The drilling of the second well of Iceland Deep Drilling Project (IDDP) was finished on January 25th. The IDDP-2 was the result of extending an existing well RN-15 (which was initially ~2 km depth) to a depth of 4,659 m. The well was drilled at the Reykjanes geothermal power plant, which is operated by HS Orka, in collaboration with Landsvirkjun, Orkuveita Reykjavíkur and Iceland’s National Energy Authority, with active participation of Statoil and substantial grants from the EU H2020 funding program as well as other international science grants from ICDP and NSF. At a triumphant informational meeting in Reykjavik on Februrary 1, results of the drilling operations were announced in Icelandic. The IDDP also released an helpful informational sheet on the project website, which describes temperature/pressure measurements, injectivity tests and geological analysis of recovered core samples.
In addition to the scientific value of collecting rock samples at such great depths, the main rationale for drilling the IDDP well is that electricity generation from volcanic geothermal systems could be greatly increased by tapping into so-called “supercritical geothermal resources”. The first IDDP well, IDDP-1, was drilled in 2009-2010 into the Krafla geothermal system in northeast Iceland. Drilling operations were ceased after the presence of quenched glass in the wellbore indicated that the well penetrated a magmatic intrusion around 2.1 km depth. Subsequent well testing indicated an a reservoir at 450 °C and 18 MPa that could potentially generate 30 MWe electricity generation (roughly an order-of-magnitude greater than a conventional well drilled into a volcanic geothermal system). Thus, the IDDP-1 showed that depths of >3-4 km (where fluid pressures would be >22 MPa, corresponding to the critical pressure of pure water) were not necessary to obtain economically attractive supercritical geothermal resources in systems containing dilute waters. The observed data and overall message was further supported by subsequent modeling of fluid flow around magmatic intrusions I performed during my PhD.
However, in contrast to the IDDP-1 well, which was located far enough from the coast and thus contained rainwater-derived subsurface fluid with less than 1% total dissolved solids, the IDDP-2 is located in close proximity to the Icelandic coast, and as a result, the subsurface fluid consists of heated seawater. Strictly speaking, supercritical conditions can only be strictly defined for single-component fluids (such as pure H2O), and do not exist in multi-component fluids (such as seawater, which is often approximated by the system H2O-NaCl). This is described in detail in the introductory paper by A. Liebscher and C. Heinrich in the really excellent “Fluid Fluid Interactions” volume published by the Mineralogical Society of America in 2007:
The term “supercritical” is widely used in the literature on fluid-fluid equilibria. However, it is not applied consistently but used to describe rather different phenomena within the different systems. In some cases, its use is misleading or even meaningless… this term therefore should be avoided. In one component systems, the term “supercritical” refers to P-T (pressure-temperature) conditions above the critical point of the system… Because in these systems P and T of the critical point are uniquely defined, the “supercritical” P-T field is likewise uniquely defined; even here, some authors use the term “supercritical” for high-temperature fluids at any pressure, because the fluid is single phase and changes in P or T do not induce phase separation. In two- or multi-component systems, critical behavior occurs along critical curves and is no longer uniquely defined in terms of P and T. “Supercritical”, as implying P > Pc and T > Tc, therefore becomes meaningless. This holds all the more if one considers that in two- or higher component systems the vapor-liquid two-phase field may open with increasing temperature as, for example, in the H2O-NaCl system. In these systems, raising T > Tc (at a given pressure) implies fluid phase separation instead of homogenization, contrary to what is intended by the term “supercritical.”
Measured temperatures and pressures at the bottom of the IDDP-2 well are 427 °C and 340 bars (the actual bottom-hole temperature could likely exceed the measured value as the logging tool was only calibrated to 380 °C). This is seen in a temperature and pressure log shown below. While this bottom-hole temperature is similar temperature to what was measured for the IDDP-1, the pressure is much higher; this is not surprising, as the well is much deeper and the hydrostatic force exerted by seawater is greater than dilute geothermal water due to it’s higher density.
The temperature and pressure log reveals a high permeability zone at 3,450 m depth, and smaller feed zones at 4,450 m and 4,500 m depth. During drilling, there was complete loss of circulation below ~3 km depth that could not be solved with lost circulation materials, or by multiple attempts to seal the loss zone with cement. The presence of high permeability structures at this depth is encouraging, as a primary concern of drilling to such depths and temperatures is a lack of permeability to sustain productive fluid resources. While loss of circulation meant that drill cuttings couldn’t be returned to the surface, thirteen coring runs were conducted that recieved a total of 37 m. The last coring run returned some 7 m of core from ~4,650 m depth! This core will be useful for learning about the petrologic and geochemical evolution of mid-ocean ridge hydrothermal systems, since the Reykjanes system is located on the tip of a peninsula where the mid-ocean ridge comes onto land.
In the coming months, production tests will indicate the generation potential of the IDDP-2 well. However, I anticipate that generation capacity will be similar to the IDDP-1 well even though the fluid is seawater. Because the bottom of the well is located at >4 km depth and fluid pressures >30 MPa, circulating seawater can be heated to vapor-like densities prior to phase separation. During phase separation, a small fraction of hypersaline brine will condense out of the intermediate-density fluid, and overall heat transport by advecting seawater is maximized. This is discussed in detail in a new paper I recently published in Geophysical Research Letters - please check it out!