Abstract

Gas wells may suffer a significant decline in gas production or may even be destroyed as a result of the influx of water into production zones. The situation may even be worse in horizontal wells. Closing off the water-producing zone may not be appropriate and the disposal of produced water is, in many cases, expensive. A substantial decline in gas production is usually associated with an increase in water production or water cut. Water shut-off/reducing water production is helpful in maintaining gas production at high values. In this study, the feasibility of reducing the water production in modelled gas reservoirs by changing the wettability of the gas zone from preferential water- to gas-wetness using a fluorine carbon surfactant has been investigated experimentally. The basic mechanism used is that the entry capillary pressure has to be overcome prior to water entering the gas zone after the wettability has been altered to gas-wetness. Naturally, water can imbibe into gas zones spontaneously because the rock in gas zones is, most probably, water wet. After wettability alteration from water- to gas-wetness, water cannot enter gas zones if the differential pressure is less than the entry capillary pressure, and so the water flux will be significantly reduced even if the differential pressure is greater than the entry capillary pressure. An artificially made, consolidated two-layer core model was used to conduct the study. The two layers had different permeabilities, and the top layer was served as the gas zone and the bottom layer as the bottom aquifer. The gas production was measured at different initial water saturations with and without wettability alteration from preferential water- to gas-wetness in the gas zone. The experimental results showed that the water breakthrough time could be postponed and that the amount of water entering the gas zone could be reduced significantly by altering the wettability of the gas zone to gas-wetness. The advantages of this approach in reducing water cut were: (1) the permeability of the gas zone was almost unaffected by the chemical treatment for wettability alteration; and (2) the chemical treatment for wettability alteration had great longevity.

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