Factors Affecting Cleanup of Water Blocks: A Laboratory Investigation

Abstract

Summary Water-blocked low-permeability gas formations with drawdown pressures comparable to capillary entry pressures can take a very long time to clean up. This work aims to study the effect of permeability, wettability, temperature, and drawdown on the cleanup of cores containing brine. Gas displacement experiments were conducted on cores fully saturated with brine. Addition of methanol, increasing temperature, and increasing core permeability by changing rock type resulted in faster cleanup after approximately 50 to 100 pore volumes (PV, the volume of vacant pores in the dry sample) of gas flow. The change of wettability of the rock from water-wet to oil-wet also resulted in faster recoveries in gas relative permeability. The cleanup of water blocks in gas wells occurs in two regimes: displacement of the fluids from the formation, followed by vaporization by the flowing gas, which becomes undersaturated as the pressure decreases. Our observations show that the cleanup of water blocks can be improved by: (a) Influencing the displacement regime (i.e., by changing the wettability) and (b) increasing the rate of vaporization by introducing volatile solvents such as methanol. The study quantifies the effects of factors such as rock type (permeability), wettability, surface tension/volatility, and temperature on gas relative permeability. The results of this study will help in selecting strategies for cleanup of water blocks created by various operations such as drilling, acidizing, and fracturing, as well as making recommendations for the use of surfactants or solvents for well treatments to remove water blocks.