Multiscale modeling and optimization of coiled tubing acidizing in carbonate horizontal wells

Coiled tubing drag acidizing is a commonly used diversion technique in carbonate reservoir horizontal wells. However, due to challenges such as long wellbore length, strong formation heterogeneity, and uneven damage distribution, predicting acidizing effectiveness remains uncertain. To address this, a radial dual-scale wormhole propagation model and a numerical model of coiled tubing drag acidizing were developed to simulate the evolution of acid etching patterns and the mechanism of damage removal under varying injection parameters. Field data from the X oilfield in Iraq were integrated for process optimization. Simulation results show that as the acid injection rate increases, acid etching morphologies evolve from conical pores to dominant wormholes, branched wormholes, and eventually uniform dissolution. Dominant wormholes require the least acid volume and exhibit the highest propagation efficiency. Wormhole length increases significantly with acid volume intensity, extending from 0.55 m to 1.42 m as intensity increases from 0.2 to 0.8 m³/m. Greater damage depth and lower permeability contrast lead to poorer acidizing outcomes. Based on the analysis, when the damage depth is less than 0.5 m, an injection rate of 0.5 m³/min and an acid intensity of 0.3~0.4 m³/m are recommended. For damage depths around 1 m, the recommended rate is 1 m³/min with an acid intensity of 0.4~0.7 m³/m. Field results confirm that combining coiled tubing drag acidizing with near-heel pinpoint injection can enhance well productivity by 2~5 times, outperforming other acid placement strategies. This study provides theoretical and practical guidance for optimizing acidizing parameters and operational design in carbonate long horizontal wells.