Two stages power generation test of the hot dry rock exploration and production demonstration project in the Gonghe Basin,northeastern Qinghai-Tibet plateau,China

The Hot Dry Rock(HDR)is considered as a clean and renewable energy,poised to significantly contribute to the global energy decarbonization agenda.Many HDR projects worldwide have accumulated valuable experience in efficient drilling and completion,reservoir construction,and fracture simulation.In 2019,China Geological Survey(CGS)initiated a demonstration project of HDR exploration and production in the Gonghe Basin,aiming to overcome the setbacks faced by HDR projects.Over the ensuing four years,the Gonghe HDR project achieved the first power generation in 2021,followed by the second power generation test in 2022.After establishing the primary well group in the initial phase,two directional wells and one branch well were drilled.Noteworthy progress was made in successfully constructing the targeted reservoir,realizing inter-well connectivity,power generation and grid connection,implementing of the real-time micro-seismic monitoring.A closed-loop technical validation of the HDR exploration and production was completed.However,many technical challenges remain in the process of HDR industrialization,such as reservoir fracture network characterization,efficient drilling and completion,multiple fracturing treatment,continuous injection and production,as well as mitigation of induced seismicity and numerical simulation technology.

Paleoclimate and Paleoenvironment of Gonghe Basin, Qinghai-Tibet Plateau, During the Lastdeglacial: Weathering, Erosion and Vegetation Cover Affect Clay Mineral Formation

With the analysis of the sources and formation mechanism of the clay minerals in the sediment core from the Dalianhai lake in the Gonghe Basin,northeastern Tibet-Qinghai Plateau,clay mineral composition proxies,grain-size and carbonate contents have been employed for high-resolution study in order to reconstruct East Asian Summer Monsoon （EASM） over the northeastern Tibet-Qinghai Plateau during the lastdeglacial.The study also extended to establish a relationship between vegetation cover changes and erosion during the last 14.5 ka with pollen record and clay mineral proxies.Smectite/kaolinite and smectite/（illite＋chlorite） ratios allow us to assess hydrolysis conditions in lowlands and/or physical erosion process in highlands of the Gonghe Basin.Before 12.9 Cal ka BP,both mineralogical ratios show low values indicative of strong physical erosion in the basin with a dominant cold and dry phase.After 12.9 Cal ka BP,an increase in both mineralogical ratios indicates enhanced chemical weathering in the basin associated with a warm and humid climate.The beginning of Holocene is characterized by high smectite/（illite＋chlorite） and smectite/kaolinite ratios that is synchronous as with deposition of many peat laminae,implying the best warm and humid conditions specifically between 8.0 to 5.5 Cal ka BP.The time interval after 5.0 Cai ka BP is characterized by a return to high physical erosion and low chemical weathering with dry climate conditions in the basin.Comparing variations of clay mineral assemblages with previous pollen results,we observe a rapid response in terms of chemical weathering and physical erosion intensity to a modification of the vegetation cover in the basin.

Holocene Moisture Variation Recorded by Aeolian Sand-Palaeosol Sequences of the Gonghe Basin, Northeastern Qinghai-Tibetan Plateau, China

The northeastern Qinghai-Tibetan Plateau(QTP) of China is located at the triple junction of the Asian winter and summer monsoons and the westerlies, where paleoclimatic evolution has an important scientific significance for recognizing the spatial-temporal pattern of Asian monsoons in the past and predicting environmental change in the future. Nevertheless, the framework of the Holocene moisture variation and related mechanisms remain controversial, owing to complex hydroclimatic conditions triggered by the landform of the large mountain-arid basin. Here, we employed geochemical proxies from typical aeolian sand-palaeosol sequences in the Gonghe Basin, northeastern QTP, together with Optically Stimulated Luminescence(OSL) dating, to reconstruct the pattern of effective moisture variation and associated mechanisms in this region. Our results indicate that the regional effective moisture was at its lowest until 9–8 ka, and approached a maximum during 8–4/3 ka of the middle Holocene. Afterwards, the climate became relatively dry in general, but with a transient humid interval around 2–1 ka. Our geochemical evidence indicates that the dry early Holocene probably can be attributed to a strong winter monsoon forced by remnant ice sheet, combined with the high evaporation caused by solar insolation. Also, shifts of humid-dry are closely linked to the Asian summer monsoonal strength and therefore the balance of evaporation-precipitation in the middle and late Holocene. Thus, the pattern of the Holocene effective moisture variation is characterized as the ‘monsoon model’ in a closed intermontane arid and semi-arid basin near the western Asian monsoonal limit.

