TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA volumetric gas reservoir reaches its economic limit when the reservoir pressure falls below the level needed to sustain the surface pressure necessary to move the gas through surface facilities and into a pipeline. Compression is considered when economically feasible. Waterflooding of low-pressure gas reservoirs is viable alternative to compression. Waterflooding reserves may in certain cases exceed that of compression. Water displaces gas efficiently because of extremely favorable mobility of low-pressure gas. The incremental recovery depends on the initial reservoir pressure and the pressure at which the waterflooding is implemented. This management option may also be used in conjunction with compression to maintain reservoir pressure, which ensures well unloading, and reduces need for well workover.The economic feasibility of waterflooding depleted gas reservoirs is controlled by the amount of water needed, which is high because of the high gas compressibility. The amount of needed water increases dramatically as the reservoir pressure decreases. Managing this option as a water disposal for the field or nearby fields would be a major additional economic incentive. The injection is not a major cost component, as the water hydraulic head would exceed the reservoir pressure in most cases.The cost of water injection can be eliminated altogether by downhole commingling of high pressure large aquifer with the depleted gas reservoir. This option would require special but simple downhole completion. Water volume supplied by the aquifer can be monitored using reservoir pressure and production data.Studies of actual reservoirs demonstrated the technical and economic feasibility of this management option.
Water floods are typically conducted using the least expensive, easily available, non-damaging brine. Very little attention is given to the possibility of changing brine composition to improve oil recovery. Over the last 20 years, there has been laboratory and field trial evidence that shows changing brine chemistry, especially to low salinity, can sometimes increase the recovery. The various mechanisms of additional oil recovery from changing brine chemistry are not entirely clear. We report here on the effect of using low salinity and divalent altered brines on oil recovery through a variety of laboratory methods and materials. More than twenty corefloods were conducted to evaluate the effect of brine chemistry and initial wettability on incremental oil recovery. We also performed phase behavior tests, contact angle measurements, and wettability index measurements to evaluate recovery mechanisms. Initial wettability of the core was altered by ageing it with different crude oil containing wide range of asphaltene content. The core flood with lowest wettability index (least water-wet) produced about 12% incremental recovery while the most water-wet core only produced ∼ 4% during the secondary low salinity waterflood.
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