Abstract

Vertical seawater injectors for a carbonate reservoir in Saudi Arabia have an injection zone (open hole) of approximately 200 ft. This reservoir has drastic permeability variations. Acid stimulation without proper diversion normally results in undesirable stimulation of the high permeability streaks. Therefore, acid diversion plays a key role in improving both the post-acid, well-injectivity and injection profile.

Various diverting agents have been used to improve the acid distribution during acid injection, including wellbore gels, foams and in-situ gelled acids. The use of polymers is an excellent means of diversion.1,2 However, evaluation of polymeric systems has demonstrated formation damage due to residual polymer, which is considered unacceptable, especially for water injection wells.3 Conventional foams have also been tested with good success.

In the present study, the use of a polymer-free viscoelastic surfactant (VES) as a diverter system is investigated and compared with in-situ gelled acid and foams. The VES is a twocomponent system, and in the presence of salts, the surfactant molecules associate to form rod-like micelles. These micelles can entangle with each other and form a three dimensional network, responsible for the high viscosity of the system.4,5 This non-damaging "surfactant gel" can be broken completely by injecting mutual solvent or injection water. This added advantage eliminates the need to flow back the well following acid treatments.

Laboratory studies demonstrated up to 2-fold increase in viscosity and nearly 20-fold increase in foam stability compared to foams containing polymers. The properties of VES solutions are independent of the salinity and pH of the mixing water.

Bullheading the acid enables the operator to pump these treatments at high rates, which are needed in some cases. Pumping pressure, post-acid injectivity history and injection flow profile of several seawater injectors indicate good stimulation results and better acid diversion across the whole target zone.

This paper presents the first application worldwide on the use of VES technology in water injection wells. A total of six stimulation treatments were performed using this technology, one treatment using coiled tubing (CT) and five by bullheading.

The VES technology provides a solution for heterogeneous carbonate reservoirs, and eliminates formation damage induced by the polymers present in the conventional diversion systems. Stimulation using the VES system on six wells resulted in an injection increase of more that 45,000 bbl of water per day, much higher than similar treatments where conventional foams were used.

Introduction

Power water injectors (PWI) provide pressure maintenance to a carbonate reservoir (Arab-D) in Saudi Arabia.6,7 These wells are vertical wells completed with casing down to the top of the injection interval and open hole across the interval. The total depth (TD) of these wells is approximately 7,500 ft. At this depth, the bottomhole temperature ranges from 110 to 150°F and the bottomhole pressure is approximately 5,050 psi static and 6,000 psi when injecting. This carbonate formation, which can be 150–200 ft thick, has been divided into four zones. Changes in permeability (0.1 to 200 mD) and porosity (5 to 20 vol%) throughout this formation results in non-uniform injection flow profile as shown in Fig. 1.

Carbonates are highly soluble in a variety of acids at all temperatures, Table 1. The carbonate solubility in HCl is the highest, but this acid is corrosive, especially at high temperatures. Organic acids are used to stimulate deep wells and those with CRA completions.

Several studies have shown that retarded acids achieve deeper acid penetration in carbonate formations.8,9 Retardation is effectively achieved using a stable acid-in-diesel emulsion.

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