Deep Geothermal Processes Acting on Faults and Solid Tides in Coastal

Physics of the Earth and Planetary Interiors 264 (2017) 76–88

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Physics of the Earth and Planetary Interiors

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Deep geothermal processes acting on faults and solid tides in coastal Xinzhou geothermal ﬁeld, Guangdong, China ⇑ Guoping Lu a,b, , Xiao Wang b, Fusi Li b, Fangyiming Xu b, Yanxin Wang b, Shihua Qi b, David Yuen c a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China b School of Environmental Studies, China University of Geosciences, Wuhan 430074, China c Institute of Supercomputing, The University of Minnesota, Twins, United States article info abstract

Article history: This paper investigated the deep fault thermal flow processes in the Xinzhou geothermal field in the Received 13 April 2016 Yangjiang region of Guangdong Province. Deep faults channel geothermal energy to the shallow ground, Received in revised form 27 December 2016 which makes it difficult to study due to the hidden nature. We conducted numerical experiments in order Accepted 27 December 2016 to investigate the physical states of the geothermal water inside the fault zone. We view the deep fault as Available online 29 December 2016 a fast flow path for the thermal water from the deep crust driven up by the buoyancy. Temperature measurements at the springs or wells constrain the upper boundary, and the temperature inferred from the Keywords: Currie temperature interface bounds the bottom. The deepened boundary allows the thermal reservoir to Solid tide revolve rather than to be at a fixed temperature. The results detail the concept of a thermal reservoir in Geothermal Reservoir terms of its formation and heat distribution. The concept also reconciles the discrepancy in reservoir tem- Deep fault peratures predicted from both quartz and Na-K-Mg. The downward displacement of the crust increases Circulation depth the pressure at the deep ground and leads to an elevated temperature and a lighter water density. Ultimately, our results are a first step in implementing numerical studies of deep faults through geothermal water flows; future works need to extend to cases of supercritical states. This approach is applicable to general deep-fault thermal flows and dissipation paths for the seismic energy from the deep crust. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction the source for the hot waters (Liang, 1993). However, the deep flow processes remain relatively unknown. With elevated temperatures (one of the thermal water vent’s The thermal springs that occur in association with a fault zone temperature is as high as 98°C), the Xinzhou geothermal field is have the potential to provide data on geochemical characteristics, known for its thermal water outflows associated with a deep fault- physical states, and thermal outflows, all of which are useful in ing. Located in the coastal region of western Guangdong, it has a understanding deep flow processes and cyclic variations. In this backdrop of the southern and south-eastern China geothermal belt, study, daily and annual variations in the geothermal waters of characterized by regional faults along the longitudinal, latitudinal, Xinzhou geothermal field were recorded and modeled to under- and northeastern-southwestern directions (Fig. 1a) (Lu and Liu, stand the water flow processes of the system. The modeling study 2015). is considered as a vital tool used in understanding the underground The deep geological structure of the Xinzhou geothermal area geo-environments for the site. has not been identified until recently. The Xinzhou geothermal area Geothermal springs are commonly associated with fast flow is classified as a medium-scale geothermal field (Tian, 2012). The paths that connect the deep crust to the surface (Curewitz and hot spring waters are slightly alkaline and salty, and of Cl-Na type Karson, 1997; Zheng and Bennett, 2002; Sun and Lu, 2009) due (Table 1; Wang, 2013), mixed with the cold water and the seawater. to the horizontal impedance and vertical enhancement of the The local area’s ground waters are generally of HCO3-Ca, HCO3-Cl- groundwater flow by a fault zone (Bredehoeft et al., 1992; Caine Ca-Na, and SO4-Cl-Ca-Na types. The atmospheric precipitation is et al., 1996; Lu and Liu, 2015). Deep faults and fractured bedrocks, as infiltration and runoff channels, are critical control factors in the ⇑ flow of hot waters to the surface (Wang, 2005; Gao et al., 2009). Corresponding author at: School of Environmental Studies, China University of Faults and fractures are potential channels for seawater to intrude Geosciences, Wuhan 430074, China. E-mail address: [email protected] (G. Lu). inland concurrently (Chen et al., 2001). http://dx.doi.org/10.1016/j.pepi.2016.12.004 0031-9201/Ó 2017 Elsevier B.V. All rights reserved. G. Lu et al. / Physics of the Earth and Planetary Interiors 264 (2017) 76–88 77

Fig. 1. Geological background map for the Xinzhou geothermal field in Yangjiang, Guangdong province: a. Regional tectonic map (Guangdong Province Geological Bureau Regional Geological Survey Brigade 1988; Chinese Academy of Sciences,1959); b. Regional geological map for Xinzhou geothermal field (Geological information drawn from 1: 250,000 outline configuration diagram of Yangjiang city, 2004); and c. Water sampling sites. New Hole was a 1000 m deep scientific borehole (Wang et al., 2015). Cross section I-I’ shown in Fig.6. 78 G. Lu et al. / Physics of the Earth and Planetary Interiors 264 (2017) 76–88

Table 1 Thermal reservoir temperatures (T) and reservoir depthsa estimated using geothermometers for Na/K, K/Mg and quartz.

No. Name of Spring Temperature (°C) Reservoir Depthb (m) Na-Kc K-Mg Quartz Maximum Minimum XZ01d New hole 219 140 153 3369.0 2921.5 XZ02 Dun well 209 158 154 3393.7 2942.9 XZ03 Ta well 222 141 157 3450.7 2992.4 XZ04 Jia well 213 158 155 3409.5 2956.6 XZ05 East wei well 214 153 156 3433.3 2977.2 XZ06 East mao 213 142 147 3202.8 2777.5 XZ07 Old hole 202 130 138 2985.4 2589.0 XZ08 West ting 211 156 151 3311.7 2871.9 XZ09 East tang 217 144 154 3379.5 2930.6

Note: a. Aqueous chemistry data from Xu (2016); b. Reservoir depth calculated by (Ta T0)/g + h. Ta means temperature of quartz thermometer, while To is the average annual temperature of Guangdong (22.5°C). Constant h represents depth of annual constant temperature (13 m), and g is geothermal gradient (3.9°C/100 m–4.5°C/100 m); c. Formulation based on Giggenbach (1988); d. All samples are of Cl-Na type.

