Journal of Petroleum Technology July 2012 : Page 37
0.05 to 0.08 mg/m 3 . This means that the purity of the filtered gas reached the level of the atmospheric air. It is believed that the filter device completely blocked dust particles that are larger than 3 μm equiv-alent diameter and the purified gas met the requirement of the compressor. In order to ensure the accuracy of the measurements, a calibration test for the on-site monitor TSI AM510 was fur-ther conducted. The calibration device PALAS 3000 is a more accurate instru-ment suitable for indoor test. A calibrat-ed contrast curve is presented in Fig. 2, which shows that the dust concentration was much lower than that of the field data. This again proves the reliability of the separation and filtration system. It was known from the data col-lected by the mud logger that the stand pipe pressure increased by only 0.1 MPa because of the separation process. This is the total pressure drop in the separa-tion system. Apparently, the separation system has negligible effect on the drill-ing pressure. Problems and Solutions The first open-loop field test of the GRS was essentially successful. All the equip-ment was in working order and the sep-aration efficiency was high. This work served as a solid base line for more closed-loop tests. The following prob-lems were found during the test. ◗ Continuous discharge design. According to the original plan, the separated cuttings should be discharged continuously by circulating water. However, the electrical motor for suction pump was not explosion-proof. It did not meet the field security requirement. Therefore, the gas discharge mode had to be adopted. All electrical equipment must be explosion-proof in the future design. In the discharge process, the working condition of the second cyclone separator was normal. However, the blooie line of the first separator was blocked for a moment. Larger size blooie line should be adopted in the future design. ◗ The height of the equipment and skid mounted design. The current separation system is about 7 m high. Collision may occur easily during the transportation and installation process. This height is also inconvenient for monitoring and maintenance of the system. Therefore, the equipment’s height should be reduced in the subsequent design without affecting separation efficiency. At this time, the design of a new horizontal separation system has been completed. After further improvements, a skid mounted system will be fabricated for easy transportation and equipment integration. ◗ System measurement and control design. When a closed-circulation is achieved, the operating parameters such as pressure and gas flow rate should be monitored in real time for safety. Valve switching should be used with both manual and automatic modes. In addition, a real-time alarm system should be added for safe operation. Due to the constraints of time and field conditions, an on-site reading method was used in this test. The automatic measurements and control systems should be emphasized in subsequent development. ◗ Compressor inlet design. The entrance of a conventional compressor is open to air. Because the separated gas needs to be introduced to the compressor through piping, a proper parallel gas distributing manifold should be designed to fit the compressor inlet. Currently, such a manifold has been conducted and tested with conventional compressors and the result is satisfactory. erator for increasing gas input volume to the system. This step will minimize drill-ing complications and ensure smooth drilling. Since nitrogen gas is highly compressible, which does not cause an immediate pressure drop in the bore-hole, it may be a good practice to select between 25% and 35% of the capacity of the membrane nitrogen generator using normal nitrogen drilling practice. Rust corrosion due to oxygen in a wet system is a concern in any nitrogen gas drilling if the oxygen filters do not perform well, whether the system is an open or a closed one. Fortunately, most membrane nitrogen generators remove oxygen to much lower than percent level and no significant risk is expected. Because CO 2 and H 2 S corrosions occur in wet systems, they should be minimized with inhibitors whenever these gases are encountered during drilling. Downhole fire/explosion can occur when drilling hydrocarbon-bearing zones in the presence of oxygen. For this to hap-pen, the oxygen/hydrocarbon ratio has to be in a certain range. In systems contain-ing natural gas and air only, the natural gas concentration needs to be between 5% and 15%, depending on pressure. Since air contains about 21% of oxy-gen while membrane-generated nitro-gen contains less than 5% oxygen, it is uncommon to see a downhole fire/explo-sion in a nitrogen gas drilling operation. Conclusions The first open-loop field test on the GRS was successful. The purity of the postsep-aration gas is superior to the atmospher-ic air in terms of particle con centration. The filtered gas met the requirement of gas compressor and circulation in the well. The success of the test has laid a good foundation for the further devel-opment of the system. It is a viable and feasible innovation for reducing drilling cost. It has a huge potential to be applied to the gas drilling operations including nitrogen drilling and natural gas drill-ing after improvement. This technology will have a huge impact on reducing the cost of gas drilling and improving drill-ing performance . JPT Operational Risks Some operational risks still exist with the new technology. They are 1) quick addi-tion of gas volume into the borehole in an emergency, 2) oxygen rust corrosion, and 3) downhole fire/explosion. Whenever the hole cleaning rais-es a concern because of drill cuttings accumulation, borehole collapse, exces-sive formation liquid influx, and/or gas leakage, it is imperative to automatically switch on the membrane nitrogen gen-JPT • JULY 2012 37
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