砂岩高温超临界CO_2水岩模拟实验及其地质意义

在中国东部的含油气盆地中发育有较多的高含CO_2天然气藏,其富含的CO_2为深层高温CO_2气体沿断层向上运聚而成。在高温CO_2的运聚过程中,与之接触的砂岩储层会发生超临界CO_2水岩反应,过去对这方面的研究报道较少。为此,利用采自柴达木盆地冷湖构造带南八仙气田新近系的砂岩样品,开展了超临界CO_2—水—砂岩反应模拟实验(温度为150℃、压力为15MPa)。结果表明:①高温超临界CO_2侵入后,主要对砂岩中的方解石、黏土、石英等矿物产生重要影响,而对长石的影响较小;②酸性流体环境导致方解石不仅被大量溶蚀,还经历了由溶蚀→单向短锥形生长→双向长锥形复三方偏三角面体生长的过程;③黏土矿物含量明显减少,石英含量则明显增加,原因在于形成了较多的次生石英,其形成演化经历了微晶石英→他形石英→柱状或团块状石英的过程;④长石含量变化很小,未发现长石明显溶蚀的现象,反而存在钠长石含量的微弱增加。结论认为:高温超临界CO_2水岩反应会推进火山侵入岩围岩中砂岩储层的致密化。

High-temperature supercritical CO2 water-rock simulation experiment of sandstone and its geological significance

The petroliferous basins in the eastern China are reported to contain CO₂-rich gas reservoirs. The CO₂ originated from the high-temperature CO₂ gas that migrated upward from the deeper strata via the faults. During the migration and accumulation of high-temperature CO₂, supercritical CO₂ water-rock reaction occurred in sandstone reservoirs that were in contact with CO₂, which was rarely studied or reported before. Thus, Neogene sandstone samples of the Nanbaxian Gasfield located within the Lenghu structural belt in the Qaidam Basin were utilized to conduct a supercritical CO₂ water-sandstone reaction simulation experiment (under 150℃ and 15 MPa). The results suggest that: ① as high-temperature supercritical CO₂ invades, some minerals in sandstone (i.e., calcite, clay and quartz) are significantly influenced, but feldspar is less influenced; ② under an acidic flow environment, calcite is greatly dissolved and experiences a process from dissolution to unidirectional short-taper growth to bi-directional long-taper ditrigonal scalenohedron growth; ③ clay mineral content reduces and quartz content increases remarkably, as significant amounts of secondary quartz are formed during the evolution process from microcrystalline to xenomorphic to prismatic/crumby quartz; and ④ the feldspar content varies little with no obvious feldspar dissolution , but a slight increase of albite. It is concluded that, high-temperature supercritical CO₂ water-rock reaction can aid the densification of sandstone reservoirs surrounded by volcano-intrusive rocks.