Abstract

The Veslefrikk field is located in block 30/3 of the Norwegian sector of the North Sea. The field has now been on production for approximately ten years, and can be described as being in the decline phase. The Brent and IDS zones contain different fluid systems, and are produced through several commingled wells. Seawater is injected for pressure support. The first water breakthrough occurred in 1992. Although the ion composition of the Veslefrikk formation waters are not extreme when compared to other North Sea formation waters, severe scale formation has been observed. Both CaCO3 and BaSO4 scale have been identified. This is probably due to high reservoir temperature, 125°C (CaCO3), and the commingled production (BaSO4). Based on the field experience a preventive scale control strategy has been implemented. Scale inhibitor squeezes, re-perforation and mechanical and chemical scale removal are applied to maintain the productivity in the wells. Recently the effect of scale formation upon the production rates and cash flow, has been assessed. Two different scenarios, ‘No scale control’, and ‘No scale’, have been studied. These cases have been compared to the baseline production profile. This consists of the historical production data and the base case prognosis for the field, which includes scale control at today's level. The work, covering the period from start of production to June 1999, shows that 9 million Sm3 less oil would have been produced if no scale control had been implemented. This represents a value of approximately 1100 million USD. The cost of the scale control operations which have been performed, including inhibitor squeezes, reperforations and mechanical and chemical well stimulations, are approximately 6.3 million USD. 3.7 million Sm3 more oil, representing a value of 400 million USD, could have been produced if the field had not experienced any scale formation. Assuming constant reserves, the accelerated production represent a 320 million USD increase in the total field lifetime NPV. In addition, the operating costs would have been reduced. The results clearly show that scale control operations are highly beneficial, and it is recommend to intensify the preventive scale control at Veslefrikk.

Introduction

The two most common types of scale in the Veslefrikk field are calcium carbonate scale (CaCO3) and barium sulphate scale (BaSO4). CaCO3 scale can precipitate if produced fluid containing formation water is exposed to a significant pressure drop when flowing into the well and in the tubing. BaSO4 scale is a consequence of mixing seawater and formation water. The composition of the two Veslefrikk formation waters that have resulted in scale formation are shown in Table 1. A common situation in the Veslefrikk field is that formation water and seawater from different zones are mixed within the liner and tubing, and causes large amounts of BaSO4 scale. As most of the producers have now experienced seawater breakthrough, BaSO4 has become the most important scale type and the main cause of the scale formation problems in the field.

The strategy for downhole scale control in the Veslefrikk field was revised during 19991, and it was changed from corrective to preventive. This is particularly important for commingled producers, i.e. wells that are producing from more than one pressure regime. To protect wells with a scale formation potential, scale inhibitor treatments (squeezes) are performed. This means that inhibitor chemicals are pumped into the well and the near well bore formation. Treatment of a commingled producer is more complicated than treatment of a single zone producer, because the correct placement of the chemicals is difficult to achieve. As of August 2000, 45–50 scale inhibitor treatments have been performed at Veslefrikk.

For a number of reasons, for instance prioritizing well operations and R&D projects, it is useful to understand the economical consequences of scale formation in the wells. Earlier some ‘qualified guesses’ were presented, but these numbers were always a subject of discussion and arguing. Hence, the idea of assessing the subject systematically, was turned into a study that was carried out at the end of 1999.

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