Thermally driven fracturing in hot dry rock systems and the role of wellbore cooling

SummaryNew ways of energy production through the use of coal seam gas plays and geothermal hot dry rock and hot sedimentary aquifer systems pose challenges in identifying and monitoring fluid in the subsurface. We propose the use of the magnetotelluric (MT) method to image static and dynamic fluid distributions in the subsurface exhausting the contrast in electrical conductivity between resistive host rock and conductive fluid-filled, porous rock. Base line MT measurements provide reference transfer functions and inverse models to characterise the electrical conductivity distribute on which is linked with bore hole and other geophysical data to obtain knowledge about fluid distribution at depth. The reference models are used to accurately forward model fluid injection or extraction temporally and spatially. This work shows results from fluid injections at a hot dry rock system at Paralana, South Australia, and its applicability to other geothermal and coal seam gas systems.

Most geothermal reservoirs and hot dry rock (HDR) systems are located in fractured low porosity geological formations. In such settings, fractures provide conduits for fluid flow through such rocks. Therefore fractures, either natural or those artificially created, are important for the successful operation of geothermal reservoirs and HDR systems.

ABSTRACT: The fluid flow and heat transfer characteristics in fracture systems are crucial to efficient geothermal energy development, directly influencing heat transfer medium leakage and long-term heat extraction efficacy. However, numerical simulations based on idealized assumptions and single-fracture flow heat transfer experiments with small rock samples struggle to accurately replicate the actual injection-production heat transfer process in hot dry rock (HDR) systems. Currently, there remains a gap in experimental research on large-scale injection-production heat transfer processes in HDR systems. This study utilizes a self-developed true triaxial injection-production physical simulation experiment system to conduct heat transfer experiments under various injection-production modes. The heat transfer process was analyzed by collecting parameters such as flow rate, temperature, and production pressure difference during the experiment, examining the conversion mechanism between heat exchange and leakage channels. Results indicate that an increase in the number of production wells enhances heat extraction efficiency but also exacerbates seepage competition and fluid loss between fractures. Further analysis reveals that heat transfer performance is constrained by two key factors: the presence of dominant channels, which limit the heat transfer area's full utilization, and fluid leakage, which directly impacts production flow. This study offers novel experimental insights supporting the long-term and efficient development of HDR geothermal resources.

Convective hydrothermal systems have been extensively studied using electrical and electromagnetic methods given the strong correlation between low conductivity anomalies associated with hydrothermal brines and high temperature areas. However, studies addressing the application of similar geophysical methods to hot dry rock geothermal systems are very limited in the literature. The Timanfaya volcanic area, located on Lanzarote Island (Canary Islands), comprises one of these hot dry rock systems, where ground temperatures ranging from 250 to 605 °C have been recorded in pyroclastic deposits at shallow (<70 m) depths. With the aim of characterizing the geophysical signature of the high ground temperature areas, three different geophysical techniques (ground penetrating radar, electromagnetic induction and magnetic prospecting) were applied in a well-known geothermal area located inside Timanfaya National Park. The area with the highest ground temperatures was correlated with the location that exhibited strong ground penetrating radar reflections, high resistivity values and low magnetic anomalies. Moreover, the high ground temperature imaging results depicted a shallow, bowl-shaped body that narrowed and deepened vertically to a depth greater than 45 m. The ground penetrating radar survey was repeated three years later and exhibited subtle variations of the signal reflection patterns, or signatures, suggesting a certain temporal variation of the ground temperature. By identifying similar areas with the same geophysical signature, up to four additional geothermal areas were revealed. We conclude that the combined use of ground penetrating radar, electromagnetic induction and magnetic methods constitutes a valuable tool to locate and study both the geometry at depth and seasonal variability of geothermal areas associated with hot dry rock systems.

Geothermal energy has been identified as a renewable and environmentally friendly energy source, however, at present commercial geothermal energy production is limited to conventional geothermal resources. Hot dry rock systems are considered as unconventional geothermal resources which require engineering techniques to enhance the factors affecting the energy production. This thesis has been aimed to evaluate the coupled effects of the hydro-thermo-mechanical and chemical behaviour of hot dry rock systems with hydraulic and thermal stimulation techniques to understand the optimum conditions to enhance the permeability of the reservoir through a comprehensive research that includes advanced laboratory testing, numerical simulations and mathematical modeling.