Effects of climate change and human activity on lake shrinkage in Gonghe Basin of northeastern Tibetan Plateau during the past 60 years

Changes in the status of freshwater resources are a topic of major global, regional and local concern. This is especially so in the arid and semi-arid regions of China, where shortage of water resources plays a crucial role in limiting sustainable socioeconomic development, as well as in sustaining natural ecosystems. Recent climate change, as well as the effects of localized human activity, such as the use of water for irrigation agriculture, may have significant effects on the status of the water resources in the region. Here, we report the results of a study of changes in the areas of lakes in Gonghe Basin, northeastern Tibetan Plateau of China, over the last 60 years. The data were acquired from optical satellite images and demonstrate that the total water area of lakes in Gonghe Basin decreased significantly from the 1950s to 1980s. The cause is ascribed mainly to human activity including exploitation of farmland, against a background of increasing population; in addition, climatic data for the region demonstrate a minor drying trend during this period as the temperature increased slightly. After the construction of several reservoirs, significant amounts of water were redistributed to promote irrigation agriculture and we conclude that this caused a significant shrinkage of the natural lakes. However, both the area of farmland and the population size remained approximately constant after 1990. We conclude that the variation of the total area of lakes during the second period was mainly controlled by climatic factors （precipitation and temperature）. As the regional temperature reached a new high, the area of some of the lakes decreased sharply before finally maintaining a relatively steady state. We emphasize that anthropogenic climate change and human activity have both significantly influenced the status of water resources in the arid and semi-arid regions of China.

The first power generation test of hot dry rock resources exploration and production demonstration project in the Gonghe Basin,Qinghai Province,China

Hot dry rock(HDR)is a kind of clean energy with significant potential.Since the 1970s,the United States,Japan,France,Australia,and other countries have attempted to conduct several HDR development research projects to extract thermal energy by breaking through key technologies.However,up to now,the development of HDR is still in the research,development,and demonstration stage.An HDR exploration borehole(with 236℃ at a depth of 3705 m)was drilled into Triassic granite in the Gonghe Basin in northwest China in 2017.Subsequently,China Geological Survey(CGS)launched the HDR resources exploration and production demonstration project in 2019.After three years of efforts,a sequence of significant technological breakthroughs have been made,including the genetic model of deep heat sources,directional drilling and well completion in high-temperature hard rock,large-scale reservoir stimulation,reservoir characterization,and productivity evaluation,reservoir connectivity and flow circulation,efficient thermoelectric conversion,monitoring,and geological risk assessment,etc.Then the whole-process technological system for HDR exploration and production has been preliminarily established accordingly.The first power generation test was completed in November 2021.The results of this project will provide scientific support for HDR development and utilization in the future.

