Summary
Wettability of two types of sandstone cores, Berea (permeability on the
order of 600 md), and a reservoir rock (permeability on the order of 10 md), is
altered from liquid-wetting to intermediate gas-wetting at a high temperature
of 140C. Previous work on wettability alteration to intermediate gas-wetting
has been limited to 90C. In this work, chemicals previously used at 90C for
wettability alteration are found to be ineffective at 140C. New chemicals are
used which alter wettability at high temperatures. The results show that: (1)
wettability could be permanently altered from liquid-wetting to intermediate
gas-wetting at high reservoir temperatures, (2) wettability alteration has a
substantial effect on increasing liquid mobility at reservoir conditions, (3)
wettability alteration results in improved gas productivity, and (4)
wettability alteration does not have a measurable effect on the absolute
permeability of the rock for some chemicals. We also find the reservoir rock,
unlike Berea, is not strongly water-wet in the gas/water/rock system.
Introduction
A sharp reduction in gas well deliverability is often observed in many
low-permeability gas-condensate reservoirs even at very high reservoir
pressure. The decrease in well deliverability is attributed to condensate
accumulation (Hinchman and Barree 1985; Afidick et al. 1994) and water blocking
(Engineer 1985; Cimolai et al. 1983). As the pressure drops below the dewpoint,
liquid accumulates around the wellbore in high saturations, reducing gas
relative permeability (Barnum et al. 1995; El-Banbi et al. 2000); the result is
a decrease in the gas production rate.
Several techniques have been used to increase gas well deliverability after
the initial decline. Hydraulic fracturing is used to increase absolute
permeability (Haimson and Fairhurst 1969). Solvent injection is implemented in
order to remove the accumulated liquid (Al-Anazi et al. 2005). Gas
deliverability often increases after the reduction of the condensate saturation
around the wellbore. In a successful methanol treatment in Hatter’s Pond field
in Alabama (Al-Anazi et al. 2005), after the initial decline in well
deliverability by a factor of three to five owing to condensate blocking, gas
deliverability increased by a factor of two after the removal of water and
condensate liquids from the near-wellbore region. The increased rates were,
however, sustained for a period of 4 months only. The approach is not a
permanent solution to the problem, because the condensate bank will form again.
On the other hand, when hydraulic fracturing is used by injecting aqueous
fluids, the cleanup of water accumulation from the formation after fracturing
is essential to obtain an increased productivity. Water is removed in two
phases: immiscible displacement by gas, followed by vaporization by the
expanding gas flow (Mahadevan and Sharma 2003). Because of the low permeability
and the wettability characteristics, it may take a long time to perform the
cleanup; in some cases, as little as 10 to 15% of the water load could be
recovered (Mahadevan and Sharma 2003; Penny et al. 1983). Even when the problem
of water blocking is not significant, the accumulation of condensate around the
fracture face when the pressure falls below dewpoint pressure could result in a
reduction in the gas production rate (Economides et al. 1989; Sognesand 1991;
Baig et al. 2005).
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
13 July 2005
- Meeting paper published:
9 October 2005
- Revised manuscript received:
10 April 2007
- Manuscript approved:
7 July 2007
- Version of record:
20 December 2007
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