超临界 CO₂ 酸压过程中碳酸盐岩多尺度力学参数衰减机制

Mechanisms of Multiscale Mechanical Parameter Degradation in Carbonate Rocks During Supercritical CO ₂ Acid Fracturing

摘要

为解决深埋高温低压系数碳酸盐储层常规酸压反应过快、作用距离短、导流能力低及压后返排困难等问题，在 110 ^◦C、30 MPa 条件下，分别以 SC-CO₂、SC-CO₂+ 地层水对马家沟组致密碳酸盐岩进行浸泡，结合 X 射线衍射（XRD）、扫描电镜（SEM）、纳米压痕与单轴抗压测试实验，构建矿物微结构微观和宏观力学的耦合演化机制。结果表明：纯 SC-CO₂ 以去白云石化并伴随方解石二次沉淀为特征；加入地层水后转为碳酸溶蚀主导，表现为方解石择优溶解、白云石相对富集并可能再结晶。浸泡过程呈阶段性演化，早期去白云石化与次生沉淀，中期方解石溶蚀加剧伴随白云石的相对富集与重结晶，后期孔隙化与连通性增强并出现通道化。SEM 显示表面由点蚀向沟槽与孔洞贯通演化，方解石反应性高于白云石。纳米压痕表明硬度与弹性模量呈幂律衰减，早期软化最强，随后短期回弹并再度软化。宏观力学同步劣化，抗压强度与弹性模量显著降低、泊松比升高，承载骨架弱化且塑性增强。

Abstract

To overcome the issues of conventional acid fracturing in deep, high-temperature carbonate reservoirs with low pressure coefficients–namely overly rapid reactions, short effective penetration distances, low fracture conductivity, and difficult post-fracture flowback–this study performed soaking experiments on tight carbonates of the Majiagou Formation at 110 ^◦C and 30 MPa using supercritical CO₂ (SC–CO₂) and SC–CO₂ plus formation water. Integrating X–ray diffraction (XRD), scanning electron microscopy (SEM), nanoindentation, and uniaxial compressive tests, we constructed a coupled mineral–microstructure-micro/macro–mechanics evolution framework. The results indicate that pure SC–CO₂ is characterized by de-dolomitization accompanied by secondary calcite precipitation; with the addition of formation water, the regime shifts to carbonic-acid-dominated dissolution, manifested as preferential dissolution of calcite with relative enrichment (and possible recrystallization) of dolomite. The soaking process exhibits staged evolution: early de–dolomitization with secondary precipitation; a middle stage of intensified calcite dissolution with dolomite enrichment/recrystallization; and a late stage marked by increased porosity and connectivity and the emergence of channelization (wormholing). SEM shows surface evolution from pitting to grooves and through-going pores, with calcite more reactive than dolomite. Nanoindentation reveals power–law decay in hardness and elastic modulus–pronounced early softening followed by a short–term rebound and subsequent re-softening. At the macroscale, mechanical properties degrade concurrently: compressive strength and elastic modulus decrease markedly, Poisson ^′s ratio increases, the load-bearing framework weakens, and ductility is enhanced.

关键词

超临界 CO₂ 水碳酸盐岩相互作用 / 矿物组分 / 微观结构 / 微观力学性质 / 宏观力学性质

Key words

supercritical CO₂-water-carbonate rock interactions / mineralogical composition / microstructure / micromechanical properties / macromechanical properties

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基金资助

陕西省重点研发计划(2025CY-YBXM-612)

新疆维吾尔自治区天山英才计划(2022TSYCCX0055)