As a realistic and emerging source, geothermal energy has surrounded by low mountains on the eastern and southeastern attracted increased attention (Baioumy et al., 2015). Furthermore, edges. The altitude of the Xinzhou geothermal field is about geothermal energy is clean, renewable, and does not produce car- 10–13 m (Fig. 1). bon dioxides (Smith, 2007; Younger and Gluyas, 2012; Craiget al., Quaternary sediments cover the Xinzhou geothermal field. The 2013). Studies on the potential of geothermal energy have implica- basement granites are buried on a shallow level and their outcrops tions in environmental geology. Deep and large faults are often can be seen in ditches and ponds. To the north of the site is the linked to potential geothermal energy, as well as potential earth- boundary of Precambrian-Cambrian light metamorphic clastic quakes (Skarbek and Rempel, 2016; Bai et al., 2013). rocks (Fig. 1c). We studied the geothermal system in terms of the high temper- Hot springs in Xinzhou are naturally exposed along the stream ature and high pressure states in the deep fault. We also investi- bed. Hot water flows from the boreholes and oozes from the pond gated the up-swelling of the geothermal water and the driving bottoms. The Jia well is located in the middle of the field and the force along the deep fault in the Xinzhou geothermal field in order thermal water outflow can reach up to 98.4 °C, with this being to find out the interplay of controlling factors. We did this in order the highest temperature among the springs. to further understand how the deep system responds to changes of The field site has a deep fault at a high angle (almost vertical at states. The results could potentially provide further insight into 97°), tipping to the south. The fault was initially revealed in dril- deep geothermal behavior in response to earthquakes. lings at an earlier time (Liang, 1993). Recently, the fault was fur- The objective of this paper is to study the physical and chemical ther characterized by the geophysical method of Audio processes in the geothermal site via field site monitoring and Magnetotelluric Sounding (AMT) (Wu, 2013) and was later con- numerical modeling. The site’s description, conceptual model, and firmed in a 1000-m scientific drilling in 2014 (Wang et al., 2015). research methods are described (Sections 2 and 3), as well as the The advanced geophysical method’s apparent resistivity and impe- daily and annual variations in the outflows (Section 4). We also esti- dance phase have revealed the faulting to extend to a depth about mated the thermal reservoir temperature and calculated the reser- 10 km into the crust (Wang et al., 2015). voir depth (Section 5). The modeling study is presented (Section 6), and the findings are discussed and summarized (Sections 7 and 8). 2.3. Conceptual model

2. Site description and conceptual model The flow system is conceptualized as being well connected to the deep heat source through the deep fault F1 (Figs. 1 and 2). 2.1. The regional geology The groundwater is recharged through the rain water infiltration, and generally flows from the mountain to the ocean, while the sea- Geothermal fields in the Guangdong province are distributed in water intrudes through the tidal zone to mix with the groundwa- such a way that reflects the pattern of the regional deep faults in ter. The geothermal flow can create a relatively low pressure the southeast coast of China (Fig. 1a). Geophysical surveys show zone around the fall zone, thus affecting the groundwater flow sys- that the Enping-Yangjiang deep fault, located on the western side tem (Fig. 2a). of the Pearl River estuary, reaches as much as 20 km in depth The local scale of the groundwater could affect the geothermal (Ren et al., 2011). flow, in addition to the regional scale of the groundwater flow from The Xinzhou geothermal field is located in the southwestern the mountainside circulating to the ocean. However, the local side of the Enping-Kaiping deep fault and the eastern side of the groundwater tends to flow shallowly and travel a short distance southern section of Yangjiang-Guangzhou-Conghua deep fault. before discharging into the nearby stream bed. Thus, the local flow The internal structure of the Xinzhou geothermal field is well can be assumed to have a relatively small effect on the geothermal developed, and the hot springs are exposed in the junction region flow in terms of the regional scale circulation. of the two faults (Wang, 2013). The deep fault, perceived as a tension fault, serves as a fast flow The Xinzhou geothermal area exposes the Yanshan II granite in path for the flow and heat transfer. It progressively gets hotter as it the southern part of the geothermal fields and sees outcrops spo- goes deeper into the crust. The hotter water from the greater radically in the north. The granite forms the northwestern edge depths drives itself up onto the ground surface because of its of the Xinzhou rock mass. buoyancy. The temperature in the deep ground can reach beyond a super- 2.2. Site description critical condition (374°C at 22.5 MPa). In addition, the geothermal temperature at the Xinzhou area reaches the Currie temperature The Xinzhou geothermal field is located on the northwestern point of 585°C(Zhang et al., 2000) at around 21 m in depth (Wu, part of the semi-circular Xinzhou basin. The Xinzhou basin is 2013). G. Lu et al. / Physics of the Earth and Planetary Interiors 264 (2017) 76–88 79

Fig. 2. Conceptual model for solid tides on geothermal flows in the Xinzhou Geothermal field: a. Geothermal flow through the deep fault and the regional flow. b. Conceptual physical states in the water column along the fault zone. 4P for pressure change, dT and dq corresponding responses from temperature T and water density q. The deep fault zone was detected by apparent resistivity and impedance phase data interpretation (Wang et al., 2015). The attraction of the Moon and the Sun induces periodic displacements of the Earth’s solid surface and typically about a few tens of centimeters. Such deformation accompanies stress variations within the lithosphere that are dominated by pressure decrease and increase at the times of ground upward and downward movements, respectively. Deep geothermal flow mixes with shallow groundwater in the upper region. Deep groundwater up-swells along the deep fault zone due to buoyancy under elevated temperatures. The solid tide causes the pressure fluctuate.

