The Krafla drill site
III. High efficient wells to decrease drilling costs
This project aims to provide geophysical and geochemical sensors and methods to evaluate deep geothermal wells up to supercritical conditions (T>380°C). Supercritical geothermal wells are presently non-conventional but may provide a very efficient way to produce electricity from a clean, renewable source. A deep geothermal well is currently being drilled for this purpose into the Iceland (an icelandic volcanic zone, Krafla) volcanic zone, Iceland as part of the IDDP (“Iceland Deep Drilling Project”) and with joint funding from Icelandic industry and science.
Aimed to explore supercritical wells and to enhance production from them, HITI is to develop, build and test in the field new surface and downhole tools and develop approaches for deep high temperature boreholes. The new set of tools and methods have been chosen to provide a basic set of data needed to describe the supercritical reservoir structure and dynamics including the evolution of the borehole condition during production. The set of new instruments should tolerate high temperature & pressure in a highly corrosive environment. Slickline tools up to 500°C and wireline tools up to 300°C will be developed due to the present limitation in wireline cables (320°C).
For reservoir characterisation, the measured quantities are temperature and pressure (for fluid characterization, thermodynamic modeling of the reservoir and thermomechanical modeling of borehole integrity), natural gamma radiation and electrical resistivity (for basement porosity and alteration), acoustic signal (with borehole wall images for reservoir fracturing and in-situ crustal stresses) and reservoir storativity and equilibrium (from geothermometers and organic tracers). Casing and cement integrity, collar location, as well as thickness changes due to corrosion or plugging from mineral precipitation (from acoustic images again) will be measured. The new tools will be tested in-situ.
Over the last few decades, increasing concerns have been directed towards the world’s hydrocarbon energy usage with eventual supply shortcoming and harmful environmental impact. Geothermal nearly - renewable energy has been considered as being one of the major alternatives in the near and distant future. Efficient use of existing geothermal fields and higher energy yields from new sources are seen as priority issues. Recently, ideas to radically improve power extraction from geothermal boreholes have been put forward. A ten-fold increase in power production has been predicted theoretically when drilled from the conventional 3 km to unconventional 5 km depth in Icelandic geothermal regions. An ongoing project is being funded by\ major local power companies with cooperation from European and international societies and companies, codenamed IDDP (Iceland Deep Drilling Project). For the first time, the scientific community will also be able to study a hydrothermal reservoir at supercritical temperatures. Supercritical fluids have higher enthalpy than steam produced from two-phase systems. Large changes in physical properties near the critical point can lead to extremely high flow rates, resulting in the projected ten-fold increase in turbine power production relative to conventional production (W.A. Elders et.al., 2003).
The main objective of this project is to develop sensors and methods to accurately determine the existing conditions of the reservoir and fluids in-situ at the base of a deep geothermal system. As well as investigating supercritical phenomena, drilling in this environment can address a wide range of scientific questions related to, for example, the origin of black smokers along mid-ocean ridges, or the deposit of hydrothermal ores. Deep drilling has been achieved previously with a world-record depth of 13 km in Kola, Russia. Drilling in geothermal areas up to supercritical temperatures has also been demonstrated in Kakkonda, Japan, reaching there 500 °C, at a depth of 3.7 km. Other reports of near-supercritical temperatures include a well site in Larderello, Italy, at 400 °C and Nesjavellir, Iceland, exceeding 380 °C.
Seismic activity in the Iceland volcanic zone and its location along the Atlantic ridge allows the prediction of supercritical fluids at depths below 3.5 km. At temperatures and pressures above the critical point, which occurs for pure water at 221.2 bars and 374.15°C (but higher in waters with dissolved components), only a single phase of supercritical fluid is present. The pressure-enthalpy diagram for pure water (Figure 1) from Fournier (1999) provides a summary of how a supercritical geothermal system might be managed to produce electricity.
Figure 1 : Pressure-enthalpy diagram for pure H2O with selected isotherms. The arrows show various different possible cooling paths (Fournier, 1999). The conditions under which steam and water coexist is shown by the shaded area, bounded by the boiling point curve to the left and the dew point curve to the right. The objective of IDDP is to follow the pathway along F-G, coloured in red.
If a supercritical hydrothermal fluid (point A) flows upward, it decompresses and cools adiabatically to reach the critical point (B). Further decompression yields two phases, water and steam (E and D). The concept behind the research program of IDDP is to follow a pathway along FG (Figure 1) from supercritical fluids at reservoir depth to superheated steam at surface, resulting in a much greater power output than from a typical geothermal well, at least by a factor of ten.
The arrows show various different
III. High efficient wells to decrease drilling costs
An important component of the cost of electricity from geothermal resources in Iceland is the expense of drilling the necessary high-temperature production wells (typically about 2 - 3 million € each). Reducing the number of wells by increasing significantly the power output of each well by producing supercritical fluids will reduce this cost. Preliminary modeling studies, by Albertsson, et al., (2003) indicate that a 5000 m deep well producing from a supercritical geothermal reservoir at temperatures greater than 450 °C could yield sufficient high-enthalpy steam to generate 40-50 MWe of electric power, an order of magnitude greater electrical power production than from a well 2 km deep producing from a 300 °C reservoir. If successful, the IDDP approach could be used worldwide in high-temperature geothermal systems.
