Compared with other enhanced oil recovery (EOR) techniques like gas flooding, chemical flooding, and thermal production, the prominent advantages of microbial enhanced oil recovery (MEOR) include environment-friendliness and lowest cost. Recent progress of MEOR in laboratory studies and microbial flooding recovery (MFR) field tests in China are reviewed. High biotechnology is being used to investigate MFR mechanisms on the molecular level. Emulsification and wettability alternation due to microbial effects are the main interests at present. Application of a high-resolution mass spectrum (HRMS) on MEOR mechanism has revealed the change of polar compound structures before and after oil degradation by the microbial on the molecular level. MEOR could be divided into indigenous microorganism and exogenous microorganism flooding. The key of exogenous microorganism flooding was to develop effective production strains, and difficulty lies in the compatibility of the microorganism, performance degradation, and high cost. Indigenous microorganism flooding has good adaptation but no follow-up process on production strain development; thus, it represents the main development direction of MEOR in China. More than 4600 wells have been conducted for MEOR field tests in China, and about 500 wells are involved in MFR. 47 MFR field tests have been carried out in China, and 12 field tests are conducted in Daqing Oilfield. MFR field test’s incremental oil recovery is as high as 4.95% OOIP, with a typical slug size less than 0.1 PV. The input-output ratio can be 1 : 6. All field tests have shown positive results in oil production increase and water cut reduction. MEOR screening criteria for reservoirs in China need to be improved. Reservoir fluid, temperature, and salinity were the most important three parameters. Microbial flooding technology is mature in reservoirs with temperature lower than 80°C, salinity less than 100,000 ppm, and permeability above 5 mD. MFR in China is very close to commercial application, while MFR as quaternary recovery like those in post-polymer flooding reservoirs needs further study.
In low oil price era, it seems that ASP flooding has little market. However, ASP progress in China shows that ASP flooding is good technology to help oil companies thrive and make profit. Since 2014, ASP flooding has entered industrial application in Daqing oilfield. ASP flooding production in 2015 is 3.5 million ton, 9% of the oilfield’s total production of Daqing oilfield. There are 22 ASP flooding blocks in Daqing, containing 7231 wells and 3 are new blocks in 2016. Obviously, more and more ASP flooding is being carried out. Another large ASP flooding field test in high temperature (80°C) in Henan oilfield has got staged incremental oil recovery of 7.7%. Latest theory and application in ASP flooding in China are reviewed to help ASP flooding go from success to more application. Weak alkali is better than strong alkali ASP flooding. Relation between viscosity and IFT is discussed. This paper also explains why weak alkali ASP flooding is promoted in Daqing after 12 field tests on strong alkali one. ASP flooding can be a great help to survive low oil price.
ASP flooding is one of the most promising EOR technologies. Lots of laboratory studies and pilot tests have been finished in Daqing oilfield which is the largest oilfield in China. Comparison of two typical strong alkali ASP (WASP) and weak alkali ASP (SASP) pilots are presented with detained information. ASP flooding could not only remarkably improve displacement efficiency but also improve sweep efficiency due to the low interfacial tension effect and mobility control technique with help of viscosity enhancement and emulsification effects. The incremental recovery of two ASP was near, while in peak oil production period after the injection took effects, WASP had high oil production rate than SASP. The emulsification effects in weak alkali ASP was weaker than strong one. The chromatographic separation was different in two pilot tests, in which weak alkali ASP had alleviated chromatographic separation. The constitution production sequence was both polymer first, then alkali and finally surfactant. The time gap between surfactant and polymer was about 0.0606 PV for strong alkali ASP, while a respective value of 0.1281PV for weak alkali ASP. Scaling was different and thus anti-scale technique adopted in two pilot tests were a little different. The overall input-output ratio for two tests was different and weak alkali ASP performed much better. Comparison was first made between strong alkali and weak alkali based ASP flooding from field tests perspective. Weak alkali based ASP is proven the development trend.