Rain water replenishes the groundwater and lowers the 3.2. Mathematical model groundwater temperature in general. The groundwater interacts with rocks along its flow path, making it possible to trace the The geothermal processes in this study require mass and energy sources of waters. balance equations for fluid and heat flows in general multiphase The solid tides cause cyclical displacements of the crust and (liquid, gas in this study) and multi-component systems. Fluid lead to pressure variations in the crust. The pressure changes cause advection is described with a multiphase extension of Darcy’s physical state change in the geothermal water, resulting in the law. Heat flow occurs by conduction and convection. Thermody- cyclical variations in outflows of the thermal spring wells. namic conditions are described on the assumption of local equilibrium of all phases. The balances of the basic mass and energy can be expressed in a 3. Methods generalized form: Z Z Z 3.1. Field work d MK dVn ¼ FK ndCn þ qK dVn ð1Þ dt Vn Cn Vn The field work measured geothermal water outflows of wells for physical parameters. Water outflows were investigated from Jan- The integration is performed on an arbitrary volume Vn uary 2013 to January 2014. Water temperatures were measured enclosed by surface Un. The M in the summation on the left side with mercury thermometers with 0.1°C resolution. Thermometers represents the mass or energy in a unit volume, with K = 1, ..., were calibrated against each other. Flow rates were measured with NK for mass components (water, air), K = NK + 1 for the heat ‘‘com- buckets and a stop watch. The well outflows provided direct access ponent”. F represents the flux for mass or heat, and q is the sink, or to geothermal water from the deep circulation for convenient source. n is the normal vector on the surface element dUn pointing water samplings and flow measurements. Because of potential to the inside of the Vn. mixing with shallow groundwater along the upward flow path to The generalized form of mass accumulation terms is summation the ground surface, geothermal outflows may differ from those in of fluid phases: X the deeper grounds. Continual measurements using instruments K K M ¼ u Sb qb Xb ð2Þ for flow rates were not feasible because of the low tolerance of b temperature for the built-in sensors in addition to the drifting that occurs on the readings. The total mass for component K is obtained from summation of The outflows were measured for flow rates and temperatures, in fluid phases b. u is porosity, Sb is saturation of fluid phase b (i.e., addition to a set of physical parameters such as the pH, Eh, dis- the fraction of pores occupied by fluid phase b), qb is density of fluid K solved oxygen, and electrical conductivity. In addition, the out- phase b,Xb is the fraction of component K in fluid phase b. Advec- flows were monitored for their annual variation and daily tive mass flux term FKjadv is the summation of fluid phases: fluctuations. The annual variation is expected to yield information X Kj ¼ K ð Þ on rain infiltration, and the daily fluctuations are relevant to the F adv Xb Fb 3 b solid tidal effect on the deep fault zone. The fluctuations or daily variations could potentially reflect the flow path, i.e., the fault zone where Fb is the flux of fluid phase b and is given by the Darcy’s law or fracture zone. This is a new approach in terms of data interpre- for multiphase flow: tation for deep geothermal flow. k b q The geothermal water flow is affected by temperature. This ¼ q ¼ r b ðr q ÞðÞ Fb bub k l Pb bg 4 warrants a non-isothermal consideration of the geothermal field. b 80 G. Lu et al. / Physics of the Earth and Planetary Interiors 264 (2017) 76–88

Here ub is the Darcy velocity (volume flux) for fluid phase b,kis wellsprings (the Jia well and the Dun well) (Fig. 1c) for variations absolute permeability, krb is relative permeability of fluid phase b, in temperatures and outflow rates (Fig. 3). The Dun well has a lb is viscosity. And g is the gravity acceleration. Fluid pressure smaller flow rate and a lower temperature, while the Jia well is (fluid phase b)Pb is given as among the highest in flow rates and temperature. The Dun well has a small flow rate of about 0.057 L/s, a temperature from Pb ¼ P þ P b ð5Þ c 94.2 °C to 94.7 °C(Fig. 3b). Outflow temperatures were on an where pressure P for reference phase (generally taking gas phase) increasing trend and were the highest from 2 PM to 3 PM, while the flow rates were at a low level. From 1 AM to 2 AM, the outflow and capillary pressure Pcb (60). þ temperature tends to decrease, followed by another ascension Heat flux FNK 1 includes conductive and convective compo- when the flow rates are rising to the maximum of the day. nents are given as X The Dun well’s outflow temperature changed in synchroniza- NKþ1 F ¼krT þ hbFb ð6Þ tion with flow rates (Fig. 3b). The flow rates change as the outflow b temperatures change, implicating that outflow volume might be controlled by temperature. The solid earth tidal wave is also plot- k where is thermal conductivity, and hb is specific enthalpy in phase ted in Fig. 3 to facilitate comparisons. The daily variations appear b . T is temperature. to be related to the tides. The flow and tides will be better matched The mathematical model was implemented with the multi- with each other if we take into account the delayed responses of phase flow simulator TOUGH2 from Lawrence Berkeley National tidal fluctuations. A tidal peak corresponds approximately to a val- Laboratory (Pruess et al., 1999). TOUGH2 is a non-isothermal flow ley in solid earth tides (IERS, 2003; Ekman, 1989). and heat simulator, which has been widely used in the world. The The Jia well, not far from the Dun well, shows much larger out- simulation uses the equation of state (EOS3). flows and higher temperatures (as high as 98 °C) than the Dun well (Fig. 3a). There are two water venting holes in the Jia well, with one 4. Daily and annual variations of geothermal flows at half a meter above the other in a steel piping structure. In order to collect the real outflow rate data, we monitored the flow rates of 4.1. Daily flow rate and temperature the upper opening and temperatures for both the upper opening and lower outlet. The temperature of the upper opening is slightly Flow variations reflect responses of the deep geothermal region higher than that of the lower outlet (Fig. 3a). The upper opening to tidal waves. Monitored wells were selected based on the flow has lower pressure, resulting in a slightly higher temperature. This rates and accessibility. Daily monitoring was carried out on two is consistent with the result in the modeling section (Fig. 10).

