Abstract

Disproportionate permeability reduction (DPR), in which water permeability is reduced to a much greater extent than oil permeability, has been observed in a number of polymer gel systems. Although several different mechanisms have been proposed, no one mechanism has been generally accepted. In the study reported here, the focus was on the effect of flow rate on DPR by varying the pressure gradient. Polyacrylamide-chromium acetate solutions were allowed to gel in Berea sandstone rocks. Flow channels were then developed by dehydration of the gels by flowing oil or water at a specified pressure gradient. After dehydration was complete, water and oil displacements were successively conducted over a wide range of pressure gradients, always maintaining the pressure gradient at a value equal to or less than the pressure gradient used for dehydration. Oil saturation of the rock was monitored from analysis of the effluent and the use of a tracer in the oil phase. The saturation profiles in some of the rocks were obtained from microwave scans at various stages of the experiment. Permeabilities and residual resistance factors at residual saturations were calculated and reported as a function of pressure gradient. RRF values for brine decreased by an order of magnitude upon increase in pressure gradient. RRF values for oil, however, decreased to a much less degree over the same increase of pressure gradient. At high pressure gradients, DPR diminished in one set of experiments. The variation of permeability with pressure gradient demonstrates that the gel structure is elastic and deforms when pressure gradient changes. The results were compared to literature data and it was found that rate effects were generally present in previously reported work.

Introduction

Produced water is a growing concern in mature fields as oil production declines and water cuts increase. Each additional barrel of produced water adds to the cost of

  • lifting the hydrocarbon in a production well,

  • separation of oil and water,

  • protection against corrosion in tubing, and

  • disposal of water.

Economic and environmental interests drive development of techniques that can reduce production of water without affecting production of oil significantly.

The treatment of a production well with a gelled polymer system is a remedial measure in which the operator would like to preferentially reduce the permeability to water with minimum effect on oil production. Although no treatment has been found that reduces water permeability without affecting oil permeability to some extent, several polymers and gels reduce permeability to water significantly more than to oil when injected into porous rock. This phenomenon, termed disproportionate permeability reduction (DPR), is potentially beneficial for water shut-off treatments in production wells when hydrocarbon-productive zones cannot be protected during gel placement or when the source of water production cannot be determined.

Many production wells have been successfully treated with gelled polymer systems.1 Treatment methodology tends to be empirical in part because there is still uncertainty on how permeability is reduced selectively. The effect of polymer treatments on the permeability to water and oil after gel treatment has been investigated for almost two decades. Research has focused on two general areas:

  • investigation of mechanism(s) leading to disproportionate permeability reduction2–19. and

  • estimation of permeability under various fractional flows.20,21

Willhite et al.14 demonstrated that disproportionate permeability is observed when oil and brine flow through the new pore structure created by dehydration of a portion of the gel structure in the treated porous rock by injection of oil. Disproportionate permeability was thought to occur because residual oil was trapped in the new pore structure when oil was displaced by water. Disproportionate permeability reduction was found to be a function of the pressure gradient applied initially to dehydrate the gel, and the flow rate (or pressure gradient) in the new pore structure created by dehydration.

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