Summary Alkali/surfactant/polymer (ASP) flooding is one of the most-promising enhanced-oil-recovery (EOR) technologies. Strong alkali (NaOH) was used in early field tests mainly because of its stronger emulsification ability and wider surfactant range, which can meet the requirements of ultralow interfacial tension (IFT). However, subsequent field tests indicated that the advantages of a strong alkali did not outweigh the disadvantages caused by serious scaling and production-capacity loss. Although a critical comparison of strong alkali ASP (SASP) and weak alkali ASP (WASP) on the basis of field tests is quite difficult and complex, considering the small differences in reservoir characteristics, injected fluid, and operational changes, the two completed field tests in Daqing provided us with valuable and important information. The petrophysical features of the two field tests were similar. The well spacings and well patterns of the two field tests were critically the same, and the same screening standards and design ideas were followed. The incremental recoveries of WASP and SASP were nearly the same, while WASP had a higher peak oil production than SASP after the injection took effect. WASP was proved to have less liquid-producing-capacity loss than SASP. The emulsification effects of WASP were weaker than those of SASP, which also lowered the difficulty and cost of the treatment of the emulsified fluid. The chromatographic separation was different in the two pilot tests, in which WASP had alleviated chromatographic separation. Breakthrough of the polymer occurred before the alkali followed by the surfactant, and this occurred at 0.06 pore volumes (PV) for SASP but was delayed until 0.13 PV for the WASP flooding. The scaling of SASP was much-more severe than that of WASP, leading to a much-higher treatment cost. The economic performances of the two tests, which are of vital importance in a low-oil-price era, were quite different, and WASP had much-better performance than SASP. The input/output ratios of WASP in B-2-X and SASP in B-1-DD were 1:3.7 and 1:2.3, respectively. The returns on investment (ROIs) of WASP in B-2-X and SASP in B-1-DD were 19.1 and 12.9%, respectively, whereas the financial internal rates of return (FIRRs) after tax were 22.3 and 18.0%, respectively. The average FIRR of local oil-industry projects is 12%. Field tests indicated that WASP is both technically and economically better than SASP under the conditions in the Daqing oil field.
Capillary number theory is very important for chemical flooding enhanced oil recovery. The difference between microscopic capillary number and the microscopic one is easy to confuse. After decades of development, great progress has been made in capillary number theory and it has important but sometimes incorrect application in EOR. The capillary number theory was based on capillary tube bundles and Darcy’s law hypothesis, and this should always be kept in mind when used in chemical flooding EOR. The flow in low permeability porous media often shows obvious non-Darcy effects, which is beyond Darcy’s law. Experiments data from ASP flooding and SP flooding showed that remaining oil saturation was not always decreasing as capillary number kept on increasing. Relative permeability was proved function of capillary number; its rate dependence was affected by capillary end effects. The mobility control should be given priority rather than lowering IFT. The displacement efficiency was not increased as displacement velocity increased as expected in heavy oil chemical flooding. Largest capillary number does not always make highest recovery in chemical flooding in heterogeneous reservoir. Misuse of CDC in EOR included the ignorance of mobility ratio, Darcy linear flow hypothesis, difference between microscopic capillary number and the microscopic one, and heterogeneity caused flow regime alteration. Displacement of continuous oil or remobilization of discontinuous oil was quite different.
Summary Although the alkali/surfactant/polymer (ASP) flooding technique used for enhanced oil recovery (EOR) was put forward many years ago, it was not until 2014 that it was first put into practice in industrial applications with hundreds of injectors and producers in the Daqing Oil Field in China. In this study, 30 ASP-flooding field tests in China were reviewed to promote the better use of this promising technology. Up to the present, ASP flooding in the Daqing Oil Field deserves the most attention. Alkali type does affect the ASP-flooding effect. Strong alkali [using sodium hydroxide (NaOH)] ASP flooding (SASP) was given more emphasis than weak alkali [using sodium carbonate (Na2CO3)] ASP flooding (WASP) for a long time in the Daqing Oil Field because of the lower interfacial tension (IFT) of the surfactant and the higher recovery associated with NaOH than with Na2CO3. Other ASP-flooding field tests completed in China all used Na2CO3. With progress in surfactant production, a recent large-scale WASP field test in the Daqing Oil Field produced an incremental oil recovery nearly 30% higher than most previous SASP recoveries and close to the value of the most-successful SASP test. However, the most-successful SASP test was partly attributed to the weak alkali factor. Recent studies have shown that the WASP incremental oil recovery factor could be as good as that of SASP but with much-better economic benefits. Screening of surfactant by IFT test is very important in the ASP-flooding practice in China. Whether dynamic or equilibrium IFT should be selected as criteria in surfactant screening is still in dispute. Many believe the equilibrium IFT is more important than the dynamic IFT in terms of the displacement efficiency; thus, it is better to choose a lower dynamic IFT when the equilibrium IFT meets the 10−3 order-of-magnitude requirement. However, it is impossible for many surfactants to form ultralow equilibrium IFT. Because of the low acid value of the Daqing crude oil, the asphaltene and resin components play a very important role in reducing the oil/water IFT and asphaltene is believed to be more influential, although more work is required to resolve this controversial issue. Whether polymer viscoelasticity can reduce the residual oil saturation is still a matter of debate. Advances in surfactant production and in the overcoming of scaling and produced-fluid-handling challenges form the foundation of the industrial application of ASP flooding. Further work is advised on the emulsification effect of ASP flooding. According to one field test, the EOR routine should be selected depending on consideration of the residual oil type to decide whether to increase the sweep volume and/or displacement efficiency. The micellar flooding failure in one ASP field test in China has led all subsequent field tests in China to choose the “low concentration, large slug” technical route instead of the “high concentration, small slug” one. ASP flooding can increase oil recovery by 30% at a cost of less than USD 30/bbl; thus, this technique can be used in response to low-oil-price challenges.
