化学刺激法提高花岗岩类岩石裂隙渗透性的实验研究

Expansion of fracture network in granites via chemical stimulation:a laboratory study

增强型地热系统(EGS)用于通过人工形成地热储层的方法从深部低渗透性岩体中开采地热能;国际上常采用水力压裂辅以化学刺激的方法改造EGS储层以提高其渗透率。本文以采自青海共和盆地的花岗闪长岩样品为对象,选用3种不同化学刺激剂(氢氧化钠、盐酸和土酸),在3组不同注入流速条件下开展了系统化学刺激实验。结果表明:注入盐酸和土酸后样品渗透率均有提高,且采用土酸时渗透率提高幅度明显大于盐酸;但注入氢氧化钠后,样品渗透率反而降低。在3类化学刺激剂中,土酸对长石类矿物的溶蚀能力最强,而氢氧化钠溶液对石英的溶蚀能力最强,但氢氧化钠溶液在溶解岩石样品裂隙表面矿物后极易形成非定形态二氧化硅或非定形态铝硅酸盐蚀变矿物并阻塞裂隙,反而对化学刺激效果造成负面影响。总体来看,土酸是青海共和盆地干热岩体的最佳化学刺激剂。在中等注入速度(3mL·min^-1)条件下,土酸对岩石样品的溶蚀程度就可达到最高;在此基础上进一步降低流速,则可能使溶解组分更易从液相中沉淀而充填于样品裂隙,导致样品渗透率有所下降。

Enhanced geothermal system(EGS)has been used to extract heat from deep hot dry rock with low permeability by creating an artificial geothermal reservoir.Hydraulic fracturing,along with chemical stimulation,was usually adopted to improve the permeability of a target EGS reservoir.In this study,we collected granodiorite samples from the Gonghe basin of Qinghai Province and subjected them to a systematic chemical stimulation test by use of three chemical agents(sodium hydroxide,hydrochloric acid and mud acid)at three different injection rates.The results show that the permeability of the rock samples increased upon injection of hydrochloric acid or mud acid,with the latter bringing larger increases;whereas the application of sodium hydroxide solution reduced the permeability.Among the three chemical agents,mud acid exhibited the strongest ability of dissolving feldspar minerals in granodiorite;in contrast,quartz was eroded most severely by sodium hydroxide solution.Nevertheless,during the chemical stimulation test using sodium hydroxide solution,we saw amorphous silica or aluminosilicates precipitation due to excess dissolution of the primary minerals on the fracture surface;and precipitation resulted in micro-fracture filling which definitely had a negative effect on improving permeability.Thus,in general,mud acid is the best chemical agent for the target hot dry rock in this study.At a moderate injection rate(3 mL·min^-1),mud acid can most effectively erode granodiorite samples and remarkably enhance sample permeability.Lowering injection rates,however,could cause secondary mineral precipitation and fracture filling therefore lowering sample permeability.