| Title | Fluid Chemistry Scenarios Anticipated for IDDP-2 to Be Drilled in Reykjanes, Iceland |
|---|---|
| Authors | Thráinn FRIDRIKSSON, Andri STEFÁNSSON, Finnbogi ÓSKARSSON, Ester Inga EYJÓLFSDÓTTIR, Ómar SIGURDSSON |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | IDDP, super critical, super heated, production properties, Reykjanes, Iceland, fluid chemistry |
| Abstract | The Icelandic Deep Drilling Project (IDDP) plans to drill well IDDP-2 to a depth of 3.5 to 5 km in the Reykjanes geothermal field, SW Iceland. The objective of this paper is to constrain the chemical composition of the fluid that may be encountered in the well and assess the implications of the predicted fluid composition for the production properties of the fluid. The fluid composition is constrained by the following: the estimated temperature at 5 km depth, the fluid is assumed to be of seawater origin, quartz solubility over a wide range of P-T conditions and phase relations in the H2O-NaCl system The temperature at 5 km is estimated to be between 380°C and 540°C. The temperature estimates considered result in very different predicted fluid conditions at 5 km depth. In the lowest temperature case a single phase 3.5% NaCl fluid is expected with SiO2 concentration of the order of 900 mg/kg. Production properties of this fluid will be similar to that of conventional high temperature geothermal solutions from this field, i.e. hot, saline, silica rich liquid that would flash upon decompression. The intermediate P/T estimate is located in the stability field of two coexisting fluid phases, i.e. brine and dilute superheated steam. The solubility of silica in the superheated steam is ~200 mg/kg assuming equilibrium with quartz. The presence of brine under these conditions depends on the P-h trajectory of the fluid on its way to this point. If superheated steam at these P – h conditions is decompressed it will condense to small degree on the way to the surface. The condensate is expected to be highly acidic and corrosive and the silica dissolved in the superheated steam is expected to partition into the liquid phase and for silica scales. Decompression of a deep brine (~10 to 20% NaCl) would result in massive deposition of NaCl, quickly rendering the production well unusable. The highest temperature estimate results in a superheated steam phase coexisting with solid NaCl. The solubility of quartz in this fluid is ~200 mg/kg. This fluid can be decompressed without condensation thus resulting in less severe casing corrosion problems than fluids from scenario 2. This fluid will, however, form silica dust upon decompression. This may result in erosion of the casing and surface pipeline. However, experience from IDDP-1 has shown that problem can be managed by steam scrubbing at high pressure. Fluid circulation to P – h conditions above the critical point will result in massive deposition of solids. This is both because of the decreasing solubility of silica at high temperature but relatively low pressure (~300 bar) and the precipitation of most of the dissolved solids. It is therefore likely that fluid circulation to these conditions will be very limited in volume and/or time. This may limit the long term productivity of the reservoir at these P – h conditions. |