Aqueous phase trapping damage in the production of tight sandstone gas reservoirs: Mechanisms and engineering responses

Abstract

Aqueous phase trapping (APT) that causes vital damage has not attracted attention in production yet. In this paper, the APT damage is evaluated based on imbibition and backflow experiments via studying six factors (imbibition pressure difference, liquid types, initial permeability and porosity, initial water saturation, clay minerals and natural crack). Experimental results show that the imbibition pressure difference is positively correlated with APT damage, and the damage extent is still medium (Permeability Damage Ratio (PDR) is 0.32) even for underbalanced conditions. Compared to the 3%KCl solution and 0.01% scale inhibitor solution, the 0.1% foam fluid presents the weakest damage (PDR is only 0.14) because of its lower interfacial tension (σ=22.1mN/m) and higher contact angle (θ=20.5°). A lower initial water saturation can result in more serious APT damage. The APT damage increases with the declination of permeability, but the increase of porosity. The content of kaolinite is positively correlated with the damage extent and plays a significant role because of the largest adsorption capacity compared to other studied clay minerals (maximum adsorption volume and specific surface area of kaolinite are 120.28 cm3/g and 24.63 m2/g, respectively). The APT damage extent of natural crack is lower than that of matrix pores (PDRs are 0.10 and 0.32 for the natural crack and matrix pores, respectively). However, the natural crack can strengthen APT damage in matrix pores because the crack face provides a larger imbibition area for the matrix pores. Three unfavorable engineering responses can be induced by APT damage in production, which are the reduction of production flow rate, increase of residual liquid in wellbore and the elevation of the pressure difference between casing and tubing, respectively. This paper provides new insights for the APT damage and presents a great significance for the diagnostics and control of APT damage for tight sandstone gas reservoirs.