Wellsite Computers-Their Increasing Role in Drilling Operations
Authors: Keenan Patrick G. , Dyson Peter M. 
Published: December 2007

Summary
Strontium sulfate scale in the Fateh field results from mixing incompatible waters. Precipitation squeezes with phosphonate scale inhibitors were shown to be effective in laboratory studies and field use of precipitation squeezes confirmed this. Continued use has resulted in additional operating and performance data.

Introduction
In May 1974 Dubai Petroleum Co. stalled the injection of seawater in the Fateh Mishrif reservoir. The first indication of the strontium sulfate precipitation problems to come was seen in laboratory compatibility studies between Arabian Gulf and Mishrif waters. These results were confirmed several years later when strontium sulfate deposits were identified in Fateh producing wells.

Strontium Sulfate Solubility
Solubility of strontium sulfate in waters and brines as reported in the literature varies rather markedly. For instance, solubilities in sodium chloride solutions as determined by Lucchesi are significantly less than results reported by Collins and Davis, Culberson et al., and Brower and Renault. Similarly, solubility product data collected by Sillen scatter widely as a function of temperature. Nancollas and Campbell suggest that problems in determining true solubility can be attributed to impurities in the water. Enustun and Turkevich credit interfacial tension between the water and the crystallite with responsibility for the problem. Under the latter explanation, minute crystals tend to be more highly soluble than larger crystals of strontium sulfate. The solubility data reported by Collins and Davis, even though limited to 109 degrees F (77 degrees C), seem to be most generally accepted. Brower and Renault's data as a function of temperature can be combined with solubilities reported by Booth and Bidwell to give a solubility-temperature curve in water (Fig. 1).

Laboratory Studies
With the expectation of strontium sulfate scale deposition in producing wells as a result of waterflood operations, laboratory studies were initiated. These included evaluation of scale inhibitors as well as adsorption isotherms for possible application to the inhibitor squeeze treatment.

Scale Inhibitor-Performance of Commercial Treating Chemicals
Scale inhibitor evaluations were carried out in glasslined stainless steel bombs containing a 50/50 mixture of Arabian Gulf water and Mishrif formation water. Scale inhibitor at 0- to 30-mg/L concentrations was added before the two waters were mixed. The bombs then were closed and maintained at the botto mhole temperature of 230 degrees F (110 degrees C) for 24 hours. At the end of this time, the liquid from the bomb was filtered hot under pressure to remove any precipitated strontium sulfate. The filtrate then was analyzed for residual strontium concentration. Loss of strontium during the test was used to calculate the effectiveness of the inhibiting chemical. A minimum effectiveness of 95% protection at 5 mg/L chemical was requested before an inhibitor was considered for further use.

Inhibitor Adsorption/Desorption
Normal squeeze techniques rely on a physical adsorption of chemical onto the rock. Adsorption isotherms were prepared with the procedure outlined by Kerver and Heilhecker-e.g., Fig. 2. Very few phosphate ester compounds were used in these tests because of the reservoir temperature limitations [230 degrees F (110 degrees C)].
JPT
P. 1302^