Abstract
During acid fracturing, acid-rock reaction heat has a significant influence on temperature profiles in fractures and consequently on etched fracture dimensions, but it is usually neglected or simplified in acid fracturing models. This can lead to misestimating of etched fracture dimensions. A model for calculating real-time acid-rock reaction enthalpy, which is a function of temperature, pressure and volumetric work of carbon dioxide produced by reactions, is coupled into a heat transfer model and a fracture growth model, and its effect on etched fracture dimensions is simulated. True experimental data from SL oilfield in China is used for simulation. The results show that acid-rock reaction heat reduces the effective etched fracture length by around 10%, and the effect of reaction heat on the etched fracture length in limestone is 10%–15% larger than in dolomite. Acid-rock reaction heat makes the etched width profile along a fracture more inhomogeneous. With consideration of acid-rock reaction heat, etched fracture widths are 15%–20% larger near the wellbore and over 20% narrower at fracture tip, and its effects are more intense in limestone than in dolomite. The influences of acid-rock reaction heat on etched fracture dimensions are stronger when the initial formation temperature is lower and when acid of high concentration is used. When the pump rate of acid fracturing is increased, the effect of acid-rock reaction heat on etched fracture dimensions is weakened. The new coupled models were used in carbonate reservoirs in Tarim Basin, China for acid fracturing optimization. A scenario comparison showed that the designed treatment parameters of acid fracturing should be different when acid-rock reaction heat was fully considered. The application of the optimized scenario resulted in at least three folds of production rate increase compared to that before stimulation.
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