Limiting Halite Deposition in a Wet High-Temperature Gas Well

1 Limiting Halite Deposition In a Wet High-Temperature Gas Well 

There are several gas-well fields that produce hydrocarbon gas associated with very high TDS connate waters. Classical oilfield scale problems (e.g., calcium carbonate, barium sulfate, and calcium sulfate) are minimal in these fields. Halite (NaCl), however, can be precipitated to such an extent that production is lost in hours. As a result, a bottom-hole fluid sample is retrieved from all new wells. Unstable components are fixed immediately after sampling, and pH is determined under pressure. A full ionic and physical analysis is also carried out in the laboratory. 

Some of the analyses were run through an ion-association model computer program to determine the susceptibility of the brine to halite precipitation. If a halite precipitation problem was predicted, the ion-association model was run in a mixing mode to determine if mixing the connate water with boiler feedwater would prevent the problem. This approach has been used successfully to control salt deposition in the well with the composition outlined in Table 8.18. The ion-assodation model evaluation of the bottom-hole chemistry indicated that the water was slightly supersaturated with sodium chloride under the bottom-hole conditions of pressure and temperature. As the fluids cooled in the well bore, the production of copious amounts of halite was predicted. 

In this case, the ion-association model predicted that the connate water would require a minimum dilution with boiler feedwater of 15 percent to prevent halite precipitation (Fig. 8.23). The model also predicted that over-injection of dilution water would promote barite (barium sulfate) formation (Fig. 8.24). Although the well produced F1,S at a concentration of 50 mg/L, the program did not predict the formation of iron sulfide because of the combination of low pH and high temperature. Boiler feedwater was injected into the bottom of the well using the downhole injection valve normally used for corrosion inhibitor injection. Injection of dilution water at a rate of 25 to 30 percent has allowed the well to produce successfully since startup. Barite and iron sulfide precipitation have not been observed, and plugging with salt has not occurred. 

It has been weU-established that ozone is an efficient microbiological control agent in open recirculating cooling-water systems (cooling towers). It has also been reported that commonly encountered scales have not been observed in ozonated cooling systems under conditions where scale would otherwise be expected. The water chemistry of 13 ozonated 

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