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Abstract

Historically, the survival rate for conventional non-damaging 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.

This paper describes the development of a sodium formate kill pill geared specifically for the demanding sgard Development in the Norwegian North Sea. The pill was engineered and successfully applied in a high-temperature application to seal a high permeability (400-500 Darcys) frac pack sand in the Sm rbukk field. In the field, the newly engineered formate-base pill proved to be thermally stable, while providing sufficient leak-off control. Furthermore, the development reinforced the critical role of proper particle size distribution in minimizing fluid invasion and formation damage.

With emphasis on particle size distribution, the authors will review the formation damage testing procedure of the new formulation on simulated frac packs. In the laboratory, the fluids demonstrated excellent thermal stability after long-term exposure to temperatures exceeding 150°C. After the pills were aged for 16 and 72 hours, leak-off tests were performed on 16/30 proppant at 165°C, resulting in minimal filtration. The authors also will examine the unusually demanding conditions at sgard relative to other Norwegian fields and will detail the application and performance of the new kill pill in this hostile downhole environment.

Introduction

Losing completion brines to the formation is both expensive and damaging to permeable reservoirs. Consequently, the industry 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 loss of fluid to a permeable zone. Kill pills should be designed using the same criteria as other drill in fluids. The product selection and concentration should be formulated to provide a stable non-invading fluid for the specific formation exposed.

As Chang, et al, points out many kill pills are designed using sized particles, such as calcium carbonate (CaCO₃), to bridge the sand face.¹ Unfortunately, he notes that sized particles may often remain in the wellbore to permanently damage the formation. In an attempt to avoid solids plugging problems, linear gels have been employed without particulates, but in high-temperature wells this fluid can invade deeply into the formation, again restricting productivity. The development of a pill capable of remaining stable under an abnormally high temperature environment was especially critical in the sgard 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 fluid design.2 There, static bottomhole temperature exceed 160°C and the downhole pressure required an equivalent fluid density of 1.25 s.g.

For this project, the kill pill would be required as a contingency to seal proppant after a frac pack treatment. However, in laboratory evaluations it was discovered that above 150°C many conventional non-damaging polymers are not thermally stable for any significant time period.

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 leak off.