The first power generation test of hot dry rock resources exploration and produc

Er-y Z, D-u W,*, Gu- W, W- Y, W-s W, C- Y,Xu- L, Hu W, X-u T, W W, Ku L, C-yu Z, M-x L,H-b Lu, H-yu Hu, W Z, S-q Z, X- J, H- Wu, L-yu Z,Q- F, J-yu X, D W, Yu- H, Yu-w W, Zu-b C, Z-u C,W- Lu, Y Y, H Z, E- Z, Yu- G, Yu Z, C-s J,S-s Z, Xu Nu, Hu Z, L-s Hu, Gu- Zu, W- Xu,Z-xu Nu, L Y

a Center for Hydrogeology and Environmental Geology Survey, China Geological Survey, Ministry of Natural Resources, Baoding 071051, China

b Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, China Geological Survey, Shijiazhuang 050061, China

c Department of Natural Resources of Qinghai Province, Xining 810001, China

d Institute of Exploration Techniques, Chinese Academy of Geological Sciences, China Geological Survey, Langfang 065000, China

e Department of Hydrogeology and Environmental Geology, China Geological Survey, Ministry of Natural Resources, Beijing 100037, China

f Chinese Academy of Geological Sciences, China Geological Survey, Beijing 100037, China

g Beijing Institute of Exploration Engineering, China Geological Survey, Ministry of Natural Resources, Beijing 100083, China

h Institute of Geomechanics, Chinese Academy of Geological Sciences, China Geological Survey, Beijing 100081, China

i Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences, China Geological Survey, Langfang 065000, China

j Institute of Exploration Technology, Chinese Academy of Geological Sciences, China Geological Survey, Chengdu 611734, China

k Geoscience Documentation Center, China Geological Survey, Ministry of Natural Resources, Beijing 100083, China

l Jilin University, Changchun 130012, China

m Oil and Gas Survey, China Geological Survey, Ministry of Natural Resources, Beijing 100083, China

n Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China

o Institute of Geophysics, China Earthquake Administration, Beijing 100081, China

p Institute of the Hydrogeology and Engineering Geology of Qinghai, Xining 810008, China

Keywords:Hot dry rock Directional drilling Reservoir stimulation Microseismic monitoring Organic Rankine cycle (ORC)Power generation test Energy geological survey engineering Gonghe Basin Qinghai Province China

ABSTRACT 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 °C 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 wholeprocess 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.

1.Introduction

Hot dry rock (HDR) is considered a significant renewable energy source with great potential in the 21stcentury due to its wide distribution, environmental friendliness, and enormous resource potential (Tester JW et al., 2006; Wang JY et al.,2012; Kelkar S et al., 2016; Wang GL et al., 2018).Since the first HDR research project was carried out at Fenton Hill in the United States in the 1970s (Kelkar S et al., 2016), some HDR development research projects and enhanced geothermal system (EGS) projects have been carried out in the United Kingdom (Richards HG et al.,1992; Rodrigues NEV et al.,1995; Parker R, 1999), Japan (Tenma N et al.,2008), France(Genter A et al., 2010), Germany (Monsees AC et al., 2020),Australia (Baisch S et al., 2015; Wyborn D et al., 2010),South Korea (Grigoli F et al., 2018; Song Y et al., 2015), etc(Sigfússon B et al., 2015; Wang Y et al., 2018; Xu TF et al.,2018; Lu SM, 2018).These projects have gained valuable experience in efficient drilling and completion, reservoir fracture network stimulation, microseismic monitoring and interpretation, and induced seismicity control system (Ayling BF et al., 2016; Norbeck JH et al., 2018; Kwiatek G et al.,2019; Kraal KO et al., 2021; Wu H et al., 2021).However,most of the projects were shut down due to a series of major challenges such as high cost, inefficient inter-well connection,and induced seismicity.The utilization technology of HDR is still in the exploratory stage, and there is still a long way to go before commercialization.

