Geothermal wells, acidizing

UNCONVENTIONAL SANDSTONE AND GEOTHERMAL WELL ACIDIZING PROCEDURES 119... 

Deep and geothermal wells are inherently hot and are well served by aluminum phosphate formulations (see Chapter 11). The dynamic temperature in these wells is 250 °F (121°C) or higher, and the static temperature is >350 °F (235°C). For these wells, berlinite-based CBPC works well with its maximum solubility at 244 °F (118°C) of alumina and phosphoric acid solution. As we have seen in Chapter 11, this reaction takes place at 302 °F (150°C) that is in the range of the temperatures of deep and geothermal wells [7]. Even at these temperatures, the solubility of aluminum oxide is too low, but addition of a small amount of microcrystalhne or amorphous aluminum hydroxide aids in increasing the soluble ions in the solution. With its large surface area, alumina provides the necessary solubility at the given downhole temperamre. 

Candidates should be selected on the basis of the same principles as discussed earlier. In geothermal wells, the strongest indication of acid-removable formation damage is a sharp drop in the production rate. In a new well, poor production is the only indication. Well-testing techniques... 

Nearly all geothermal wells that are acidizing candidates have been damaged by the following ... 

For carbonate scale removal, 15% HCl is most common. HF should not be pumped in such cases. Unfortunately, the scales formed in geothermal wells may be mixed and layered scales, composed of carbonate, sulfate, and silicate. Therefore, a combination of HCl and HF (or of acid stages) is required in order to at least partially dissolve the scale. 

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