The working environment in hot dry rock boreholes, encountered in deep
geothermal investigation drilling and ultra-deep geological drilling
(up to 5000
With the recent rapid development of the national economy,
shortages in resources and energy have become major barriers to
economic development. In order to obtain more resources and
energy, we now need to explore deeper into the Earth's
crust. Therefore, the depth of drill holes for mineral resource
exploration and extraction is constantly increasing. Furthermore,
geothermal energy and shale gas are considered to be new “green
energies” that can maintain sustainable development, which is of
both domestic and foreign concern. Therefore, the development of
drilling engineering, in the field of high-temperature geothermal
energy and shale gas, is included in the development plan of China
(Chen et al., 2015). With increases in borehole depths, there is
also an increase in temperature. In general, the temperature
gradient for a normal formation is about
One of the main technical problems in ultra-deep borehole drilling construction is how to measure the drilling trajectory in a high-temperature environment while ensuring measurement accuracy. At present, the drilling trajectory measurement technique is to measure the zenith angle of the hole's (well's) body axis (the angle between the tangent of the wellbore axis and the vertical), the azimuth angle (the direction of the horizontal projection of the tangential line of the wellbore axis), and the depth of the hole (the depth position of the measuring point in the well). These three main geometric parameters are then used in an appropriate calculation method to calculate the spatial position of the measured point indirectly, so as to obtain the trajectory data for the hole (well) (Xiao et al., 1989). Figure 1 shows the coordinate system of a borehole trajectory.
Coordinate system of a borehole trajectory.
As shown in Fig. 1, the curve OAC is the trajectory of the borehole
in the coordinate system, the zenith angle (
The hole depth can be measured by the length of the drill pipe or cable under the hole. The measurement of zenith angle generally uses a sensor based on the measurement principle of liquid level, suspension principle and gravitational acceleration, and the precision is relatively high. The azimuth measurement is usually of the following two types: one uses the principle of the Earth's magnetic field, and the other uses the principle of inertial navigation. The borehole inclinometer, which is based on the principle of geomagnetic field orientation, is only suitable for non-magnetic interference or weak magnetic mining areas (Sedlak, 1994). In strongly magnetic mining areas or in magnetic interference drilling, due to the interference of the Earth's magnetic field or metal shielding (magnetic mining area, instrument casing, and drill pipe casing), the accuracy of this type of instrument does not meet the measurement requirements (Yamaguchi et al., 2015). In order to solve the problem of borehole inclination in the case of strong magnetic interference (inside the drill pipe, inside the casing, etc.), the azimuth is generally measured by optical fiber or dynamic tuning gyroscope, based on the principle of inertial navigation. Some existing fiber-optic gyro (FOG)-based drilling trajectory measuring instrumentation (DTMI) are shown in Table 1 (Mass et al., 2007).
Technical parameters of some DTMIs based on FOG.
From Table 1, we can know that the instrument that can withstand the
highest temperature and pressure is the Keeper type produced in the US,
and the maximum temperature is 200
Here, we look at working environments with a maximum temperature not
exceeding 270
DTMI is an important instrument for investigating drilling construction quality and trajectory parameters in anti-magnetic interference drilling engineering. The DTMI is mainly composed of an external confining tube, a metal vacuum flask, and an FOG measuring probe, shown in Fig. 2.
Photos of external confining tube, metal vacuum flask, and FOG measurement probe.
The external confining tube is the outermost layer, a threaded
interface is equipped with a high-pressure metal sealing ring to
ensure that the maximum pressure reached is 120
Internal structural diagram of DTMI.
The measurement flowchart of the DTMI is shown in Fig. 4. The DTMI is lowered and lifted by a wire rope connection, and the data measured, during operation, are stored in the probe's memory, in real time. When the measurements are completed, the trajectory measuring probe is taken out of the borehole, the storage module in the FOG probe is connected to the ground laptop, through the data line, and the data stored in the probe are read by the upper computer measurement software. Data processing and display are performed, thereby giving the results of the trajectory measurements.
Hardware diagram of DTMI.
The measurement module consists of a three-axis fiber-optic gyroscope (inertial measurement unit) and a three-axis accelerometer (acceleration measuring unit), which are orthogonal to each other, as shown in Fig. 5.
Composition diagram of the three-axis measurement module.
The measurement module uses a module design method; it consists of a three-axis accelerometer sensor module, a three-axis FOG sensor module (IMU), a temperature sensor module, a signal conditioning module, a high-precision A/D conversion module, a navigation calculation processing module, and a high-temperature power module, which are shown in Fig. 6.
Figure of measurement principles.
The key component is the FOG measurement component. This is composed of an interference fiber gyro (I-FOG), an optical path portion, and a circuit portion. According to the three-axis integrated design, three interferometric fiber-optic gyroscopes (I-FOG) are fixed, respectively, on the three coordinate axes of the carrier coordinate system, which are orthogonal each other. As shown in Fig. 7, each single-axis optical path is partially composed of a light source (divided by a super-luminescent diode (SLD) tube), a coupler, an integrated optical modulator (referred to as a waveguide-Y), and an optical fiber ring (a special process used for a polarization-maintaining optical fiber). The detector consists of five major components, and three FOGs share one SLD light source. Other functional modules adopt a mature all-digital closed environmental biasing scheme. This design has advantages of low cost, small size, and high stability. Meanwhile, the design of the inertial navigation measurement module focuses on the assembly process, low thermal power, overall electromagnetic compatibility, and anti-interference (Titterton and Weston, 2004; Savage and Paul, 2013).
