Effects of Reservoir Anaerobic, Reducing Conditions on Surfactant Retention in Chemical Flooding
- F.H.L. Wang (Exxon Production Research Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1993
- Document Type
- Journal Paper
- 108 - 116
- 1993. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 1.6.9 Coring, Fishing, 5.4.1 Waterflooding, 1.2.3 Rock properties, 5.7.2 Recovery Factors, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 2.5.2 Fracturing Materials (Fluids, Proppant), 5.6.5 Tracers, 4.3.4 Scale, 2.4.3 Sand/Solids Control, 5.4.10 Microbial Methods, 1.8.5 Phase Trapping, 5.5.2 Core Analysis, 5.2.1 Phase Behavior and PVT Measurements, 5.2 Reservoir Fluid Dynamics, 5.3.4 Reduction of Residual Oil Saturation
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Surfactant retentions observed in four microemulsion-flooding pilot tests at the Loudon field were substantially lower than predicted from conventional laboratory coreflood experiments. This paper presents research results that explain this discrepancy. The oil reservoir was in an anaerobic, reducing condition, whereas laboratory corefloods were normally conducted under aerobic, oxidizing conditions. The difference in redox condition was shown to have a serious effect on surfactant retention. Laboratory corefloods conducted under reservoir-like, anaerobic, reducing conditions gave surfactant retention results significantly closer to those observed in field tests. The effect of redox conditions on surfactant adsorption was substantiated further by results from static adsorption experiments with various clay types. Exposure of preserved cores from a reduced reservoir to aerobic conditions can cause high surfactant retention in corefloods. Methods were developed to restore such oxygen-contaminated core material to its original, anaerobic, reduced state. These coreflood procedures simulate actual reservoir conditions better and give meaningful surfactant-retention results for process design optimization.
Two primary criteria that determine whether a chemical-flooding process will be successful are the amount of oil recovered and the amount of chemical required to achieve such recovery. Naturally, the quantity of surfactant needed is a strong function of the level of retention loss in the reservoir rock as the microemulsion bank propagates through the reservoir.
Exxon Production Research Co. has completed several microemulsion-flooding pilot tests at the Loudon field in Fayette County, IL. The field was in an advanced stage of depletion after about 13 years of primary production and 30 years of waterflooding. The main reservoir in the pilots was the Mississippian (Chester) Weiler sandstones. Most reservoir conditions listed in Table 1 were favorable for chemical flooding except for the high brine salinity. Table 2 shows the composition of Tar Springs brine (TSB). Tar Springs is a younger aquifer above the Weiler reservoir, and the TSB, used in the waterflood for many years, had become the resident brine of the reservoir.
The results of two small pilots (0.7 acre [0.28 ha] each in the Ripley lease) and two expanded pilots (40 acres [16 ha] in the Griffith lease and 80 acres [32 ha] in the Heckert and Hackert leases) are available in the literature.1-4 One of the most surprising observations in these field tests was that the surfactant retention was less than one-half of that measured in conventional laboratory coreflood experiments. The large difference in surfactant retention between the field and laboratory floods complicated the use of corefloods to optimize economics of a chemical-flooding process. This paper presents research results that explain the discrepancy between the field and laboratory observations.
Loudon Microemulsion-Flood Process
The microemulsion-flood process used in the Loudon field included the injection of a small microemulsion bank, followed by a large polymer-drive bank, and then by chase brine until pilot termination.1-4
The microemulsion was formulated by mixing two surfactant concentrates and a xanthan biopolymer broth with the TSB to achieve a target of 0.023 g/100 mL surfactant and 28-cp [28-mPa·s] viscosity at a shear rate of 11 seconds-1 under the reservoir temperature of 78°F [25.6°C]. A refined oil was added to incorporate polymer into the microemulsion.5,6 The salinity of the microemulsion was 96% that of the TSB. The surfactant used was a mixture of oxyalkylated alcohol sulfates, i-C13H27O(C3H6O)m(C2H4O)n SO3Na.6,7 Average values of m and n for the surfactant blend used in the first Ripley pilot were 3.60 and 2.80, respectively, and 3.78 and 2.44, respectively, for the other three pilots.
The polymer drive contained xanthan bipolymer dissolved in TSB and fresh water to achieve a salinity of 70% TSB (60% in the first Ripley pilot) and a viscosity of 38 cp [38 mPa·s] at 11 seconds-1 and 78°F [25.6°C]. The chase brine had the same salinity as the polymer drive. Lowering the drive-water salinity helped remobilize part of the adsorbed surfactant and minimize microemulsion phase-trapping problems.5
All injected fluids in the first Ripley pilot also contained 75 mg/L of sodium dithionite (an oxygen scavenger), 10 mg/L of active dibromonitrilopropionamide (DBNPA) biocide, a small amount of acetic acid to maintain the pH near 5.0, and various tracers to monitor fluid propagation.
All injected fluids in the three other pilots contained about 1,000 mg/L formaldehyde as a biocide, 90 mg/L citric acid to maintain the pH near 5.2 (to control iron precipitation and to prevent polymer crosslinking), and various tracers to monitor fluid propagation.
Surfactant Retention Observed in Field Tests
Numerous laboratory coreflood experiments were conducted in the late 1970's and early 1980's to help design the Loudon microemulsion-flooding process. These corefloods yielded surfactant retention from 0.5 to 0.9 mg/g rock after the drive-water salinity reduction (60% or 70% TSB). Surfactant retentions at the resident brine salinity (100% TSB) were about 50% higher than these values. An average retention value of 0.7 mg/g rock was used in the design of the field pilot tests.
However, surprisingly low surfactant retention was observed in all Loudon field tests. Table 3 lists measured retention levels in these tests. Surfactant retentions at the resident salinity were determined from frontal delay of surfactant relative to the tracer. Retentions at the drive-water salinity were determined by material-balance calculation. Retentions in postpilot cores were measured by extraction with a 50/50 mixture of isopropyl alcohol (IPA) and 60% TSB brine.
The first Ripley pilot had a relatively high surfactant retention (0.35 mg/g rock) by material-balance calculation. Some of this surfactant probably was retained by phase trapping at a later stage of the flood because bacterial degradation of xanthan biopolymer caused a loss of mobility control in the polymer-drive bank.1,2 The postpilot core was taken from a site that had been swept adequately by the polymer-drive bank and therefore yielded a smaller surfactant-retention value (0.18 mg/g rock) than the pattern average value. The bacterial problems were eliminated in the three subsequent pilots by use of formaldehyde.
The second Ripley pilot had better polymer transport, sweep efficiency, and oil recovery throughout the test.3 Surfactant retention (0.08 mg/g rock from material-balance calculation) was also the lowest among the four pilots. Oil displacement efficiency in the postpilot core exceeded 99%.
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