Thermal Stresses Around EGS

Transcription

1 GRC Transactions, Vol. 40, 2016 Thermal Stresses Around EGS Mahmood Arshad, Masami Nakagawa, Kamran Jahan Bakhsh, and Lucila Dunnington Department of Mining Engineering, Colorado School of Mines, Golden CO Keywords EGS, enhanced geothermal systems, stress distribution, reservoir rock, in-situ stress, stress distribution ABSTRACT Developing an Enhanced Geothermal System (EGS), is a complex process and is dependent on range of design parameters, geological and operating variables. Stresses around EGS are believed to be either sound and non-harming or violent and catastrophic among different groups involved, directly or indirectly, in EGS. This paper explains how thermal inflow from country rock to the reservoir rock induces temperature changes in the zone, herein referred to as impact zone, around the reservoir. The growth behavior of impact zone depends on aforementioned sets of variables and is explained in this paper. Thermal stresses are generated during and after working cycle of the EGS within this impact zone due to thermal changes. This paper also gives an insight in understanding the behavior of stresses distribution around reservoir rock. As the reservoir rock is thermo-elastically connected to the country rock, newly generated stresses interact with the existing in-situ stresses under prevailing conditions of geological, design and operating variables. A simple model has been used for the demonstration of facts stated in this paper. Later on, variables dictating the continuous, safe and efficient working of EGS are also outlined. 1. Introduction Enhanced Geothermal Systems (EGS) is considered as a clean source of energy that can significantly contribute to the national energy security. It is a developing technology and there are many debates about working and effects of EGS, during and after working life of EGS. This paper aims at explaining heat flows, behavior of stresses and impact zone which in this paper is referred to as a zone around the EGS reservoir where rocks experience thermal changes and resulting effects that follow these changes. Variables participating safe and efficient EGS operation are highlighted. A simplistic model is also added as part of this paper to demonstrate the relevant arguments. Enhanced Geothermal System, EGS (HWR - Hot Wet Rock, HDR - Hot Dry Rock or HSR - Hot Sedimentary Rock are similar concepts used to engineer geothermal system), is a subsurface heat exchanging system where fractured rock at optimal depth with appropriate temperatures is stimulated and a fluid flow is set up through injection and production borehole(s) to utilize earth s heat. Heat in EGS is sourced by a combination of natural radioactivity, earth s heat of formation and combined flow of heat by conduction, advection and radiative transport (R. Jeanloz, 2013). Energy won by EGS or similar methods, may be used in power generation, industrial applications (aquaculture, agriculture, greenhouse, process heat, and drying) and domestic uses (district heating, snow melting, space heating, and bathing and swimming). Efforts have been made to make use of geothermal heat in cascading manner (Lund, 2010) and to treat acid mine drainage as well (Dunnington, et al., 2016). Some issues need monitoring and precautionary measures to be taken, for EGS to function well for the designed life. Fluid short circuiting (Ghassemi, et al., 2008) (Ghassemi & Kumar, 2007), thermal breakthrough time (Bakhsh, et 293