In 2017, an HDR well of 236 °C was drilled at a depth of 3705 m in the Gonghe Basin, Qinghai Province, China(Zhang SQ et al., 2018; Feng YF et al., 2018).In 2019, China Geological Survey (CGS) launched the HDR resources exploration and production demonstration project in the Gonghe Basin.In 2021, the first power generation test of HDR was successfully realized in the Gonghe Basin.Several critical technological breakthroughs were made, including the genetic model of HDR in the Gonghe Basin, directional drilling and completion in high-temperature hard rock,reservoir stimulation, reservoir characterization, and productivity evaluation, reservoir connectivity, and fluid circulation, thermoelectric conversion, monitoring, geological risk assessment, etc.Then, the whole-process technological system for the exploration and production of HDR was preliminarily established.

2.Geological background

The Gonghe Basin is located in the central and eastern part of Qinghai Province, northwest China, covering an area of about 15200 km2.The average altitude of the basin is about 3200 m, with the Yellow River flowing through it from southwest to northeast.The basin is rich in wind, solar,hydropower, and geothermal energy resources.

2.1.Regional geothermal geology

Geotectonically, the Gonghe Basin is located on the northeastern margin of the Qinghai-Tibet plateau.The basin boundaries are defined by surrounding faults, including the NNW-trending Duohemao and the Wahongshan-Wenquan dextral strike-slip faults to the east and west, the NWW-trending southern margin of the Qinghai Nanshan and the Heka-Guinan Nanshan thrust and dextral strike-slip faults to the north and south, respectively (Zhang SQ et al., 2018; Su Q et al., 2017).The geological map of the Gonghe Basin can be referred to Fig.1 by Feng YF et al.(2018).Since the Paleozoic, this basin has been affected by multiple periods of compression with principal stress being closed to north-south,east-west, north-east, etc., forming the compression-torsional faults dominated by NW- and NWW-trending in the early stage and the secondary tension-torsional faults dominated by NE-trending in the late stage.

The U-Pb zircon age shows that the granite, which constitutes the basin’s deep HDR reservoir, was mainly formed during the Triassic (Chen XJ et al., 2020; Tang XC et al., 2020; Yun XR et al., 2020).The granite was uplifted,denuded, and exposed to the surface during the Jurassic to Paleocene.The rock cooled rapidly and the residual heat of the magma was lost due to the lack of effective cap rock.The current distribution of the HDR is mainly controlled by tectonic activity since Neogene.In Miocene, the Gonghe Basin was in a rift-depression stage, with active tectonic movements and the deposition of huge thick fluvial-lacustrine clastic sediments.Thermal history revealed by the zircon (UTh)/He shows that the rock temperature increased continuously during this period.Since the Pleistocene, the basin has undergone a significant differential uplifting and denudation (Jia LY et al.2017).The granite in the northeast was uplifted to the shallow surface to form the current available HDR resources.Comprehensive geophysical data such as gravity, geomagnetism, and 2D seismic data combined with drilling temperature measurement reveal the burial depth of granite in the northeastern part of the basin is generally less than 3000 m.The temperature of the granite typically reaches 180 °C in the depth range of 3000-3500 m.

2.2.Geothermal geological characteristics of the site

The production test site (PTS) is located in the Qiabuqia area, northeast of the Gonghe Basin.The Middle to Late Triassic granite forms the basement in this area, overlaid by the post-Neogene fluvial-lacustrine sediments with a thickness of about 1360 m.The average heat flow value in the PTS is about 101.6 mW·m-2.The average geothermal gradient of the basement and sedimentary cover are estimated to be 40.5°C/km and 64.2°C/km, respectively (Fig.1).

Typical hydrothermal alteration minerals can be found in the rock core of the PTS.The alteration mineral combination in the granite core is quartz + chlorite + epidote + zeolite +calcite, accompanied by pyrite formation.This indicates that the HDR has experienced hydrothermal intrusion with a temperature above 250 °C.

