of natural gas with formation temperature and pressure as independent variables is established to provide abnormal pressure and temperature environment in log interpretation.Gas layer evaluation provides reference.