一种利用中子瞬发伽马能谱确定含油饱和度方法及数值模拟

Study on Oil Saturation Determining Method Based on Prompt GammaSpectrum Induced by Neutron and It’s Numerical Simulation

剩余油饱和度的定量解释对于提高油气产能及指导油田后期开发具有重要意义。脉冲中子饱和度测井通过记录地层中C、O原子核与高能中子作用产生的特征伽马射线来确定剩余油饱和度。骨架矿物组成和粘土矿物类型等因素的变化会使剩余油饱和度解释结果存在误差。为确定岩性对非弹能谱中碳窗和氧窗伽马计数比(C/O)、俘获能谱中硅窗和钙窗伽马计数比(Si/Ca)的影响,采用蒙特卡罗方法(MCNP)模拟不同岩性、孔隙度和饱和度地层条件下的伽马能谱。模拟结果显示:传统碳氧比解释模型进行碳氧比差值计算时未考虑岩性影响,是造成剩余油饱和度解释存在误差的主要原因。通过在碳氧比差值计算时引入岩性校正因子,提出一种改进的碳氧比解释模型,提高饱和度解释精度。模拟算例和实测结果表明,该改进碳氧比解释模型满足砂岩地层饱和度解释精度要求的同时,在混合岩性地层计算饱和度误差小于10%。

The quantitative interpretation of remaining oil saturation has a great significance for improving oil production and guiding the follow-up development of oil fields.The characteristic gamma rays are recorded to determine the remaining oil saturation,which is from the interaction of high energy neutron emitted by the pulsed neutron logging tool and the element of carbon and oxygen.The interpretation results of the remaining oil saturation are affected by the mineral composition of rock and clay types.In order to understand the influence of lithology on the inelastic gamma ratio of carbon to oxygen and the capture gamma ratio of silicon and calcium,the MCNP was employed to obtain the gamma energy spectrum response of formation with different lithology,porosity and saturation.The results show that when the difference of C/O is calculated by the traditional interpretation model,the influence of lithology on C/O difference results in the interpretation error of remaining oil saturation.So an improved interpretation model was proposed to enhance the precision of saturation by introducing the lithological effects.Meanwhile,the results of the simulation example and field example show that the improved C/O interpretation model guarantees the saturation interpretation accuracy requirements of the sandstone formation and the error of calculation saturation is less than 10%in the mixed formation.