ExperimentalStudyon theInfluenceof Supercritical CO₂ on Shale Fracture Morphology

Wu Hua’Wang Xiaoqiong'Ge Hongkui1Wang Haige²He Jixiang³Zhang Yanlong1，²Wang Jianyong' （1.UncomventionalRetroleumResearchInstitute，ChinaUniersityofPetroleum（Beijing）；2.CNPCEngineeringTechnologyamp; DCompanyLimited;3.ExplorationandDevelopmentResearch Institute，PetroChina Xinjiang OilfieldCompany）

Abstract： In the ahead fracturing process of supercritical carbon dioxide （SC-CO ² ），the injected CO ² is easy to enterthe porous media ofshale，and combined with the formation water or water-based fracturing fluid to form a slightlyacidic fluid，thus stimulating the initiationand propagationof shale microfractures.Toclarifythe influence of CO₂ on the initiation and propagation of shale microfractures，a combined experiment of pressurized soaking and uniaxial compression was cariedout，and the influence offluid type，soak time and soak sequence on the initiation and propagation of shale microfractures was studied. The study results show that the ahead injection of CO₂ can promote the opening of bedding fractures，and the longer the CO₂ action time，the more bedding fractures are opened. When the pressure drops，the bedding fractures opened by SC-CO ² are prone to close. However，when the“SCCO2→water”composite action occurs，water easily enters the bedding fractures opened by CO to form effective support，reacts with CO ² to generate carbonic acid to dissolve carbonate minerals，weakens the mechanical strength of shale，promotes the propagation of microcracks，and forms a complex and stable fracture network.When the \"water + SC-CO ” composite action occurs，water infiltrates into the small pores of shale under the action of pres2 suredifferenceandcapillry force，formingalarge amountofwaterfilmon the porewall，seriouslyhindering thediffusion of CO ² into the shale，and making it difficult to effectively open the bedding fractures.Moreover，the fracture propagation of vertical bedding samples is mainly controled byloading stress，and the influence offluid is weak. Aftercracking，atilted fracture surfacethat penetrates the bedding is generall formed.The study results providea theoretical basis for optimizing the technological parameter design of CO₂ ahead fracturing of shale，and are of great significance for improving the development efciency of shale oil and promoting the geological storage of CO ² ：

Keywords： pre-fracturing;supercritical CO ² ; shale oil; CO₂ soak mode；soak sequence；uniaxial compression; fracture morphology

0 引言

中国页岩油资源丰富，是重要的战略接替资源[1]，但其开发受限于水平井 + 体积压裂技术的局限性，面临压力衰减快、稳产期短、采收率低等问题。矿场试验结果表明， CO₂ 前置压裂技术结合了超临界 CO₂ （ SC-CO₂ ）的物理化学作用与水力压裂的机械作用，可显著提高储层裂缝复杂性和原油流动性，从而提升采收率并实现 CO₂ 地质封存[2-5]。然而， SC-CO₂ 前置注人增强裂缝复杂性的机理尚不明确，亟需深入研究。

近年来，随着双碳目标的提出，国内学者围绕非常规储层 CO₂ 提采机理开展了大量研究，包括 CO₂ 与原油相互作用、 CO₂- 水-岩相互作用以及CO₂ 压裂造缝机理等。苏玉亮等通过高压PVT试验发现， CO₂ 注入可增加原油密度、降低原油黏度并提升地层压缩性。宋兆杰等［7研究了页岩油与 CO₂ 相间传质规律，揭示纳米限域效应对 CO₂ 与原油混相行为的促进作用。致密砂岩和页岩储层的 CO₂⋅ -水-岩反应试验表明， SC-CO₂ 作用下岩石力学性质显著劣化，其中碳酸盐岩矿物（如方解石、白云石）的溶蚀是关键机制，溶蚀后孔隙度和渗透率增加，岩样表面粗糙度增大，抗张强度降低［8-11]。LIL.等[12]的研究指出， SC-CO₂ 注入后可能因溶蚀作用增大孔渗，也可能因次生矿物沉淀堵塞孔喉而降低孔渗。易勇刚等[13]发现，玛湖砾岩岩心的矿物溶蚀主要发生在前5天内，之后溶蚀量显著减少。叶亮等[14通过真三轴压裂试验发现，致密砂岩 sc-CO₂ 压裂的破裂压力最低，其次为液态 CO₂ ，分别比滑溜水压裂降低28.2% 、 22.1% 。刘卫彬等[15研究表明，SC-CO浸泡后页岩的破裂压力比未浸泡页岩降低 45.7% 。……