Recently, there are increasing interest on polymer flooding due to its high enhanced oil recovery (10%- 20% OOIP in field tests in Daqing) and much lower cost compared to surfactant-polymer (SP) flooding and alkali-surfactant-polymer (ASP) flooding. How to select polymer molecular weight (Mw) and concentration to improve mobility ratio and avoid blocking in less permeability strata is important but difficult issue. Maximum allowable polymer Mw and concentration for given permeabilities by using different techniques is studied. Maximum allowable polymer Mw and concentration for a given permeability in polymer flooding varies according to different methods. Even in Daqing Oilfield, where the largest polymer flooding industrial application in the world has been carried out since 1996, and the annual oil production from polymer flooding is more than 10 million tons, no agreement on the maximum allowable polymer Mw and concentration exists. Different criteria of resistance factors (Fr) and residual resistance factor (Frr) used in China are reviewed and compared. These criteria are generally based core flow tests with less or no consideration on heterogeneity and oil saturation effect. Recent studies shows that Frr is misleading and incorrect because of insufficient injection volumes. Too high viscosity and molecular may block low/less permeability layer. Polymer molecular is seen as a sphere and the average diameter ratio to average pore-throat should be higher than 5 or 10 to avoid blocking. Though the pore- throat of a core can be got from mercury intrusion method, the average diameter of hydrolyzed polymer (HPAM) is subjected to change with different method, not to say the scattered range of Mw. Fr and Frr was frequently used in determining whether a certain Mw or concentration polymer is blocked in a certain permeability. However, the critical values varies. Another theory connects the polymer Mw and concentration with allowable flow rate in field test.
After decades of development, great progress has been made in capillary number theory and it has important but often incorrect application in EOR. Investigation into progress on capillary number theory and some misuse of capillary number theory helps to make better use of it. Latest progress concerning with capillary number theory and its application in chemical EOR is reviewed by studying the experiments data and checking its model hypothesis. Classic Capillary Desaturation Curves (CDC) are summarized and new CDC is introduced. Typical classical CDC showed larger capillary number lead to lower residual oil saturation and when capillary number increased to a certain critical value(first critical value), the residual oil saturation could drop to a minimum value even zero. CDC shapes were different in water-wet and oil-wet media. Recovery can be improved by increasing flooding rate, though invalid and impractical, displacement phase viscosity or/and reducing oil/water interfacial tension, which are actually adopted by chemical flooding. Guided by this theory and also first critical capillary number value requirement, it lead to the pursuit of low interfacial tension to largest extent and the requirement of ultra-low interfacial tension (10 -3 mN/m) in surfactant screening. However, experiments data showed that residual oil saturation was not always decreasing as capillary number increased. After capillary number increased to a certain value (second critical value), the residual oil saturation may increase or decrease as capillary number increase. What was more, the final residual oil saturation was quite more than zero and this CDC was regarded as the new CDC. Experiments in heavy oil laboratory tests showed that smaller injection rate lead to higher recovery which seemed contrary to capillary theory.
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