Summary
Liquid blocking in some gas-condensate reservoirs is a serious problem when
the permeability is low (for example, of the order of 10 md or less). The
current practice centers mainly on hydraulic fracturing to improve gas flow. In
most cases, the frequency of application results in high costs. An alternative
is the permanent alteration of wettability from liquid-wetting to
preferentially gas-wetting. In this work, we present an experimental study of
wettability alteration to preferential gas-wetting using a multifunctional
surfactant and polymer synthesized for this particular application. The
treatments are performed with an alcohol-based-surfactant/polymer solution. We
treat Berea cores and reservoir-rock samples from two gas-condensate
reservoirs. In one of the reservoirs, water blocking has resulted in a
significant reduction of well deliverability. The treatment provides
significant improvement on the phase mobility. In this study, our focus is the
investigation of water/gas two-phase flow at high temperatures (80 and
140°C).
Basic measurements such as contact angle, spontaneous imbibition, and the
effect on the absolute permeability are discussed. The initial liquid
saturation at the time of treatment may have an influence on the wettability
alteration. The results of the treatment on oil-saturated and water-saturated
cores are presented. The treatment by alcohol without using the polymer is
compared and discussed. Two-phase-flow tests in single-phase and two-phase
injections are performed before and after the treatment using brine and gas.
Relative permeabilities of gas and water are measured, and the improvement
after the treatment is presented. Various measurements in our work show that
water and gas relative permeability increase significantly in a wide range,
especially at high liquid saturation.
Introduction
The exploitation of low-permeability gas-condensate reservoirs has received
increased emphasis in recent years. Liquid trapping (water or condensate
blocking) around a wellbore is one of the major causes of reduced productivity
in low-permeability gas-condensate reservoirs. Formation damage by the loss of
aqueous fluids by operations such as drilling, fracturing, or acidizing is a
potential source of reduced productivity of gas reservoirs because it causes
water accumulation (water blocking) near the wellbore.
Water blocking is caused by capillary pressure, which tends to imbibe and
hold the liquid phase, resulting in a reduction of gas mobility. The decrease
in water saturation from hydraulic fracturing around the wellbore is a slow
process when the permeability is low and the capillary forces are high. The
investigation of the gas/water relative permeabilities can determine the
gas-mobility loss associated with the change of water saturation.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
1 June 2005
- Revised manuscript received:
17 March 2008
- Manuscript approved:
19 March 2008
- Version of record:
20 August 2008
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