Department of Civil Engineering, Tsinghua University

Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences

地热井经常面临初期产能不足或运营期产能下降等生产性难题，酸化压裂作为一种柔性激发技术，是深层碳酸盐岩热储增产改造的首选技术，其基本原理是通过地热井向热储层注入化学压裂液，利用化学溶蚀作用来提高岩体基质孔隙率和裂隙开度。由于地热领域的酸化压裂工艺以借鉴油气领域的成熟经验为主，尚未形成完整的技术体系，特别是针对深部热储层增产的设计方法、工艺参数优选、预测模拟、效果评价等方面缺乏理论指导。基于裂隙岩体热水力化多场耦合理论框架，通过引入裂隙刚度和最大闭合量的酸蚀修正系数，提出了考虑力学与化学耦合作用的非线性裂隙本构模型，发展了适合于深层碳酸盐岩热储酸化压裂增产改造的数值模拟方法。依托雄安新区D22井酸化压裂增产改造现场试验，验证了所提出的模拟方法的正确性，研究了射孔位置、质量分数、排量等酸化压裂施工参数对增产改造效果的影响规律。结果表明：深层碳酸盐岩热储酸化压裂增产改造主要发生在初始开度较高的裂隙中，且酸蚀裂隙逐渐沟通了周边高渗储层，形成了酸化改造网络；增产改造效果不仅与酸−岩反应和水压作用下裂隙开度增加量显著相关，也取决于改造后裂隙的闭合路径；射孔位置距离高渗储层越近，酸化压裂改造效果越好；高浓度、大排量的施工方案有助于形成长裂缝，有利于连通高渗储层，酸化压裂改造效果更好。

Geothermal wells often suffer from production difficulties such as disappointing injectivity/productivity right from the start or declining injectivity/productivity during long-term operation, Acid fracturing, as a soft stimulation technology, is becoming the preferred technology to stimulate the carbonate geothermal reservoir of low permeability in large depth. The basic mechanism of acid fracturing is to inject acid into the carbonate geothermal reservoir through geothermal wells, and then the matrix porosity and fracture apertures can be significantly improved through acid corrosion. Since the acid fracturing technology in geothermal reservoirs is mainly based on the experiences obtained from oil and gas fields, a complete technical system has not yet been established. To be specific, its design method and field implementation process lack theoretical guidance. In this paper, two correction coefficients are proposed to describe the acid etching induced changes in fracture normal stiffness and the maximum closure, based on which a nonlinear fracture constitutive model considering the coupled mechanical-chemical mediations on fracture apertures is developed. The developed nonlinear fracture constitutive model is incorporated into the theoretical framework of coupled thermal-hydraulic-mechanical-chemical processes in fractured rock mass, and a numerical modeling method suitable for acid fracturing in deep carbonate geothermal reservoir is developed. Based on the field test of acid fracturing in the geothermal well D22 in Xiong’an New Area, Hebei, China, the accuracy of the developed modeling method is verified, and the influences of human-controlled parameters including perforation position, acid concentration (mass fraction) and injection rate on the stimulation efficiency of acid fracturing is investigated though sensitivity analysis. The results show that acid fracturing mainly occurs within the fractures with high initial apertures in the carbonate geothermal reservoir of low permeability in large depth. The acid-etched fractures gradually propagated into the surrounding reservoir rocks with high permeability, and a stimulated network is formed after acid fracturing. Stimulation efficiency of acid fracturing is dependent on not only the increase in fracture apertures induced by acid etching and hydraulic pressure, but also the closure path of fractures after stimulation. The closer the perforation position is to the reservoir rocks with high permeability, the better stimulation efficiency of acid fracturing. The high acid concentration and injection rate are helpful to stimulate long fractures, by which the reservoir rocks with high permeability may be connected, and thus the better stimulation efficiency of acid fracturing is obtained.

国家重点研发计划资助项目(2019YFB1504103)；国家自然科学基金(联合基金)重点支持资助项目(U2067203)

赵志宏，徐浩然，陈思聪，等. 雄安新区碳酸盐岩热储酸化压裂增产改造数值模拟研究[J]. 煤炭学报，2023，48(7)：2691−2699.

ZHAO Zhihong，XU Haoran，CHEN Sicong，et al. Numerical modeling on acid stimulation in carbonate geothermal reservoir in Xiong’an New Area[J]. Journal of China Coal Society，2023，48(7)：2691−2699.