Geochemical evidences of dry climate in the Mid-Holocene in Gonghe Basin, northeastern Qinghai-Tibetan Plateau

Previous research on climatic change in the Mid-Holocene in China indicates that it was a warm and humid period, accompanied by stronger summer monsoons, and it is defined as the Megatherrnal in the Holocene, or the Holocene Optimum period. However, this conclusion is mainly dh＇eeted at the monsoonal region in eastem China. In this research, we chose the Gonghe Basin in the northeastern Qinghai-Tibetan Plateau as the study area. Geochemical analysis of the profiles of paleosols and aeolian sand in the Santala area in the middle of the Gonghe Basin, along with OSL （optically stimulated luminescence） dating, indicates that the regional climate has experienced several warm-humid and cold-dry cycles since 11.8 ka. In particular, the Mid-Holocene （8.14.6 ka） was relatively cold and dry as evidenced by drastic fluctuations in chemical weathering degree and humidity, a higher aridity index, and sparse vegetation, accompanying increased winter monsoonal strength. In order to clarify whether this is an individual or local signal, we compared our geochemical analysis results with lake and peat records and aeolian de- posits of the monsoonal boundary region. The results indicate that the climate deteriorated widely, with declines in temperature and moisture, in the Mid-Holocene in the modem monsoonal boundary zone. Furthermore, the duration of climate deteriora- tion （relatively dry period） generally decreased from west to east in the aforementioned regions. Therefore, this dry phase in Gonghe Basin may be representative of dry events in Mid-Holocene in northem China. In addition, we discuss the reasons for this dry climate from several perspectives： （1） it probably can be attributed to a decline in summer monsoonal strength; （2） the regional evaporation loss （forced by high temperature） was not compensated by regional precipitation; （3） the thermal dynamic effect of the Qinghai-Tibetan Plateau.

Variations in aeolian landform patterns in the Gonghe Basin over the last 30 years

Wind erosion,or the transportation and deposition of sand into desert dunes and aeolian loess,is one of the most important aeolian activities.The progression of aeolian landforms expands arid and barren landscapes,leading to the degradation of adjacent areas.The Gonghe Basin,as a typical plateau with abundant sand sources,is highly sensitive to changes in the local climate conditions.In order to quantify the spatial-temporal variations in the aeolian landforms in the Gonghe Basin,we conducted field surveys and also analyzed twelve remote sensing(Landsat5 TM and Landsat8 OLI)images that sample the Gonghe Basin from 1989 to 2019.In the Gonghe Basin,we identified aeolian landforms such as climbing dunes on the windward slopes of the foothills,checkerboard dunes in the southeastern part of the basin,flat dunes,parabolic dunes and crescent dunes on the east and west sides of Longyangxia Reservoir,shrubby sandbanks on the valley slope in Shazhuyu,Tanggemu,and Indel,and sandy thickets at the bottom of the valley near the Dalian Sea,the Longyangxia Reservoir,and the tributaries of the Yellow River.From 1989 to 2005,the area of the aeolian regions expanded by 816.7 km2,with an annual conversion rate of 0.05%.From 2015 to 2019,the area of the aeolian regions shrunk by 2411.9 km2,with an annual conversion rate of−0.15%.The number and size of the fixed and semi-fixed dunes(e.g.the shrubby sandbanks on the valley slope and the sandy thickets at the bottom of the valley)were more stable than those of the mobile dunes(e.g.the checkerboard dunes,the flat dunes,the crescent dunes,the parabolic dunes,and the climbing dunes).The fixed and semi-fixed dunes were arranged in an irregular ring shape,and the location of the center of gravity of this ring did not change significantly from 1989 to 2019;in this time,the mobile dunes migrated to the northwest.

Evaluation of water resources carrying capacity of Gonghe basin based on fuzzy comprehensive evaluation method

Gonghe Basin belongs to arid and semi-arid climatic zone, where water resource is relatively scarce and desertification is serious, so it is necessary to find out water resources carrying capacity. By using fuzzy comprehensive evaluation method, the assessment on water resources carrying capacity in Gonghe Basin was conducted: Water resource carrying capacity of Gonghe County is less, while that of Guinan County and Chaka Town of Wulan County are greater; water resources of Gonghe Basin could sustain cultivated land of 652.6 thousand acres and grassland of 2 368.6 thousand acres respectively in 2020, water resources of Gonghe Basin could sustain cultivated land of 948.2 thousand acres and grassland of 2 247.6 thousand acres respectively in 2030.

