Strategy for Alkaline/Polymer Flood Design With Berea and Reservoir-Rock Corefloods
- D.E. Potts (Chevron Oil Field Research Co.) | D.L. Kuehne (Chevron Oil Field Research Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1988
- Document Type
- Journal Paper
- 1,143 - 1,152
- 1988. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.1 Reservoir Characterisation, 5.2.1 Phase Behavior and PVT Measurements, 4.3.4 Scale, 1.6.9 Coring, Fishing, 4.1.2 Separation and Treating, 5.7.2 Recovery Factors, 2.4.3 Sand/Solids Control, 1.2.3 Rock properties, 5.3.4 Reduction of Residual Oil Saturation, 5.4.10 Microbial Methods, 5.6.5 Tracers, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 2.7.1 Completion Fluids, 5.4 Enhanced Recovery, 5.3.2 Multiphase Flow, 5.1.2 Faults and Fracture Characterisation, 5.4.1 Waterflooding, 5.2 Reservoir Fluid Dynamics
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Summary. This paper discusses the differences between fired Berea and reservoir-rock corefloods using an alkaline/polymer chemical system to recover an acidic California oil. Integrating Berea and reservoir-rock results was necessary in this study because of a limited supply of reservoir core material. Berea corefloods were adequate for initial screening and comparing different injection strategies but gave unrealistically high tertiary oil recoveries.
Both sodium/hydrogen and sodium/calcium ion exchanges were higher in reservoir rock than in Berea. Higher alkali concentrations were required in the reservoir cores to prevent chromatographic separation of alkali and polymer. Excessive exchange of hardness ions was a major cause of lower polymer. Excessive exchange of hardness ions was a major cause of lower oil recovery in reservoir corefloods. Recovery improved dramatically when extensive preflushing was used to remove hardness.
Berea sandstone is widely used as a standard porous medium for laboratory coreflooding experiments. Properly selected and prepared Berea cores offer distinct advantages over field cores, such prepared Berea cores offer distinct advantages over field cores, such as greater availability, lower cost, and more uniform rock properties. The cores are often fired before use to reduce freshwater properties. The cores are often fired before use to reduce freshwater sen-sitivity, to lower the waterflood residual oil saturation, or to improve flood reproducibility. Reservoir cores are still necessary for designing chemical processes such as alkaline/polymer flooding because fluid/rock interactions may have a significant effect on oil recovery efficiency.
This paper compares the rock properties and flooding characteristics of Berea sandstone with an unconsolidated reservoir sand. Reservoir core material was obtained from the Whittier field in California, which has a medium-gravity oil exhibiting good reactivity with caustic. A pilot caustic flood was conducted at Whittier in 1966 and was followed in 1983 by fieldwide caustic injection. The alkaline/polymer research reported here was an extension of earlier caustic development work for this field.
Because only a limited amount of core material was available, we planned to use Berea cores for preliminary screening and for much of the process development work. Promising formulations would then be tested in field cores for a more realistic assessment of oil recovery. In preliminary tests, we found major differences in flood performance between the two types of porous media, even when permeabilities were closely matched. Only about one-half as much tertiary oil was recovered from field cores compared with fired Berea when identical chemical formulations and slug sizes were used.
To explain these differences. we initiated a thorough comparison of fired Berea sandstone with the unconsolidated reservoir sand to understand Berea the limitations of using Berea cores in the design of alkali-related EOR processes. Specifically, we wished to address two questions: (1) how the physical and chemical properties of the two rock types compare and (2) how these properties properties of the two rock types compare and (2) how these properties affect flood performance. The problem of long-term alkali consumption from rock dissolution was not considered in this study. Alkali loss from slow reactions has been discussed elsewhere.
The paper is organized into four major sections. In the first, we compare the mineralogy and rock properties of fired Berea samples with representative field cores. Properties considered include wettability, heterogeneity, relative permeability, and sodium/ calcium and sodium/hydrogen ion-exchange capacity. The second section discusses flooding procedures and gives results of preliminary screening floods in Berea cores. In the third section, matched pairs of corefloods in both porous media demonstrate the importance of accounting for fluid/rock reactions. The fourth section outlines a general strategy for laboratory optimization of the alkaline/ polymer process, discusses the problem of polymer retention, and polymer process, discusses the problem of polymer retention, and examines alternative chemical-injection strategies.
Mineralogy and Core Properties
Reservoir core material was obtained from the third zone of the Whittier field at a depth of about 640 m [2, 1 00 ft]. The third-zone reservoir is an unconsolidated Lower Pliocene sand, deposited by turbidity currents. Core plugs were cut from uniform, high-permeability intervals of the core with liquid nitrogen. Berea sandstone cores were cut from a large block of medium-permeability rock obtained from Cleveland Quarries, Amherst, OH. All Berea cores were fired at 550deg.C [1,022deg.F] for 18 hours before use.
Mineralogy. Table 1 compares the mineralogy of the reservoir sand with that of Berea sandstone before and after firing. Whittier sand is composed primarily of plagioclase, potassium feldspar, and quartz, with biotite and smectite as the major clays. Berea sandstone is obviously quite different in mineralogy; it is composed mainly of quartz, with kaolinite and chlorite as the major clays. Both rocks contain carbonate in the form of dolomite, with siderite (iron carbonate) also present in Berea.
Fired Berea sandstone bears even less resemblance to the reservoir sand. Dolomite, siderite, and kaolinite structures are changed by firing so that they are no longer detected by X-ray analysis. Thin-section analysis indicates that the siderite is completely decompos to ferric oxide and that other iron-bearing minerals are oxidized to the ferric state. *
The higher surface area and cation-exchange capacity (CEC) of the Whittier sand, given in Table 1, are attributed mainly to the presence of smectite. The surface area and CEC of this clay range presence of smectite. The surface area and CEC of this clay range from 600 to 900 m2/g and 80 to 120 meq/100 g, respectively. Chlorite, the only detectable clay in the fired Berea sample, is a medium-surface-area clay with CEC ranging from 10 to 30 meq/100 g.
Wettability. The Amott wettability index was measured on a 2.54-cm [1-in.] -diameter by 7.62-cm (3-in.] -long plug of reservoir sand encased in heat-shrinkable teflon.
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