IDDP fluids and their handling
|Location||International Geological Congress,oslo 2008|
|Author||ءrmannsson, Halldَr۱; Reed, Mark H.۲|
|Holding Date||08 October 2008|
Elders and Fridleifsson and Fridleifsson and Elders (this volume) have described the IDDP project and the plans for the first well to be drilled. As there is no information available about similar drilling the composition of the potential fluid needs to be assessed. Two approaches are taken, i.e. theoretical computation of fluids arising from water-rock reaction at the expected conditions, and the composition of fluids that have been observed from similar situations.
Two types of fluid have been considered for the computations, i.e. a saline fluid as is known in the proposed Reykjanes drilling area, and a dilute fluid as is observed in the proposed Krafla and Hengill drilling areas. Both are subjected to addition of volcanic gas, and a reaction with Icelandic basalt is simulated. The results suggest that at high salinity and after addition of volcanic gas at high or low salinity the fluid will become acidic, whereas an alkaline fluid would result from the reaction of a dilute fluid with basalt with no gas added. If the fluids are cooled down there is a considerable danger of deposition of silica and sulphides.
Excess volcanic gas was observed in the Krafla geothermal system during and subsequent to the Krafla fires of 1975-1984, but had mostly disappeared by about 1990. Well KG-12, drilled in 1978, was different as it was not affected by excess gas but it discharged superheated steam that contained significant amounts of hydrogen chloride, which upon condensation became acid and very corrosive. Well KJ-36, drilled in 2007, close to the proposed IDDP drilling site, had similar characteristics, i.e the fluid was superheated at the well head and contained chloride. The pH of this fluid when condensed was too high to be due to hydrogen chloride alone and the fluid contained a significant amount of iron which was interpreted as a result of temporary condensation of the fluid causing corrosion of pipes during its passage to the surface. It is possible that fluid close to supercritical conditions has been found there. Studies of supercritical water-rock interaction, e.g. in granite in Japan, have suggested that the supercritical region may be divided into liquid-like and a vapor-like regions where gas-rich hypersaline brines are generated, possibly producing hydrothermal fracturing but this process is probably countered by deposition.
Thus it is prudent to be ready for an acidic fluid and probably extensive deposition. A fluid handling group is in operation considering methods to counteract potential problems, including special welding and cladding of the wellhead, as well as hydroxide and/or amine scrubbing methods that have been practiced and tested in the Geysers, USA and Larderello, Italy. Suitable inhibitors against deposition may also be examined.