Application status and research progress of shale reservoirs acid treatment technology

In recent years, shale oil and gas development has been thriving in China. However, the shale oil and gas production always suffers a rapid decline. Based on the analysis of a large amount of former theories and experiences, a summary of acid treatment stimulation methods in shale oil and gas is presented, and the acid stimulation mechanism is analyzed. The mainstream technique in acid treatments includes: acid wash, matrix acidizing, prop fracturing with acid preflush, and multi-stage alternate-inject acid fracturing. The main stimulation mechanism of acid treatment can be summarized into 3 categories: a) the influence on shale matrix, namely the acid-induced increase of porosity and permeability, and reduce of wetting property of shale; b) the influence on rock mechanical properties, namely shale brittleness and toughness, and even Young Modulus to some degree; c) the influence on fractures' conductivity, caused by the fact that acid dissolves calcite-enrichment area in priority, and then increases roughness on fracture surface. In room temperature and atmospheric pressure, acid reduces fractures' conductivity, while in pressurized condition, the acid-soaked fractures' conductivity is higher than the conductivity of non-acid-soaked fractures. These knowledges would provide useful reference for furthering stimulation techniques and processes in shale oil and gas development.