Experiments and Analysis of the Hydraulic Fracture Propagation Behaviors of the Granite with Structural Planes in the Gonghe Basin

Hydraulic fracture(HF) propagation behavior is significant when building enhanced geothermal systems(EGS). HF geometry is closely related to the structural planes(SPs) in hot dry rock(HDR), such as natural fractures(NFs), quartz veins(QVs) and lithologic interfaces(LIs). However, the HF behaviors in HDR have not been well understood, especially the influence of multiple SPs on the HF geometry. To clarify this mechanism, several groups of physical simulation experiments of hydraulic fracturing were conducted to investigate the intersection relationship between the HFs and the SPs. Results show that the HF geometry shows great differences when intersecting with different SPs. In summary, the HF geometry displays four basic patterns, namely, propagation along the SPs, branching, capture, penetration/non-dilation. The fluctuation degree of the pressure-time curve and the HF complexity show a positive correlation. The cementing strength of the SP and their different mechanical properties from rock matrix influence the HF behaviors significantly. Therefore, the HF shows diverse geometries when intersecting with the NFs and LIs, while propagating along the QV when intersecting with it. For the complex networks, it is favorable for the HF to penetrate through and dilate several SPs, rather than simply cross or propagate along the SP.

Winter and summer monsoonal evolution in Gonghe Basin, northeastern Qinghai-Tibetan Plateau since the Last Glacial Maximum

Geochemical and grain size analysis on the DQ （Dongqi） profile from Gonghe Basin, northeastern Qinghai-Tibetan Plateau, indi- cates that regional climate has experienced several cold-dry and warm-wet cycles since the last glacial maximum （LGM）. The cold and dry climate dominated the region before 15.82 cal. ka B.E due to stronger winter monsoon and weaker summer monsoon, but the climate was relatively cold and wetter prior to 21 cal. ka B.E. In 15.824.5 cal. ka B.E, summer monsoon strength in- creased and winter monsoon tended to be weaker, implying an obvious warm climate. Specifically, the relatively cold and dry condition appeared in 14.7-13.7 cal. ka B.E and 12.14.5 cal. ka B.R, respectively, while relatively warm and wet in 13.~12.1 cal. ka B.E. The winter and summer monsoonal strength presents frequent fluctuations in the Holocene and relatively warm and wet conditions emerged in 9.5~.0 cal. ka B.E due to stronger summer monsoon. From 7.0 to 5.1 cal. ka B.E, the cycle of cold-dry and warm-wet climate corresponds to frequent fluctuations of winter and summer monsoons. The climate becomes warm and wet in 5.1 2.7 cal. ka B.E, accompanying increased summer monsoon, but it tends to be cold and dry since 2.7 cal. ka B.R due to en- hanced winter monsoonal strength. In addition, the evolution of regional winter and summer monsoons is coincident with warm and cold records from the polar ice core. In other words, climatic change in the Gonghe Basin can be considered as a regional re- sponse to global climate change.

Fracture Network Volume Fracturing Technology in High-temperature Hard Formation of Hot Dry Rock

It is more difficult for a hot dry rock to form a fracture network system than shale due to its special lithology, physical and mechanical properties under high temperature. The essential characteristics, rock mechanics and in-situ stress characteristics of a hot rock mass have been systematically studied by means of laboratory tests and true tri-axial physical simulation. The fracture initiation and propagation characteristics under different geological and engineering conditions are physically simulated, and the main controlling factors for the formation of a complex fracture network are revealed. The technology of low displacement for enhancing thermal cracking, gel fluid for expanding fracture and variable displacement cyclic injection for increasing a fracture network has been applied in the field, and good results have been achieved. Microseismic monitoring results demonstrate that complex fractures were formed in the field test, and the stimulation volume for heat exchanging reaches more than 3 million cubic meters. The research results play an important role in the stimulation technology of an enhanced geothermal system(EGS) and realize a breakthrough for power generation.