1、2009 ASHRAE 959ABSTRACT Two third of the reservoir oil left by the traditional primary and secondary oil production methods can be recovered by techniques employed at the third phase, commonly known as Enhanced Oil Recovery (EOR). Appreciable decline in the new reservoirs discovery and increase in t
2、he petroleum demands has forced oil companies to develop various EOR methods. Nitrogen injection is one of the emerging EOR technologies that is receiving serious attention most recently among various EOR methods. In general, the choice of the EOR method and the expected recovery depends on many con
3、siderations, economic as well as technical. In this paper, Nitrogen gas injection for reservoir enhanced oil recovery is discussed and compared with other available methods. Nitrogen injection provides an emerging opportunity for the cryogenics industry to adopt latest developments in this field for
4、 cost effective utili-zation of N2injection in reservoirs, thus its establishment as a competitive enhanced oil recovery technique. While the pres-ent paper does not intend to get into technical details and the challenges involved in a typical cycle, it does provide an intro-duction of an important
5、emerging application for the cryogen-ics industry, its comparison with other EOR techniques and the challenges involved. Review of previous research, including implementation projects on Nitrogen injection for EOR purposes on a global level is also discussed.INTRODUCTIONWhile the petroleum demand is
6、 continuously increasing day by day, petroleum production worldwide is in a steady state. Due to various emerging technological developments, it can be expected that substantial portion of otherwise neglected oil can be recovered. The life of an oil well goes through three distinct phases (primary,
7、secondary and tertiary recovery) where various techniques are employed to maintain crude oil production at maximum levels. Techniques employed at the third phase are commonly known as Enhanced Oil Recovery (EOR) and they can substantially improve extraction effi-ciency. In the recovery of oil from r
8、eservoirs, it is usually possible to recover only minor portions of the original oil by the primary recovery methods which utilize the natural forces present in the reservoir and the major parts, nearly 2.0 1012barrels of conventional oil and 5.0 1012barrels of heavy oils remain in reservoirs worldw
9、ide after conventional recovery methods have been exhausted (Thomas 2008).Much of these oils would be recovered by various EOR methods which involve the injection of a fluid, or series of fluids, into the reservoir through an injection system. Over the years, interest in EOR has been growing due to
10、the increase in oil reserves. Although large volumes of oil remain in the mature reservoirs, the oil production in large quantities by EOR processes will not be possible unless these processes can compete economically with the cost of oil production from conventional sources. So it is important to f
11、ind economically suitable EOR methods for oil production from the reservoir. Thermal, Chemical and Gas injection are three major EOR methods developed during the last years (Moritis 2004). Natu-ral gas (lean or rich) has been used successfully for many years as a primary choice of the operators for
12、gas injection (miscible or immiscible). The limited availability and increasing value of natural gas has made its use for conventional cycling economically unattractive, especially in offshore environ-ments where initial capital investment is large. Therefore, the use of less expensive substitutes,
13、such as inert nitrogen, has been suggested (Donohoe and Buchanan 1981). Nitrogen was Emerging Applications in Cryogenics Nitrogen Injection for Reservoir Enhanced Oil RecoveryMd. Didarul Islam, PhD Mohamed Alshehhi Michael Ohadi, PhDStudent Member ASHRAE Fellow ASHRAEMd. Didarul Islam is a research
14、associate in the Department of Mechanical Engineering, Petroleum Institute, Abu Dhabi, UAE. Mohamed Alshehhi is a PhD candidate in the Department of Mechanical Engineering, University of Maryland at College Park, MD. Michael Ohadi is a professor of Mechanical Engineering and Provost at the Petroleum
15、 Institute in Abu Dhabi, UAE.LO-09-091 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or dig
16、ital form is not permitted without ASHRAEs prior written permission.960 ASHRAE Transactionsselected as a substitute makeup gas on the primary basis of pioneer work reported by Koch and Hutchinson on miscible displacement of reservoir oil using flue gas (Koch and Hutchinson 1958). They correctly conc
17、luded that flue gas (88% N2) could be substituted for hydrocarbon gas without sacrificing miscibility. Moreover, nitrogen provides a higher reservoir displacement volume per standard volume of nitro-gen than any other gas injectant; that is, it provides the lowest volume requirement for pressure mai
