Nitrogen Drilling
On-Site Generated Nitrogen (Membrane Filter)
Figure 1 illustrates the on-site nitrogen generation setup. Onsite generation of nitrogen is typically achieved using the membrane filtration technique.
In this case, there is no need for transporting and storing large quantities of cryogenic nitrogen liquid. The economic impact of increased N2 rates no longer exists, since consumption and cost are not directly related. The generated N2 is charged at a flat rate per day up to the unit capacity.
In this technique, conventional air compressors deliver the air; then it is cooled down to approximately 80°F as it passes through a number of primary filters. The function of these filters is to remove contaminants, such as lubricating oil, water moisture and dust particles. The next stage uses a membrane filter, which consists of an array of very fine hollow polymeric fibers. At this filter nitrogen is separated from the heavier oxygen molecules and it is delivered to the booster unit and the standpipe. The purity of the nitrogen can be controlled by the air injection rate into the filter and the back pressure on the unit. Field observations have shown that ambient temperature and ground elevation can have a significant effect on compressor deliverability, thus on nitrogen purity.
Nitrogen Drilling Procedures and Operating Considerations
The operating procedures for nitrogen drilling are the same as those for dry air drilling. The risk of downhole fires is much lower. However, the release of oxygen at the membrane filter must be conducted with care to avoid possible accumulations, since even a modest increase in oxygen concentration can cause significant changes in the combustibility limit of materials.
The formation of mud rings is still a concern in nitrogen drilling. However the major limitation is the cost factor. Regardless of the generation method used to obtain nitrogen, the operation is more expensive compared to dry air drilling. It might be possible to justify these high costs, if there are long productive intervals to be drilled through either multiple zones or horizontal wells.
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