The lithology of the HDR reservoir is mainly composed of medium to coarse-grained granodiorite, with high content of brittle minerals.The X-ray diffraction analysis shows that the average content of the main minerals, including plagioclase,potash feldspar, and quartz are 50.1%, 19.6%, and 18.7%,respectively.

Fig.1.Comprehensive chart of the lithology, reservoir temperature, and temperature gradients logs in the study area.

The HDR in the PTS has high strength.The tri-axial rock mechanic experiments at high temperature and high pressure show that under the reservoir condition, the compressive strength of the granite reaches about 410 MPa, Young’s modulus is about 24 GPa, and the cohesion is about 34 MPa,and the internal friction angle is about 32°.

2.3.Characteristics of geostress and natural fractures development in the PTS

The Gonghe Basin is in the strike-slip and thrust faulting regime, and the reservoir is relatively intact.Thein-situstress field of the HDR in the PTS is complex.Thein-situstress measurements, such as core anelastic strain recovery (ASR),diametrical core deformation analysis (DCDA), and imaging logging, show that thein-situstress regimes from shallow to deep are normal faulting (≤1500 m), strike-slip faulting(1500-3477 m) and thrust faulting (≥3477 m), respectively.The reservoir is characterized by significantly high compressive stress, with the maximum principal stress direction being approximately NE35°.The maximum horizontal principal stress (SHmax) reaches 131.7 MPa, the minimum horizontal principal stress (SHmin) is about 103.6 MPa, and the horizontal differential stress is about 28 MPa(Fig.2).

High-temperature formation microscanner image logging,acoustic imaging logging, and far-sounding acoustic logging were employed to collect more accurate data on the natural fractures.The acoustic and electrical imaging results show that the attitudes of natural fractures in the HDR reservoir are complex and the fracture density is significantly inhomogeneous.The natural fractures are mainly NW- and NE-trending, with high, medium, and low dip angles.Fractures in the shallow part of the reservoir are relatively abundant, with the maximum fracture density above 2200 m reaching 105 per 10 m.Fractures in the target fracturing section are relatively rare, where except for the bottom hole section, the natural fracture density below 3500 m is less than 10 per 10 m (Fig.3).

3.HDR resources exploration and production demonstration project

3.1.The roadmap of HDR R&D by CGS

China Geological Survey (CGS) has planned to promote the HDR’s exploration and production test in three stages.The first stage for the realization of the power generation test and the preliminary establishment of the whole-processtechnological system based on the existing technology.The secondary stage for the research on the large-scale exploration and production technology.And the third stage for the industrialization promotion of HDR exploration and production.This paper is a summary of the first stage.The site view of the first stage is shown in Fig.4.

Fig.2.Geostress profile of the PTS using the ASR method.

3.2.General overview

In 2019, the first HDR production test well GH-01 was designed and drilled.This vertical well was completed, and then the fracturing tests were carried out in the same year.A microseismic monitoring system deployed on the ground and in shallow and deep wells was established to monitor microseismic events effectively.

In 2020, the fracturing of GH-01 was carried out.According to the fracture distribution characteristics, two directional wells (GH-02 and GH-03) were designed and drilled.To improve the inter-well connectivity, each directional well was set with two target sections at a depth of 3500-4000 m.By this time, the HDR well group, which consisted of one vertical well and two directional wells, was finally completed, as shown in Fig.5.The three wellheads were arranged in a straight line, with GH-02 and GH-03 located 60 m on either side of GH-01, and the boreholes’distance in the HDR reservoir ranged from 180 m to 380 m.

In 2021, the multi-stage and multi-process fracturing of the HDR reservoir was carried out, and the inter-well connectivity was finally achieved.Two organic Rankine cycle(ORC) generator units with capacities of 340 kW and 1200kW were designed and constructed, and the synchronized grid-connected equipment was carried out simultaneously.

Fig.3.Formation microscanner image of GH-01(a), GH-02(b), and GH-03(c) among the reservoir part.

Fig.4.The site view of HDR Exploration and production Demonstration Project in the Gonghe basin, China.