18、ntenance. In addition, nitrogen is also non corrosive. Therefore, no special metal-lurgy is required for the injection equipment. A cryogenic (air liquefaction and distillation) process can produce 99.999% pure nitrogen. Non-cryogenic processes employ membranes or adsorbents (PSA/ VPSA) to remove th
19、e unwanted compo-nents of air. They produce nitrogen which is typically 95 to 99.5% oxygen-free (pure). Non-cryogenic plants are less energy efficient than cryogenic plants (for comparable product purity) but may cost less to build, in particular when the required production rate is relatively small
20、. Non-cryogenic plants are relatively quick and easy to start up, which is useful when product is not needed full time. At high production rates, cryogenic processes are the most cost-effective choice. Cryo-genic processes can produce very pure end products; and must be used to produce liquid nitrog
21、en, oxygen and argon. Using either process, nitrogen can be generated at almost any loca-tion. Specific guideline on the required purity of N2for injec-tion in the reservoir is unavailable in literature.Depending upon the pressure, quantities, and location, nitrogen may cost one-quarter to one-half
22、the price of natural gas (Clancy et al. 1980). Here, this cost range of nitrogen mentioned above is the nitrogen generation cost (Cryogenic air separation or inert gas generation) compared to the natural gas price depending on pressure, quantities, and location. Because of the increasing cost of nat
23、ural gas, nitrogen injec-tion is becoming more popular and attractive. For example, in 1983, over 500 million cubic feet per day of nitrogen was being injected into thirty oil or gas reservoirs (Clancy et al. 1985). In 1985, this number becomes 600 million cubic feet per day (Clancy et al. 1985) and
24、 by 1990 this number grew to 800 million cubic feet per day in over forty oil and gas reser-voirs. Produced cryogenically from air with an established and proven technology, nitrogen can be made available for contin-uous trouble free injection in vast quantities at any location. Bath et al. (1980) f
25、irst highlighted the scope for EOR in the North Sea. One of the more promising processes identified was nitrogen injection, which could be considered as either a secondary process to replace the water flood in certain parts of the fields or as a tertiary process to recover water flood residual oil.
26、The drawbacks of nitrogen injection are that the produced hydrocarbons sooner or later become contaminated with nitro-gen, which then require additional separation facilities. This technical problem can be solved easily with some extra cost. Then, the plants should have the additional separation fac
27、ili-ties in order to separate the hydrocarbons contaminated with nitrogen. So, the investors have to pay extra cost for this addi-tional separator in the plant and this cost will depend on the available technology, existing separator and plant size.In fact, Nitrogen injection into subsurface reservo
28、irs does not present any major problems. It is being applied success-fully in an increasing number of EOR projects (Ahmed et al. 1983, Peterson 1978, Koch and Hutchinson 1958). Also the condensate gas displacement by nitrogen in the reservoir is a simple process that has proved to be very efficient
29、(Moses and Wilson 1981). Hence the feasibility of nitrogen injection as a substitute for gas cycling is mainly an economic problem.EOR METHODSMany EOR methods have been used in the past, with vary-ing degrees of success, for the recovery of light and heavy oils. Figure 1 shows the classification of
30、these methods (Sarma 1999). As it is shown, EOR is divided into two major types: thermal and non-thermal. Thermal methods supply heat to the reservoir, and vaporize some of the oil. The major mechanisms involve a large reduction in viscosity, and hence mobility ratio. They are best suited for heavy
31、oils and tar sands. Non-thermal methods (Selby et al. 1989) are best suited for light oils. In few cases, they are applicable to moderately viscous oils, which are not suitable for thermal methods. Lowering the interfacial tension and improving the mobility ratio are two major objec-tives in non-the
32、rmal methods. In non-thermal EOR, there are two types of gas injection, miscible gas injection and immis-cible gas injection. In miscible gas injection, the gas is injected at or above minimum miscibility pressure (MMP). Various gases (i.e.CO2gas) and inert gas (i.e.N2gas) are commonly employed to e
33、stablish oil displacement by conditional misci-bility. For example, M.D. Rushing et al. (1977) describes a miscible oil displacement process involving the injection of high pressure nitrogen. They stated that pure nitrogen is injected into the reservoir and functions to initially strip rela-tively l
34、ow molecular weight hydrocarbons from the reservoir oil. As the light hydrocarbons are absorbed, a two-phase equi-librium point is established between the reservoir oil and nitro-gen at a location near the injection well. The liquid phase is composed initially of significant quantities of light and
