Study to Rapidly Improve Rock Strength during Acid Fracturing with an In-Situ Mineral Alteration

Summary In deep and ultradeep geothermal or natural gas reservoirs, the fracture conductivity generated by acid fracturing stimulation plays a critical role in determining geothermal energy extraction efficiency and gas production rates. However, extremely high closure stress and the rock weakening effect caused by acid-rock reactions often result in the fracture conductivity declining rapidly. Mineral alteration technology aims to enhance rock strength by in-situ transforming existing minerals into new, harder ones. However, the current mineral alteration process is excessively slow (72 hours). In this study, we investigate the effects of 15 wt% hydrochloric acid (HCl), varying concentrations of phosphoric acid (H3PO4), and sodium phosphate (Na2HPO4) on rock strength at 200°C, and propose an innovative pH-regulation strategy to accelerate mineral alteration. First, experimental results show that 15 wt% HCl and H3PO4 significantly reduce rock strength, with reductions of up to 33.1% and 32.1%, respectively, while Na2HPO4 enhances rock strength, with a maximum increase of 63.7%. Second, the mechanisms of both rock weakening and strengthening, as well as the positive influence of high pressure, are elucidated. The 15 wt% HCl solution induces abundant irregular dissolution pores and microcracks, severely damaging the rock structure and significantly reducing strength. H3PO4 accelerates calcium ion (Ca2+) release. As H3PO4 concentration increases, although a higher hydrogen ion (H+) concentration aggravates acid damage, the products formed by phosphate ions and Ca2+ evolve from disordered, amorphous forms to well-ordered, highly crystalline forms, leading to a reduced loss in rock strength. Na2HPO4 enhances strength through dual mechanisms: transforming carbonate minerals into harder phases and repairing acid-damaged rock structures. Its strengthening effect improves with increasing concentration. High pressure promotes crystal formation, further increasing rock hardness. Finally, the mixed solution of H3PO4 and Na2HPO4 with a pH of 5.6 and a concentration of 0.8 M, referred to as PPN for short, combines the advantages of both components, enabling the hardening treatment to be completed in just 1 hour, representing a 98.6% reduction in time. This study presents a new pathway for rapid in-situ modification of reservoir fractures to enhance rock strength.