3.3.Drilling and completion of the HDR Wells

3.3.1.Drilling speed-up technologyDuring the drilling process, the upper sedimentary formation was rapidly drilled with the PDC bit, and the lower granite formation was mainly drilled with the roller bit.To improve the rate of penetration (ROP) in the high-temperature granite, various technical measures were taken in terms of downhole tools, high-efficiency drill bits, bottomhole assembly (BHA), and the HDR composite drilling process were developed.The improved average ROP in granite reached 2.5 m/h with a maximum of 5.8 m/h.As a comparison, the average ROP of conventional drilling is only 1.2 m/h.The maximum footage per trip was 176 m during the drilling.In addition, the jet suction hydraulic hammer, double action hydraulic hammer, wear-resistant rotary drilling tool,high-temperature-resistant turbo drilling tools, and highenergy hydraulic hammer were developed and tested.

3.3.2.Air DTH hammer drillingThe air down the hole (DTH) hammer drilling technology,with ϕ215.9 mm drill bit, and impact frequency of 20-25 Hz were tested in the production test well.In granite, the cumulative footage was 748 m, with 11 trips of the cumulative drilling times.The average ROP of the air DTH hammer drilling reached 5.35 m/h, which was 4 times faster than conventional mud drilling, and the maximum footage per trip attained more than 200 m.It shows that the air DTH hammer technology significantly improved the granite drilling speed in the Gonghe Basin.However, there are still technical challenges in the following aspects.Firstly, the service life ofmany drill bits is short, due to teeth breakage and teeth falloff.It is necessary to develop high wear resistance and longlife air DTH hammers to better adapt to granite.Secondly,formation water greatly increases the auxiliary time of the air DTH hammer drilling, further aggravating the borehole instability and downhole risks.Last but not the least, an additional reaming procedure is required due to the wear of the outer diameter, since the air hammer drilling could not perform reaming operations.

Fig.5.Schematic diagram of the trajectory of HDR well Group.

3.3.3.HDR directional drilling technologyTwo subsequent directional wells were drilled into the fracture network.Each directional well was set with two target sections at a depth of 3500-4000 m.The downhole composite-controlled drilling technology was adopted,combined with the specially developed high-temperature measuring while drilling (MWD) system.The top drive system, sliding directional drilling, and downhole motors were alternately used to adjust and control the borehole trajectory.The inclined drilling started from a depth of 3000 m with a maximum build-up rate of 5°/30 m and a maximum apex angle of about 27°.The horizontal displacement was 400 m, and the continuous directional drilling reacheed 1000 m under the high-temperature hard rock at 160-209 °C.The two directional wells have achieved dual-target hits with the precision of 1.5-9.8 m.In directional drilling, the drilling fluid and assembly were optimized, and low-density drilling fluid was used as much as possible to ensure borehole stability and drilling safety.

3.3.4.High-temperature resistant drilling fluidThe bottomhole temperature of the HDR wells in the Gonghe Basin is exceptionally high (> 200 °C).Affected by the geostress and hydraulic fractures in the target section, the stability of the surrounding rock is poor, and the rheology and filtration of the drilling fluid are difficult to control, which brings significant challenges to the drilling fluid technology.According to the characteristics of HDR drilling in the Gonghe Basin, a 240 °C high-temperature tackifier was developed.A filtrate reducer and a defoamer were selected to form a high-temperature resistant polymer drilling fluid system suitable for directional drilling in high-temperature granite.During the drilling process of the HDR, the system has a thin filter cake, strong toughness, slight damage to the sidewall, and fine particle carrying capability, which is conducive to maintaining borehole stability and can meet the high-temperature condition at the bottomhole.The composition of the high-temperature drilling fluid system consists of environmentally friendly materials, with no irritating odor and less environmental pollution.

3.3.5.Drilling fluid cooling devicesTo lower the circulating temperature and meet the operating conditions of the MWD, air-cooled and spiral-plate drilling fluid cooling devices were developed.The air-cooled drilling fluid cooling device comprises a spray system, a cooling packing layer, and an axial flow fan.The solid control system pumps the high-temperature drilling fluid to the spray system, which sprays the drilling fluid downward onto the cooling packing layer.The axial flow fan forces the air into the cooling packing layer and exchanges heat with the hightemperature drilling fluid to cool the drilling fluid.

