Study On The Evaluation Method Of Tight Gas Reservoir Parameters Based On Pulsed Neutron Logging

Tight gas is one of the three major unconventional gases（tight gas,shale gas and coal bed methane）.It is now the most exploited unconventional gas in the world.At present,tight gas plays a pivotal role in energy supply.Accelerating the exploration and development of tight gas resources is of great strategic importance to improving China’s energy structure,easing the pressure on energy demand and promoting the sustainable development of the oil and gas industry.Although logging techniques are widely used in the evaluation of tight gas formations,the low porosity,low permeability,complex rock structure,complex pore structure and gas-bearing characteristics of tight gas formations make the accurate evaluation of the saturation and porosity of tight gas reservoirs challenging and difficult to evaluate by well logging method.Pulsed neutron logging provides formation neutron cross section parameters that are sensitive to low porosity and natural gas,and not to oil or water.The neutron cross sections for gas and matrix differ significantly.The neutron cross-section method for evaluating gas formations is little influenced by factors such as formation lithology and shale content,providing a new solution for accurate evaluation of saturation and porosity of tight gas formations.Therefore,it is necessary to carry out research on the evaluation method of tight gas reservoir parameters based on pulsed neutron logging.The paper uses Monte Carlo numerical simulation techniques to analyze the response relationships between different high-energy neutron cross sections and fast neutron count rates,fast neutron count ratios,inelastic gamma count rates and inelastic gamma count ratios;the response relationships between low-energy neutron cross sections and thermal neutron count rates,thermal neutron count ratios,capture gamma count rates and capture gamma count ratios are analyzed.On this basis,from the neutron diffusion theory and gamma transport theory,a calculation model for the total fast neutron cross section is established based on the inelastic gamma count ratio,epithermal neutron count ratio and formation density,and a calculation model for the thermal neutron elastic cross section is established based on the inelastic gamma count ratio,capture gamma count ratio and formation density.The calculation accuracy of different types of neutron cross sections is compared and analyzed and the influence of borehole and formation factors on the calculation accuracy of neutron cross sections is analyzed.Based on the analysis of the gas sensitivity of the total fast neutron cross section,thermal neutron elastic cross section,neutron porosity and formation density,the slope of the change in response between different parameters is used to construct cross-plots for the qualitative identification of dense gas layers.The influence of different environmental factors on the identification effect is analyzed.The cross-plot method is proposed to distinguish hydrocarbon gas from carbon dioxide.A gas saturation calculation model based on a single gas formation sensitive parameter,a gas saturation calculation model based on the slope of response between different sensitive parameters and a gas saturation calculation model based on multiple parameters are proposed.The porosity calculation model based on the secondary gamma hydrogen content index and the porosity calculation model based on multiple parameters of the formation are proposed.The validity of the saturation and porosity calculation models is verified using actual and model wells.The results show that the inelastic gamma count ratio has the best effect in characterizing the total fast neutron cross section;the capture gamma count rate is greatly influenced by the thermal neutron elastic cross section and less influenced by the thermal neutron capture cross section.The calculation error of the total fast neutron cross section is basically less than 0.1 m^-1.The lithology and shale content have an influence on the calculation accuracy;the calculation error of the thermal neutron elastic cross section is generally less than 0.05 cm^-1,and the influence of stratigraphic factors on the calculation accuracy is smaller for dense formations.At porosities below 10 p.u.,the slope cross-plots still identifies gas formations relatively well,and the identification ability is largely unaffected by lithology and mud content.As the water saturation increases,the slope cross-plots becomes less capable of identifying hydrocarbon gases.In addition,slope cross-plots have a good differentiation effect between hydrocarbon gas and carbon dioxide.The results of practical well applications show that the slope cross-plot method can effectively identify hydrocarbon gas formations,and the error between the saturation calculated by the slope method and the fluid test results is less than 10%,which is more accurate than the traditional Alchie’s formulas method.The error of saturation calculated by the joint multi-parameter evaluation method in dense formations is basically less than 10%,which verifies the effectiveness of the method.In terms of porosity evaluation,the dynamic range of the secondary gamma hydrogen content index is much higher than the thermal neutron count ratio and the capture gamma count ratio,and the calculation error of this method is generally less than 1.5 p.u.under different formation conditions.The calculation error of the combined multi-parameter method in tight reservoirs is generally less than 1 p.u.,which meets the evaluation requirements for tight gas reservoirs.The method in this paper has a broad application prospect in the evaluation of saturation and porosity of tight gas reservoirs,and can provide strong support for the exploration and development decisions of unconventional gas reservoirs.