Abstract

Injecting stable, preformed microgels as relative permeability modifiers to reduce water production minimizes the risk of well plugging or the absence of efficiency inherent to a technology based on in-situ gelling. Recent investigations showed that microgels formed by crosslinking a polymer solution under shear are soft, size-controlled, quasi-insensitive to reservoir conditions, stable over long periods of time and can control in-depth permeability by adsorbing onto all types of rock surface. The new laboratory studies reported in this paper aimed at knowing how to control the kinetics of crosslink formation by ionic strength and at determining the role the interactions between microgels on their propagation in porous media. The reported experiments include:

  1. gelling tests at different ionic strengths,

  2. measurements of viscoelastic properties of solutions,

  3. determination of both microgel density and microgel-microgel interaction parameter for different conditions of stabilization,

  4. the relation between the interaction parameter and the mode of adsorption of microgels.

Partly attractive microgels were found to adsorb by forming multilayers and thus to induce drastic permeability barriers. Fully repulsive microgels adsorb as a monolayer and propagate easily in porous media at long distances depending only on the quantity of microgel injected. Thus, by controlling both gelling and stabilization processes, microgels can be produced to be either diversion agents or disproportionate permeability reducers to control water permeability at long distances from the wells.

Introduction

The reduction of water production becomes an increasingly important objective for oil industry, particularly because new environmental regulations impose severe limitations on the disposal of produced water.

To reduce water production, a commonly used technique is to inject a polymer solution together with an organo-metallic crosslinker (1,2). The success of such well treatments would imply a good control of the in-situ formation of weak gels capable of reducing water permeability without affecting oil permeability. However, both gelation kinetics and final gel strength are very sensitive to the physico-chemical environment prevailing around the wells (pH, salinity, temperature, shear rates...). Since all these parameters cannot be known with the required accuracy, the results of such well treatments are hardly predictable: no gelling implies no effect on water production whereas the formation of a strong gel can affect drastically well productivity.

To minimize these risks inherent to all well treatments based on in-situ gelling, we proposed to use soft, size-controlled "microgels" formed and stabilized before injection (3–6). Such microgels can be prepared on-site in a unit specifically designed to control precisely both shear rate during gelling and physico-chemical conditions. Ideally, microgels designed for water shutoff or profile control should be:

  1. insensitive to shear and reservoir physico-chemical conditions,

  2. size-controlled to prevent face plugging,

  3. small enough to ensure an in-depth treatment and large enough to reduce significantly water permeability,

  4. soft enough to be collapsed onto pore wall by capillary pressure in presence of oil flow in order to be disproportionate relative permeability modifiers,

  5. strongly adsorbing onto pore surface and stable over time, and

  6. non-toxic for the environment.

Using non-toxic crosslinkers was a strong incentive to investigate the gelling properties of zirconium complexes (9–21).

The main aim of this study was to improve the conditions of microgel formation and stabilization in order to obtain the properties required for different types of water shut-off operations. In the first section are described new gelling experiments carried out to elucidate the role of electrostatic forces on crosslinking kinetics. In the second section, the viscoelastic properties of microgel solutions are analyzed, giving information on the deformability of microgels under hydrodynamic forces. In the third section, the relation between the Huggins constant (a commonly used interaction parameter) and the propagation of microgels in porous media is established. In the fourth section, the behavior in porous media of fully repulsive microgels has been investigated. Finally, the main conclusions as well as the perspectives are drawn.

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