Effect of Acidification Pretreatment on the Deep Shale Mechanical Parameters of Longmaxi-Wufeng Formation in China

Abstract

As the shale gas exploitation in the Sichuan Basin of China is developing to the depth of 3,500m ~ 4,500m, the hydraulic fracturing treatment is facing the problem of high breakdown pressure and treatment pressure, which could case safety issues. For some shale rich in calcium carbonate (gray mudstone), such as the shale at the bottom of the Longmaxi - Wufeng formation of the Weiyuan gas field in the south Sichuan Basin studied in this paper, the acid can change the mechanical properties of the shale by dissolution while flowing in the natural fractures and artificial fractures. Therefore, the study of the mechanism of acidification pretreatment for micro fractures, lithology, and shale pore structure is very important for the design and application of shale acidification pretreatment.

In this paper, X-ray diffraction and scanning electron microscopy are used to describe and compare the changes of rock mineral composition and crystal structure before and after acid treatment. Experiments such as nuclear magnetic resonance, mercury intrusion and nitrogen adsorption are carried out to describe and compare the changes of microstructure characteristics such as porosity and pore throat before and after acidification. Finally, the triaxial rock mechanics test is used to analyze the changes of rock mechanical properties before and after acidification pretreatment.

It is found that in the acidic environment, the interplanar spacing in the illite increases, and its strength property decreases; the hydrophily of feldspar is enhanced; chlorite crystal structure is susceptible to high concentration of hydrochloric acid. After acid treatment, the compressive strength is reduced by 51.53%, Young's modulus is reduced by 60.53%, and the brittleness is reduced by 78.7%. The change of the shale mechanical properties varies with the mineral composition, pore structure and pore-permeability relationship under different acid concentrations at different treatment times. As the increase of acid concentration, the carbonate mineral content, compressive strength, Young's modulus and brittleness index of shale decreases after acidizing, while the porosity and the permeability increases. The results of this study can help to predict the breakdown pressure after acidification pretreatment and provide parameters and guidance for acid fracturing design.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018