Abstract

This study explores the potential of dual polymer-colloid systems to improve polymer placement. The approach consists of an analysis of microscopic flow aspects. We show that dual bridging adsorption of polymer-colloids may occur similarly to that of polymers alone. However, in the former case, the ensuing permeability reduction is much stronger. Therefore, dual polymer-colloid adsorption may improve considerably polymer placement.

Introduction

Adequate polymer placement is crucial in water control by polymers and polymer gels. Satisfactory placement can be achieved by:

  1. self selectivity of flow based on high permeability contrast (fractured rock), or

  2. mechanical zone isolation (layered reservoirs).

The latter method applies when layers of distinct permeability are clearly identified and readily accessible.

Zone isolation is costly and the operators prefer to avoid it. Moreover, the above placement methods are inappropriate when permeability contrast is moderate, or zones of different permeability are poorly described (complex heterogeneous structures, horizontal wells, etc). Those cases are quite frequent and highly significant for oil recovery.

Previously, we described the adsorption of stretched polymer molecules (bridging adsorption) and showed how this effect can improve polymer placement. We observed that bridging adsorption occurs:

  1. under high velocity gradients,

  2. in the presence of adsorption and

  3. when the permeability is small.

The main effect of this phenomenon is a strong permeability reduction. The above observations were consistent with the concept that bridging adsorption takes place when the polymer stretched length (R) and pore diameter (Dp) are of the same order of magnitude, or when the polymer length is larger than the pore diameter. This suggests the existence of a pore diameter DMAX, above which no bridging adsorption occurs. Pores such that Dp >DMAX seldom contribute to permeability reduction.

The bridging adsorption phenomenon was first identified and investigated using packs of well calibrated and edge shaped grains. These packs had narrow pore size distributions, which apparently maximized the permeability reduction. Natural rocks have a much broader pore size distribution. The theory predicts that adsorption of only polymers may produce insufficient permeability reduction in those rocks. This paper explores the potential of polymer and colloid properties to strengthen the effects of bridging adsorption. We examine the flow of those suspensions in bulk and in porous media. The analysis indicates that the simultaneous bridging adsorption of polymers and colloids must occur similarly to that of polymers alone. However, the permeability reduction may be much larger than for only polymers. Our preliminary results support the idea that dual polymer-colloid systems may improve considerably polymer placement, and thus water control.

Bridging Colloids

1. General properties. Colloids are dispersions of liquid or solid particles, with an major length L ranging from 10 nm to 1 m, in another liquid. These systems are inherently unstable because attractive forces between particles favor flocculation. In non-polar or slightly polar materials those forces arise mainly from the London-van der Waals (LVDW) dipole-induced dipole interactions. At short inter-particle distances, LVDW forces are associated with an attractive potential of the form

(1)

where AH is the Hamaker constant, and n is an exponent dependent on the shape of the particles; for spherical particles we have n = 1.

The stability of colloids is due to surface repulsive forces. In electrolyte solutions, surface electric charges arise from unequal adsorption of positive and negative ions.

P. 501^

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