Effects of combined dynamic-static loading and acidic corrosion treatment on the mechanical properties and microstructure of shale

Abstract

A critical scientific gap exists in quantifying the intrinsic mechanisms of shale mechanical property degradation induced by the combined effects of perforation (impact) and acidization—two core techniques for shale reservoir permeability enhancement. To address this gap, this study proposed an innovative coupled experimental framework integrating dynamic-static cyclic loading (to simulate perforation impact) and acid erosion. Static uniaxial compression tests were performed on treated damaged shale samples, with microstructural characterization via X-ray diffraction (XRD) and scanning electron microscopy (SEM). Key findings include: (1) The damage factor (characterized by longitudinal wave velocity) showed a significant positive correlation with acid concentration; (2) Combined damage (impact + acidization) caused far more severe mechanical deterioration than single damage modes—for instance, samples under combined damage with 20% hydrochloric acid exhibited a strength reduction to 158.97 MPa, with sharp decreases in peak strength and elastic modulus; (3) Damage reduced total energy and elastic strain energy of samples while increasing dissipated energy proportion, leading to more developed internal fractures and severe failure in combined damage samples; (4) Acidization promoted sample fragmentation into smaller debris, resulting in significantly higher fractal dimensions of acidized shale than other damage types under the same acid concentration; (5) XRD and SEM analyses confirmed that high-concentration acid erosion reduced shale carbonate content, and the synergy of mechanical pre-damage and chemical dissolution in combined damage accelerated acid-rock reactions, significantly increasing micro-interfacial pores and degrading shale structural integrity. This study’s innovation lies in establishing a coupled experimental framework that reproduces the actual “perforation-acidization” sequence, quantitatively revealing the synergistic degradation mechanism of shale mechanical properties under combined damage—providing a novel theoretical basis for optimizing shale reservoir stimulation parameters.

1. Introduction

With the continuous growth of global energy demand and the accelerated depletion of conventional fossil energy reserves, the development of unconventional oil and gas resources, represented by shale gas and tight oil, has become a strategic choice for ensuring energy security [1]. According to the 2024 report by the International Energy Agency, the global technically recoverable reserves of shale gas exceed 2000 trillion cubic meters. Among them, the proven geological reserves of China and the United States reach 540 trillion cubic meters and 3200 trillion cubic meters, respectively, accounting for 35% of the global total [2]. Since the early 21st century, the United States has achieved a surge in annual shale gas production from 12.2 billion cubic meters in 2000 to 960 billion cubic meters in 2023 by leveraging innovations in horizontal well staged fracturing technology. This production accounts for over 70% of its total natural gas output, dramatically transforming the global energy supply pattern [3].

In contrast, shale gas development in China faces core challenges of low reservoir porosity, low permeability, and strong heterogeneity: the main development formations are deeply buried, reservoir porosity and permeability are extremely low, and single-well production remains at a low level for a long time [4]. As a key technology to improve the seepage capacity of shale gas, the perforation-acidizing technology enhances reservoir conductivity through the synergistic effect of “mechanical ro...