IV. HITI targets
In order to achieve this industrial and societal objective, HITI is aimed at solving the technological problems associated with the characterization and production of supercritical geothermal reservoir. This implies developing downhole instruments capable of tolerating temperatures over 300 °C, and preferably up to 500 °C. The applicants agreed on a minimum set of new instruments and methods needed to achieve within HITI part of the needed scientific and industrial goals of supercritical reservoir description and production.
The key parameters to be measured for thermodynamic modeling of the reservoir and production evaluation are wellbore fluid parameters: temperature (T), pressure (p) and nature (i.e. ionic charge).
First of all is temperature and, for this, three main types of downhole instruments are being considered as part of HITI:
1) wireline instruments, where a cable with electrical wires is constantly connecting the downhole gauge to a surface computer,
2) slick line instruments where a metallic wire is used to lower the instrument and the data is gathered onto a memory chip inside the instrument, without real-time readout at surface,
3) monitoring instruments, where distributed temperature sensors along a fiber-optic cable is installed inside the borehole and quasi-continuous temperature profiles are obtained during all phases of production.
4) In-situ reservoir temperature might also be obtained from Na-Li geothermometers.
These approaches are complementary and should provide a needed cross-calibration. The most appropriate overall approach will be determined as part of HITI.
Also a key issue is that of reservoir petrophysical parameters evaluation. Since permeability cannot be measured in boreholes in a continuous manner, porosity and overall storativity of the reservoir are the most important parameters. The electrical resistivity of basaltic rocks is controlled by the pore fluid nature and temperature, the presence of fractures and rock mass alteration due to hydrothermal circulation, as well as the pore space topology (Archie, 1942; Flovenz et al., 1985; Pezard, 1990). With temperature, the electrical resisitivity of the rock are often considered as the most important parameters for geothermal reservoir evaluation (Ryback and Muffler, 1981).
For fracture contribution to porosity and reservoir production, the crustal stress regime and orientation, as well as reservoir pressure also plays a key role, as only fractures aligned with minimum horizontal stress will remain open for fluid to circulate through them. Fracture opening, hence permeability, is directly proportional to pore pressure. The presence of open fractures, as well as the nature and orientation of the stress field at reservoir depth might be assessed independently from acoustic imaging of the borehole surface (with a wireline tool already developed by ΗΙΤI partner ALT).
In volcanic rock, alteration also contributes strongly to the overall electrical signal (Pezard, 1990). It can be identified from core (as routinely collected in Iceland, and planned for IDDP), and natural gamma radiation logging. With independent measurements of temperature and fluid sampling, either at surface or in-situ, the electrical resistivity of the reservoir may be related directly to porosity, elsewhere assessed from core and in-situ acoustic images. Near and beyond supercritical conditions, the response of electrical properties at varying pressure and temperature is described by Holzapfel (1969), but has not been specifically studied for basalts and gabbros and as a function of fluid nature. As fluids with high ionic charges are expected in supercritical geothermal systems, a laboratory calibration of electrical properties as a function of p, T and fluid nature is required to analyze downhole data. As for fluids in the hole, reservoir parameters may be measured from different downhole instrumental approaches. This new approach for hot geothermal systems should lead to extraction of detailed downhole results about the porosity structure and pore fluid salinity in the near vicinity of the borehole.
V. 300°C : a limitation from wirelines
Wireline instruments are standard in the oil industry and allow an interactive look at data while being recorded. However, these sensors depend on availability of reliable wirelines, which currently are not labelled above 316 °C. Also, the highest temperature electronics available can suffer 250 °C for a few hours, and will have to be heat shielded to deal with a supercritical environment. Normally, heat shielding is achieved either from cylinders placed over the instrument (with "Dewar" design), reducing the thermal conductivity of the tool and protecting it from the surroundings, or with the presence of distributed heat sinks meant to delay the internal heating of the tool. Off-the-shelf technology presently reaches 250 °C in this domain. All proposed instruments in HITI will be tested up to 300 °C, either in existing hot wells in Iceland, or during the coring phase in the IDDP hole, when substantial cooling will be achieved. It includes fluid temperature, rock electrical resistivity and natural gamma radiation, and acoustic imaging of the borehole surface for fracturing and stress (existing), and pipe integrity (to be developed as part of HITI).
Slick-line instruments require no wireline, hence may provide access to higher temperatures, into the supercritical domain. Data sampling and processing is made at surface after autonomous recording in the hole. A self-contained fluid characterization package is planned for pressure, temperature, electrical conductivity and flow measurements. Monitoring instruments with fiber-optics belong to the most recent addition to measure temperatures in boreholes. The system currently available to this project was tested in 4.2 km depth at temperatures of 146 °C (Henninges et al., 2005). HITI aims to progress beyond this fiber optic temperature limit in a comparable depth range, using a new fiber optic cable design.
VI. Conlusion
In summary, the main instrumental objectives of HITI can be stated as follows:
• develop and field test downhole instruments and methods tolerating temperatures above current limits. These instruments include: temperature, pressure, fluid and rock electrical resistivity, natural gamma radiation, televiewer acoustic images, casing collar locator, casing monitoring, fluid flow, chemical temperature sensing and organic tracers,
• adapt an existing HPHT (High Pressure, High Temperature) laboratory facility to the measurement of electrical resistivity at appropriate reservoir rocks, conditions and varying fluid nature, with a procedure to extract and isolate the contribution of surface electrical conduction from rock mass alteration,
• validate the new instruments from the analysis of downhole data and samples (either core or fluid) from field tests in either hot existing wells, or the new IDDP hole.
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