INTRODUCTION

ABSTRACT

Calcium carbonate is one of the most common scales found in oilfields. Scale inhibitor treatment is a traditional method to control the calcium carbonate deposition. Scale acid treatment is another common way to remove/dissolve carbonate scale. However, high corrosion rate of scale dissolvers at high temperature and the environmental requirement for scale dissolvers have been two major challenges in the development of scale dissolvers and applications under HTHP conditions.

In this paper, a low corrosive environmental friendly calcium carbonate scale dissolver has been developed to provide a treatment to remove calcium carbonate scale formed in HTHP wells. The characteristics of this product are as following:

  • Yellow Y1 environmentally acceptable chemical in Norway.

  • Both the aged (170°C) and non-aged samples showed similar CaCO3 dissolving capacity.

  • A high dissolving rate in the initial stages of scale dissolution.

  • No re-precipitation risk after cooling.

· Low corrosiveness at temperatures up to 170°C. General corrosion is low and no pitting corrosion observed.In addition, a winterized version of this dissolver has been developed. The performance tests have been carried out and reported as well.

Calcium carbonate, CaCO3, is one of the most common scale depositions found in oilfield production wells and surface facilities. It can be deposited all along the water paths from injectors through the reservoir to the surface equipment, especially in high temperature and high pressure (HTHP) wells, where temperature is up to 250oC and pressure is up to 20000psi.

Calcium carbonate scale forms when the solution is supersaturated with respect to Ca2+ ions and HCO3 -ions. The two major factors causing CaCO3 deposition in the oil and gas industry are reduction in pressure and high temperature during production1,2. Pressure drop leads to the loss of carbon dioxide (CO2) from aqueous solution. This causes an increase of pH and an associated increase of supersaturation. High temperature is another driving force causing CaCO3 self-deposition. The solubility of calcium carbonate decreases with temperature increase, hence CaCO3 crystallization frequently occurs at high temperature3. In addition, the kinetics of calcium carbonate scale formation is a function of temperature, i.e. slow kinetics at low temperature. As the temperature increases, the formation of calcium carbonate will accelerates and precipitation may occur at an earlier stage4.

Carbonate scale formation can impair production by blockage of tubing and flowlines, fouling of equipment and concealment of corrosion. The effects of carbonate scale can be dramatic and costs can be enormous6.

Effective techniques are needed to solve the scale deposition and keep producing wells healthy. In most cases, scale prevention through chemical inhibition is the preferred method of maintaining well productivity. In order to minimize the formation of scale deposits, scale inhibitors treatment with polycarboxylates or phosphorous containing compounds (such as phosphonates or phosphate esters) are common practice in oil industry5. However, while the most polymer scale inhibitors showed a good thermal stability, some phosphonate inhibitors had a limited usage for being applied at temperatures over 170oC.

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