Research On Optimization Of Vertical Resolution Of Natural Gamma Measurement Based On Core Scale

With the old oil fields in the east of our country gradually entering the late stage of waterflooding development,in order to improve the old oil recovery rate,the thin mezzanine layer of about 10 cm in the sedimentary layer becomes the key of understanding the distribution of remaining oil in the layer.Natural gamma logging is a necessary technology to evaluate the longitudinal distribution of reservoir physical properties.The function of natural gamma logging is to classify lithology,formation correlation,and determine the shaliness content.The conventional sampling rate of open hole logging during the completion stage is 3points /ft,so the application of these data cannot meet the needs of effectively identifying thin interlayers.Therefore,it is very important to improve the longitudinal resolution of natural gamma logging instrument to improve the recognition of thin interlayer in cased-well(old well).Through MCNP5 numerical simulation,supplemented by field experimental verification,the relationship between four NaI(Tl)crystals with the sizes of φ2.7cm×5.3cm,φ2.7cm×12cm,φ2.7cm×18cm and φ2.7cm×22cm and the count rate,statistical fluctuation and longitudinal resolution was studied.The results are as follows: As NaI(Tl)crystal size(length)increases,NaI(Tl)crystal count rate also increases,statistical fluctuation decreases,and longitudinal resolution increases.Therefore,NaI(Tl)crystals with larger count rate,smaller statistical fluctuation and higher longitudinal resolution should be selected as far as possible in actual logging operations.According to the above research results,among the four NaI(Tl)crystals studied,NaI(Tl)crystals with sizes of φ2.7cm×12cm and φ2.7cm×18cm should be selected as appropriate.Based on the relationship between the thickness of the thin sandwich and the allocated detection points,the theoretical calculation of the sampling rate is carried out,which lays a foundation for selecting the sampling rate more suitable for velocity measurement.Field experiments were designed to study the influence of logging NaI(Tl)crystals withφ2.7cm×12cm and φ2.7cm×18cm velocity measurement at 300m/h,600m/h and 900m/h on longitudinal resolution.Meanwhile,in the recognition of thin interlayer by different NaI(Tl)crystals,The sampling rate and velocity measured by thin interlayer are more close to the real value of thin interlayer.The results are as follows: the larger the sampling rate is,the better the response curve will respond to thin interlayer.However,when the sampling rate reaches a certain value,the sampling rate will not affect the response curve.The larger the velocity measurement is,the less the measured value of thin interlayer can reflect the real value of thin interlayer.According to the above research results,in order to identify the thin interlayer with a high source of 10 cm,the sampling rate should be no less than 20 points /ft and the velocity should be no more than 600m/h for logging,so as to better reflect the true thickness of the thin interlayer.If the 6cm low-source thin interlayer is to be identified,the sampling rate should not be lower than 50 points /ft and the velocity should not be higher than 300m/h for logging,so that the true thickness of the thin interlayer can be better reflected.Based on the above conclusions,the size(length),logging speed and sampling rate of NaI(Tl)crystals were optimized through numerical simulation and experimental verification.In field logging,it is recommended to select NaI(Tl)crystal with the size of φ2.7cm×12cm,with a sampling rate of 50 points /ft and a velocity measurement of 300 m/h.Compared with the longitudinal resolution of the traditional gamma instrument,which is only about 0.2m,the longitudinal resolution of the optimized instrument has been significantly improved,and can reach about 0.1m.The ability to identify thin interlayer is stronger,which makes the measured value of thin interlayer more close to the real value.