Effects of Polymer Adsorption and Flow Behavior on Two-Phase Flow in Porous Media
- C.G. Zheng (BDM Petroleum Technologies) | B.L. Gall (BDM Petroleum Technologies) | H.W. Gao (BDM Petroleum Technologies) | A.E. Miller (BDM Petroleum Technologies) | R.S. Bryant (BDM Petroleum Technologies)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- June 2000
- Document Type
- Journal Paper
- 216 - 223
- 2000. Society of Petroleum Engineers
- 1.8 Formation Damage, 5.3.1 Flow in Porous Media, 5.3.2 Multiphase Flow, 5.3.4 Reduction of Residual Oil Saturation, 1.6.9 Coring, Fishing, 5.2.1 Phase Behavior and PVT Measurements, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.4.1 Waterflooding, 5.1 Reservoir Characterisation, 4.1.2 Separation and Treating, 5.6.2 Core Analysis, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation
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To determine the effect of water-soluble polyacrylamide polymer adsorption and flow behavior on oil recovery, relative permeability and mobility were determined from flow experiments at various polymer concentrations. A selective reduction of the relative permeability to water with respect to the relative permeability to oil was observed for both Berea and reservoir sandstone cores. Adsorbed polymer layer increases water wettability. Relative permeability reduction could be attributed to both wettability change and pore-size restriction due to the adsorbed polymer layer. An empirical model was proposed to correlate the relative permeability reduction and the amount of polymer adsorption. Depletion-layer effect results in a reduced polymer viscosity in porous media with respect to bulk solutions. Modification of the existing shear-rate model allows for accurate prediction of this effect. The integration of the new models in UTCHEM provides a more accurate tool for engineering design of polymer applications.
Water-soluble polyacrylamide polymers have been used to reduce water production in oil wells and for mobility control in injection wells for decades.1 One of the attractive properties of polyacrylamides is their ability to reduce the relative permeability to water more than the relative permeability to oil in porous media. From the published field tests on well treatments by polymer adsorption in the 1970's and 1980's,1,2 only a few jobs were considered to be economically successful. Results could not be interpreted due to the lack of detailed information. At present, the importance of laboratory research and simulation study are emphasized for successful field design.
The selective permeability reduction by polymer adsorption was traditionally termed as "permeability reduction" or "residual resistance factor," which is equivalent to the endpoint relative permeability. Previous laboratory results1 indicated that the maximum reduction of the endpoint relative permeability to water caused by polymer adsorption can be as high as a factor of 10, while the reduction of the endpoint relative permeability to oil is less than 2. If crosslinking or swelling agents are applied, the maximum reduction of the relative permeability to water could be more than two orders of magnitude. The mechanisms of this selective reduction have been explored by several researchers.3-5 An understanding of these mechanisms could be obtained from the selective permeability reduction by gels.6 Measurement of the residual resistance alone may provide a qualitative estimation. To model the effect of polymer adsorption, however, a measurement of the relative permeability is necessary, especially when residual saturation and the shape of the relative permeability curves change after polymer adsorption.
Modification of the relative permeability by polymer adsorption has been intensively studied recently.3-5,7-10 Ali et al.4 and Barrufet and Ali,5 derived the relative permeability from drainage capillary pressure measured by an ultracentrifuge and showed that the reduction of the relative permeability caused by starch-based polymers is dependent on saturation. The reduction was interpreted as a change in lubrication along the pore walls. Direct measurement of relative permeability after polymer adsorption was also seen in Refs. 3 and 7 through 10. In water-wet porous media, it was found that the residual oil saturation remained almost the same after polymer treatment. At residual oil saturation, the quantity of adsorbed polymer per gram of rock was also found to be almost the same as at 100% water saturation, but the endpoint relative permeability reduction to water was increased in the presence of residual oil.
Based on a capillary bundle model, a correlation of the relative permeability curves with polymer-layer thickness was proposed by Zaitoun and Kohler.3 However, the relationship of the polymer-layer thickness with the quantity of adsorbed polymer is still unknown, and further modification of the capillary bundle model may also be needed to model complex pore matrices.
On the other hand, as polymer propagates through porous media, polymer solution will be diluted in the propagation front due to dispersion and adsorption, and the dilution could extend to the entire slug if the slug size is too small. So far, few researchers have related the variations of the relative permeability curves as a function of polymer concentration or the quantity of adsorbed polymer. Therefore, one of the objectives in the present study is to measure and correlate relative permeability curves as a function of polymer adsorption.
Polymer solution mobility was also studied as a function of polymer concentration. Effective viscosity at low shear rate in porous media is lower than that in bulk solution at the same shear rate. The dependence of the depletion-layer effect on polymer concentration as well as porous media will be examined in this paper.
Finally, the numerical models of relative permeability and mobility as a function of polymer concentration developed in this study will be incorporated in UTCHEM. Several cases will be studied to compare incremental oil recovery predicted by these new models with that predicted by previous descriptions of polymer behavior in porous media. A simplified layered reservoir model will be used for comparative simulation runs. Polymer flooding and near-wellbore polymer treatments will also be simulated. Results from these simulations should provide guidelines for future field strategies.
Both strongly water-wet and mildly oil-wet cores were chosen to study the influence of wettability on polymer adsorption, two-phase relative permeability, and polymer solution mobility. The mildly oil-wet medium is a Warden reservoir sandstone core from Santa Fe field, Stephens County, Oklahoma. Two strongly water-wet media are Berea sandstone cores with different permeabilities. Table 1 summarizes the petrophysical properties of these sandstone samples.
Synthetic brines were prepared to represent reservoir brine (produced water) composition and makeup water (injection water) composition used in the Warden reservoir. Produced water has a total dissolved solid (TDS) of 31,300 ppm which contains 29 g/L NaCl, 0.94 g/L CaCl2, 0.77 g/L MgCl2 , 0.11 g/L KCl, and 1.1 g/L NaHCO3, and injection water has a TDS of 1,490 ppm which contains 0.343 g/L CaCl2, 0.252 g/L MgCl2 , 0.176 g/L Na2SO4, and 0.72 g/L NaHCO3
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