Fig. 3. Daily variations of temperatures and ﬂow rates for the Xinzhou geothermal ﬁeld: a. Jia well, and b. Dun well (Fig. 1c). Temperatures for Jia Well measured at two venting holes above the ground. The monitoring events held in August 26–27, 2013. Tidal data from China National Sea Service (CNSS). Solid tide not calibrated, calculated from a program from International Earth Rotational and Reference Systems Service (IERS). G. Lu et al. / Physics of the Earth and Planetary Interiors 264 (2017) 76–88 81

Fig. 4. Annual variations of temperatures (T) and flow rates of Jia Well and Dun Well outflows in the Xinzhou geothermal field. The time counts start from January 1, 2013. Letter ‘‘a” marks November 7, 2013 the time that water started flowing out from XZ01 (New Hole the scientific drilling hole). The rain data based on public domain’s Yangjiang historical weather information.

Fig. 5. Graphical evaluation of the water-rock equilibrated temperatures (Giggenbach, 1988) using relative Na, K, and Mg concentrations (mg/L) from Xinzhou, Ruihai and Shenzao thermal waters. The spring samples from Ruihai (the largest and most active hydrothermal area in Tengchong) are located closely around the full equilibrium line. Data for Ruihai geothermal field in Yunnan Tengchong indicate parent geothermal fluid from a magma chamber (Guo and Wang, 2012). All the hot spring samples collected from Xinzhou and Shenzao geothermal fields plotted in the partially equilibrated or mixed area, while the cold waters collected from Xinzhou geothermal field located in the immature waters area and very close to Mg.

When pressure is lowered, the temperature rises slightly. An alter- 4.2. Annual flow rates and temperature nate explanation is that the upper opening is directly on the pipe wall, leading to a higher measurement in temperature. The lower The annual variations were monitored for the Jia well and the opening is running through a tunnel-like passage of about 30 cm Dun well (Fig. 4). The temperature differs in summer and in winter long of calcite and sodium-chloride salt, leading possibly to a read- for both wells. The fluctuation is about 2°C. The temperature and ing of temperature lower than that of the upper opening. In addi- flow rate for the Dun well decreases from April to October and tion, the lower opening is under higher pressure due to its lower slowly rises after October, which results from the decline of rain- position and the out-gushing venting of thermal water. fall. However, the flow rate of the Jia well decreases in May. Fol- In contrary to the Dun well, temperatures of the Jia well are lowing this, the flow rate increases from October to December, reduced between 2 PM and 3 PM, but increases in the morning of followed by a slight decline past December. The outflow tempera- the next day. Although the trend seems to be the opposite of that tures for the Jia well trends opposite to the flow rates. of the Dun well, the variations are well synchronized with the The New Hole Well was a thousand-meter deep scientific changes of the solid earth tide. drilling borehole. The hot water started to flow out on November 82 G. Lu et al. / Physics of the Earth and Planetary Interiors 264 (2017) 76–88

Fig. 6. The discretization and boundary conditions of deep geothermal water flows at the Xinzhou geothermal spring field: a. Regional shallow groundwater flow, and b. Deep geothermal water flow. The shallow groundwater is recharged by rainfall and discharges to the sea through shallow ground water flow by gravity. The deep groundwater, which has a lighter density at a higher temperature, is driven by buoyancy to a shallow position through fast flow path, such as faults and fractures. F1 is the tension fault which has been revealed by the 1000 m scientific drilling (Fig. 1b, c). F2 is the fault which was revealed in fieldwork (Wang et al., 2015). The bottom boundary was set as a constant temperature boundary. Every dot in the pictures represents a middle points of one gridblock. Different kinds of rocks are color-coded, and the keys are provided in Table 2 as layer numbers. The profile location marked as I-I’ in Fig. 1b.

