Waterflooding mechanisms

B. licheniformis JF-2 and Clostridium acetogutylicum were investigated under simulated reservoir conditions. Sandstone cores were equilibrated to the desired simulated reservoir conditions, saturated with oil and brine, and flooded to residual oil saturation. The waterflood brine was displaced with a nutrient solution. The MEOR efficiency was directly related to the dissolved gas/oil ratio. The principal MEOR mechanism observed in this work was solution gas drive [505]. 

This section summarizes and discusses several mechanisms proposed in the literature regarding low-salinity waterflooding. 

Apparently, mechanisms of low-salinity waterflooding are related to the DLVO theory, which is named after Derjaguin, Landau, Verwey, and Overbeek. The theory describes the force between charged surfaces interacting through a liquid medium. It combines the effects of the van der Waals attraction and the electrostatic repulsion due to the so-called double layer of counter ions. 

It was also observed that the compaction did not stop in the waterflooded areas, even though the reservoir was repressurized to the initial condition. Thus, seawater appeared to have a special interaction with chaUc at high temperatures, which has an impact on oil recovery and rock mechanics (Austad et ah, 2008). Austad and his coworkers started to work on the issues related to seawater flooding in carbonate reservoirs in 1990s. In the next section, the salinity effect on oil recovery is briefly summarized. 

One obvious mechanism in polymer flooding is the reduced mobility ratio of displacing fluid to the displaced fluid so that viscous Angering is reduced. When viscous Angering is reduced, the sweep efficiency is improved, as shown in Figure 1.2. This mechanism is discussed extensively in the waterflooding literature it is also discussed in Chapter 4. When polymer is injected in vertical heterogeneous layers, crossflow between layers improves polymer allocation in the vertical layers so that vertical sweep efficiency is improved. This mechanism is detailed in Sorbie (1991). 

One important point in the proposed OSP is that the salinity in the chase slug after the guard slug in OSP must be lower than Csei. One of the main mechanisms to justify such salinity is surfactant desorption. Liu et al. (2004) found that, in an extended waterflood following an alkaline-surfactant slug... 

In this section, simulation results are compared with the information from the literature for different polymer and surfactant-polymer injection schemes. We expect that UTCHEM simulation of a core-scale chemical process is the best simulation approach to study mechanisms. In this study, we use a ID core flood model with 100 blocks to represent a 1-foot-long core. The permeability is 2000 md, and the water and oil viscosities are 1 and 2 mPa s, respectively. To optimize injection schemes, we compare the incremental oil recovery factors over waterflooding and chemical costs. Chemical costs are evaluated using the amounts of chemicals injected per barrel of incremental oil (Ib/bbl oil). 

Johnson (1976) summarized several proposed mechanisms by which caustic waterflooding may improve oil recovery. In alkaline flooding, emnlsification is... 

One natural core was used to compare the performance of waterflood (W), AP flood, and ASP flood. The recovery factors for W, AP, and ASP were 50%, 69.7%, and 86.4%, respectively. These core flood tests were history matched, and the history-matched model was extended to a real field model including alkaline consumption and chemical adsorption mechanisms. A layered heterogeneous model was set up by taking into account the pilot geological characteristics. The predicted performance is shown in Table 11.3. In the table, Ca, Cs, and Cp denote alkaline, surfactant, and polymer concentrations, respectively. After the designed PV of chemical slug was injected, water was injected until almost no oil was produced. The total injection PV for each case is shown in the table as well. The cost is the chemical cost per barrel of incremental oil produced. An exchange rate of 7 Chinese yuan per U.S. dollar was used. From... 

The injection scheme for this test was 0.25% A1 + 0.5% A2 + 0.06% S -e 0.15% P. The pilot was started in October 1997 and was stopped in June 1999 because the casing for the injector 125 was broken. The test was resumed in October 1999, and a chemical injection was ended in June 2000. Only the wells on the southern and northern sides of the injector 125 responded to the chemical injection (water cut reduced and oil rate increased), not the wells on the eastern and western sides, because the main water injection stream was oriented in the eastern-western direction so that the wells in this direction were well flushed by waterflood before the ASP injection. The ASP injection improved sweep efficiency on the southern and northern sides. Thus, the wells on these sides responded to the ASP injection. The injected surfactant concentration was low (0.06%), and the crude oil had a low acid number. The main mechanism in this pilot was probably the sweep efficiency improvement by polymer injection. 

The caustic method as a means of improved waterflooding for enhanced oil recovery is a complex process. Johnson has outlined four recovery mechanisms (6). Presumably, besides the ultralow tension mode, there are other requirements to ensure efficient and stable recovery of an oil in a given reservoir. To name a few spontaneous emulsification, entrainment, entrappment, wettability reversal in both directions, etc. In order to maintain a particular set of pro-... 

