A Rheological Model for pH-Sensitive Ionic Polymer Solutions for Optimal Mobility Control Applications
- Chun Huh (U. of Texas at Austin) | Suk Kyoon Choi (U. of Texas at Austin) | Mukul Mani Sharma (U. of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 9-12 October, Dallas, Texas
- Publication Date
- Document Type
- Conference Paper
- 2005. Society of Petroleum Engineers
- 4.3.4 Scale, 1.8 Formation Damage, 7.1.9 Project Economic Analysis, 5.1.1 Exploration, Development, Structural Geology, 5.4 Enhanced Recovery, 5.4.5 Conformance Improvement, 3 Production and Well Operations, 4.1.1 Process Simulation, 2.5.2 Fracturing Materials (Fluids, Proppant), 7.1.10 Field Economic Analysis, 5.4.1 Waterflooding, 5.3.1 Flow in Porous Media, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 2.4.5 Gravel pack design & evaluation
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Polyelectrolytes are water-soluble polymers that form molecular networks by association in solution. The properties of such polymers are very sensitive to the pH and ionic content of the solution.Polyelectrolytes such as poly(acrylic acid) can swell up to ~1,000 times of its own volume by retaining a huge volume of water, or they may reversibly de-swell to their original volume.Their solution viscosity can accordingly be changed by several orders of magnitude in a controlled manner by adjusting the solution pH.This remarkable property of pH-sensitive polymers can be exploited for a number of improved oil recovery (IOR) applications, as proposed by Al-Anazi and Sharma (2002).A key advantage for polyelectrolytes such as poly(acrylic acid) is their very low cost, because immense quantities are manufactured annually for consumer and industrial applications.For IOR processes, such as surfactant flooding, CO2 flooding and alkaline flooding, a major technical challenge is to bring the injected chemicals into contact with the oil and to properly herd the mobilized oil to producers, minimizing the undesirable influence of reservoir heterogeneity.Placement of the mobility control polymer in desired locations in the reservoir for controlled viscosification can significantly alleviate the problem of volumetric sweep.For the optimal control of the polymer viscosity at the desired locations, the dependence of the polymer rheology on pH, salinity, polymer concentration and molecular structure needs to be known accurately.The rheology correlation can then be employed for IOR process simulations for optimal design of mobility-control bank placement and viscosification, and for evaluation of its effectiveness.
A comprehensive rheology model is developed by combining the ionic hydrogel swelling theory of Brannon-Peppas and Peppas (1988) with the Mark-Houwink equation that relates the polymer intrinsic viscosity with polymer molecular size; the Martin equation that relates the characteristic Newtonian viscosity with polymer concentration and intrinsic viscosity; and the Carreau equation that relates polymer viscosity with shear rate.The ionic swelling theory had been independently validated with laboratory swelling experiments earlier, and the Mark-Houwink, Martin, and Carreau equations have been employed to characterize aqueous polymer solution rheology.The polymer solution viscosity can, therefore, be reliably predicted as a function of pH, salinity, polymer concentration, and shear rate.The model predictions match the laboratory-measured viscosity data reasonably well, for a range of pH, salinity, polymer concentration, and shear rate.The model is presented in a form that can be implemented in an IOR process simulator for optimal mobility control applications, as described above.
For various improved oil recovery (IOR) processes, it is generally feasible to develop injectant chemicals that are effective in displacing oil once the chemical contacts oil in the reservoir.A major technical challenge, however, is to bring the chemicals to all pore volumes where oil exists and, when the resident oil is mobilized by the chemical, to herd the oil to producers without loss by re-trapping on the way.High-viscosity polymer solutions (Cannella et al. 1988) and foams (Rossen 1996) have been used as mobility control banks to push the injectant bank and the mobilized oil bank for effective oil displacement and recovery.There are, however, two technical difficulties with use of polymer and foam: First, the high-viscosity of polymer (and low-mobility of foam) requires a large pressure drop at the injection well, and maintaining a good injectivity is sometimes difficult.Second, to compensate for adsorption/retention of polymer (and foam-forming surfactant) in a reservoir, a large quantity of polymer needs to be used for effective mobility control, making the chemical cost a significant deterrent for project economics.A broad objective of the present study is to investigate the feasibility of employing a pH-sensitive polyelectrolyte to alleviate the above two difficulties, and to develop a new way of mobility control that is significantly more effective than currently used.
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