Summary

Matrix acidizing and water control are usually addressed as two separate issues. Associative polymers can be used to simultaneously achieve effective acidizing and water control during a single treatment. A polymer-based treatment was applied in an offshore, perforated vertical well with two sets of perforations in a carbonate reservoir in Saudi Arabia. The acid treatment was needed to restore the productivity of the upper set of perforations and reduce water production from the lower set of perforations.

Experimental studies were carried out to investigate the potential use of associative polymers to control water mobility and act as an acid diverter. Coreflood experiments were conducted on reservoir cores at downhole conditions (temperature of 200°F and pressure of 3,500 psi). Extensive laboratory testing showed that associative polymers had no significant effect on the relative permeability to oil. However, the relative permeability to water was significantly reduced.

This paper presents a case history where an associative polymer was applied during matrix acid treatment of a damaged well. The treatment included two stages of associative polymer solutions and 20 wt% HCl with additives. Post-stimulation treatment production data showed that oil rate increased 11.18-fold, whereas water rate decreased 1.7-fold, resulting in a reduction in the water cut from 75 to 14 vol%.

The production logging tool (PLT) results indicated that the associative polymer was effective in diverting the acid into the oil producing zone. The upper set of perforations was producing most of the fluid, which further confirmed that the associative polymer significantly reduced water production from the lower zone.

Introduction

Matrix acidizing and water control are two important treatments conducted to enhance well performance. These treatments are commonly addressed as two separate issues. Associative polymers can be used to simultaneously achieve effective acidizing and water control utilizing a single treatment (Eoff et al. 2005).

Acid diversion is an important issue contributing to the success of any matrix acid stimulation treatment. For this reason, extensive laboratory studies and field applications have been performed on several acid diverting agents as reported in the literature. Among the techniques that have been applied to improve acid coverage are: mechanical (packers, ball sealers, and particulate diverting agents) and chemicals (foam, polymers, and in-situ-gelled fluids). More recently, viscoelastic surfactants have been used extensively for diversion during matrix acid treatments, and have shown a tendency to reduce water production as reported by Nasr-El-Din et al. (2006).

Relative permeability modifiers, commonly used for water control, can also be utilized for acid diversion. They can act simultaneously to enhance diversion during matrix acid treatments and impair water mobility. Eoff et al. (2005) presented laboratory and field tests, which showed that associative polymers could provide both goals in sandstone reservoirs. However, a few studies considered application of associative polymers to divert and control water production in carbonate formations. Therefore, the objectives of the present study are to:

  1. assess the effectiveness of associative polymers in reducing brine permeability in carbonate cores,

  2. design a polymer-based treatment to control water and divert acid in matrix treatments, and

  3. evaluate the use of associative polymers based on field application.

This paper presents laboratory data that support the use of this new technology in carbonate reservoirs. It will also give for the first time field results on the application of associative polymers in a carbonate reservoir in Saudi Arabia. Field data were in good agreement with laboratory results.

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