Evaporative Cleanup of Water Blocks in Gas Wells

Summary The flow of a gas toward the wellbore of a production well will result in the evaporative cleanup of water blocks, which occurs primarily because of gas expansion. This paper presents for the first time a model for calculating the rate at which such water blocks are removed, for either fractured or unfractured gas wells. The model can be used to compute the impact of evaporative cleaning on well productivity. The removal of water first occurs by gas displacement. Evaporative cleanup is caused by gas expansion. The resulting saturation profile is qualitatively different for low-or high-permeability rocks. As a consequence, the increase in gas relative permeability, or the well productivity, with time can vary depending on the rock permeability and the well drawdown. High-permeability (e.g., fractured) rocks clean up significantly faster. By contrast, low-permeability unfractured wells may require a very long time to clean up. Large pressure drawdowns, as well as the use of more volatile fluids, such as alcohols, also result in faster cleanup. A distinctive feature of the work presented is that the model equations are formulated and solved completely without the assumption of skin factors for the damage zone. Thus, the prediction of cleanup rates can be made more accurately.