高压条件对页岩吸附水赋存特征的影响:以重庆涪陵礁石坝地区页岩储层为例

Influence of high-pressure conditions on the occurrence characteristics of shale adsorbed water: A case study of a shale reservoir in the Jiaoshiba area, Fuling, Chongqing

页岩普遍含水,明确页岩吸附水的赋存特征对提高页岩气的排采效果具有重要意义。以重庆涪陵焦石坝地区JY11-4井和JY41-5井页岩岩心为研究对象,通过自主设计页岩冷冻氮吸附实验和核磁共振实验流程,分析了高压条件对页岩吸附水赋存特征的影响。结果表明:(1)常压条件下,由“称重法”计算得到的样品单位质量吸附水体积平均值0.017 3 mL/g。赋存于微孔和介孔中的水体积总占比(平均值90.94%)明显高于赋存在大孔中的水体积占比(平均值9.06%),这可能与相对压力较小时,水分子无法占据页岩所有孔隙中的吸附位点,大部分水分子凝聚在微孔和介孔中,只有较少的水分子进入大孔中,以及富黏土页岩在水吸附过程中小孔隙被水分子“充填堵塞”有关。(2)30 MPa饱和水压力条件下,由“称重法”计算得到的样品单位质量吸附的水体积平均值为0.021 6 mL/g。赋存于微孔和介孔中水体积总占比(平均值40.26%)低于赋存在大孔中的水体积占比(平均值59.74%),这可能与相对压力显著升高时,水分子在毛细管力的作用下占据微孔和介孔内表面的吸附位点后,仍能占据更多大孔内表面的吸附位点有关。(3)相比于常压,高压条件会导致页岩单位质量吸附的水体积增加(实验中约增加25%)、大孔中的水体积占比高于微孔和介孔中的水体积总占比。(4)注水压裂时,储层相对压力显著升高,压裂液在毛细管力的作用下可能进入之前在原始储层压力下未能进入的大孔中来“缓解”原始页岩储层的“非饱和状态”。压裂完成后,储层周围压力逐渐被释放,原先进入页岩吸附孔隙中的压裂液可能难以克服其孔喉处的毛管阻力而难以返排。

[Objective]Shale generally contains water,and it is highly important to clarify the occurrence characteristics of adsorbed water in shale to improve the drainage effect of shale gas.[Methods]This study analysed shale cores from the JY11-4 and JY41-5 wells in the Jiaoshiba area of Fuling.The influence of high-pressure conditions on the occurrence characteristics of adsorbed water in shale was investigated using custom-designed frozen nitrogen adsorption experiments and nuclear magnetic resonance experiments.[Results and Conclusion]The results show that(1)under atmospheric pressure,the average volume of adsorbed water per unit mass calculated by the"weighing method"is 0.0173 mL/g.The volume proportion of water in the micropores and mesopores(average of 90.94%)significantly exceeds that in the macropores(average of 9.06%).This may be because water molecules cannot occupy the adsorption sites in all the shale pores when the relative pressure is relatively low.Most water molecules condense in micropores and mesoporous pores,and only a few water molecules enter the macropores.In addition,the small and medium pores in clay-rich shale become"filled and blocked"by water molecules in the process of water adsorption.(2)Under a saturated water pressure of 30 MPa,the average adsorbed water volume per unit mass of the sample calculated by the"weighing method"is 0.0216 mL/g.The volume proportions of water in the micropores and mesopores(average of 40.26%)is lower than that in macropores(average of 59.74%),which may be related to the fact that water molecules can occupy more adsorption sites on the inner surface of micropores and mesoporous pores under the action of capillary forces when the relative pressure is significantly increased.(3)Compared with normal pressure,high-pressure conditions increase the volume of water adsorbed per unit mass of shale(approximately 25%increase in the experiment),and the proportion of water volume in large pores is greater than that in micropores and mesoporous pores.(4)During hydraulic fracturing,the relative pressure of the reservoir increases significantly.Under the action of capillary forces,fracturing fluid may enter large pores that cannot enter under the pressure of the original reservoir to"relieve"the"unsaturated state"of the original shale reservoir.After fracturing is completed,the pressure around the reservoir is gradually released,and the fracturing fluid that originally enters the adsorption pores of the shale may have difficulty overcoming the capillary resistance at the pore,making it difficult to return.