Also, according to Charles et al. (1985), specific oilfield waterflood operations have used mechanical and chemical technologies in an attempt to remove hydrogen sulfide from active waters. Some mechanical methods involve aeration, anion exchange resins, degassing, distillation, steam reforming, and zeolite softeners. Most of these methods are viewed currently as being expensive or impractical. 

The final conclusion was that acrolein can be used effectively to scavenge hydrogen sulfide from the oilfield waterflood. Good performance results depend on the natnre of the system and the mechanism of the scavenging process. Once these concepts are identified within a given system, economic jndgments can be made (Charles et al. 1985). 

With these figures we calculate that the surfactant bank should have arrived at the effluent end of the core at 0.87 pore volumes of fluids injected provided that adsorption of surfactant on the rock was the only mechanism of retardation. The surfactant bank actually arrived, as measured by C/C = 0.5, at 0.98 pore volumes of chemical slug injected. To attribute that additional 0.11 pore volumes of surfactant retardation to increased adsorption requires a mechanism by which the presence of oil in the rock, at waterflood residual saturation and less, more than doubles the amount of surfactant absorbed by the rock. Furthermore, since the concentration of surfactant in the effluent liquids rises above the concentration of surfactant in the chemical slug, any mechanism based on adsorption would have to include a condition under which previously adsorbed surfactant could be produced in the presence of full-strength chemical slug. We believe, rather, that the greater retention of surfactant in the presence of oil is due to the formation inside the core of microemulsion phases which are richer in surfactant and often are more viscous than the chemical slug. 

Run DY-15 shown in Figure 4, In this experiment, the flow rate decreased to zero just before breakthrough of the driving fluid (at about T = 0.6 PV in a nominal waterflood). Substantial differences in the upstream pressure of the flow apparatus (400 psi at T = 1.2 PV) and the differential pressure across the core (75 psi at T = 1.2 PV) indicate that most of the emulsification and entrapment occurred in the entrance region of the sandpack. This mechanism was repeatedly observed in high pH, non-saline floods of moderate acid number (> 2.0) oils. 

Ultra-low tension In the alkaline flooding of acidic acids, some reduction in interfacial tension (from 30 to approximately 10 1 dynes/cm) is necessary for the emulsification and subsequent mobilization of waterflooded residual oil by the previously discussed phase alteration mechanisms. The residual oil may also be mobilized and produced by a low-tension displacement process which is similar to surfactant flooding if the interfacial tension can be further reduced to ultra-low values (10 to 10 dynes/cm). 

Emulsification and coalescence The possibility of enhancing oil recovery from porous media by a spontaneous emulsification mechanism has been examined by Schechter and coworkers (6). These researchers postulated that residual oil, which is entrapped after a conventional waterflood, can be mobilized by spontaneous emulsification and subsequent coalescence of small droplets with other... 

Recovery of acidic oils with alkaline agents by an emulsification and coalescence mechanism Calcium hydroxide [Ca(0H)2] was used to verify the emulsification and coalescence concept since, as suggested by the theoretical and experimental evidence of an earlier section, the carboxylic salts of divalent ions form unstable emulsions of water-in-oil. The emulsification and coalescence concept was quantitatively verified by secondary and tertiary flooding of partially oil-saturated sandpacks. A tertiary chemical flood with Ca(0H)2 (pH = 12) recovered 44 percent of the waterflood residual oil from a 3.5-darcy Ottawa sandpack the oil had an acid number of 2 and a viscosity of 1.5 cp. A secondary caustic flood with Ca(0H)2 (pH = 12.32) recovered 82.3 percent of the original oil in place from a 0.25-darcy Ottawa sandpack the oil phase in this secondary flood had the same physical and chemical properties as the oil phase used in the tertiary mode flood. It should be noted that the microscopic mobilization efficiencies of these... 

The entrapment and mobilization of residual saturations is pertinent to many aspects of enhanced recovery (1-4). Entrapment mechanisms determine what proportion of oil is recovered from the swept zone of a waterflood, and, consequently, how much oil remains for possible recovery by tertiary methods. The volume of oil remaining in swept zones and the fraction of this oil that is recovered are critical economic factors in application of enhanced recovery processes. 

When waterflooding a reservoir proves to be inefficient, in the sense that there is early water production and low oil recovery at breakthrough, polymer flooding may be considered as a possible remedy. However, exactly how the polymer flooding process is applied and how the design of the flood is determined depends on the nature of the recovery mechanism that is in operation. 

An important application of polymers in several areas of the world is in the improvement of waterflooding performance in highly inhomogeneous reservoirs. In particular, polymers can reduce the harmful effects of high-permeability layers in a vertically stratified system and so reduce watercut and improve vertical sweep efficiency. Polymers can act by a combination of two mechanisms ... 

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