Numerical-based Study of Spent Acid Blockage Damage in Acidized Gas Wells
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Aqueous fluids introduced by different stimulation treatments cause water blockage in the near-wellbore region of wells. This water blockage acts the same as formation damage when the well is put back on production. One of the examples is when gas wells in carbonate reservoirs are acid-stimulated; the wormholes that propagate into the formations might be surrounded by a region of high aqueous fluid saturation created by the leakoff of spent acid. The spent-acid blockage damage could be severe, especially in lower permeability regions where capillary forces are relatively high. This research presents studies that investigate the spent-acid damage in wormhole region of acid-stimulated gas wells. We start the investigation with lab-scale coreflood experiments. With the experimental study, this work verifies the spent acid blockage phenomenon accompanying the acid stimulations. A model that simulates a gas flowback experiment is then developed to match with the results from the experiments. From this numerical simulation, we are able to obtain the information of the properties that cannot be measured directly from the experiments. We then extend the research to a field-scale study by approximating the wormhole as a long, slender half-ellipsoid centered in an ellipsoidal flow field. Models that can capture both the displacement and evaporation regimes of spent acid recovery process are developed. These models are solved numerically to predict the pressure behavior and spent acid distributions during the gas flowback process. With the numerical models, we study the effects of several key factors, such as pressure drop, pore-size distribution, and addition of additives on the efficiency of spent acid recoveries. The results show that the time needed to recovery the same amount of spent acid increases exponentially with decreasing pressure drop and absolute formation permeability. Besides, common additives routinely added to acid systems may aid, or hinder, spent acid recovery, depending primarily on their effects on rock wettability. With the studies performed on the model developed, we provide recommendations for minimizing spent acid damage to gas well productivity.
Zhang, Qi (2013). Numerical-based Study of Spent Acid Blockage Damage in Acidized Gas Wells. Doctoral dissertation, Texas A & M University. Available electronically from