Analysis of atmospheric water locks and water lock solutions in tight sandstone gas reservoirs in the Ordos Basin

Abstract The target layer of the tight sandstone gas reservoir in Area J of the Ordos Basin is the Lower Shihezi Formation, and the well depth is greater than 3,000 meters. The porosity of the reservoir ranges from 4% to 16%, and the permeability ranges from (0.1 to 3.0) × 10 −3 μm 2 . It is a low-porosity and low-permeability reservoir with developed micro-fractures, high clay mineral content, high bound water saturation, and high capillary forces under the dual medium of pores and micro-fractures. It is extremely vulnerable to water lock damage during development. Based on the geological characteristics of the study area, water-lock experiments using the saturating displacement method and the imbibition method, as well as water-lock release experiments using the reverse injection of nitrogen and the reverse injection of methanol were carried out to compare the degree of water-lock damage and the degree of water-lock recovery using different methods. The research found that the water-lock damage under the saturating displacement method was 67.15%, and the water-lock damage under the imbibition method was slightly higher at 69.63%, both of which showed significant water-lock damage. The reverse injection of nitrogen to relieve water locks has a high rate of permeability recovery; the reverse injection of methanol to relieve water locks has the second-highest rate of permeability recovery. The lower the permeability, the higher the water lock damage rate and the rate of permeability recovery. A deeper understanding of the water lock mechanism in tight sandstone gas reservoirs and the improvement of water lock evaluation and water lock relief methods are of great significance for the efficient development of tight sandstone gas reservoirs.