The Effect of Wormholing on the Fluid-Loss Coefficient in Acid Fracturing

Abstract

Summary In most acid fracture designs, a leakoff model developed for proppant fracturing is used to calculate the acid volume lost into the formation. Because wormholing in acid fracturing results in a few large channels near the fracture, the leakoff velocity of the fracturing fluid can be higher in acid fracturing than in proppant fracturing. This paper presents a new leakoff model that includes the effect of wormholing on the overall fluid-loss coefficient. The leakoff model is based on a volumetric model describing wormhole growth in acid fracturing. Conveniently, the leakoff velocity based on the volumetric model for wormholing is still inversely proportional to the square root of treatment time. Laboratory coreflood results are presented that support the use of the volumetric model for the linear flow region near the fracture walls. The results of the model show that the overall fluid-loss coefficient depends strongly on the number of PV required for wormhole breakthrough in a coreflood. A large number of PV to breakthrough means a slow wormhole growth rate, which occurs more commonly in dolomite compared with limestone. The effect of wormholing on the overall fluid-loss coefficient is insignificant in acid fracturing of dolomite. However, for limestone, the number of PV to breakthrough is small and wormholing increases the fluid-loss coefficient significantly, especially for a gas well. An example in the paper shows that the overall fluid-loss coefficient can be more than 100% higher because of wormholing in a typical case for a gas well in a limestone formation.