Polymer Retention during Flow of Polymer Solutions through Porous Media

Abstract

Polymer solution flow and retention through porous media is of interest to many applications in the oil industry such as drilling, water shut-off and enhanced oil recovery. Operators of mature oil and gas fields are faced with the problem of excessive water production (EWP), which can cause a premature abandonment of some oil and gas wells. It has been found that the injection of high molecular weight polymer solutions through the pay zones of the oil and gas wells would induce a sharp decrease of the water production without affecting the oil and gas production. This effect is called disproportionate permeability reduction (DPR) and the polymer solutions inducing such an effect are called relative permeability modifiers (RPM). Hence, the DPR effect has been utilized in the water shut-off or conformance control of oil and gas wells suffering from EWP. In spite of the extensive research of the DPR effect, there is still a lack of agreement on the mechanisms controlling such an effect and relatively high percentage failures are observed during conformance control field applications. Polymer retention in porous media has been attributed to mechanisms such as bridging-adsorption, adsorption-entanglement, and flow-induced adsorption. These mechanisms have been proposed to account for the increase in flow resistance during or after the flow of polymer solutions through porous media. The DPR effect has been attributed to effects induced by this retained polymer such as steric and lubrication effects, wettability change, segregated oil and water pathways, and swelling and shrinking of the adsorbed polymer layer. The aim of this study is to add knowledge on the effect of polymer solution flow on polymer retention in porous media. In this study, the rheology of high molecular weight polymer solutions was studied using a cone-and-plate setup. Moreover, the characteristics and the effective hydrodynamic thickness of adsorbed polymer layers on glass from these polymer solutions under static conditions were investigated using atomic force microscopy (AFM). Also, quartz crystal microbalance with the dissipation monitoring (QCM-D) was used to investigate the effect of increasing the flow rate of polymer solutions on the adsorbed amount on silica and gold surfaces. Additionally, the mobility reduction and the residual resistance as a result of polymer solution flow through single glass capillaries, 2D and 3D models of porous media were studied. The implementation of the above techniques was used to relate the microscopic effect of the flow of the polymer solutions to the polymer retention in the porous media. The anti-thixotropic behaviour of the polymer solutions, which can be attributed to the shearinduced formation of micron-size transient entanglement networks (TEN), is expected to play a major role in the polymer retention in porous media. These microscopic structures can adsorb on the solid surfaces if the adsorption energy of the polymer/solid system is sufficient. Also, in porous media in which mechanical entrapment is possible, these structures can be entrapped in the small pores and pore throats. Two new mechanisms for polymer retention are proposed in this study: transient-entanglement networks adsorption (TENA) and transient-entanglement networks entrapment (TENE). The TENA is the retention mechanism of the TEN structures in flow systems in which mechanical entrapment is not possible provided that the adsorption energy is sufficient. If mechanical entrapment is possible, then the retention by adsorption and mechanical entrapment are lumped in the TENE mechanism. The results from this study have given a new insight on the flow and retention of polymer solutions through porous media. Hence, it is believed that the improved understanding will improve the design of high molecula