鄂尔多斯盆地天环北部致密砂岩气藏地层水微观赋存特征

应用铸体薄片观察、场发射扫描电镜、高压压汞、恒速压汞及核磁共振等实验技术，开展储层微观孔喉结构和地层水微观赋存状态研究。研究认为鄂尔多斯盆地天环北部盒8段—山1段储层以粒内溶孔、晶间孔和残余粒间孔为主要孔隙类型，分别占33%、31%和16%，孔隙半径分布在80~300 μm之间，平均值为154.18 μm，喉道半径分布在0.01~1.60 μm之间，主流喉道半径平均值为0.55 μm，为微米级孔隙和纳米级喉道，喉道半径是储层渗流能力的主要控制因素。地层水具有束缚水、毛管水、自由水和吸附水4种微观赋存状态，大孔喉的粒间孔和溶蚀孔内，低压充注呈气水混合状，含气量较高，赋存自由水，较高充注压力下为纯气残余少量膜状吸附水；中小孔喉控制的粒间孔和溶蚀孔内，低压充注下呈现气水混合，含气量低，赋存大量毛管水，高压充注下呈气水混合或纯气，含气量高，赋存少量毛管水；微小孔喉的粒间孔内，低压充注下为纯水，高压充注为气水混合，但含气量低，赋存束缚水；晶间微孔内，低压和较高压力充注下均以纯水为主，为束缚水。4种地层水微观状态的孔喉半径截止值分别为0.10 μm、0.26 μm和0.28 μm，渗透率截止值分别为0.21×10^-3 μm²、0.51×10^-3 μm²和0.55×10^-3 μm²，孔隙度截止值分别为5.86%、7.99%和8.18%。启动压力梯度和小于0.10 μm的孔喉是地层水微观赋存状态和残余含水饱和度的主控因素，在天然气成藏过程中，随气驱水强度增大，大孔喉控制的地层水百分比逐步降低。研究区自由水占50%，毛管水占18%，束缚水占30%，吸附水占2%，残余含水饱和度为32%左右。

Micro-occurrence of formation water in tight sandstone gas reservoir of north Tianhuan in Ordos Basin

In this paper， the casting thin section observation， field emission scanning electron microscopy， high pressure mercury injection， constant velocity mercury injection and nuclear magnetic resonance were used to study the pore-throat microstructure and formation water microscopic occurrence in the Upper Paleozoic tight sandstone gas reservoir in northern Tianhuan. The results show that the main pore types in the reservoir of the He8-Shan1 members in the north of Tianhuan are intragranular pore， intergranular pore and residual intergranular pore， the proportions were 33%， 31% and 16%， respectively. The distribution of pore radius is between 80 μm and 300 μm， with an average of 154.18 μm， the distribution of throat radius is between 0.006 μm and 0.598 μm， the average of main throat radius is 0.552 μm. It is micron pore and nano throat controlled reservoir， and throat radius is the main controlling factor of reservoir seepage ability. Formation water has four microscopic occurrences： Bound water， capillary water， free water and adsorbed water. In the intergranular pore and dissolution pore controlled by the macro-pore throat， the mixture of gas and water was found under the low-pressure charging， with high gas content and free water， a small amount of membrane adsorption water was found under the high-pressure charging； In the intergranular pore and solution pore controlled by the small pore throat， the mixture of gas and water under the low-pressure charging presents low gas content and a large amount of capillary water， while the mixture of gas and water or pure gas under the high-pressure charging presents high gas content and a small amount of capillary water； In the intergranular pore controlled by the tiny pore throat， the low-pressure charging is pure water， and the high-pressure charging is gas-water mixture， but the gas content is low and the bound water occurs； In the intergranular micropore， pure water is the main component under low pressure and high pressure charging， which is the bound water. The cut-off values of pore throat radius of the four micro-occurrence of formation water are 0.10 μm， 0.26 μm and 0.28 μm， the cutoff permeability values are 0.21×10^-3 μm²， 0.51×10^-3 μm² and 0.55×10^-3 μm²， the cutoff values of porosity are 5.86%， 7.99% and 8.18%， respectively. Starting pressure gradient and pore throat less than 0.10 μm are the main controlling factors of formation water micro-occurrence and residual water saturation. In the process of natural gas accumulation， the percentage of formation water controlled by large pore throat decreases gradually with the increase of gas displacement intensity. In this study area， the free water is 50%， capillary water is 18%， bound water is 30%， adsorbed water is 2%， and residual water saturation is about 32%。