Successful Water Shutoff Chemical Treatment in Horizontal Openhole Completion in Carbonate Reservoir

The development of well completion strategy at the subject field has evolved from simple vertical completions to extended horizontal wells with selective openhole completions (Etuhoko et al. 2014). Normally, well completions in this field involve large volume acidizing treatments. Matrix acidizing and acid fracturing stimulation are widely used in carbonate formations to improve oil recovery arising from the high solubility of the carbonate formation in hydrochloric acid. The ability to create wormholes in the near-wellbore vicinity provides a stimulation effect by connecting to secondary porosity (natural fractures, voids, and vugs). The connection of the natural fracture network to the wellbore utilizing acid fracturing treatments provides a stimulation effect because the hydraulically fractured reservoir increases the effective wellbore radius and results in a negative skin factor. However, massive acid treatments raised certain issues after beginning to develop oil-bearing zones near water/oil contacts. Significant water production occurred in some flank wells after acid stimulation. High permeability channeling is suspected between the aquifer and the wellbore. There is a high probability of water flow through natural fracture networks because no barrier exists to prevent drainage from the aquifer, which is connected to the wellbore with acid-stimulated wormholes or via acid etched fracture faces. Because production facilities are not designed to handle a high water volume, production wells with an unfortunate water cut are shut down. Obviously, field economics suffer and an effective water shutoff solution is now a vital component of future field development.

Although many advances have been made in both mechanical and chemical methods to reduce unwanted water production, horizontal wellbores present challenges. If water breakthough is recorded, the field operator conducts a multiphase production log (MPLT) survey to identify the influx depth. The first choice for water shutoff is usually a mechanical isolation in which the water-producing interval is either shut in by a sliding sleeve (SS) or the entire wellbore is isolated by bridge plugs. Unfortunately, mechanical isolation does not help because the water flow easily bypasses the restriction by flowing through the carbonate formation and enters the wellbore in the upper interval (Fig. 1). The most recent approach includes a total shutoff of the water-producing interval by placing a polymer sealant in the targeted zone. Several chemical water shutoff jobs have already been performed by the field operator. Although some of treatments can be considered successful as a result of proven reduced water cut over an extended time period, some treatments did not provide the expected results, despite a large chemical volume pumped into the formation.

Figure 1

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Schematic of horizontal well before the chemical water shutoff treatment.

Figure 1

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Schematic of horizontal well before the chemical water shutoff treatment.

This document describes the materials used for chemical water shutoff purposes and the history of the technology application in the subject field. Avoiding known pitfalls and following best operational practices may help to increase the success rate of chemical water shutoff treatments.