致密砂岩气藏充注模拟实验及气藏特征——以川中地区上三叠统须家河组砂岩气藏为例

致密砂岩气藏岩石孔喉细小、孔隙结构复杂、含水饱和度较高且主控因素不明,制约了对天然气规模成藏机制的认识和气水分布的预测。为此,以四川盆地中部（以下简称川中地区）上三叠统须家河组致密砂岩气藏为研究对象,应用基于低场核磁共振与高压驱替装置有机结合的模拟实验设备,开展致密砂岩在不同驱替压力下气驱水过程的在线动态模拟,研究不同压力下气、水在岩石中的赋存及流动特征,定量表征流体饱和度与充注压力、岩石孔径等的关系,探讨致密砂岩气富集机理。研究结果表明：①决定须家河组气藏含气饱和度高低的储集主体是孔径介于0.1～10.0 µm的储集空间;②含气饱和度总体上有随孔隙度和渗透率增大而升高的趋势,孔渗条件相近时,含气饱和度高低主要受控于孔径大于1.0 µm的储集空间,大孔径占比越高,含气饱和度越大;③须家河组致密砂岩在3.0～5.5 MPa充注压力下达到总含气饱和度的70%,此后随充注压力增大,含气饱和度增幅缓慢且总量小;④“小压差驱动、相对大孔径空间储集的耦合”是低生气强度区致密砂岩形成较高含水饱和度大中型气田的重要因素。结论认为,川中地区须家河组储层“孔径小、生气强度低、近源聚集”的特点决定了其主要以小压差驱动、相对大孔径空间储集,天然气可以规模富集成藏,但储层含水饱和度高。

Charging simulation experiment and characteristics of tight sandstone gas reservoirs: a case study of the Upper Triassic Xujiahe Formation sandstone gas reservoir in the central Sichuan Basin

Tight sandstone gas reservoir has small pore throat, complex pore structure and high water saturation with the main controlling factors unknown, which makes it difficult to investigate the large-scale hydrocarbon accumulation mechanism of gas reservoir and predict gas and water distribution. By taking the tight sandstone gas reservoir of Upper Triassic Xujiahe Formation in the central Sichuan Basin as the research object, on-line dynamic simulation was carried out on gas-driving-water process in tight sandstone under different displacement pressures using the simulation experimental equipment based on the organic combination of low-field NMR and high-pressure displacement device. Then, the occurrence and flow characteristics of gas and water in rocks under different pressures were investigated, and the relationships between fluid saturation and charging pressure and pore diameter were characterized quantitatively. Finally, the enrichment mechanisms of tight sandstone gas were discussed. And the following research results were obtained. First, the main reservoir body controlling the gas saturation of Xujiahe Formation gas reservoir is the reservoir space with a pore diameter of 0.1–10.0 μm. Second, on the whole, gas saturation presents an increasing trend with the increase of porosity and permeability. When porosity and permeability are similar, gas saturation is mainly controlled by reservoir space with a pore diameter larger than 1.0 μm. And the larger the proportion of large pore diameter, the higher the gas saturation. Third, the gas saturation of the Xujiahe Formation tight sandstone reaches 70% of the total value under a charging pressure of 3.0–5.5 MPa and then it increases slowly and the total increase is small with the increase of the charging pressure. Fourth, "coupling of small differential pressure drive and reservoir space with relatively large pore diameter" is an important factor for the formation of large- and medium-sized gas fields with high water saturation in the tight sandstone of low gas generation intensity region. In conclusion, due to the characteristics of "small pore diameter, low gas generation intensity and proximal accumulation", the Xujiahe Formation reservoir in the central Sichuan Basin is mainly driven by small pressure difference and has the reservoir space with relatively large pore diameters. Natural gas can be enriched and accumulated into reservoirs on a large scale, but the reservoir has high water saturation.