2 al., 2016), mechanical alteration of reservoir rock (MIT, 2006) are few to mention that can interfere heat recovery process. Induced seismicity is considered a big drawback during initial development and production times of EGS. While this concern may only be true for predominantly only small magnitude earthquakes or microearthquakes (Majer, et al., 2007) (MIT, 2006) other subsurface applications (such as waste water injection associated with flow back water disposal from shale gas and oil production, carbon dioxide sequestration in geologic formations and hydropower dams and other large structures that could influence earth stress distributions) are more directly connected to serious concerns about induced seismicity. Magnitudes of induced seismicity in EGS can be successfully managed but just needs to be carefully assessed and monitored. The primary objective of EGS is heat recovery from earth s crust, which defined the dynamics of the EGS vicinity over next few years. Thermal flows define the new stress state that ultimately is responsible for all changes and effects that take place during and after working life of EGS. All EGS factors are connected to stress distribution whether directly or indirectly (Arshad, et al., 2016). 2. Stress Changes in EGS 2.1 Basic Hypothesis The EGS reservoir, itself, has been the focus of most scientific and research work. The focus of this research is the picture outside the EGS reservoir. The question answered here is, what happens in surrounding of EGS reservoir when heat inflows generate tensile stresses and causing stress distribution during and after working life of EGS. The assumption in answering this question is that a fair degree of homogeneity exists in general in the surrounding country rock and that the rocks are thermoelastic in nature. The starting point of the answer is that the EGS is setup with inlet and outlet boreholes in place, fractures have been stimulated, flow through fractures is established, and the heat recovery process has started. At this point, the reservoir rock experiences slow but continuous thermal and eventually structural changes. Due to variable nature of geology over different regions and depths in the real world and the fact that EGS can, ideally, be setup anywhere with appropriate ground conditions, new stress state of the area cannot be concluded with absolute certainty. Bringing in the basic assumption of homogeneity, structural and stress transition can be predicted, based on the basic reaction of the rock to flow through fractures, and thermal changes. Rock shrinks as a result of cooling due to injection of cold water. The rocks at depths of 3-5 km (~10,000 to 16,000 ft) are thought to have thermoelastic properties, and so, this shrinking generates tensile stresses in country rock. Insitu stresses interact with these newly generated tensile stresses and develop a new stress pattern in the EGS zone. It is this stress pattern that dictates next set of resulting events. Mode, time and magnitude of the next event of any geological significance, are dependent on this very stress state. One event can also redefine the stress state and lead to another. Complete EGS process during its full span of life, is illustrated in figure 1 and is dependent on variety of sets of variables involved in EGS. Estimating the span, timeline and magnitude of eventual events, and extent to which it may be affecting the Figure 1. EGS process flow chart (Arshad, et al., 2016). 294

3 surrounding geological settings is not a simple deal (Khademian, et al., 2012). For EGS operation to run efficiently, there has to be a net balance between heat outflow (heat recovery) and heat inflow towards EGS reservoir, developing a continuous heat flow from the country rock towards reservoir. These heat flows develop different thermal zones and cooling fronts expanding in the vicinity, in more like a distorted spherical shape (figure 2). Diffusion front outlines the rock still at ambient temperature from diffusion zone where rocks have temperatures between ambient reservoir temperature and injection water temperature). A Cooling front differentiates between diffusion zone and cooled zone where the temperature of rock has dropped down to injection temperature and doesn t actively participate in adding heat to water (R. Jeanloz, 2013). Two basic facts connected to heat flows in Figure 2. Heat zones in EGS reservoir rock (Arshad, et al., 2016). EGS, first, existence of a continuous heat inflow from the country rock to EGS reservoir, second, existence of earth s natural thermal gradient imply that temperature above reservoir is already less than the temperatures being dealt with and that most of the heat inflows are from the rocks in all directions but from the rock above the EGS reservoir rock. It is not that there is absolutely no inflow from upper level; it will, however, be just low or negligible inflow compared to other directions. To explain it better, assume that the reservoir rock is a perfect homogeneous cube. There is only negligible heat inflow from upper side. Four sides, perpendicular to horizontal plane, have ideally same temperature and heat flow conditions in the surrounding. Bottom side of the cube has highest temperature difference for heat flow but rest of the heat flow conditions are still same. Out of two sides for vertical heat inflow; upper side providing only negligible heat inflow, leaves only one side for vertical heat inflow. Whereas four sides are proving for horizontal heat inflow. This brings into realization, a fact that most of the heat inflows, thermal changes and its stress and structural effects have to occur in lateral direction and under EGS reservoir; more in the surrounding of inlet borehole compared to the outlet. Replacing the cube with real world reservoir with distorted spheres and curves, gives more like a horizontal fat penny shaped volume for heat inflows. Figure 3(a) is the illustrative explanation of the fact and figure 3(b) is the illustration of what ultimate diffusion or ultimately, impact zone may look like (Arshad, et al., 2016). Occurrence of diffusion or ultimately, impact zone more in lateral plane suggests the impacts of the EGS like thermal stress build up, redistribution and geological effects later on, to be majorly in lateral dimension. Another factor to be considered here is the timeline of the whole process. Considering the life of EGS to be years, it is going to take even longer, with earth s thermal diffusion constant, for the impact zone to nullify the temperature changes and regain the ambient temperature. Such a slow and time taking process that supposedly has major impacts laterally, may be suggested to safely accommodate our energy needs in future (Arshad, et al., 2016). 2.2 Simulation of a Simple Model A simple model is developed to study the heat inflow behavior from country rock to the Figure 3. (a) Heat flows in EGS reservoir rock (b) Ultimate diffusion/impact zone (Arshad, et al., 2016). 295

