This paper describes a new secondary oil recovery method that will make the world’s producible petroleum reserves last decades longer. It makes water and WAG pattern floods passé. Its novel element is a unique completion configuration for simultaneous water and gas injection in a pattern flood. The gas is injected through one site located near the bottom of the formation, and the water is injected through a second separate site located directly above the first. High-rate water injection in the upper part of the formation greatly impedes the vertical upward flow of gas, causing it to finger horizontally far into the reservoir before flowing upwards. This mode of simultaneous injection can give a 3-4-fold greater vertical gas sweep of the formation than does WAG, the alternate injection of water and gas into a single completion interval. Such a flood can be initiated at any time in the life of a field, including after complete depletion.

When combined with the use of horizontal wells, the new approach is effective even in thin reservoirs. Also, calculations for a Prudhoe Bay-similar reservoir indicate that an additional recovery of 5 million barrels of oil above WAG recovery can be recovered from each 80-acre pattern, using the existing vertical wells

Injecting gas along with water gives higher oil recovery than water flood alone, because the presence of residual gas saturation at the end of a flood results in lower residual oil saturation than would result from a water-only flood. If the gas miscibly displaces the oil, the water-gas residual oil will approach zero. Even for gases that are completely immiscible with the oil, a number of investigators have found the residual oil after a water-gas flood to be in the range of 0-50 % of the water flood residual, in water-wet and intermediate-wet reservoirs. These studies lead to the conclusion that both miscible and immiscible gases are effective in lowering the residual oil in a water-gas flood. However, there will likely be many reservoirs for which the cost or lack of availability of the miscible fluid will make immiscible gases more economic than miscible ones.

An approximate quasi-steady state reservoir simulator was developed and used to identify this superior mode of gas and water injection. Transient simulation could not be used for this purpose because of the numerical dispersion (truncation error) inherent in it. The quasi-steady state simulator is validated by a favorable comparison of its solution to one resulting from a finely gridded transient reservoir simulation. The two methods yielded very nearly the same quasi-steady state shape and size of the gas-flooded zone. The gas saturations in constant saturation zones established by the flood were also matched.

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