鄂尔多斯盆地东部上古生界致密砂岩超低含水饱和度气藏形成机理

The formation mechanism of Upper Paleozoic tight sand gas reservoirs with ultra-low water saturation in Eastern Ordos Basin

鄂尔多斯盆地东部上古生界石盒子组8段、太原组致密砂岩气藏存在超低含水饱和度现象,研究该现象的形成机理,对制定合理的储层保护和压裂改造的技术措施、提高气藏单井产量等均具有重要的意义。为此,采用密闭取心岩样,借助气水相渗、核磁共振、流体包裹体测温等技术手段,分析了致密砂岩气藏的含水饱和度特征和天然气成藏特征。结果表明:①应用气水相渗、核磁共振2种实验技术方法所测定的束缚水饱和度均高于密闭取心测定的原始含水饱和度值,说明该区盒8段、太原组致密砂岩气藏普遍存在超低含水和度现象;②气藏形成过程中由于温度、压力增大,随着烃源岩在过成熟演化阶段干气的注入,天然气的携水能力不断增强,束缚水不断蒸发气化,并随着天然气的大规模运移及气藏后期的泄漏散失而带出储层,从而形成超低含水饱和度气藏。因此,针对超低含水饱和度储层吸水易形成永久性水锁的问题,制订了合理的水锁伤害预防与补救措施,以期有效提高储层气相渗透率和单井产量。

Ultra-low water saturation occurs in the tight sand gas reservoirs of the 8th member of the Shihezi Fm and the Taiyuan Fm of the Upper Paleozoic in the eastern Ordos Basin. The study of its forming mechanism is of great significance to the selection of ra- tional technical measures of reservoir protection and stimulation and to the improvement of single well production. The gas-water rel- ative permeability measurement, NMR and thermometry of fluid inclusions were performed on sealed coring samples to analyze the characteristics of water saturation and hydrocarbon accumulation features of tight sandstone reservoirs. The irreducible water satura- tion measured with techniques of gas-water relative permeability and NMR is higher than the initial water saturation obtained from the sealed coring sample measurement, indicating that ultra-low water saturation is common in the tight sand gas reservoirs in the 8th member of Shihezi and Taiyuan Fins. Along with the increasing of temperature and pressure during the formation of gas reservoirs and the input of dry gas during the over mature stage of source rocks, the water carrying capacity of natural gas gets enhanced progressively. The irreducible water evaporates continuously and gets out of the reservoirs along with large-scale gas migration and gas leakage at later stages, resulting in the formation of ultra-low water saturation gas reservoirs. Considering that permanent water locks are easy to form due to water absorption in ultra-low water saturation gas reservoirs, rational measures can be taken to prevent and remedy water-lock damage, so as to improve the gas phase permeability of reservoirs and single well production.