4 reservoir. A 10 m wide disk at a temperature of 480 K is place in a cube of 100 m in dimension, with a temperature gradually increasing from 500 K (227 C/440 F) at the top to 510 K (237 C/458 F) in the bottom (figure 4 is model schematics). The system is simulated for a period of 100 years during which a net heat flow exist from cube towards the disk (figure 5). An imaginary cube of 40 m side length placed around the disk to measure the heat flux from each side (figure 6). Individual heat fluxes from all 6 sides of the cube over the time are plotted in figure 7. As explained in the previous section, individual heat flux from the upper side is the lowest, it highest from the bottom side, whereas intermediate flux exists from the vertical sides of the cube (all 4 curves overlap in illustration). Figure 8 illustrates the net vertical and horizontal heat flow. Net vertical heat flow (sum of fluxes from top and bottom) is far less compared to net horizontal heat flow (sum of fluxes from 4 sides of cube perpendicular to XY plane). This is a clear indication that heat flow is dominant in the horizontal plane suggesting that the impact zone to be dominant in horizontal plane as well. It is to be noted that planer heat flows are to be radially symmetric because of the assumed homogeneity. Figure 4. Model geometry. Figure 5. Temperature profile once the heat inflow is setup. During this simulation, tensile stresses are generated as the rock cools down and shrinks due to net flow of heat towards the disk. Figure 9 shows the behavior of thermal stress as heat flow is setup in the cube. As shown in figure 9, tensile stresses grow radially around cooling zone in the subsequent years of simulation. Figure 10 is illustration of how tensile stresses along the horizontal cutline (0 to 45 m in length from side of the actual cube to the start of the disk) grow in the subsequent years as more and more rock cools down and loses heat. It is important to notice here that these illustration only show the amount of tensile stresses generated, not the stress redistribution. The purpose of studying just the tensile stresses is to realize the actual magnitude of stresses generated and their behavioral change along the line moving farther from the zone of disturbance in the upcoming years. These stresses, as expected, decrease moving away from the disk but tend to increase in subsequent years keeping the same trend. Figure 6. Virtual cube to measure heat flows. 296

5 Heat Flux (W/m 2 ) Time (years) Figure 7. Avg. flux from sides of virtual cube. Vr Bottom Flux (XY Pl) Vr Top Flux (XY Pl) Hr Front Flux (XZ Pl) Hr Back Flux (XZ Pl) Hr Left Flux (YZ Pl) Hr Right Flux (YZ Pl) Heat Flux (W/m 2 ) Time (years) Net Vr Flow Net Hr Flow Figure 8. Net vertical and horizontal heat flow. Figure 9. Behavior of thermal stresses in subsequent years. 1st Principle Stress (Mpa) Line B (25 m away) Distance (m) Line A (5 m away) Year 1 Year 2 Year 5 Year 10 Year 50 Year 100 Figure 10. Comparison of stress buildup over time along a horizontal line between a side of cube and disk. If stress behavior is compared at two points 5 m and 25 m away from the disk (marked by dashed lines A and B respectively in figure 10) in different times, stress levels are closer to each other in the early years at 25 m mark and comparatively farther at 5 m mark. This trend is not dominant in later years of simulation. This may be an indication that stress build up isn t dominant in early years of EGS at a point away from reservoir but stress starts to build up as soon as the point experience either thermal changes or stress redistribution. This conclusion may be expanded to heat flows in the EGS surrounding, i.e., as soon as any zone in the country rock experiences temperature changes and starts contributing to conductive thermal flow towards EGS reservoir, there will be a stress build up in the zone proportional to heat flows and thermal flux in the zone. This also means that thermal flow zone and stresses around EGS reservoir will continue to expand with time as EGS operation continues to recover heat by fluid movements. 297

