页岩气储层核磁共振测井理论基础与响应机理研究

Nuclear magnetic resonance (NMR) logging is an effective method for detecting the underground oil and gas resources, it can better solve many problems of complex reservoir well logging evaluation, which has been widely recognized in the oil industry. But the special fluid occurrence state of shale gas reservoirs, nanometer pore, rich in clay minerals and paramagnetic pyrite make the existing NMR logging theory and petrophysics experimental methods facing many challenges. The fluid diffusion in nanometer pore shows restricted diffusion, paramagnetic minerals and micropores result in the dramatic decay of NMR signal in shale gas reservoirs, NMR response characteristics is not clear, these are the basic scientific problems. This application studies the theoretical foundation of NMR logging in shale gas reservoirs surrounding the above basic scientific issues. It attempts to solve these problems from four aspects: digital core construction in the nanometer pore scale, shale gas restricted diffusion mechanism, the distribution of internal magnetic field gradient and NMR logging response characteristics in shale gas reservoirs. This study tries to develop new theories and methods of NMR logging in the shale gas reservoirs to promote the progress of exploration technology in unconventional oil and gas reservoirs.

核磁共振测井是一种有效的地下油气资源探测方法，已经得到石油工业界的广泛认可，较好地解决了复杂储层测井评价的许多疑难问题。但是页岩气储层特殊的流体赋存状态、纳米级孔隙、富含黏土矿物及顺磁性黄铁矿，使得现有的核磁共振测井理论和岩石物理实验方法面临许多挑战，纳米级孔隙中流体扩散表现为受限扩散、顺磁性矿物及微小孔隙造成页岩气储层核磁共振信号的急剧衰减、核磁共振响应特征不明确等是其基本科学问题。本申请围绕页岩气储层核磁共振测井的基本科学问题开展理论基础研究，试图从页岩气储层纳米孔隙尺度下数字岩心的构建、页岩气储层孔隙流体受限扩散机理、内部磁场梯度分布、以及页岩气储层核磁共振测井响应特征等四个方面开展研究，发展页岩气储层核磁共振测井新理论和新方法，推动非常规油气资源探测技术的进步。

页岩气储层是重要的非常规油气储层之一。页岩储层岩石致密，岩石物理实验难度大，且耗时长。页岩储层纳米级孔隙中流体受限扩散显著，岩石中富含顺磁性矿物，存在较强的内部磁场梯度。核磁共振测井数据信噪比低，使得核磁共振T2反演结果偏离地层真实情况，现有的核磁共振测井理论和方法面临挑战。本研究围绕页岩储层核磁共振测井面临的难题开展理论基础研究，构建了页岩储层三维数字岩心，利用随机游走方法模拟了孔隙大小对流体受限扩散的影响，在二维D-T2分布图中给出了油、气和水的扩散系数与横向弛豫时间的关系曲线，并分析了岩石表面弛豫率和胶结指数对润湿相流体受限扩散系数线位置的影响。利用等效磁偶极子理论研究了页岩储层内部磁场梯度，提出了一种消除岩石内部磁场梯度对T2分布影响的方法。发展了提高核磁共振数据信噪比的降噪方法，优选出小波包域自适应滤波方法用于低信噪比核磁共振数据的降噪处理。针对核磁共振数据量大，反演速度慢，分别提出了联合压缩方法和基于主成分分析的核磁共振回波数据的压缩方法。对比分析了不同的正则化参数选取方法选取的最优正则化参数对核磁共振数据反演结果的影响，提出了一种基于求解最大熵目标函数的低信噪比核磁共振数据反演新方法。