关井时机对页岩气井返排率和产能的影响

页岩气井压后返排率普遍较低,大量的压裂液永久赋存于储层中,对页岩气井的生产有可能造成不利影响。为此,以实际生产数据为基础,分析了页岩气井早期生产返排特征,并根据典型数据建立相应的数值模型,研究了不同时机关井持续时间、生产制度对页岩气井返排率和产能的影响。结果表明:①在返排前关井期间,极窄的相渗曲线共渗区急剧降低压裂液在储层中的渗吸运移速度,关井100 d后移动距离小于3 m,随着关井持续时间增加,压裂液返排率呈指数降低,开井的初始产气量先减小后增大,对长期产气量的影响则恰好相反,因此,并不能简单得出关井时间越长,越有利于生产的结论 ;②而在生产返排后关井期间,随着关井持续时间增加,返排率减小,开井的初始产气量增大,长期产气量则会减小,但相比之下,返排后关井效应弱于返排前;③对于生产制度而言,生产压差增大会掩盖应力敏感导致的渗透率降低效应,最终表现为累计产气量、累计产水量都增加,同时,高生产压差人工缝底部积液,而低压差含水饱和度则几乎为0。该研究成果为认识压裂液的滤失机理及其在储层中的赋存方式、确定页岩气多段压裂水平井的最佳关井时间与生产制度,提供了技术支撑。

Effects of shut-in timing on flowback rate and productivity of shale gas wells

After fracturing is performed in a shale gas well, the flowback ratio is commonly lower and a large amount of fracturing fluid is permanently detained in the reservoir, leading to an adverse effect on the production performance of the well. In this paper, the early- time flowback characteristics of shale gas wells were analyzed based on the actual production data, and a numerical simulation model was established based on the typical data to study the effects of shut-in timing and production mechanisms on the flowback rate and productivity of shale gas wells. It is shown that in the shut-in period before the flowback, the imbibition migration of fracturing fluid in the reservoir is slowed down sharply because the common percolation zone in the relative permeability curves is extremely narrow. When a shale gas well is shut in for 100 days, the fracturing fluid will migrate less than 3 meters. As the shut-in lasts, the flowback rate of fracturing fluid decreases exponentially, and the initial gas production rate decreases first and then increases. However, the effect on the long-term gas production rate is just opposite to that on the initial ones. Therefore, it is inappropriate to draw a conclusion that longer shut-in time is more favorable for production. In the shut-in period after the flowback, the flowback rate decreases, the initial gas production rate increases and long-term gas production rate decreases as the shut-in continues. The shut-in effect after flowback is weaker than that before flowback. As for the production mechanisms, the drawdown pressure can conceal the reduction of permeability caused by stress sensitivity, which is ultimately presented as the increase of cumulative gas and water production. In addition, liquid loading occurs at the bottom of the hydraulic fractures in the case of high drawdown pressure, while the water saturation is nearly zero in the case of low drawdown pressure. These research results provide a technical support for understanding the filtration mechanism of fracturing fluid and its existence forms in reservoirs and determining the optimal shut-in time of shale-gas horizontal wells with multi-stage fracturing.