6 2.3 Variables Involved in EGS There are many variables involved in defining the final stress state around EGS, and any combination of these variables may play role in a given EGS scenario. These variables may include subsets of geology of the area, rock physical and chemical properties, fluid and flow attributes, and design parameters. These variables are outlined below. 3. Conclusion/ Remarks Table 1. Variables involved in EGS. Fluid Related Operating Variables Rock/ Geological Variables Fluid viscosity Specific Heat Fluid Compressibility Temperature Points Fluid flow rate Rate of heat removal Chemical alteration Fatigue/ mechanical breakage Design Parameters Pattern & No. of Injection /Production Boreholes Injection /Production Well Geometry Flow Regime Characteristics Porosity and Permeability Thermal conductivity Heat flux Stratigraphy Constituent Composition Heterogeneity Porosity and permeability of rock Fracture pattern: size, span, width, distribution, active or inactive Pore pressure Pore volume To summarize, the basic points of hypothesis are as follows: When an EGS system is put in action, it recovers heat from the reservoir rock depending on the permissibility of reservoir. Following the 2 nd law of thermodynamics, heat removal from reservoir rocks initiates a net heat inflow from the surrounding rock to the reservoir rock. Continued heat flow creates different thermal zones in the area, like symmetric zones shown here. Net cooling of the rock causes rock to shrink. Shrinking rocks generate tensile stresses in the surrounding country and reservoir rock. Generated tensile stresses interact with existing stresses to create a whole new stress pattern. New stress pattern impacts and affects the surrounding depending on geological variables and timeline of the system. The impacts and effects may occur in a chain or cyclic manner which is not necessarily shown here. Thermal flow and in turn, ultimate impact zone is suggested to exist dominating in horizontal plane for homogeneous scenario. Stresses are developed in the country rock as soon as it is part of heat and flow dynamics of EGS. Stress growth is more dominant at a point far away from EGS reservoir in later years of EGS operation than in the beginning. Major changes are shown to occur in lateral direction. As this whole process is a slow and time taking process that is shown to have major impacts laterally, with the assumption of general homogeneity and thermoelasticity, EGS may be suggested to successfully and safely fulfil our future energy needs. 4. Future Research Validating this theory and quantifying involved stresses and their impacts, through more rigorous physical modeling and, if possible, field results of different ongoing projects is going to be the starting point. Studying the behavior and effects of stress redistribution due to thermal changes in the surrounding of EGS reservoir is mainly the aim of this research project. What happens outside the reservoir and time, extent and cyclic behavior of the impacts, depending on the EGS variables, is the focus of my research. Another interesting question related to this research is looking into the possibilities without the basic assumption of this proposed theory. That is, what would be the scenario if the surrounding of EGS reservoir is non-homogenous in general? This theory may also be improved using experts opinions and comments and an improved version may be presented or published on a later date. References Arshad, M., Nakagawa, M., Jahanbakhsh, K. & Dunnington, L., An Insight in Explaining the Stress Distribution in and around EGS. Stanford, California, Stanford University. Bakhsh, K. J., Nakagawa, M., Arshad, M. & Dunnington, L., Modeling Thermal Breakthrough in Sedimentary Geothermal System, Using COMSOL Multiphysics. Stanford, California, Stanford University. 298

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