Fundamental Study On Three-detector Pulsed Neutron Capture Logging Method

With the deepening exp loration of oil and gas field,pulsed neutron logging technology plays an important role in the dynamic monitoring of residual oil and gas reservoir identification.However,natural gas exploration is difficult because of its characteristics that is in buried depth,poor physical properties and complex traps.The most widely used in gas saturation with pulse neutron logging technology is neutron lifetime logging(TDT)and carbon oxygen spectral logging(C/O)etc,which generally only suitable for medium or high salinity and porosity formation,and the limitation of the unconventional reservoir evaluation is relatively large.Three detector pulse neutron capture logging technology can be used to detect thermal neutron and gamma and obtain the time spectrum information,reduce the effect of background gamma,can be applied to the formation conditions of lower salinity and porosity.In this paper,through the survey of foreign and domestic pulse neutron logging technology,the current development of logging method and tool is understood,the theoretical basis of the research of the three-detector pulse neutron capture logging method is established.The measurement system of three-detector pulse neutron logging instrument is mainly composed of D-T neutron generator,two He-3 detectors and one NaI detector,the NaI detector is located between two He-3 detectors.Using Monte Carlo numerical simulation method,the response sensitivity and measuring statistics such as basis to determine the neutron pulse width,timing measurement and the source detector distance,and the method of measuring time spectrum has be formed based on the D-T neutron source.Wth use thermal neutron time spectrum information which is recorded form He-3 detector and gamma spectrum information which is recorded form NaI detector,the value of Sigma,Ratio and RIC can be calculated,in response to a change in these parameters is studied by numerical simulation with different borehole strata and lithology conditions,obtain the evaluation method of formation fluid,study the response of RPN under different well condition,to indicate the change of fluid in the well,thermal neutron time spectrum attenuation imaging and capture imaging can be used to visually display the reservoir fluid and litholo gy changes.Through the combination of three-detector ratio information,improve the sensitivity of gas reservoir identification and obtain the method for determining gas saturation.Through the calculation model of fluid saturation,the curve of water saturation and gas saturation can be calculated at different depths,and compared with the actual data.In this paper,we study the form of three-detector pulse neutron capture logging method that can overcome the deficiency of traditional one in a certain extent,and provide an important theoretical basis for the following equipment development and oil /gas field exploration.