Summary: CO2 flooding is emerging as a pivotal technique used extensively for carbon capture, utilization, and storage (CCUS) strategies. Acid stimulation is one common technique widely used to improve well-formation connectivity by creating wormholes. This work is motivated to investigate the gas-fingering behavior induced by acid stimulation during CO2 flooding. We present an integrated simulation framework to couple the acid stimulation and CO2 flooding processes, in which the two-scale continuum model is used to model the development of wormhole dissolution patterns. Then, sensitivity case simulations are conducted through the equation of state (EOS)–based compositional model to further analyze the CO2 fingering behavior in acid stimulation formations separately under immiscible and miscible conditions. Results demonstrate that for acid stimulation, the typical dissolution patterns and the optimal acid injection rate corresponding to the minimum acid breakthrough volume observed in the laboratory are prevalent in field-scale simulations. For CO2 flooding simulation, the dissolution patterns trigger CO2 fingering (bypassing due to the high conductivity of wormholes) in the stimulated region, and a lateral boundary effect eliminating fingers exerts its influence over the system through transverse mixing. The optimal acid injection rate varies when the focus of interest changes from the minimum acid breakthrough volume to CO2 flooding performance. The best CO2 flooding performance is always observed in uniform dissolution, and the dissolution patterns have a greater influence on the performance under miscible conditions. This work provides technical and theoretical support for the practical application of acid stimulation and CO2 flooding.

Summary: This paper presents the productivity formulas of a perforated vertical well on the basis of the dual-radial-flow model and then presents the productivity formulas of horizontal wells on the basis of disturbing elliptical flow theory. The threshold pressure gradient (TPG) is used to characterize the seepage characteristics of low-permeability reservoirs. By combing the above formulas, the authors proposed the productivity formulas of perforated horizontal wells in a low-permeability reservoir on the basis of the triple-radial-flow model. The formulas are derived considering the cases of the damaged zone being penetrated and being partially penetrated. At the same time, the flow patterns of single-phase flow and oil/water two-phase flows are considered. Sensitivity analysis on the performance of the perforated well shows that the productivity increases with the increase of the length of the horizontal well, penetration depth, shot density, perforation diameter, and phasing, and it decreases with the increase of TPG, compaction thickness, and crush degree of the crushed zone; the influence of the length of the horizontal well, TPG, penetration depth, shot density, compaction degree, phasing, perforation diameter, and compaction thickness on the productivity is in descending order.