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.
Invasion of aqueous drilling, completion, or fracturing fluids during well
completion, workover, or stimulation operations can reduce the relative
permeability to gas and thereby cause a water block. The gas phase relative
permeability depends on the water saturation in the porous medium and the
fractional flow characteristic of gas in the presence of water. The water block
can be removed by reducing the saturation of the invaded fluid in the wellbore
region and/or by affecting the fractional flow characteristics of the gas.
Simulation studies show that the cleanup of water blocks in gas wells is
faster if the absolute permeability of the formation is high.1 In the case of
low-permeability formations, the capillary pressure tends to be high because of
the smaller pore sizes. However, Holditch2 showed that when the drawdown
pressures are very high, capillarity is not important. Studies by Parekh and
Sharma3 show that the ratio of pressure drawdown to the capillary pressure and
the relative permeability curve exponents has a significant effect on the
cleanup of water blocks, with faster cleanup for high values of Corey
exponents. Abrams and Vinegar4 show that additives such as alcohol and/or
surfactants do not significantly improve the final gas flow when the drawdown
pressures are significantly greater than the capillary entry pressures.
© 2005. Society of Petroleum Engineers
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- Original manuscript received:
11 February 2004
- Revised manuscript received:
20 April 2005
- Manuscript approved:
1 May 2005
- Version of record:
15 September 2005