The problem of stimulating wells that have a low bottom hole pressure gradient has been one that the industry has met with mixed results over the years. For the purpose of this paper, low gradients are anything less than a 0.35 psi-ft. gradient. We have picked this number since under normal stimulation conditions, few wells will flow their load back without some means of artificial lift. Clearly, mechanical means of lift have been historically the more widely used, the most successful, but intrinsically the least attractive of all alternatives. This is, of course, due to the inherent mechanical risks, depth limitations, and pressure considerations of bringing a well back in. To simplify, swabbing in a gas well at 8000′ with 2000′ psi bottom hole pressure (0.25 gradient) is at best a delicate chore. Therefore, we have for years attempted to use other means of providing the energy for these wells to be able to come back on their own.

Mixing nitrogen with various fluids has been done with varying degrees of success over the years. In this author's experience, nitrogen seems to work best at higher pressure gradient levels and that lower pressure gradient levels tend to interfinger back through the fluid causing loading conditions. Additionally, wellhead pressure using nitrogen will significantly increase due to the lower average density of the total system. This, of course, increases the required hydraulic horsepower and potentially, the required working pressure of the surface equipment.

With the increasing availability of carbon dioxide for oil field use, we have been able to successfully unload two wells with extremely low pressure gradients. One of these wells is slightly below 5000′. The other was slightly below 22,000′. The jobs were designed similarly with the obvious difference in scaling factors. We chose carbon dioxide due to the density and the similarity of the gas in its compressed state to a liquid. We designed the jobs so that at no time would there be enough total fluid volume in the tubing to reach loading conditions. Additionally, we preceeded and followed the acid jobs with a full tubing volume of carbon dioxide.

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