Have We Discarded Promising Squeeze Chemicals For High-Temperature Applications?

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 105505, "Squeeze Chemical for HT Applications - Have We Discarded Promising Products By Performing Unrepresentative Thermal- Aging Tests?," by Rex Wat, SPE, Lars- Even Hauge, SPE, Kare Solbakken, Kjell Erik Wennberg, SPE, Linda Merete Sivertsen, and Berit Gjersvold, Statoil ASA, prepared for the 2007 SPE International Symposium on Oilfield Chemistry, Houston, 28 February-2 March. The paper has not been peer reviewed.

Selecting an effective scale inhibitor for a squeeze application at 170°C is no simple task. The traditional thermal-stability test by aging the chemical in bulk often is perceived as too harsh. An alternative of conducting the aging test inside core materials could be more representative of downhole conditions. The results from a recent investigation in which a scale inhibitor was aged by two methods, one in bulk as commonly practiced in the industry and one inside a sandstone core, demonstrated that the conventional method is unrepresentative. The study showed an unexpected relationship between desorption and inhibition effectiveness. 

Introduction

In November 2005, Statoil commenced production from the Kristin field. Kristin is a high-pressure/high-temperature (HP/HT) gas/condensate field in the Haltenbanken area of the Norwegian Sea. The reservoir temperature is 170°C, and reservoir pressure is 911 bar. Producing by natural depletion and with the formation water containing in excess of 2,500-ppm calcium and 900-ppm bicarbonate, downhole CaCO₃-scale deposition is a major production problem. An active program to qualify suitable scale-control chemicals includes chemicals for squeeze treatment, wellhead continuous injection, and dissolver. Many squeeze chemicals were discarded because of their apparent thermal degradation at test conditions.

The screening technique relied upon aging chemicals in a sealed container for a period of 7 to 21 days. The extent of degradation was measured by their performance relative to the fresh products. In earlier studies, the focus was on the effect of carrier-brine composition, pH, and oxygen level. The main degradation mechanisms were considered backbone scission and functional-group degradation that were caused by hydrolysis and free-radical attack.

While this approach was considered reasonable for the different products, doubt remained if this was representative of the field because the test was not within a rock matrix. The degradation mechanism of the molecules in a physically trapped environment was different from that being hindered by a surface-binding interaction. The conventional approach could overlook the most critical part of the degradation process for a squeeze chemical—the combined effect of thermal aging and surface-retention mechanisms. It is on this combined effect that this study focused.

Planning the Experiment

The aim of this project was to prove or disprove that a scale-inhibitor chemical that is adsorbed and thermally aged inside a core will retain its effectiveness more so than if it is aged in bulk. Considerations included the selection of chemical, pore volume of the core assembly, choice of core materials, type of brines, flooding sequence, and system integrity over an extended period of time.