A spiral plate drilling fluid cooling device comprises a spiral plate heat exchanger, cooling tower, refrigerant, etc.The returned high-temperature drilling fluid passes through the spiral plate heat exchanger and the refrigerant to achieve efficient heat exchange and cooling, and the cooling tower continuously cools the refrigerant.The above two sets of devices were separately used for GH-02 and GH-03.The practice showed that they could both lower the circulation temperature of drilling fluid by more than 25 °C, which ensures that the re-entry temperature is less than 55 °C and the bottomhole circulation temperature is approximately 120°C, enabling the 175 °C resistant while-drilling measurement system to work stably and measure the entire well trajectory.

3.3.6.Completion structure of HDR wellsThe bottomhole temperature of the HDR production test wells in the Gonghe Basin is all over 200°C, which is considered an ultra-high temperature formation.After completion, it is necessary to meet the needs of subsequent high-intensity fracturing and long-term high-temperature circulating operation.Therefore, the requirements for completion methods, cement slurry, casing, and cementing processes are extremely stringent.Three wells were completed with three sections and small gaps well structure optimization.According to the injection and production requirements, the final hole size was designed to be ϕ215.9 mm.For the first section, the size of the drill bit was ϕ444.5 mm, and the ϕ339.7 mm casing was run to seal the Quaternary unconsolidated layer.For the second section, the size of the drill bit was ϕ311.2 mm, and the ϕ244.5 mm casing was run to the complete granite section.For the third section,the ϕ215.9 mm open-hole drilling was carried out over 2500 m, the ϕ177.8 mm casing was run into this section after drilling.Ultra-high temperature slurry cementing was applied.And sieve tubes were used in the HDR reservoir section to meet hydraulic fracturing demands.

3.4.Reservoir stimulation of well GH-01

The geological characteristic of the PTS is characterized by highin-situstress with strike-slip and thrust types.To form a large-scale reservoir with a complex fracture network safely and efficiently, the reservoir stimulation process of well GH-01 was divided into three phases (minifrac test, fracturing pilot test, and large-scale stimulation) based on the geological characteristics of the PTS and the experience of HDR reservoir stimulation around the world.

The primary purpose of the minifrac test, which included a typical diagnostic fracture injection test (DFIT), acidizing test, step-rate injection test, continuous and intermittent fracturing test, temporary plugging agent, and gel test, was to analyze the physical characteristics of the target reservoir, the availability of fracturing fluid, the suitability of the equipment, etc.The cumulative injection volume of fluid was 1491 m3, the maximum pump rate was 2.5 m3/min, themaximum wellhead pressure was 42.6 MPa and the effective stimulated reservoir volume was 6×105m3, during the six days’ fracturing test.The result showed that thein-situpermeability of the reservoir was about 2.6×10-2mD.

The main purpose of the fracturing pilot test was to make technical preparation for large-scale stimulation.The shear fracturing tests of HDR under different pump rates and injected liquid volume conditions were carried out by analyzing the characteristics of fracture propagation under a certain scale of injected liquid volume.The cumulative injection volume of liquid was 4300 m3, the maximum pump rate was 3.05 m3/min, the maximum wellhead pressure was 48.1 MPa, and the effective stimulated reservoir volume was 2.41×106m3.

