Nitrogen Drilling

Nitrogen Drilling Overview Nitrogen may be used in place of air drilling in underbalanced drilling operations. Its major advantage is that mixtures of nitrogen and hydrocarbons are not flammable. The risk of downhole fires in underbalanced drilling can be reduced if nitrogen is used as the drilling fluid. Flammability limits are basically controlled by the oxygen concentration. The following correlation can be used to estimate the minimum oxygen concentration required for a flammable mixture in the presence of nitrogen and methane: Equation 1 [latex]Q_{min} = 13.98 - 1.68 \log \left ( p \right )[/latex] where Qmin= minimum oxygen concentration, % p= absolute pressure, psia The nitrogen density is slightly lower than the density of air. However, under downhole conditions its cuttings transport efficiency is effectively the same as that of air. Because of the cost factor of nitrogen (N2), selection of the injection rate needs to be Currently there are two major techniques available to supply nitrogen for underbalanced drilling operations: Liquid (Cryogenic) nitrogen supplyOn-site generated nitrogen (Membrane filter) These two types of nitrogen supply options are explained in the following sections. Liquid (Cryogenic) Nitrogen Supply Besides drilling, nitrogen is widely used at the wellsite for various completion, stimulation and production operations. Because of its very low boiling temperature (-321°F), nitrogen needs to transported and stored in cryogenic tanks. At wellsite locations, pumps and heat exchangers are needed to convert the liquid nitrogen into a usable gaseous form. Various sized nitrogen conversion units are available for different size operations. Field experience shows that the cost of liquid nitrogen can be as high as 25% of the total drilling cost. Since nitrogen is converted from a liquid phase, it is more straightforward to measure the delivery rate for drilling. Normally, each gallon of liquid nitrogen yields approximately 100 scf of gaseous nitrogen. It is essentially pure completely eliminating potential corrosion problems in the presence of connate water and oxygen. 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. FIGURE 1 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. Recommended for You Dry Air Drilling Assessment 1. What is the major advantage of nitrogen drilling? A .Enhanced cuttings transportB .Cheaper than air drillingC .Not flammable with hydrocarbons ✔D .A, B and C 2. Which of the following is NOT a technique used to supply nitrogen for underbalanced drilling operations? A. Liquid (Cryogenic) nitrogen supplyB. On-site generated nitrogen (Membrane filter)C. Synthetic generation ✔ 3. What is the boiling temperature of nitrogen? A. -321 oF ✔B. -121 oFC. -21 oFD. 21 oF 4. How much gaseous nitrogen is obtained from each gallon of liquid nitrogen? A. Approximately 20 scfB. Approximately 50 scfC. Approximately 100 scf ✔D. Approximately 500 scf 5. Ambient temperature and ground elevation normally do NOT have a significant effect on nitrogen purity in membrane filtration technique. A. TrueB. False ✔ 6. What is the major limitation of nitrogen drilling operation? A. RegulationsB. Risk of downhole firesC. Mud ring formationD. Cost ✔ Recommended for You Dry Air Drilling