A known technique.

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Note added at 6 hrs (2014-12-17 00:58:31 GMT)
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Repressuring is the method by which the pressure inside a crude oil well is increased so as to increase the output of the well. Pumping crude oil from a reservoir causes its pressure to drop, which further reduces pumpability. Returning the natural gas or other inert gases to the oil well increases the pumpability of the crude present inside the well by decreasing its viscosity and density. In one instance, slower rate of oil production and recycling of natural gas at 9.6 MPa enabled a net recovery of 70% of the oil originally present.
Although recovery can be increased even by repressuring with air, air is seldom used because it causes an inferior quality of product. Instead carbon dioxide or natural gas itself is used so as to not only increase the recovery from the oil well but also to maintain the high quality of the product.
http://en.wikipedia.org/wiki/Repressuring

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Note added at 6 hrs (2014-12-17 01:00:46 GMT)
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Method for repressurizing an oil reservoir
When water is pumped into an oil bearing formation, the specific gravity of the ... in place in an oil field, at a well site, under conditions of high temperatures and ...
http://www.google.com/patents/US3342259