Study on the Performance Degradation of Sandstone under Acidification.

In most oilfields, acid fracturing is widely used for oil production. Understanding the relationship between the individual factors (i.e., carbonate rock types, acid rock reaction kinetics, and deterioration of rock mechanical properties) can provide practical guidelines that can be used for the design and optimization of acid fracturing operation. This paper takes hydrochloric acid, acetic acid, and citric acid as the main research objects and carries out acidification experiments on sandstone in Changqing Oilfield, China. In addition, the effects of tribasic, dibasic, and monobasic acids on the mechanical properties of sandstone were studied. Results show that in this study area, the most obvious effect was seen with the use of dibasic acids (hydrochloric acid + acetic acid), which effectively reduced the sample quality, uniaxial compressive strength, and elastic modulus. Citric acid and Mg promote the conversion of amorphous calcium carbonate to high-crystallinity calcite, forming a white precipitate. Furthermore, it is found by scanning electron microscopy analysis that experimental group 5 (hydrochloric acid + acetic acid) has the most ideal rock erosion effect. Inductively coupled plasma emission spectrometry analysis shows that the acid rock is present in the solution. X-ray diffraction qualitative analysis of the composition and concentration of ions shows that the formation of white precipitates is citric acid and Mg promotes the conversion of amorphous calcium carbonate to high-crystallinity calcite, forming a white precipitate. The findings of this study can help to better understand the erosion, failure state, and failure mechanism of different acid types on sandstone, which may provide certain references and guidelines for sandstone acid fracturing oil production.