7, 2013, causing slight decreases in the flow rates of the Dun well mixing with the Mg-rich cooling water when flowing up to the and Jia well. For the Dun well, the flow rate changed from about ground surface. The Mg content of geothermal water decreases 0.58 L/s to 0.44 L/s after November 7, and rose back to 0.56 L/s as temperature increases. Therefore water with elevated Mg on November 11. The same changes can be seen in the Jia well. concentrations would have been involved in low-temperature The flow rates of the upper opening in Jia well were about water-rock equilibriums, probably in a near surface environment. 0.50 L/s before November 7 and after November 11. However, in According to the reservoir temperatures of quartz, we calcu- between those two dates, the flow rate fell to 0.46 L/s. The drop lated the reservoir depth (Table 1) using the annual average tem- in flow rate for both wells may indicate that the creation of the perature of Guangdong at 22.5°C, constant temperature zone at New Hole well had impact on flow rates of the other two wells, 13 m and geothermal gradient 2.49–4.5°C/100 m. Chalcedony but the effect reduced in value after 3–4 days. was also used in validating the reservoir temperatures, yielding very close values to the ones from quartz (Fournier, 1977). Na-K 5. Thermal temperature at deep groundand groundwater combination would possibly yield higher values. Albite and circulation K-feldspar release Na and K under high temperature. Considering that Xinzhou field is in the coastal region and possibly affected Groundwater circulation of the deep ground is enhanced at a by seawater components, the applicability of thermometers is geothermal field. Geothermal water gets hotter and lighter in dee- therefore complicated. Discussion of the limitations or applicabil- per ground and is driven up due to buoyancy. Temperatures of the ity for these thermometers is beyond the scope of this work. 1/2 reservoir fluid can be estimated based on temperature-dependent Field water samples were also evaluated in the Na-K-Mg tri- mineral fluid equilibrium. angular diagram (Fig. 5) for equilibrium states and different water Thermal reservoir temperatures of Xinzhou geothermal system types. The cation geothermometers (Na-K temperatures and K-Mg were estimated by several commonly used thermometers. The temperatures) were used to calculate the reservoir temperature. In geothermometers specifically include quartz (Verma and the triangular diagram, the geothermal water samples from Xinz- Santoyo, 1997), Na/K geothermometer (Giggenbach et al., 1983), hou and Shenzao are plotted in the partially equilibrated or mixed and K-Mg geothermometer (Giggenbach, 1988). The estimated area of the triangular diagram, while the cold water samples are thermal reservoir temperatures are listed in Table 1. plotted in the immature area. This indicates that Na-K tempera- The estimated K-Mg temperatures are generally lower than that tures and K-Mg temperatures can be applied to calculate the of Na-K and Quartz temperatures, which can be explained due to reservoir temperature, but the results might be affected by mixing. G. Lu et al. / Physics of the Earth and Planetary Interiors 264 (2017) 76–88 83

Fig. 7. Simulated results of regional shallow ground water flow at the Xinzhou Fig. 8. Numerical simulation results of regional deep geothermal water flow at the thermal spring field: a. temperature field, b. pressure field, c. the density of Xinzhou thermal spring field: a. temperature field distribution, b. pressure field geothermal water. d. temperature gradient, and e. pressure gradient. See Fig. 8 for distribution, c. the density of geothermal water distribution, d. temperature the results of the corresponding deep region. gradient, and e. pressure gradient. See Fig. 7 for the temperature distribution of the corresponding shallow region. Geothermal fluids under Xinzhou geothermal field most likely achieved only partial balance with wall rocks; there is also a chance that parent geothermal fluid is mixed by regional ground- 6. Modeling study water in the process of rising to the Earth surface. In comparison, two samples from Ruihai (Guo and Wang, 2012; the largest and Single continuum is assumed in generating the grid (Pruess most active hydrothermal area in Tengchong) were plotted around et al., 1999). This rock model for granites does not distinguish frac- the full equilibrate line. It has been confirmed that the magma tured medium from porous medium. The reasons are twofold- one chamber exists below Tengchong, and the parent geothermal fluids being our singular focus in the steady state flow process and the makes it possible to reach the balance of water-rock interaction. second due to lack of fracture characterization data. The Van 84 G. Lu et al. / Physics of the Earth and Planetary Interiors 264 (2017) 76–88

stream segment is 9 km long, and the offshore portion is 10 km into the seawater from the coast. The transverse span of the stream reach is set to 222 m. The model is 7.2 km deep, including the high- temperature and high-pressure zone (Fig. 6a). This site is conceptualized as a quasi three-dimensional model. The shallow groundwater can be simplified as mainly longitudinal flow advecting to the sea, and lateral-flow is simplified as being uniformly treated for its eventual convergence to longitudinal- flow as it flows towards the sea. Considering the key focus in simulation of deep geothermal processes, this approximation is deemed appropriate. The simulated domain is divided into 99 columns along the stream flow, with 19–37 rows across the stream and 75 total layers, creating a total number of 176,493 gridblocks, including those for boundary conditions (Fig. 6). In the plan view, the upstream area has 37 rows and the downstream has 19 rows. The gridding is structured to effectively reflect specific simulation objects, such as fault zones, top layers and boundary nodes. The minimum interval of each column is 0.5–1 m in fault zone and 250 m on the boundaries on both sides. The maximum interval of seawater intrusion in the sea section and the low-lying stream segment is Fig. 9. Simulated results for physical states of geothermal water along deep fault F1. The thermal reservoir is considered as being located at where pressure increase 1000 m. The interval decreases gradually from the sea to F1 and slows down to at a linear pace. Quartz temperatures commensurate with fault F2 fault zones, which is represented as 1.0 m wide dividing the walls. And fault zone provides the highest temperature level at the reservoir area. opposite fault walls. In the vertical direction, from the surface layer Subscript f for fault, + or for upstream or downstream distance from the fault; to a depth of about 120 m, the grid is made up of small spacings; field data for New Hole from Wang et al. (2015); quartz data listed in Table 1; Na-K the smallest of which could be down to just 0.5 m thick. At the thermometer data in Fig 5 with depths calculated in the same way for quartz data. same time, to give consideration to the heat source, gridblocks Genuchten-Mualem model (van Genuchten, 1980) is used for the are constructed down into 1 m thick sections. In order to prevent relative permeability function. The parameters in van Genuchten potential iteration convergence problems, the variation of adjacent notation are 0.47, 0.07, and 1.0, for the van Genuchten m, the liquid grid spacings is generally less than a ratio of 1.75 for a smoother residual value, and the liquid saturation value, respectively. Capil- transition from side to side. lary pressure is calculated with the van Genuchten function, and Boundary settings have taken full advantage of natural bound- the maximum of negative capillary pressure is set at 1.0E aries. The land part of the upper boundary serves both as a con- +06 Pa. The model parameters of permeability are summarized in stant pressure boundary fixed at atmospheric pressure and as an Table 2. infiltration boundary, set at an infiltration rate of 357.5 mm/y, which accounts for 15% of the average annual rainfall of the region 6.1. The model settings and boundary conditions (2383.2 mm/y). The low-lying stream of the estuary and the seabed both serve as the boundary for the fixed water level. The depth of The Xinzhou Hot Spring model covers the recharge area of sea at 10 km away from the coast is 10 m. The bottom boundary, at upstream, the discharge area of downstream, the coastal area, a constant temperature of 300 °C, is set at a depth of 7200 m. The and bottom part of the deep geothermal area (Profile I-I’ in bottom temperature value is approximated according to the curie Fig. 1b). Taking a full view of the recharge and discharge process, temperature interface (about 585 °C) at a depth of about 21 km the rainfall infiltration segment is 11 km long, the low-lying deep (Wu, 2013). Rainfall infiltration is given to the top gridblocks

