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ABSTRACT
Base gas is considered as an important factor in the storage operation as it remains permanently in the reservoir and maintains the reservoir pressure along the production cycle. Depending on the reservoir under consideration, base gas may occupy as little as 15% or as much as 75% of the total underground gas storage (UGS) reservoirs. Providing and injecting the cushion gas has the most contribution to the cost of the storage operations. Therefore, part of the base gas can be replaced by a cost-effective gas such as nitrogen, flue gas or air to reduce the costs of the investment. Some degree of mixing takes place when two miscible gases come into contact with one another that affects the quality of the produced natural gas. Therefore, the process needs to be studied and controlled. In this study, the feasibility of underground gas storage and the substitution of the base gas by a cheaper gas, i.e., nitrogen, flue gas, and air, are investigated in a partially depleted dry gas reservoir with very low initial pressure. To do so, a comparative study is performed among nitrogen, flue gas and air as the alternative gases to the base gas. In addition, the effect of flue gas composition on the performance of base gas replacement and ultimate gas recovery is investigated. Pure CO2 is considered as flue gas with zero mole% N2. In the end, the effect of the reservoir properties on mixing between the gases is studied. The results indicated that it is possible to substitute 24.8% of the base gas by nitrogen to obtain a 16.2% increase in the gas recovery of the reservoir. In this case, the ultimate recovery reaches 50.90%. Using flue gas as the alternative gas, the results showed a 15.6% increase in the gas recovery of the reservoir, obtained by substituting 23.9% of the base gas. The ultimate recovery using flue gas is 50.31%. According to the results, flue gas can be used as an appropriate option to replace the base gas of the UGS reservoir under consideration, and hence, there would be no more need for separation and purification of N2 and CO2. Increasing the CO2 composition in the flue gas up to 46.6 mole% leads to a decrease in the base gas replacement amount. When the composition increases above 46.6 mole%, the amount of the replaced gas does not change. However, in this composition range, more flue gas is injected into the reservoir, which has environmental advantages. The highest injection rate of the flue gas is obtained when the flue gas contains 100 mole% of CO2. The main problem in using air as the base gas is the high viscosity of air which requires a high injection pressure. According to the results, using air as the replacement gas, 21.3% of the base gas is substituted by air. In this case, gas recovery increases by 13.9% with respect to the reservoir depletion scenario and the ultimate recovery reaches 48.62%.
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Notes on contributors
Hamed Namdar
Hamed Namdar received his MSc degree in Reservoir Engineering from Sahand University of Technology. His areas of interest are underground gas storage, CO2 capture, production optimization, gas lift, low salinity water injection, heavy oil and thermal EOR, data mining and machine learning methods. He is the author of several peer-reviewed journal and conference papers. He has served as a member of the Sahand Oil and Gas Research Institute from 2012. Namdar would be in touched via: [email protected].
Elnaz Khodapanah
Elnaz Khodapanah is an Associate Professor of Petroleum Engineering at Sahand University of Technology. She has published more than 60 technical articles. She holds a BSc, MSc and PhD degrees from Sahand University of Technology, Iran. She has been a member of Society of Petroleum Engineers since 2003. She has received several awards, including; the best selected paper in the SPE sub regional paper contest, taking place on 3rd Jan. 2011, Tehran, Iran and Fractured Reservoirs Development conference, held in Ahwaz, December 2008. She has taught different courses in petroleum and chemical engineering. She has also involved in 10 research projects regarding petroleum matters.
Seyyed Alireza Tabatabaei-Nejad
Seyyed Alireza Tabatabaei-Nejad is Professor of Petroleum Engineering at Sahand University of Technology. He is the dean of Petroleum and Natural Gas Engineering faculty and the head of Sahand Oil and Gas Research Institute. He has published more than 140 technical articles and conference proceedings, and presented more than 10 workshops. He holds a BSc degree from Penn. State Univ., MSc degree from NMIMT, USA, and PhD from Heriot-Watt Univ., UK. He has served on SPE of Iran section technical committee, and is a chairperson of the SPE as well. He has received 7 honors and awards, including; the best selected research project. He has taught different courses in petroleum engineering and basic sciences for more than 20 years at SUT and other universities in Iran.Tabatabaei-Nejad may be contacted at Petroleum and Natural Gas Engineering Faculty, Sahand Oil and Gas Research Institute, Sahand University of Technology, Sahand New City, Tabriz, Iran. Email : [email protected]