The large-scale reservoir stimulation was carried out based on the minifrac and fracturing pilot tests.It took“generating shear fracture, promoting multi-fracture propagation, and improving reservoir stimulation volumes” as the general thought for the large-scale reservoir stimulation stage.The reservoir stimulating technology of HDR under safe conditions has been formed with multistage, small pump rate, long period, multi-liquid phase, and soft shut-in.To realize the effective shear and fracture propagation in HDR with low energy microseismic events, a small pump rate of less than 3 m3/min and continuous injection with 8-24 hours were adopted during the stimulation.The continuous pump injection and soft shut-in operated by the step-less variable speed fracturing equipment were adopted to mitigate the pressure shock effect.The large-scale fracturing was gradually completed through seismic mitigation technology,fracture propagation process optimization, and stimulation by multi-stages operation.The cumulative injection volume of liquid exceeded 2×104m3.The maximum pump rate was 3.05 m3/min, and the maximum wellhead pressure was 48.1 MPa(Fig.6).The effective stimulated reservoir volume exceeded 1.2×107m3around GH-01.

3.5.Fluid circulation and inter-well connectivity

The connectivity test and the fracturing of production wells accomplished the well group’s connectivity.Through alternate injection and production, acid injection dissolution,perforation, and other technological measures, the HDR reservoir was further expanded, and inter-well connectivity was strengthened continually.The problems of high reservoirin-situstress, difficult fracture evaluation, complex liquid inlet channels, severe fracture plugging, and elevated circulating pressure of the well group were overcome.In the end, the well groups were successfully connected in circulation, the effective stimulated reservoir volume was more than two times as much as that after the large-scale reservoir stimulation of GH-01.The flow rate of the interwell-groups circulation test was 15.4-22.5 m3/h (Fig.7), and the temperature was 110.4-125.7 °C.The circulation pressure significantly declined, and the injection well pressure dropped from 65.3 MPa to 54.6 MPa.The recovery rate of circulating liquid was continuously increased by two times.

3.6.Stimulated reservoir evaluation

3.6.1.Stimulated volume evaluationGeophysical methods, including microseismic monitoring and time-frequency electromagnetic prospecting, were adopted to evaluate the stimulated reservoir during hydraulic fracturing and circulation.A surface-downhole-shallow borehole combined microseismic monitoring system was deployed around the PTS.The acquired continuous microseismic data was transmitted to the data server via a wireless network, and microseismic events were detected and located in real-time.Microseismic event attributes, including event location, magnitude, strike, dip information derived from moment tensor inversion, etc., were used to interpret and evaluate the fracture networks and calculate the effective stimulated reservoir volume (Fig.8).

At the same time, the electromagnetic monitoring system was deployed in and around the PTS.The migration of circulating fluid in the reservoir was obtained by comparing the abnormal relative amplitude change before and after stimulation (Fig.9).

By comparing various calculation methods of the stimulated reservoir volume (SRV), three levels of stimulated volume were proposed, including the stress sweep volume characterized by microseismic events, the effective fracture volume derived from discrete fractures, and the fracturingfluid distribution volume characterized by resistivity difference.They correspond to the microseismic event envelope volume, fracture network modeling volume, and fluid sweep volume, respectively.

Fig.6.Injection pressure, rate, and volume curves during the reservoir stimulation.

Fig.7.Injection rate and pressure curves during the circulation.

3.6.2.Heat exchange areaThe heat exchange area is an important parameter for evaluating inter-well connectivity.A tracer test was carried out to estimate the heat exchange area of the stimulated reservoir.A tailing phenomenon of the tracer breakthrough curve was observed.It indicated that the dispersion effect was stronger, which presented a larger heat exchange area.

3.6.3.Reservoir permeabilityThere are several methods to calculate the hydraulic fracture permeability of the stimulated reservoir, one of which is calculating fracture permeability based on a discrete fracture network (DFN).The corresponding micro-seismic event location determines the location of each hydraulic fracture in the DFN, and the radius of the micro-seismic source determines the fracture length and height.The fracture width is calculated from the moment tensor.Thus, the geothermal reservoir can be divided into many equal-sized geocell.The full permeability tensor is calculated for each geocell containing those hydraulic fractures.Its value is subject to the number, strike and dips, and size of fractures inside each geocell.Another method is evaluating the flow characteristics of the fracture system in the stimulated reservoir and calculating the effective hydraulic fracture length and the hydraulic fracture permeability based on the well test data.The calculated results indicated that the range of the reservoir equivalent permeability ranged from 2.2 mD to 322 mD after stimulation and injection-production test(Fig.10).

