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Summary

Historically, the survival rate for conventional, nondamaging polymer/ carbonate kill pills under elevated temperatures was limited. Accordingly, a development program was instituted to develop a new high-temperature kill pill that would possess superb bridging characteristics on a high-permeability proppant while remaining thermally stable at temperatures exceeding 165°C (329°F).

This paper describes the development of a sodium formate kill pill geared specifically for the demanding Åsgard development in the Norwegian North Sea. The pill was engineered and successfully applied in a high-temperature application to seal a high-permeability (400 to 500 darcys) frac-pack sand in the Smørbukk field. In the field, the newly engineered formate-based pill proved to be thermally stable while providing sufficient leakoff control. Furthermore, the development reinforced the critical role of proper particle-size distribution (PSD) in minimizing fluid invasion.

With an emphasis on the PSD, this paper reviews the relative formation-damage potential testing procedure of the new formulation on simulated fracture packs. In the laboratory, the fluids demonstrated excellent thermal stability after long-term exposure to temperatures exceeding 150°C (300°F). The aging process was started simultaneously with two samples of each pill and concluded at the end of 16 and 72 hours, respectively. Leakoff tests were performed on 16/30 proppant at 165°C (329°F), resulting in minimal filtration. This paper also examines the unusually demanding conditions at Åsgard relative to other Norwegian fields and details the application and performance of the new kill pill in this hostile downhole environment.

Introduction

Losing completion brines to the formation is expensive and may be damaging to permeable reservoirs. Consequently, the industry has developed kill pills for use during completion or workover operations to seal the formation face, thereby preventing wellbore fluids from intruding into a productive zone. These pills provide a barrier that reduces the potential for formation damage by limiting the fluid loss to a permeable zone. Kill pills should be designed with the same criteria as other drill-in fluids. The product selection and concentration should be formulated to provide a stable, noninvading fluid for the specific formation exposed.

As Chang et al.¹ point out, many kill pills are designed with sized particles, such as calcium carbonate (CaCO₃), to bridge the sand face. Unfortunately, they note that sized particles may often remain in the wellbore and permanently damage the formation. In an attempt to avoid solids-plugging problems, linear gels have been employed without particulates; however, in high-temperature wells, these fluids can invade deeply into the formation, again restricting productivity. The development of a pill capable of remaining stable in an abnormally high-temperature environment was especially critical in the Åsgard development in the Norwegian North Sea.

Unique Requirements of Åsgard

The Åsgard development, which lies on the Halten Bank off mid- Norway, comprises three fields - Smørbukk, Smørbukk Sør (South), and Midgard. For the purpose of this discussion, the author will focus on the technically demanding Smørbukk Sør reservoir, which presented unique challenges that, in turn, impacted the kill pill's fluid design.² In this reservoir, the static bottomhole temperature exceeds 160°C (320°F), and the downhole pressure requires an equivalent fluid density of 1.25 s.g. (10.4 lb/gal). For this project, the kill pill was required as a contingency to seal proppant after a frac-pack treatment. However, in laboratory evaluations, it was discovered that many conventional nondamaging polymers are not thermally stable for any significant time period at temperatures greater than 150°C (300°F).

For qualification, the ultimate pill would have to remain thermally stable for 72 hours at Smørbukk Sør bottomhole temperatures, as well as seal the face of a 16/30 proppant bed with minimal leakoff.

Fluid Design

The first two aspects to be considered were the thermal stability of the polymers and the proper PSD. Sized-salt pills with xanthan gum and modified starch have frequently been used to seal formations, taking the form of either kill pills or drill-in fluids.³ The organic polymers used in these fluids are considered nondamaging to the formation, as they break down into simple sugars with chemical treatments and/or thermal oxidation. The thermal stability of these products can be controlled to a certain extent by adjusting the chemistry of the fluid.

Maintaining the thermal stability of organic polymers can be seen as a four-step process.

1. Reduce the water activity with soluble chemicals (salts). This reduces the potential for dissolved oxygen to be available in the fluid. A fluid saturated at 20°C (68°F) can provide good thermal stability. Because sized-salt fluids are supersaturated with the presence of excess salt, they maintain saturation at elevated downhole temperatures. This supersaturation assists with thermal stability up to 150°C (300°F).

2. Buffer the pH at 9.0 or greater. This reduces the risk of acid hydrolysis. Elevated temperatures tend to decrease pH rapidly.

3. Use an oxygen scavenger. This consumes any oxygen that may enter the fluid during mixing, and air can easily entrain in high-viscosity polymer fluids. In the laboratory, along with the oxygen scavenger, the aging cell must also be purged and pressurized with nitrogen because even a small amount of oxygen can be detrimental in the small, sealed chamber, especially at elevated temperatures.

4. Use an antioxidant under extreme thermal conditions. The antioxidant interferes with the oxidation process, thereby preventing catastrophic oxidation.⁴

Fluids are more affected by thermal degradation in the laboratory than in the field. This is largely a result of the limited volume of fluid being evaluated and the use of stainless steel aging cells. Conversely, carbon steel casing and tubulars may act as oxygen scavengers in the field, resulting in enhanced thermal stability.