基于铀裂变中子探测的页岩气有机碳和含气量测井理论基础研究

Shale gas reservoirs have the characteristic of extremely low porosity and permeability, strong heterogeneity and self-contained source-reservoir. Organic carbon content and gas content are the core evaluation content. However, the present logging technology cannot address the need of shale gas exploration. It has become important to develop the pulsed neutron logging technology which can accurately measure organic carbon content and gas content simultaneously. Our approach consists of theoretical analysis, numerical simulation and laboratory measurement. Based on the diffusion theory of uranium fission neutron and slowing-down neutron produced by pulsed neutron source, we will study energy, time and spatial distribution of secondary neutron in shale gas reservoir. Through this study, we want to describe the logging mechanism of determining organic carbon content with uranium fission neutron, and the pulsed neutron response characteristics in shale gas reservoir. With the results, we will design a pulsed neutron measuring system composed of a D-T pulsed neutron source, epithermal neutron and thermal neutron detectors. The analysis will develop a multi-parameter joint inversion algorithm with the count rates and neutron lifetime of fission neutron and slowing-down neutron, and a computation method for optimizing the key parameters of the logging instrument. The numerical simulation and physical experiment will further rationalize designed logging method and optimized instrument parameters. The purpose of this project is to propose an implementation scheme of pulsed neutron logging technology, determining organic carbon and shale gas content simultaneously. The project results will provide a theoretical foundation for the development of pulsed neutron technology in shale gas reservoir.

页岩气储层具有极低孔渗、非均质性强且“源储一体”的特征，有机碳含量和含气量是其核心评价内容，现有测井技术无法满足页岩气勘探开发的要求，能同时准确测量有机碳含量和含气量的脉冲中子测井方法成为研究的关键。本项目拟采用理论分析、数值模拟和实验测量相结合的研究方法，以脉冲中子源在地层中产生的铀裂变中子场和慢化中子场分布理论为基础，研究页岩含气地层中中子能量、时间、空间分布规律，阐明利用铀裂变中子确定有机碳含量的测井响应机理，明确页岩气储集层的脉冲中子响应特征，设计由D-T脉冲中子源、超热中子和热中子探测器组成的脉冲中子测量系统，建立裂变中子和慢化中子计数率、寿命等多参数联合反演算法及仪器参数优化方法，结合数值模拟和物理实验，进一步验证测井方法和仪器关键参数的合理性，最终提出能同时测量页岩气有机碳含量和含气量的脉冲中子测井实施方案，为适用于页岩气层的脉冲中子测井仪器研发奠定理论基础。

页岩气储层具有极低孔渗、非均质性强且“源储一体”的特征，有机碳含量和含气量是其核心评价内容。.本项目采用理论分析、数值模拟和实验测量相结合的研究方法，以脉冲中子源在地层中产生的铀裂变中子场和慢化中子场分布理论为基础，研究页岩含气地层中中子能量、时间、空间分布规律，阐明利用铀裂变中子确定有机碳含量的测井响应机理，构建了一种基于“中子直接测铀”技术的页岩气层有机碳含量评价方法。考虑实际测井环境，提出不受地层中子吸收特性和中子慢化能力影响的铀含量评价参数-含铀指数，设计了能实现含铀指数测量的测井方法。利用“中子直接测铀”技术得到页岩地层含铀量信息，结合铀含量和有机碳在页岩（尤其是黑色页岩）中共生共储的特性，在探测铀裂变中子的基础上，为实现铀资源和页岩油气资源评价的共同勘探开发提供新的技术方案。.针对低孔低渗页岩气层中含气量评价存在的困难，明确页岩气储集层的脉冲中子响应特征，基于气和油、水在中子作用特性方面的差异，优选含气响应灵敏的中子物理参数，提出了一种高含气响应灵敏度的测井评价参数，定义为D值。利用蒙特卡罗数值模拟方法，研究含气饱和度的测井响应，建立不同井眼、地层环境影响关系，对于确定页岩气层含气饱和度具有重要意义。.设计由D-T脉冲中子源、超热中子、热中子探测器和伽马探测器组成的脉冲中子测量系统，及源工作时序和探测器测量时序，研制了适用于铀-页岩油气共同勘探开发的脉冲中子测井仪器样机，并在铀矿模型井中进行了实验验证，实验结果证实了铀含量计算的可行性，在此基础上，进行了测井仪器工作参数的优化设计，为下一步实现铀资源和页岩油气资源共同勘探开发的新测井仪器研发提供了理论基础和技术支撑。