Characterizing The Geothermal System In The Gonghe-Guide Basin By Coupled Fluid-Heat-Chemical (Isotope) Transport Modeling

As the global energy and environmental crisis has become more serious,the search for environmentally friendly new energy has become a hot topic in research around the world.Geothermal energy has become a new energy source for large-scale promotion due to its large resources,renewable,stable mining and clean and environmental protection.Since China entered the"Thirteenth Five-Year Plan"?2016-2020?,geothermal energy has ushered in important development opportunities under the dual pressures of controlling haze and energy conservation and emission reduction.The premise of effective development of geothermal resources is to fully understand the characteristics of geothermal reservoirs,including the distribution of reservoir temperature,the source of geothermal fluids and their flow processes,and the water-rock interactions in reservoirs.However,the analysis of geothermal reservoir characteristics still faces many challenges:the drilling method is directly effective but the economic input is high;the geophysical exploration has the multi-solution problem;the laboratory research is difficult to directly simulate the site-level geothermal fluid motion process.Based on the acquisition of parameters such as water chemistry,isotope and petrophysical properties of exposed hot springs and rocks in the field geological survey,the establishment of fluid seepage-chemical?isotope?coupling model will be an effective and economical method for understanding the characteristics of geothermal reservoirs.This paper will take the typical geothermal fields in China as examples?Gonghe-Guide Basin in Qinghai Province,Tengchong geothermal field in Yunnan Province?,and study the geothermal reservoir characteristics in the following three aspects:?1?Based on the superficial water chemistry data collected in the field geological investigation,the geothermometry is improved to determine the temperature field distribution;?2?Establish a hydro-thermal-chemical field-level numerical model to study the cyclic process of geothermal fluids and the evolution of water chemistry,and give the chemical composition changes of the geothermal fluid from bottom to top and its influence on the surface response.The self-sealing area of the mineral formed by hydrothermal alteration is finally determined.?3?Establish a natural model of stable isotope to analyze the migration law of D and ¹⁸O isotopes in geothermal system,and study the relationship between temperature field and stable isotope field under the condition of deep fluid recharge.In the improvement of geothermometry,three types of geothermal systems,such as volcanic type,sedimentary basin type and fault-controlled type,are used as examples to illustrate the application of Integrated Multicomponent Geothermometry?IMG?method in predicting reservoir temperature,as well as new ideas and reconstruction strategies for dealing with surface chemical responses of different types of geothermal water.To reconstruct deep fluid chemical composition and increase the confidence in estimated reservoir temperatures,a more integral geothermometry method was compared to other classical geothermometers.Here we apply the integrated multicomponent geothermometry?IMG?method using the GeoT code to estimate reservoir temperatures at the Tengchong geothermal field in Southwestern China.Results show reservoir temperatures calculated using the quartz geothermometer are closest to those estimated with the IMG method.The concentrations of Al and Mg,as well as selected minerals for geothermometry computations,are key factors for successfully using the IMG.Using the IMG method together with classical geothermometers can significantly increase confidence in reservoir temperature estimations.The methods presented and simulation program used here may be useful for analysis of other geothermal fields under similar conditions.The Gonghe geothermal field,northeastern Tibetan Plateau,China,is considered to be a potential target development area for Hot Dry Rock?HDR?resources,where the first Enhanced Geothermal System?EGS?research project is likely to be located.This area has formed typical layered geothermal reservoirs in a sedimentary basin with an anomalously high geothermal gradient?more than 55°C/km?,and the hydrothermal systems at different depths are connected by hidden faults.To investigate the geothermal structure and geochemical characteristics,water samples from the Gonghe geothermal field were collected and measured.The method of Integrated Multicomponent Geothermometry?IMG?was used to analyse the mixing processes and to estimate the reservoir temperatures.Due to significant differences in the Al and Mg concentrations between shallow and deep reservoirs,the IMG method,which uses measured and optimized values to reconstruct the composition of shallow and deep geothermal water,was successfully applied to constrain the mixing processes.This study predicted three typical geothermal reservoirs at different depths,and it quite accurately estimated the reservoir temperatures.The methodology of the present work can also be used in other geothermal fields with similar geological and geothermal conditions,to reconstruct the original deep and shallow water compositions from mixing processes,estimate reservoir temperatures,and build conceptual geothermal reservoir models.The Zhacang geothermal field in Guide Basin belongs to the fault-controlled geothermal system,the study of its reservoir temperature comprehensively utilizes the methods summarized in Tengchong and Gonghe work.The results show that the reservoir temperature range of the Zhacang geothermal field is 144¹61°C,and the average temperature and standard deviation are 153.6±7.4°C.The obtained result is similar to the measured temperature at the bottom of the ZR1 borehole in the study area?151.34°C?,so that the depth of the borehole ZR1?3050.7 m?is the maximum circulation depth of the deep fluid in this geothermal field.This conclusion provides the boundary conditions for the establishment of the hydro-thermal-chemical?isotopic?model of the Zhacang geothermal field.The determination of the temperature field has made it possible to study the chemical evolution in geothermal systems.Strong chemical reactions in geothermal systems may cause sealing of fractures,which reduces the permeability in the reservoir and subsequently affects the heat production.However,it is difficult to reveal the sealing range in a deep buried reservoir based on limited number of downhole logs.This study recreated the sealing processes of fault-controlled geothermal system in the Guide Basin,China,by reactive transport modeling.The modeling domain was discretized based on multiple interacting continua?MINC?approach,to address the non-equilibrium heat transport processes between the matrix and conduit in the fractured fault damage zone.Once the model was validated by observations of major ions in spring water and downhole temperature logs in the discharge area,it was used to determine the coupled processes of fluid,heat and chemical transport in the reservoir and the resultant sealing ranges.It was found that the dissolution of albite and k-feldspar lead to the precipitation of smectite-ca and illite in the middle and bottom of the fault under the condition of high concentration of Ca²⁺and Mg²⁺in the recharge water.Calcite veins were formed in discharge zone,because the horizontal fast flow in shallow subsurface zone supplied abundant Ca²⁺and HCO₃^-.As a consequence,the permeability in the discharge zone reduced by 15%when compared to the original permeability of 100 mD.Moreover,another three self-sealing areas were formed near the recharge zone,deep up-gradient zone,and the down-gradient area where the fast upward fluid flow occurred.Self-sealing subsequently prevented the deep circulation of the flow and heat absorption,which tends to make the fault-controlled geothermal system inactive.Stable isotope ²H and ¹⁸O can effectively determine the water source,rate and temperature of groundwater recharge.In order to study the recharge process of geothermal reservoirs,this paper establishes a natural model of hydro-thermal-isotope(²H and ¹⁸O)in the Zhacang geothermal field of Guide Basin,and analyzes the natural model to clarify the migration law of stable isotopes in the geothermal system.Then,based on this,the changes of stable isotope under the condition of additional deep fluid recharge are discussed,and some insights of the variation of isotope in the discharge area to predict the surface response of reservoir characteristics are obtained.In summary,the improved geothermometry method and hydrothermal chemical?isotopic?coupling simulation technology established in this study provide a new wayto analyze the reservoir temperature,porosity and permeability characteristics,water source and heat source based on shallow water chemical data,and provide technical support for geothermal resource evaluation.