35、heavy residual hydrocarbons, whereas the gas phase is comprised primarily of nitrogen and light hydrocarbons. Since the gas phase has a higher mobility within the reservoir, it moves ahead of the liquid phase to contact additional reservoir. As nitrogen injection continues, the liquid phase is conta
36、cted with additional nitrogen with an attendant decrease in the concen-tration of light hydrocarbons in the liquid phase until ulti-mately the liquid phase is reduced to the heavy residual hydrocarbons. On the other hand, in immiscible gas injection, flooding by the gas is injected below MMP. This l
37、ow pressure injection of gas is used to maintain reservoir pressure to prevent production cut-off and thereby increase the rate for production oil.In the past, most of the EOR projects were active in the USA and the bulk of the production came from that country. Figure 2 shows the EOR production tre
38、nds over the last ASHRAE Transactions 96120 years (Thomas 2008) in USA. The total EOR production in USA is declining (Moritis 2006). In the figure it is shown that the majority of the EOR production was done by thermal method which is also on decline. Production from EOR method using gas injection i
39、s increasing. Production from the Chemical methods is non- existent at present. So, Gas injec-tion plays a significant role on EOR at present and will continue to have an important place in oil production in future. Nitrogen as Gas InjectantNitrogen has been used successfully for more than 20 years
40、in pressure maintenance and has now gained promi-nence over the last decades as the injectant choice for enhanced oil recovery. Nitrogen is using as a driving force for costly and limited carbon dioxide, and in some cases for miscible displacement. Nitrogen can be produced at any reservoir using pro
41、ven cryogenic, non-cryogenic technology and various energy sources. Nitrogen provides the lowest volume requirement for pressure maintenance. The injected nitrogen does not react with the reservoir fluids to produce undesirable by-products and precipitates. It should be mention here that carbon diox
42、ide gas used as injectant breaks through in the oil and appears in the associated gas and thus increase the volume of the produced gas that must be treated Figure 1 Classification of EOR methods (adapted from Sarma 1999).962 ASHRAE Transactionsto remove the CO2prior to sales. Nitrogen gas generated
43、from compressed air is readily available, extremely dry, non corro-sive and environmentally safe. It is an inert gas and therefore, does not combine with hydrocarbons to form sludge or emul-sions down hole. Nitrogen injection has proven its effective-ness in reducing the natural decline of aging res
44、ervoirs, thus maintain production capacity for both oil and natural gas. Nitrogen is a non reactive and nonflammable gas, which makes up more than 78 percent of the air. The costs and limi-tations of the natural gas and CO2have made nitrogen an economic alternative for reservoir oil recovery by misc
45、ible gas displacement. The production of nitrogen by the cryo-genic separation of the components of air has been in use since the early part of this century and is the most economical method of producing pure nitrogen (Peterson 1978). COMPARISON OF NITROGEN WITH OTHER GAS INJECTANTSNatural gas, CO2a
46、nd N2are three candidate injectants considered for comparison. Disregarding economics and availability, hydrocarbon gas is the best choice for most gas injection plants for EOR. In practical case, miscibility can be attained by gas injection (rich or LPG) to establish the misci-ble bank. High recove
47、ries have been noticed for pressure maintenance, immiscible displacement and miscible displace-ment projects where hydrocarbon gas has injected. The produced gas would require less treatment than the other gases (CO2and N2). A corrosion problem is minimal for projects using natural gas injection. Th
48、e long term dedicated supply of natural gas (long term reliability) may be questionable and in addition, supplemental gas supply requires pipelining from field to the project for EOR. The increasing natural gas price is one of the major factors that limit its use in EOR.Large reserves of naturally o
49、ccurring CO2, fossil-fueled power plants, coal-fired power plants etc. are the potential sources of CO2. Except for on-site generated CO2gas, dedi-cated pipelines are needed to transport the gas from source to the oil field. The major factors limiting CO2 injection as an oil recovery process are the availability of CO2and the cost to build pipelines to carry CO2into oil producing regions. Advantages of carbon dioxide flooding are: Miscibility can be attained at low pressures; displacement efficiency is high in miscible cases, useful over a wider range of crude oils than hydrocarbon inj
