Introduction

The occurrence of calcium sulfate scale in wells has recently become a sporadic major problem in producing wells in some CO2 floods. The problem has also been observed in wells where a potential scaling factor exists and the produced water has more than a fraction of a percent of carbon dioxide gas. Where calcium ion and sulfate ion concentrations are sufficiently large, accelerated deposition of calcium sulfate has been seen in the CO2 floods. Deposition occurs most frequently on pumps and where outgassing of CO2 or turbulence is involved.

This paper reports the results of a study to determine the cause of accelerated formation of calcium sulfate scale in CO2 environments and find scale inhibitors which can be used to prevent precipitation of calcium sulfate. An additional goal of the study was to gain a more complete understanding of factors affecting the inhibitor squeeze process, so that longer squeeze life could be obtained. The data reflects findings of research into factors affecting scale precipitation, scale inhibitor screening tests, and initial results on inhibitor placement and flowback in cores.

Discussion

CaSO4 Precipitation Acceleration of calcium sulfate precipitation has been confirmed in Amoco's West Texas CO2 floods in areas where

  1. CaSO4 scaling was previously minor,

  2. a significant CaSO4 scaling tendency existed, and

  3. levels of calcium and sulfate ions rose temporarily, apparently resulting from chemical scale removal followed by acid stimulation.

Of these problem areas, the highest level of CaSO4 precipitation and greatest treatment success have occurred in the areas of highest CO2 content in the produced fluid.

The CaSO4 precipitation resulting from calcium sulfate removal treatment followed by stimulation was sporadic and severe. In some cases, extreme buildups occurred within days of the stimulation. The scale, which according to Carlberg and Matthews is probably in the form of either CaSO4.2H2O or CaSO4/2H2O, was found on both the interior and exterior portions of the downhole equipment and would, in instances, completely cover the entire exterior portion of the downhole Dump (plunger or electric submersible) and various lengths of tubing, Figure 1. Inside the tubing, the scale formed in locally heavy quantities, with the most severe cases forming solid plugs within the tubing just downstream of a pressure drop, figure 2. Scaling was also evident in production test vessels following stimulation, and significant quantities of the pure CaSO4 was recovered during wellbore cleanouts. The scale recovered in these situations had the appearance of finely divided amorphous particles with little or no crystalline structure, Figure 3. This indicates that precipitation occurred extremely rapidly, not in the normal controlled manner in which regular crystals are formed.

Acceleration of an existing scaling problem is not always apparent from production information in a tertiary flood, but physical evidence of the increased scale growth in the West Texas CO2 floods has been found repeatedly. Massive scaling has been seen throughout the production system. from the wellbore to the production vessels. The type of scale found in these situations may be anything from pure CaSO4 to a mixture of gyp and other types of scale historically found in West Texas. The scaling has been heavier in the more mature areas of the CO2 floods; the quantity and frequency of occurrence appears to increase as areas continue to mature with respect to miscible flooding.

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