3.6.4.Reservoir productivity evaluationThe heat production rate of the stimulated reservoir is an essential parameter for evaluating reservoir productivity and providing basic data for power generation and comprehensive utilization.The heat production rate is a function of the temperature, flow rate, and specific heat of the produced water at constant pressure.The temperature and flow rate of the produced water can be measured at the wellhead of the production well.The specific heat at constant pressure is determined by the temperature and pressure of the production water.The temperature of the produced water from the reservoir in the Gonghe Basin is relatively stable.Thus, the productivity is approximately proportional to the flow rate of the produced water.

Fig.8.Microseismic events during the large-scale stimulation at different stages.The figures sequentially show the different stages of microseismic development over time.The event points from large to small represent the energy from strong to weak.

3.7.HDR power generation system

Based on the analysis of injection-production cycle characteristics of the HDR reservoir in the Gonghe Basin,organic Rankine cycle (ORC) power generation process was adopted.Generating units with capacities of 340 kW and 1200 kW were developed according to wellhead pressure, flow rate,and temperature, which overcome the difficulties such as high pressure, low initial flow rate, and large load change of hot water produced from the reservoir, etc.The units adopted series ORC power generation process, turbo-expander, andsynchronous grid connection, which can meet the various needs of the pilot demonstration and scientific research of HDR and can well operate at 110% of the rated load.

Fig.9.The liquid sweep range by the time-frequency electromagnetic method after stimulation in 2021 (A_relative: Abnormal change of relative amplitude).

As a whole, the two HDR power generating units were arranged in a two-stage series connection form.At the same time, a single unit can operate independently to meet the needs of different working conditions.The designed temperature, flow rate, and pressure of the heat source are 100-180 °C, 10-180 m3/h, and 10 MPa, respectively.The 340 kW and 1200 kW ORC power generating units mainly consist of a heat exchanger, turbo-expander, generator, working fluid pump, working fluid, air cooling island, grid-connected system, and ancillary facilities.The ground operation system mainly includes a ground injection-production cycle system,electrical control system, monitoring system, and other ancillary equipment.

4.Power generation test

In 2021, after the inter-well connection, the 340 kW ORC unit was used to carry out the power generation test in two steps.To confirm the stability of the ORC unit, the first step was conducted from September 26thto 29th, which lasted 56.8 hours.For the second step, the power generation test was run from November 9thto 15th, based on the well group consisting of two injection wells and one production well.Nano-aerogel thermal insulation pipes were installed in the boreholes for thermal insulation, and the ground geothermal water pipelines were covered with insulation.The water temperature into the ORC unit was 110.4-125.7 °C, and the inflow rate was 15.4-22.5 m3/h.The power generation test was continuously operated for 72 hours, and the second step lasted 132.2 hours,The specific grid-connected technique was also tested.Totally, the first power generation test was run for 189 hours.

5.Monitoring and evaluation of the geological environmental impact

A whole-process system for safe geological developmentof the HDR and environment monitoring has been established based on investigation, monitoring, evaluation, prediction,and management in the PTS, which provides security for the power generation test of the HDR.

Fig.10.Different depth slices of hydraulic fracture permeability from the inversion of micro-seismic data.

According to the principle of “gridding monitoring the key areas and multi-parameter mutual verification”, a monitoring system of “one network with multi parameters”has been formed.The high-precision seismic monitoring network mainly did the real-time location of earthquake and magnitude determination.A force-balance strong seismometer was installed to fit the attenuation relationship of the shallow source area and provide safety zoning data.Micro Electro Mechanical System (MEMS) simple seismic intensity meters were installed for different buildings to monitor the possible impact on buildings in real-time.The stability of faults around the site was studied, and Radon in soil gas was measured to monitor fault activation.The deep confined groundwater in wells was observed for the long term to monitoring the abnormal fluctuation of deep groundwater parameters around the site, and the water quality and tracer of confined aquifers and springs were tested regularly to analyze the impact on the groundwater environment.On-site Unmanned Aerial Vehicle(UAV) and household surveys were conducted to evaluate the seismic fortification capability of local buildings.The monitoring results show that the ground motion, water table,groundwater quality, and radon gas generated around the site were all normal.

