Effects of Brine Concentration and Pressure Drop on Gypsum Scaling in Oil Wells
Authors: Fulford Richard S. 
Published: December 2007

Gypsum scale is a problem in oil wells that produce from reservoirs containing calcium sulfate. It becomes particularly troublesome in waterflood operations where large volumes of water are being produced.

The solubility of gypsum or anhydrite in water increases with pressure due to a slight decrease in total volume as the salt dissolves. Upon release of pressure a supersaturated solution is obtained that deposits gypsum for a period of time. This is believed the most reasonable explanation for gypsum deposits at the bottom of producing wells.

The study reveals that the amounts of scale formed at a given pressure drop and temperature depend on the amount of sodium chloride and other salts dissolved in the brine. The quantity of gypsum deposited increases with salt concentration to a maximum, then decreases until, with strong brines, no scale is formed.

Introduction

Secondary oil recovery from reservoirs, containing calcium sulfate often is hampered by gypsum scale. There are numerous causes of gypsum scale. Temperature changes can alter solubility and cause scale. Water incompatibility is also a common cause of gypsum deposition. If water containing calcium is mixed with another water containing sulfate, the solubility of calcium sulfate can be exceeded, resulting in the formation of gypsum scale. Another cause is water evaporation. When water evaporates the calcium sulfate concentration increases until the saturation concentration is reached; if the water continues to evaporate, the result is gypsum scale deposition. It has been suggested that the presence of large surface areas of solid calcium sulfate with relatively small volumes of water increases solubility, and thus causes calcium sulfate precipitation when the water flows into the wellbore.

Gypsum scale also can be caused by pressure decreases, which cause a lowering of calcium sulfate solubility. Experiments show that there is a net loss of volume when anhydrite is dissolved in water (Appendix A). Calculations show that an increase in pressure would tend to shift the equilibrium toward an increase in anhydrite solubility (Appendix B). Experiments have confirmed this.

Apparatus

The experimental apparatus consisted of a 1-liter steel pressure bomb containing crushed anhydrite (mean diameter 0.5 in.). The bomb was kept in a drying oven for temperature control. The anhydrite was washed free of all fines with distilled water. The pressure was raised using a Blackhawk hand pump. Another steel bomb (0.5 liter) was placed between the pump and the bomb containing anhydrite to separate the pumping fluid (kerosene) from the water used in the experiments. This was done to prevent corrosion of the pump by water. A pressure gauge was placed between the separating reservoir and the pump. The pump and bombs were connected by 1/8-in. stainless steel tubing.

After removing the water from the steel bomb following pressure release, the water was placed in 125-ml glass bottles that were covered to prevent evaporation. Small aliquots of water were removed from time to time to determine the amount of calcium sulfate remaining in solution.

Procedure

The bomb was filled with water, and the pressure was increased. During the first several hours after the pressure increase, the pressure decreased somewhat, and it was necessary to pump back to the desired pressure. This pressure loss was caused by the volume loss when the solid anhydrite dissolved in the water. After the first day the pressure remained constant, and it was not necessary to adjust for the remaining period at high pressure. The increased pressure was maintained for at least 5 days to permit the calcium sulfate solubility to reach equilibrium. Experiments showed that this was sufficient time for solubility maximum to be reached (Fig. 7). Anhydrite dissolves in water very fast initially, and continues to dissolve at a slowly increasing rate, reaching a maximum value in 5 to 7 days with no agitation (Fig. 7). Over 90 percent of the total anhydrite solubility is reached in 5 hours.

The dissolution rate was ascertained by removing small amounts of water from the bomb and determining the amount of calcium sulfate in the water. EDTA (ethylene-diamine tetraacetic acid) titration and atomic absorption spectrophotometric procedures were used in these tests.