Optimization of acid fracturing process in naturally fractured tight sandstone reservoirs

Naturally fractured tight sandstone reservoirs in the Keshen Gas Field of the Tarim Basin are featured by low porosity and low permeability. Widespread calcite filling within natural fractures limits the inherent productivity, which necessitates optimized fracture-network acid fracturing for reservoir stimulation. In this paper, a fully coupled stress-flow-reaction numerical model is established based on the extended finite element method (XFEM). The model couples calcite dissolution in mineral-filled fractures by hydrochloric acid preflush with acid-rock reactions of hydrofluoric acid-based main acid in sandstone formations, enabling quantitative simulation of the whole evolution of complex fracture network propagation, acid transport, mineral dissolution and secondary precipitation. Systematic single-factor sensitivity analyses are performed on six key parameters, including preflush viscosity, preflush volume, injection rate, natural fracture density, acid system type and main acid volume, to quantitatively compare and evaluate the stimulation performance of various acid fracturing parameters and acid systems. The results indicate that the established multi-stage acid flow-reaction coupled model supports quantitative comparison and mechanistic analysis of different acid systems within a unified computational framework. It clarifies the control mechanisms of geological and operational parameters on natural fracture activation, fracture network growth and secondary precipitation-induced formation damage, thereby providing theoretical support and technical guidance for the optimization of acid fracturing treatments in fractured tight sandstone reservoirs of the Keshen Gas Field.