Fig. 10. Numerical simulation results of physical state changes along the deep fault zone F1 in response to pressure variation (4P 1.8e4 Pa) in the regional deep geothermal water flow at the Xinzhou thermal spring field: a. top layers, b. complete profile.+4 stands for quantity after pressure increases one 4P, 4 after pressure drops one 4P. Subscript notations referred to Fig. 9. G. Lu et al. / Physics of the Earth and Planetary Interiors 264 (2017) 76–88 85

Table 2 Model parameters for Xinzhou geothermal ﬁeld and regional groundwater and heat ﬂows.a

2 2 2 o Layer No. Rock types Lithology Porosity kx (m )ky (m )kz (m ) Heat Conductance (W/m/ C) 1Qedl Fine sandy clay 0.25 8.5E14 8.5E14 8.5E15 1.6 2Qmc Clayey ﬁne sands 0.18 8.5E13 8.5E13 8.5E14 1.8 3Qmc Course sandy gravels 0.20 8.5E12 8.5E12 8.5E13 1.9 4Qmc Sandy ﬁne sands 0.17 8.5E11 8.5E11 8.5E12 1.7 5 Granite Strongly weathered granite 0.15 1.5E13 1.5E13 1.5E14 2.1 6 Granite Intermediately weathered granite 0.12 1.5E14 1.5E14 1.5E15 2.3 7 Granite Slightly weathered granite 0.08 1.5E15 1.5E15 1.5E16 3.5 8 F2 Fault Fault zoneb 0.20 5.9E13 5.9E13 5.9E14 3.5 9 F1 Fault Deep fault zoneb 0.35 1.3E12 1.3E12 1.3E13 3.5

Note: a. Permeability data refer to site report (Wang et al., 2015), rock density 2100 kg/m3 for Quaternary sediments, and 2650 kg/m3 for granite; heat conductance takes common value. b. k for fault zones calibrated by this model. in the land section. The interface density is calculated by using upper part was constrained by the 1000 m borehole data. The tem- upstream weighting. The upstream boundary, utilizing the local perature data was field measured during drilling for a couple of watershed divide, is set as a no-flow boundary. Marine boundary scenarios: temperature recovering period for temperature mea- nodes were set at a fixed pressure, based on a freshwater head con- surements and flowing condition. The medium height of the ther- verted from seawater pressure at the center of each gridblock. The mal field is validated by the quartz temperature at corresponding boundary nodes below the seabed are set as no-flow boundaries depths, which is calculated using a certain range of thermal gradi- and are allowed to adjust their pressures. The marine boundary ents. The bottom temperature is supported by a temperature inter- is set far away enough from the coast so that the seawater salinity preted from the Currie temperature interface. is stable enough to form a waterhead to stop the fresh water from blending with the ocean. It is worth noting that the geothermal 7. Discussions and implications water is allowed to flow out according to the filtration conditions of the top boundary, so the boundary conditions are similar to 7.1. Deep geothermal water those of natural geothermal waters under no exploration. Deep geothermal water in states of high pressure and high tem- 6.2. Model result and calibration perature is distinctive in low density (Fig. 8). It has a tendency to move up owing to its low-density-inducing buoyancy. The regional The model ran for a sufficient enough time to be considered to deep fault extends deeply underground and becomes a fast flow have a steady state. The modeling results are presented in shallow path for the rising deep geothermal water, which changes the pat- portion (Fig. 7) and deep portion (Fig. 8), in terms of temperature, tern of regional groundwater flow. Geothermal water owing to pressure and water density. The profiles are drawn along the cen- driving buoyancy can effectively flow to the shallow parts through tral gridblocks in the stream-flow direction. the deep fault (Qi et al., 2017). The model was set up as a simplified 3D model for the deep Density distribution of thermal waters can be demonstrated by geothermal system. The model scale is 30 km long and 7.2 km the profile of a static water column (Table 3). Because of the low deep. In a planner view, the geothermal field is only about 500 m density at the bottom of the water column, the cumulative buoy- along the stream and about 75 m across. In this aspect, some sim- ancy of the thermal water could be so large that a small change plification is necessary. From the regional flow direction, ground- in pressure could result in buoyancy fluctuation. The geothermal water circulation would have led to deep circulation, which water around the fault has a density of about 700 kg/m3 at the would quench the geothermal flux. However, the geothermal flow deep-seated area (Fig. 8c). The temperature is slightly higher in along the fault zone presents a cone of a higher pressure zone in the deep fault zone compared to the outside at the same depth, the upper part (above 100 m depth) of the fault zone and a funnel where the pressure is relatively lower. This is the main reason of a lower pressure zone in lower part the fault zone (Fig. 7b). This why the density is abnormally low. The deep fault partially makes leads to the circulation of regional groundwater to a deeper depth the pressure dissipate in the deep-seated area, resulting in signifi- than what a normal groundwater flow otherwise would with no cant drops in pressure. geothermal effect involved. In the field site, the stream reach (from At the middle and higher portion of the profile, the pressure dis- north to south) is narrow in the cross-sectional spread and just as tribution of geothermal water is lower in the hot spring region wide as the size scale of the geothermal field. This means that the than in the other regions. This tendency is due to the regional effect regional groundwater along the stream has to go through the ther- caused by the deep fault (Fig. 8c). The affected region is about sev- mal field. In this sense, a quasi 3D approximation for the flow along eral kilometers deep and may extend up to just under the low- the regional direction is reasonable. lying riverbed area, making it favorable to potential intrusion by The flow in the thermal field is simulated as zero outflows occur the seawater. Regional groundwater forms a low pressure field in from individual boreholes. This is equivalent to the boreholes being the hot spring area, which makes the regional water flow to the plugged. This corresponds to the flow field before drillings (Liang, hot spring area at a depth of 1000 m. This result is obtained using 1993). This is warranted, given that the flow rates are equivalent the hydraulic conductivity field of granites approximated by a sin- to the waterhead for each water-venting borehole. For example, gle continuum. Because the granites contain partial fractures, a during the drilling of the 1000 m New Hole, the water column in more realistic two continua representation of the fractured gran- the new borehole was cooler and created pressures equivalent to ites could widely spread the low pressure zone in the hot spring those surrounding rock walls of elevated temperature. area. The model calibration was performed by adjusting the fault The temperature of the fault zone has a large gradient value at properties, specifically in permeability, by matching the tempera- the shallow region within one hundred meters, while it has a smal- ture profile data for the geothermal field (Table 2; Fig. 9). The ler value in deep area (Fig. 8d). The fault and fractures are high in 86 G. Lu et al. / Physics of the Earth and Planetary Interiors 264 (2017) 76–88

