酸性压裂液防治低渗煤层水锁损害实验研究

Mitigating water blockage in low-permeability coal seam by acid-based fracturing fluid

压裂液易渗吸进入煤基质,大幅增加水力裂缝-煤基质界面水饱和度,严重降低甲烷气体产出速率,为此防治水锁损害是提升煤层压裂改造效果的重要手段。以阜新盆地刘家区低煤阶煤层气富集区为研究背景,选用阜新组富含方解石(体积占比约4.5%)低渗煤样与质量分数6%乙酸溶液,测试了水相返排过程原煤样与酸化煤样气相渗透率变化规律,并结合扫描电镜、核磁共振岩心分析及CT成像,分析了方解石溶蚀对煤样渗流通道与毛管力影响。结果表明:(1)原煤样孔缝壁面倾向于强亲水,水相易渗吸侵入,加之煤基质储集空间以直径<100 nm的微、小孔隙为主,导致煤基质存在高毛管阻力,不利于渗吸水相返排,测试表明0.1,0.5 MPa气体驱替压差下仅少量水被返排出,滞留水饱和度大于90%,造成气相渗透率降低90%以上,当驱替压差增加至2.0 MPa(接近阜新组煤层最大返排压力梯度),部分煤样水锁损害率仍大于60%;(2)质量分数6%乙酸渗吸流动过程能够有效解除方解石对天然裂缝的充填堵塞,使煤岩心内部形成低毛管力、高渗透率的裂缝性酸溶蚀通道,为此0.1,0.5 MPa气体驱替压差即可有效返排主要渗流通道中的渗吸水相,返排过程气体渗透率达(6.9~82.3)×10^(-15)m^(2),较酸化前增加10~79倍,有效实现了防水锁与增渗目的;(3)借助酸溶蚀孔缝扩展延伸数值模拟、岩心柱尺度渗流实验等评估结果,优化压裂液中乙酸浓度、酸化溶蚀时间,适量解除方解石对煤层裂缝的充填堵塞,在煤层水锁损害带形成数量合理的低毛管力、高渗透率的裂缝性渗流通道,可最大程度防治水力裂缝-基质界面水锁损害,并降低酸溶蚀对煤体强度及煤粉运移影响。对于富含方解石的低渗煤层,采用乙酸改性的活性水压裂液,既可防治压裂液渗吸诱发的水锁损害,又不污染、破坏煤质,有利于煤层气抽采后煤炭绿色开采。

Fracturing fluid is easy to imbibe into coal matrix, which significantly increases the water saturation within the hydraulic fracture-coal matrix interface and severely reduces the gas permeability. Therefore, it is important to remove water blockage for improving gas production in hydraulically fractured coal seams. Coal samples, rich in fracture-filling calcite(The volume fraction is 4.5%),were collected from Liujia District of Fuxin Basin to evaluate the effect of 6%(mass fraction) acetic acid induced calcite dissolution on imbibition water removal and gas permeability recovery. The flow channels and capillary pressure alteration caused by calcite dissolution were analyzed by SEM,NMR characterization and CT imaging. The results show that:(1) The pore surfaces of original coal samples tend to be strongly hydrophilic, and thus the water is easy to invade. The storage space of coal matrix is dominated by micro and small pores of less than 100 nm in diameter, which leads to high capillary resistance in coal matrix and is not conducive to the recovery of imbibition water. The test shows that only a small amount of water is returned under the displacement pressure difference of 0.1 and 0.5 MPa, and the saturation of retained water is greater than 90%,resulting in a more than 90% reduction of gas permeability. When the displacement pressure difference increases to 2.0 MPa(close to the maximum flowback pressure gradient of Fuxin Group coal seam),the water lock damage rate of some coal samples is still greater than 60%.(2) The injection of 6% acetic acid can effectively remove the fracture blockage caused by calcite, so that the acid dissolution channels with low capillary force and high permeability can be formed in the coal core samples. Then the imbibition water in the main flow channels can be effectively returned under the displacement pressure difference of 0.1 and 0.5 MPa. The gas permeability during the imbibition water recovery reaches(6.9-82.3) ×10^(-15)m^(2),which is 10-79 times higher than that before acidizing, effectively achieving the purpose of mitigating water lockage and increasing permeability.(3) With the help of numerical simulation of acid dissolution pore extension and core-scale seepage experiments, the acetic acid concentration in the fracturing fluid and acid dissolution time should be optimized to form a reasonable number of seepage channels with low capillary force and high permeability in the coal seam water lock damage zone, so as to prevent water lock damage within the hydraulic fracture-matrix interface to the maximum extent and reduce the influence of acid dissolution on coal strength and coal fines migration. For calcite-rich low-permeability coal seams, the use of acetic acid-modified fracturing fluid can mitigate water blockage in coal matrix without coal quality pollution, which is beneficial to the co-mining of coal and methane.