6.Research and development of equipment and materials

The research and development of critical equipment have been strengthened during the implementation and promotion of the HDR resources exploration and production demonstration project.The research team has improved the 175 °C resistant wireless MWD tools, high-temperature logging tools, high-temperature water-based drilling fluids,drilling fluid forced cooling devices, high-temperature and high-efficient downhole drilling tools, high-temperature and hard-rock coring tools, high-temperature packers, highprecision microseismic monitors, vacuum insulation pipe,efficient and stable ORC power generation equipment, hightemperature directional drilling equipment and so on.

The high-temperature fracturing materials such as hightemperature temporary plugging agents, cross-linking fluids,and variable viscosity slick fluid was developed for hydraulic fracturing by applying the high-temperature molecular design concept.The carbon-carbon backbone is used as the framework, the side groups are connected with benzene ring rigid groups, and the sulfonic acid and quaternary ammonium salt plasma groups are added to improve the high-temperature resistance and plugging effect of the material.The multi-level complex cross-linking can achieve long-term temperature resistance.The cross-linked liquid rubber plug plugging technology was used to avoid the possible sensitive cracks in the reservoir, accurately control the propagation scale and extension direction of fracturing fractures, and achieve the purpose of uniform reservoir stimulation.

7.Conclusions

The HDR resources exploration and production demonstration project in the Gonghe Basin has successfully realized the power generation test and the electric gridconnected technique.This project has preliminarily established a whole-process technical system for the exploration and production of HDR, which provides a technical foundation for promoting the large-scale development of HDR.Many engineering breakthroughs have been made in various areas.

(i) The genetic model of HDR resources in the Gonghe Basin was proposed, in which the heat source is mainly controlled by neotectonic activity, and the sedimentary cap layer is thermal insulated.

(ⅱ) The granitoid buried in the PTS is 1360 meters deep and formed in the Middle to Late Triassic.The depth of the HDR reservoir target section is mainly 3500-4000 meters, and the composition is primarily medium to coarse-grained granodiorite.The near steady-state temperature measurement shows that the temperature of the target section is about 184-209℃.

(iii) Through overcoming various technical challenges such as high stress, slow drilling, and poor precision of deep directional drilling in hard rock at high temperature, one straight well and two directional wells were successfully drilled and completed.

(iv) The large-scale reservoir stimulation with a stimulated reservoir volume over 2×107m3was completed, achieving several technology and material breakthroughs.

(v) Innovative monitoring systems such as microseismic and electromagnetic methods were used, effectively supporting the environmental impact monitoring and the reservoir fracture evaluation during reservoir stimulation.

(vi) Integrated and innovative multi-well high-efficiency circulation technologies and reservoir evaluation methods were created and operated.

(vii) The high-efficiency power generating equipment was developed, and the power generation test and the specific grid-connected technique were realized.

Although the power generation test of HDR was preliminarily realized in the Gonghe Basin, there are still many challenges to overcome before the practical utilization of HDR.

CRediT authorship contribution statement

Er-yong Zhang conceived the presented idea, and wrote and reviewed the manuscript.Er-yong Zhang, Dong-Guang Wen, Gui-ling Wang, and Wei-de Yan prepared and reviewed the manuscript.Hai-dong Wu, Lin-you Zhang, Xian-peng Jin,Zheng-pu Cheng, Dan Wang, Hui Zhang, Li-sha Hu, and Li Yang designed the figures and edited the manuscript.All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgement

The research was funded by the “Hot Dry Rock Resources Exploration and Production Demonstration Project” of the China Geological Survey (DD20190131, DD20190135,DD20211336).Sincerely thanks to everyone who participates and supports the project.