随钻脉冲中子—伽马密度测井响应数值模拟

Numerical Simulation on Pulsed Neutron-Gamma Ray Density Logging Response in Logging while Drilling

随钻过程中采用D-T可控中子源和2个NaI晶体探测器系统,记录两个探测器的非弹性散射和俘获伽马射线,采用俘获伽马计数比值进行含氢指数校正后,建立非弹性散射伽马计数比和地层密度的响应关系,从而实现脉冲中子-伽马密度测井.利用蒙特卡罗方法模拟地层条件下非弹性散射和俘获伽马分布,得到非弹伽马计数与地层密度和含氢指数都有关,但近、远探测器俘获伽马计数比反映含氢指数灵敏度高,利用其对含氢指数校正后就可以得到非弹伽马计数与地层密度的关系;通过二元回归方法得出地层密度校正后的响应公式,校正后视密度和真密度值相差很小.研究结果表明,在随钻过程中利用脉冲中子伽马测井方法可以确定地层密度.

The inelastic and capture gamma ray at different spacing can be recorded with the system of D-T pulsed neutron source and two NaI crystal detectors.The response of inelastic gamma ray counting ratio from near and far detectors and formation density can be obtained after hydrogen index was corrected by the capture gamma ray counts ratio.As a result,the formation density can be measured with pulsed neutron-gamma logging method.The distributions of inelastic and capture gamma ray under different formation conditions are simulated using Monte Carlo method,and then the inelastic gamma ray counting is related to formation density and hydrogen index.Owing to the higher sensitivity of capture gamma ray reflecting to hydrogen index than inelastic gamma ray,the response of formation density and the inelastic gamma ray counting ratio from near and far detectors is founded,which hydrogen index is corrected by capture gamma ray counting.In addition,the binary regression method is utilized to get the density response formula,with which the apparent density after correction is very close to the real one.It is concluded that the formation density can be determined by pulsed neutron gamma logging method while drilling.