Polymer flooding (PF) and alkaline/surfactant/polymer (ASP) flooding have been applied throughout the world for more than 20 years.

The experimental analysis of 21 crude oil samples shows a good correlation between high molecular-weight hydrocarbon components (C 40 + ) and viscosity. Forty-four remaining oil samples extracted from oil sands of oilfield development coring wells were analyzed by high-temperature gas chromatography (HTGC), for the relative abundance of C 21 − , C 21 -C 40 and C 40 + hydrocarbons. The relationship between viscosity of crude oil and C 40 + (%) hydrocarbons abundance is used to expect the viscosity of remaining oil. The mobility characteristics of remaining oil, the properties of remaining oil, and the next displacement methods in reservoirs either water-flooded or polymer-flooded are studied with rock permeability, oil saturation of coring wells, etc. The experimental results show that the hydrocarbons composition, viscosity, and mobility of remaining oil from both polymer-flooding and water-flooding reservoirs are heterogeneous, especially the former. Relative abundance of C 21 − and C 21 -C 40 hydrocarbons in polymer-flooding reservoirs is lower than that of water-flooding, but with more abundance of C 40 + hydrocarbons. It is then suggested that polymer flooding must have driven more C 40 − hydrocarbons out of reservoir, which resulted in relatively enriched C 40 + , more viscous oils, and poorer mobility. Remaining oil in water-flooding reservoirs is dominated by moderate viscosity oil with some low viscosity oil, while polymer-flooding mainly contained moderate viscosity oil with some high viscosity oil. In each oilfield and reservoir, displacement methods of remaining oil, viscosity, and concentration by polymer-solution can be adjusted by current viscosity of remaining oil and mobility ratio in a favorable range. A new basis and methods are suggested for the further development and enhanced oil recovery of remaining oil. remaining oil, viscosity, mobility ratio, water-flooding, polymer-flooding, heterogeneity, polymer-solution viscosity Citation: Xu Z S, Zhang J H, Feng Z H, et al. Characteristics of remaining oil viscosity in water-and polymer-flooding reservoirs in

In the February 2006 issue of JPT, H.L. Chang et al. reported on some of the pilot and commercial-scale field activities of polymer flooding and alkaline/surfactant/polymer (ASP) flooding that were performed in China (Chang et al. 2006). Unquestionably, polymer flooding and ASP flooding are proven processes, and China should be proud of their work in these areas. Unfortunately, the article also advocated a controversial technology [flooding with aqueous colloidal-dispersion gels (CDGs)] as being superior to polymer flooding. I submit that this claim is misleading and generally incorrect. CDGs [i.e., relatively low concentrations of partially hydrolyzed polyacrylamide (HPAM) crosslinked with aluminum citrate] should not be applied without carefully examining the purported science and engineering behind this process. Chang et al. speculated that low-concentration aluminum citrate/HPAM microgels propagate through porous rock like superpolymer solutions. Specifically, they suggested that these CDG formulations penetrate deep into porous matrix reservoir rock and subsequently provide higher resistance factors (effective viscosities in porous media) and residual resistance factors (permeability-reduction factors) than comparable HPAM polymer solutions without cross-linker. However, independent results from three university research laboratories demonstrated conclusively that this assertion is not correct (see Seright 2006 and Wang et al. 2006 for details). Another assertion made in the February article (Conclusion 3 on page 87) is that "a large amount of CDG would preferentially enter the high-permeability or thief zones and divert polymer or water into medium- and low-permeability zones." This assertion contradicts basic calculations with Darcy's law (Seright 2006). Furthermore, the field evidence given to support CDG gels can readily be attributed to other, more plausible, factors. In many cases, questions arise whether the aluminum citrate had a positive effect on the field results. More details can be found in Seright 2006 and Wang et al. 2006.

The paper researched the measurement ways of the weight average MW (WAMW) and MWD of the HPAM. In our lab the procedure to measure the WAMW of the HPAM by the low angle laser light scatting (LALLS) is established. The WAMWs of HPAM that WAMW is 8–9 millions were measured with a precision better than 5% at a 95% confidence level. The procedure to measure the MWD of the HPAM by gel permeability chromatography (GPC) and LALLS has been established also. The MWD of the HPAM that WAMW is about 5–6 million can be measured, the sample is provided by POLYSCIENCE INC. For the HPAM that WAMW is more than 6 million, the measurement result is not good. To measure the MWD of the higher MW HPAM, we try to fractionate the HPAM by selecting the suitable precipitating agent and procedure. The MWD of the WAMW 8–9 million HPAM is measured, the result is very good. Introduction Polyacrylamides are the most widely used polymer in many aspects such as flocculants sealants, friction reducing agents, medicine, and water treatment agents. High MW water soluble, partially hydrolyzed polyacrylamides have been currently used as mobility control agents for enhanced oil recovery process. In Daqing Oilfield, the largest oilfield in China, more than 70 thousand tons HPAM has been injected into the formation every year now. The outputs of the oil by using the HPAM flooding technology are about 10 million tons every year. The oil recovery is increased 10% compared with water flooding. Major concerns in the design of such operations generally include injectivity, mechanical stability, rheological properties, and loss or retention of HPAM in the porous medium. The MW and MWD of the HPAM directly influence these properties. Again, the MW and MWD of the HPAM affect the oil recovery from the different permeability porous media1. If the MW and MWD of the HPAM is suit to the permeability of the porous media that is good mobility control in all oil saturated zones, higher permeability zones or lower permeability zones, the maximum oil recovery can be get. In our research work, high MW and wide MWD HPAMs are demonstrated to have a high oil recovery compared to low MW and narrow MWD HPAMs. For Daqing oil field it is important to develop the analytical techniques that can measure the MW and MWD of high MW HPAMs. The analytical technique is essential to research the HPAM degration mechanism, HPAM flooding mechanism and develop the best HPAM products. The determination of the MW and MWD of high MW water-soluble HPAMs is a difficult task. The report molecular weight of commercial based on their intrinsic viscosity is viscosity average MW (VAMW), that is a relative MW. The intrinsic viscosity is related to the VAMW with a two-parameter correlation commonly referred to as the Mark-Houwink relationship. For a particular polymer, this relation may vary with brine composition, polydispersity of the HPAM sample, and the hydrolyzed degree of the sample2–4. The accurate real MW of HPAM is not expressed by the VAMW. Normally, the larger MW, the more different between real MW and VAMW. GPC fails to give well-defined average MW of HPAM, because of a lack of proper packer material and calibration standards5,6. Polydispersity is the main properties of HPAM, expressed by the MWD; any kinds of average WM of HPAM can not provide information about polydispersity. Without MWD data, it is difficult for researchers to design HPAM for a specific use. GPC is commonly used to measure MWD of HPAM that MW is not more than one-million because there are no standard samples that MW is more than one-million. If the LALLS as a detector is connected to GPC, MWD of HPAM that MW is about 3–4million can be measured7,8. At present there is no GPC column that can separate the higher MW HPAM. MWD of HPAM that MW is more than 4-million had not been published before.

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