Appl Microbiol Biotechnol DOI 10.1007/s00253-017-8307-4
MINI-REVIEW
Polymers for enhanced oil recovery: fundamentals and selection criteria Sandeep Rellegadla 1 & Ganshyam Prajapat 1 & Akhil Agrawal 1
Received: 25 January 2017 / Revised: 20 April 2017 / Accepted: 24 April 2017 # Springer-Verlag Berlin Heidelberg 2017
Abstract With a rising population, the demand for energy has increased over the years. As per the projections, both fossil fuel and renewables will remain as major energy source (678 quadrillion BTU) till 2030 with fossil fuel contributing 78% of total energy consumption. Hence, attempts are continuously made to make fossil fuel production more sustainable and cheaper. From the past 40 years, polymer flooding has been carried out in marginal oil fields and have proved to be successful in many cases. The common expectation from polymer flooding is to obtain 50% ultimate recovery with 15 to 20% incremental recovery over secondary water flooding. Both naturally derived polymers like xanthan gum and synthetic polymers like partially hydrolyzed polyacrylamide (HPAM) have been used for this purpose. Earlier laboratory and field trials revealed that salinity and temperature are the major issues with the synthetic polymers that lead to polymer degradation and adsorption on the rock surface. Microbial degradation and concentration are major issues with naturally derived polymers leading to loss of viscosity and pore throat plugging. Earlier studies also revealed that polymer flooding is successful in the fields where oil viscosity is quite higher (up to 126 cp) than injection water due to improvement in mobility ratio during polymer flooding. The largest successful polymer flood was reported in China in 1990 where both synthetic and naturally derived polymers were used in nearly 20 projects. The implementation of these projects provides valuable suggestions for further improving the available processes in future. This paper examines the selection criteria of
* Akhil Agrawal
[email protected] 1
Department of Microbiology, Central University of Rajasthan, NH-8, Bandarsindri, Kishangarh, Ajmer, Rajasthan, India
polymer, field characteristics that support polymer floods and recommendation to design a large producing polymer flooding. Keywords Polymer flooding . Enhanced oil recovery . Surfactants . Oil fields . Biopolymers . Bioplugging
Introduction During the first stage of oil production, the differential pressure between the reservoir and wellbore is responsible for driving oil out of the production well. This process recovers only about 10% of the original oil in place (OOIP) and is referred as primary production (Planckaert 2005). Later, with a decline in reservoir pressure, oil recovery also decreases, leading to the implementation of secondary recovery. Secondary recovery involves the injection of an external fluid (such as water and gas) through the injection wells for maintaining reservoir pressure and displacing oil towards the wellbore. During this process, there is a physical sweeping of oil by water, which produces 15–60% of the OOIP. Further, the oil industries adopt enhanced oil recovery (EOR) process to increase oil production by improving oil flow and sweep efficiency in the reservoir. The EOR method comprises four methods: chemical, thermal, miscible and immiscible gas flooding, and microbial to increase recovery of remaining oil (Planckaert 2005). Chemical EOR (cEOR) is one of the promising methods of EOR in which surfactants, polymers, and/or alkali are used. Polymers have been used regularly in the oil field for wide range of application including polymer-based cEOR (Lucas et al. 2009; Agrawal et al. 2011). Polymer-based cEOR can improve the sweep efficiency by lowering water to oil mobility ratio.