The potential of energy extracted from hot dry rock (HDR) was investigated as a commercailly feasible alternate energy source. Run Segments 3 and 4 were completed in the prototype reservoir of the Phase I energy-extraction system at Fenton Hill, New Mexico. Results of these tests yielded significant data on the existing system and this information will be applicable to future HDR systems. Plans and operations initiating a Phase II system are underway at the Fenton Hill site. This system, a deeper, hotter commercial-size reservoir, is intended to demonstrate the longevity and economics of an HDR system. Major activity occurred in evaluation of the national resource potential and in characterizing possible future HDR geothermal sites. Work has begun in the institutional and industrial support area to assess the economics and promote commercial interest in HDR systems as an alternate energy source.

The dependence of permeability on effective stress from flow tests at hot dry rock reservoirs at Rosemanowes (Cornwall) and Fenton Hill (New Mexico)

During Fiscal Year 1987, emphasis in the Hot Dry Rock Geothermal Energy Development Program was on preparations for a Long-Term Flow Test'' of the Phase II'' or Engineering'' hot dry rock energy system at Fenton Hill, New Mexico. A successful 30-day flow test of the system during FY86 indicated that such a system would produce heat at a temperature and rate that could support operation of a commercial electrical power plant. However, it did not answer certain questions basic to the economics of long-term operation, including the rate of depletion of the thermal reservoir, the rate of water loss from the system, and the possibility of operating problems during extended continuous operation. Preparations for a one-year flow test of the system to answer these and more fundamental questions concerning hot dry rock systems were made in FY87: design of the required surface facilities; procurement and installation of some of their components; development and testing of slimline logging tools for use through small-diameter production tubing; research on temperature-sensitive reactive chemical tracers to monitor thermal depletion of the reservoir; and computer simulations of the 30-day test, extended to modeling the planned Long-Term Flow Test. 45 refs., 34 figs., 5 tabs.

Nature and technology of geothermal energy: A review

This paper demonstrates the potential role of policy and management analyses at early stages of technology development as exemplified by the case of hot, dry rock (HDR) geothermal energy resources used for electricity generation. Management issues concerning initial drilling depths, power plant design, redrilling strategies and periodic choices for well-flow rates are analyzed as they relate to R and D policies concerning first, the staging of R and D within the HDR R and D program and, second, priorities for the HDR research program relative to other R and D programs. Results reported here suggest that commercially attractive HDR systems do not require the development of new, very high temperature drilling technology (>275°C). Priorities for research concerning reservoir renewal and completion methods and baseload dimensions of an HDR system are emphasized. The desirability of periodic variations in well-flow rates as a means to reduce reservoir cooling is shown to be questionable. The commercial potential of HDR-produced electricity is assessed using calculations of the busbar cost of electricity from optimally-managed HDR geothermal systems. Busbar costs for a plausible set of base case conditions were found to vary between 26–76 mills kWh (1978 dollars) as geothermal gradients varied between 30–60°C per kilometer of vertical depth. Commercial feasibility for HDR-produced electricity then involves a comparison of these costs with one's expectations for future busbar costs for electricity produced with other fuel sources.

Geothermal reservoirs and hot dry rock systems

From the determinations of surface heat flow based on silica geothermometry, residual heat flow, radiogenic heat production of rocks, thermal conductivities of rocks and so on, the temperatures at different depths of the lithosphere in Fujian Province are estimated by using a one‐dimensional steady‐state model. According to these and the research on geological structures, the possibility of existence of various geothermal systems in Fujian is discussed.The authors consider that in Fujian, geothermal systems related to shallow magmatism do not exist and hot dry rock systems can not be exploited for the time being. However, intermediate‐ and low‐temperature hydrothermal systems are well developed. They are controlled by active faults and distributed widely in southeastern Fujian, but do not have an anomalous heat source. The groundwater circulates at great depths and has a temperature usually less than 150°C with less discharge but good quality. It can be used directly as a valuable geothermal water resource but not suitable to be developed as an energy source.

This administrative report is of historical interest. It covers research on Hydrothermal, Geopressured Geothermal, Hot Dry Rock systems, and the Long Valley Experimental (occasioned originally by the Magma Energy Program). (DJE 2005)

Overview of naturally permeable fractured reservoirs in the central and southern Upper Rhine Graben: Insights from geothermal wells

Geothermal energy in the Spanish energy plan: Present status of the most advanced projects