Modeling of Spent - Acid Blockage Damage in Stimulated Gas Wells

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 water 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 paper presents a model that investigates the spent-acid damage in wormhole region of acid-stimulated gas wells. The phenomenon of spent-acid blockage was first investigated in the experimental study to identify the problem. A labscaled model was then developed to characterize the capillary pressure and relative permeability behavior by matching the results from the model to the experimental observation. We then extended the study to field-scale by approximating the wormhole as a long, slender half-ellipsoid centered in an ellipsoidal flow field. The simulations that focused on the displacement regime of spent acid recovery process were developed. These models were solved numerically to predict pressure behavior and spent acid distributions for the flow-back process. With the models, we studied the effects of several key factors, such as capillary pressure, relative permeability, and addition of additives, on the efficiency of spent acid recovery. The results show that 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.