Table 3 Cumulative buoyancy for a high-temperature water column.a,b

Depth (m) Pressure P (MPa) Temperature T (°C) Density q (kg/m3) Change in cumulative buoyancyc (m) (well mouth to the depth) P 0.01 MPa P + 0.01 MPa 0 0.095 50 988.0688 2.197E06 2.197E06 100 1.064 55.0 986.1520 1.287E03 1.287E03 200 2.034 60.0 984.0808 4.817E03 4.816E03 500 4.942 75.0 977.0366 2.907E02 2.906E02 1000 9.789 100.0 962.9667 1.173E01 1.172E01 2000 19.483 150.0 927.9995 5.039E01 5.026E01 2266.25d 22.064d 163.31 917.3670 0.6639 0.6618 3000 29.177 200.0 885.3768 1.264 1.258 4000 38.871 250.0 835.4390 2.591 2.569 5000 48.564 300.0 778.0041 4.845 4.776 6000 58.258 350.0 712.1820 8.685 8.477

Notes: a. Assume water in closed stable condition; b. The temperature-pressure-density data from Wagner (1999) (calculation not considering changes due to temperature adjustment); c. The range of pressure change approximates crustal pressure change because of solid tides; d. Super critical pressure. permeability, so geothermal water can ascend due to its own in the whole domain, including the boundary gridblocks. The buoyancy. Taking all this into account, it follows that the fault exception was the top boundary gridblocks, which remained at and fractures are the fast flow channels for efficient geothermal atmospheric pressure. flow ascending to the shallow layer. The bottom part of the fault zone would see a rise in water den- The pressure gradient is believed to have led to the above tem- sity as pressure increased and vice versa. This can also be said for perature profile of higher gradients at the top and lower gradients the other bottom, lower and middle parts of the whole regional at the bottom (Fig. 8e). The pressure gradient presents the highest flow field. value where the top part of a lightly weathered layer meets the Dun well reaches 200 meters deep and could provide insight bottom part of a heavily weathered shallow sedimentary layer. into deep thermal processes (Figs. 3 and 10). The fault zone would Due to the low permeability of lightly weathered granite and the have lighter waters at a greater pressure, leading to greater buoy- dependence of geothermal flow on conduction, the pressure gradi- ancy and thus translate into greater flow rates for Dun well. As the ent is large. However, the strong water-transmitting ability of buoyancy goes up, the thermal water acquires higher temperature; faults and fractures causes heat to transfer mainly through advec- taking into consideration the delay of thermal water reaching the tion of geothermal water, resulting in a small geothermal temper- ground surface, this explains the peaking of temperature immedi- ature gradient at the bottom region (Fig. 8d). In comparison, the ately after the bottoming out of the downward displacement. top region sees an abrupt steepness in geothermal temperature, a The top layers have depth-sensitive responses quite opposite to result of conduction dominant heat transfer. the middle and lower parts of the fault zone (Fig. 10a). When pressure decreases in the uplift, the temperature increases and the thermal water gets lighter. The Jia well is shallow and could be 7.2. Solid tides and pressure effect on the geothermal water affected by shallow groundwater. This could provide clues to the dynamics observed in Jia well. When the uplift nearly reaches Tidal surface displacement moves the ground periodically and the peak, the thermal temperature is at its lowest; when the uplift acts upon the crust accordingly at shallow and deep depths. is down, the thermal temperature then reaches the highest tem- Upward movement of the ground creates tension within the crust, perature. Looking at the flow rate, it becomes higher and then corresponding to reductions in pressure within the rock. Down- peaks when uplift increases and reaches the top. ward movement compresses the crust, resulting in an increase of In the top layers, the temperature rises significantly when the pressure (Métivier et al., 2009). Tidal deformations vary in direc- pressure decreases (Fig. 10a). This could be a result of lowering tions and magnitudes. However, they are predominantly vertical pressure (Pruess et al., 1999). within the lithosphere and therefore are mostly radial and larger close to the surface. This means that tidal pressure within the lithosphere coincides with tidal surface displacement (e.g., Smith, 7.3. Implications and geothermal reservoir 1974; Wahr, 1981). As a rule of thumb, the increments of pressure are around a magnitude of less than 4e3–1.8e4 Pa (compared to The simulation has shown that temperature increases rapidly at earthquake stress drops of 1e5–1e7 Pa) (Vidale et al., 1998). the top part and increase at a slower pace below the top (Fig. 9). Field observations show changes in flow rate and temperature The top part is believed to be affected by groundwater flow circu- for geothermal water responding to solid tides. An increase in lation which effectively removes heat by advection. At the deeper downward displacement pressure leads to lower flow rate and zone, temperature increases linearly; the heat is dissipated higher temperature for the Jia well (Fig. 3a); whereas for the Dun through upward thermal diffusion. In between lies the transition well, the opposite result of higher flow rate and lower temperature zone that marks the weakened dominance of groundwater circula- is true (Fig. 3b). When pressure decreases due to uplifting ground, tion and top of the thermal diffusion zone supplying heat from the Jia well responds with a higher flow rate and lower temperature, deep source down under. The so-called transition zone possesses whereas Dun well responds with lower flow rates and higher tem- properties of a geothermal reservoir. peratures (Fig. 3). The geothermal reservoir in the Xinzhou geothermal field, con- Pressure changes and the corresponding responses of the sisting of the fault zone and neighboring wall rocks, is implicated geothermal system were simulated using the site model discussed in temperature distribution and possibly aqueous chemistry. The in Section 6. Pressure variation was implemented with an addi- reservoir temperature has its core temperature in the fault zone tional pressure which would be added or subtracted to the model and bulk heat dissipating area around it. The reservoir occurrence domain. Specifically, the pressure was modified for the gridblocks is consistent with the predicted depth from the water chemistry of G. Lu et al. / Physics of the Earth and Planetary Interiors 264 (2017) 76–88 87

