High-precision Pulsed-neutron Residual Oil Saturation Logging Interpretation Study And Its Applications

After long-term production and development,the residual oil saturation of reservoir may have changed significantly from the original stage.Evaluating precisely the current residual oil satruration is the basis of formulating and adjusting development plans.It is important for stabilizing and increasing production,reducing water content,and reusing shutdown wells.Because steel casing is used in wells,it is very difficult to measure the residual oil saturation through tubing in production wells.The resistivity well logging used in open hole wells cannot be used in steel cased wells.In the mean time,gamma rays and neutrons can easily penetrate through the casing and then enter the formation,so that measure the formation properties.At present,the nuclear well logging based on a pulsed neutron source is the main method applied in measuring residual oil saturation,however,the precision is poor in complex geological environments,such as low porosiy and low salinity formations.In order to precisely measure saturation parameters in complex formations,it's necessary to improve instrumental hardwares,spectrum processing methods and interpreting technologies.Reservoir Data Miner(RDM)is a new innovative residual oil saturation instrument using a pulsed neutron generator.RDM includes advanced hardware and the novel design.We conducted studies to improve the performance of RDM.Firstly,the 53 H method proposed by Tukey was applied in removing the abnormal data.The Savitzky-Goloy filtering method was employed to smooth data and reduce noises.A peak-finding algorithm,PeakUtils,and a derivative method were combined in finding peaks.The B-spline method was used in peak-drift correction.The subtraction factor method was applied to obtain pure inelastic scattering spectrum.A new evaluation index of subtraction of H peak was proposed.According to the mechanical structure of RDM,the ratio of near over far neutron count rates was calculated to indicate neutron porosity information.The verification of the processing methods above was done by the logging data of two real wells and of three calibration wells which have 0%,50%,100% oil saturation,respectively.Last,the logging curves of RDM and open-hole data have been compared,and cross-plots were applied in calculating their correlations.With all the enhanced methods above,the correlation between RDM data and open-hole ones becomes stronger.The dynamic range of the carbon-oxygen ratio curve was increased by 14% which correspondingly improves the ability of distinguishing oil and water of reservoir.The increased dynamic range of the silicon-calcium ratio curve also greatly improved the ability of identifying different lithologies.As a result,RDM can provide not only the ratio curves of near-and far-detector gamma count rates,but also the ratio curves of near-and far-detector neutron count rates.RDM provided much more curves than similar instruments on the market.In the end,some qualitative methods for identifying oil and water in reservoir were also established and their application effects under different geological envirments were discussed and summarized.