Appl Microbiol Biotechnol
Many EOR projects are launched every year to recover the residual oil. Oil companies are also filing a number of patents concerning EOR and polymer used to protect their future interest. This reflects the significance of research in this area (Raffa et al. 2016). Since 1996 to 2010, 69 polymer flooding-based EOR operations had been conducted in different countries worldwide like the USA, China, Germany, Canada, France, Russia, Argentina, India, and Indonesia. A summary of enhanced oil recovery projects performed worldwide is shown in Fig. 1. Learning from these polymer-based EOR projects implemented in one field can provide knowledge to design a product for another application. However, the lack of understanding of polymer characteristics severely restricts the choice of the polymer for specific application. Currently, two different types of polymers are used: (i) a synthetic polymer such as polyacrylamide and (ii) a biologically produced polymer (biopolymer) such as xanthan gum and cellulose. Typical synthetic polymers are partially hydrolyzed polyacrylamide (HPAM) and its derivatives. HPAM have been used for large-scale production in many fields because it is less costly (Sheng 2013). Other polyacrylamide-derived polymers used in EOR include hydrophobically associating polymers, salinitytolerant polyacrylamide (KYPAM), and 2-acrylamide 2-methyl propane sulfonate (AMPS) (Sheng et al. 2015). Application of biopolymers like xanthan gum and cellulose for EOR has been demonstrated in few fields (Abbas et al. 2013; Standnes and Skjevrak 2014). Biopolymer not only lowers the water to oil mobility ratio but also selectively plugs the high-permeability thief zones and leads to a redirection of the water-flood to inaccessible oil zones (Sen 2008). Laboratory results showed that this selective plugging is due to the action of resident bacteria which can feed either on hydrocarbons or on biopolymer such as xanthan gum (Gassara et al. 2015; Sen 2008). Additionally, polymer gels, polymer-enhanced foams, and foamed gels have also been used to improve oil recovery by plugging the highpermeability thief zones (Kantzas et al. 1999). Fig. 1 A summary of EOR projects performed worldwide including polymer flooding (AlAdasani and Bai 2010; Koottungal 2008; Moritis 2000; Taber et al. 1996). The pie chart [I] shows a number of different EOR projects reported between the years 1996 and 2010. Out of the total 69 waterflooding projects undertaken, 53 are polymer flooding projects and 16 are carried out using surfactant polymer flooding as shown in pie chart [II]
Out of 69 polymer floods conducted in the past, few of them showed limited success. Polymer flooding major limitations include the loss of polymer to the porous medium, polymer degradation, and, in other cases, loss of injectivity or applied too late after water flooding (Thomas 2008). Earlier published reviews on related topics were more focused on field characteristics for polymer flooding but lack in defining the polymer characteristics for the particular application (Al-Adasani and Bai 2010; Levitt and Pope 2008; Sheng et al. 2015). In this review, we have described essential polymer characteristics influencing the polymer flooding operations. We have also made an effort to put polymer screening criteria together and strategies to resolve problems faced during polymer flooding. The overall objective of this paper is to analyze the factors that aid the economy and efficiency of polymer flooding using various polymers. Principle and mechanism The first step in EOR process is to identify the factors, which limit production of oil during the recovery process. Once these limiting factors are being identified, the selection of the type of EOR process to be implemented could be decided. As the physicochemical properties of the reservoirs are highly variable, a generic process for oil recovery will not be successful (McInerney et al. 2005). A more in-depth knowledge of the factors responsible for limiting oil recovery should be studied before applying polymer flooding. Craig in 1971 suggested that the efficiency of oil recovery is often dominated by volumetric sweep efficiency, which is defined by an equation for the efficiency of oil recovery (Craig 1971): Er ¼ Ed Ev
ð1Þ
where, Er is the recovery efficiency, expressed as a fraction of original oil in place; Ed is the microscopic oil displacement
Appl Microbiol Biotechnol
efficiency, expressed as a total volume of oil displaced from a unit segment of rock; and Ev is the volumetric sweep efficiency, expressed as the fraction of the total reservoir that is contacted by the recovery fluid. Hence, lower sweep efficiency limits the amount of oil produced. The difference between the oil and aqueous phase mobilities is one of the factors responsible for poor sweep efficiency. Compared to oil, water tends to move rapidly resulting in an irregular front with water reaching the production well first. The relative mobility ratio of oil and aqueous phase is expressed as . M ¼ ðk w μ o Þ ðk o μ w Þ ð2Þ where M is the mobility ratio, kw is the relative permeability of water in the waterflooded zone, ko is the relative permeability of oil in the oil-saturated zone, μo is the viscosity of the oil, and μw is the viscosity of water. Higher mobility ratio results in water channeling through the same pathway across the oil phase leaving behind regions of unswept oil (Abidin et al. 2012). However, mobility ratio 10