field samples. The fault zone is at the core of the thermal reservoir The daily and annual variations in flows might provide leads in and is consistent with the Na-K-Mg temperature. The fault wall, better understanding the processes inside the crust, in term of immediately adjacent to the fault, is the wing of the reservoir at identifying fluid behaviors such as pressure and temperature a temperature predicted by quartz temperature (Table 1 and changes in response to seismic events. These changes are believed Fig. 9). In other words, this could imply that reservoir mineral- to be responses to solid earth tides. This might hold the key to fur- water interactions are kinetic in nature. In addition to the current ther understanding conditions and processes in deep grounds. heat and flow modeling, a reactive transport approach is warranted The shortcoming of the modeling is in using the quasi 3D mod- in a further study on reservoirs. eling to approximate the 3D complex system. The well-circulating The calculated result indicates that invasive seawater can geothermal flow may have been under-represented in terms of invade deep underground under certain favorable hydrodynamic intensity; the results might have underestimated the permeability conditions, and could seep further into the region where the hot of the fault zone. spring geothermal water cycle occurs. Finally, the sea water intru- Our results show that a thermal root needs a fast flow path such sion could make the salinity of the hot spring water increase. This as a fault zone several kilometers down into the crust to allow a could theoretically explain why the hot springs have a relatively significant large thermal outflow to reach out to the shallow high salinity in coastal regions, caused by modern seawater intru- ground. The fault zone with elevated temperature is providing a sion facilitated by geothermal systems. key driving force in powering the geothermal water up to shal- Temperature gradients are the largest at the shallow part and lower loci. the smallest at the bottom part (Figs. 7d and 8d). The gravel layer The fault zone is characterized in terms of hydraulic conductiv- has a larger gradient than the clay layer in the shallow area. ity or permeability, which controls the geothermal water flow and Although the thermal conductivity coefficient of the gravel layer heat transfer in the geothermal field. The model is calibrated is a little higher than the clay layer, a greater porosity makes the against the temperature profile in the spring system. It was shown gravel have a higher moisture content, leading to a less effective that the temperature distribution is quite sensitive to the perme- heat transfer. The general trend in the temperature gradient of ability of the deep fault. deep geothermal water is smaller toward the bottom, and becomes The deep fault property obtained from this study is conducive the smallest at the bottom of the fault (Fig. 8a). to further research studies in energy transfer and system response At the bottom of the fault, the temperature profile has shown a to perturbation in the deep crust. Specifically, the porosity wave significant drop from the fault to the upper stream side at about propagates favorably in a high porosity zone, which is the charac- 6500 m depth. This could indicate the lowest point that the water teristics of either tension or a tension-twist fault. circulation has reached. The deep circulation point could have been Our work represents the first of few studies with numerical affected by the bottom boundary which was set at 7200 m at experiments of the deep geothermal system. It provides a basis depth. It would require an improved representation of a full for further numerical studies of deep geothermal flow resulting three-dimensional (3D) regional groundwater flow to substantiate from elevated heat conditions, including the effect of chemical spe- in future work. cies, dehydration and chemical precipitation. The results have The deep fault property was obtained from matching the tem- implications for modeling of energy transfer in terms of porosity perature profile. A fault property representing regional flow in wave formation in seismic studies. the fault zone and deep ground could provide data for further research in the study of physical states in the deep crust. A high Acknowledgment porosity fault zone favors propagation of porosity waves, which could provide keys to prediction of earthquakes (Veveakis et al., This work was partially supported from The 211 Teaching Star- 2017; Yarushina and Podladchikov, 2015; Yarushina et al., 2015). tup Fund to the first author from China University of Geosciences, Wuhan, China Geological Survey Project (No. 1212011220014) Research and investigation of geothermal controlling geological 8. Conclusions structures in Pearl River delta and neighboring regions from Hydrogeology and Environmental Geology Division, and Natural We have conducted field measurements and numerical investi- Science Foundation of China NSFC Grant (No. 41572241). We gations to characterize the deep fault associated with coastal Xinz- express thanks to the help from Guangdong Provincial Hydrogeol- hou hot springs in Yangjiang, Guangdong Province, China. 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