Composition diagram of IMU.
Due to the narrow space inside the borehole, it is very difficult to install a stable physical measurement platform. Therefore, the instrument uses the strapdown inertial navigation technology to realize the function of navigation by using the fiber-optic gyroscope, accelerometer, and trajectory calculation model (Grewal et al., 2007). Among them, the three-axis accelerometer measures the acceleration in three directions, and the acceleration value can be used to obtain the displacement, in three directions, by double integration with respect to time; the three-axis fiber-optic gyroscopes measure the rotational speed of the carrier in three directions, and the corresponding rotation angle can be obtained by integrating with respect to time.
Using the three-axis displacement and rotation angle, the attitude
matrix is solved by the space coordinate system variation, the
drilling trajectory calculation model, the Euler angle coordinate
transformation, and the quaternion method; when the rigid body is
rotated around an axis, the angular position of the rotating rigid
body can be calculated (Çelikel and Sametoğlu, 2012). The
rotation of the carrier coordinate system, relative to the
navigation coordinate system, is shown in Eq. (
The fourth-order Runge–Kutta (R–K) method is used to solve the
ordinary differential equation (Bernardo and Shu, 1989); the
azimuth
The technical parameters of the DTMI based on the FOG are shown in Table 2.
Key technical parameters of the DTMI. MTBF is the mean time between failure.
This section focuses on the temperature resistance and measurement accuracy of the DTMI. For this, we look at three aspects: mechanical design of the external confining tube, structural design of the vacuum flask, and FOG error compensation. This is to ensure that the DTMI can work for a long duration with high performance in the ultra-deep hole trajectory measurement process and meets the design parameters.
The main function of the external confining tube is to ensure that
the DTMI can withstand an external pressure of up to
120
Joints of drilling trajectory measuring instruments, at home and abroad, widely use tapered thread, trapezoidal buckles. One of the reasons for this is high strength; the second is that they are simpler than rectangular buckles. In addition, they are wear-resistant and easier to unscrew than circular threads (Kielbassa et al., 2009). Sealing and connection of the DTMI used tapered threads; the simplified schematic of a threaded-tooth-type, trapezoidal buckle is shown in Fig. 8a. The connection and sealing of the DTMI are achieved by the pre-tightening force on the thread end face and the shoulder surface of the threaded shoulder. The pre-load force, generated by the threaded buckle, has an important effect on the critical load of the confining probe joint, the fatigue damage resistance, the axial load resistance, and the sealing of the shoulder contact surface, as shown in Fig. 8b.
For the material of the external confining probe tube, 17-4PH precipitation-type hardened stainless steel was selected. This has good mechanical properties, good temperature resistance, and slow heat conduction. The sealing joint is made of 30CrMnSiA using heat treatment. The technical parameters of the two materials are shown in Table 3 (Wen et al., 2010).
Material parameters of the confined tube.
We used the Ansys workbench platform to build a simplified model of the thread, accurately assemble it, and finally import it into the Ansys platform, for accurate mechanical simulation analysis, as shown in Fig. 9.
This section focuses on optimizing the design parameters of the
probe, from the internal and external diameter (wall thickness),
thread taper, thread height, and pitch. From field experience, the
outer diameter of the probe tube was designed to be 73
The stress nephogram of the external thread of the confined probe
joint (the inner diameters from top to bottom are 68, 67.5, 67, 66.5, and 66
Local thickening of the ends of the joint thread is one common
processes for improving the strength of the confining probe joint
(Vasudevan et al., 2013). According to the work experience and the
actual situation, the outer diameter of the probe tube is designed
to be 73
From the finite element analysis results (Fig. 11), the following
conclusion can be obtained: with a decrease in the inner diameter of
the probe joint (i.e., the thickness component is thickened), the
maximum equivalent stress at the joint thread is gradually reduced
and joint strength is increased by 20.8 %. At the same time, the
maximum total deformation of the confining probe and joint is
gradually reduced, indicating that the stress distribution inside
the thread is more balanced. Therefore, the inner diameter set at
67
The thread parameters of the confining tube are selected from the
national standard equipment standard GB/T 16951–1997 (Neq ISO
10098: 1992, 1997). For the simulation analysis, five sets of taper
were selected; these were
We comprehensively analyzed the maximum equivalent stress of the
pipe body and the joint with different tapers. When the taper of
the thread is around
Overall, seven thread heights (0.8, 0.9, 1.0, 1.1, 1.2, 1.3, and
1.5
The stress nephogram of the external thread of the confining
probe joint (the tapers from top to bottom are
The stress nephogram of the external thread of the confining
probe joint (the heights from top to bottom are 0.8, 0.9, 1.0, 1.1, 1.2,
1.3, and 1.5
From the results of the finite element analysis (Fig. 15), it can
be seen that different thread heights have a large influence on the
joint strength. We comprehensively analyzed the maximum equivalent
stress value and maximum total deformation of the pipe body, with
the tooth height set at 0.9
A total of five pitches (6, 7, 8, 9, and 10
The stress nephogram of the external thread of the confining
probe joint (the pitches from top to bottom are 6, 7, 8, 9, and 10
The trend diagram of the maximum equivalent stress of tube body with pitch increases.
From the finite element analysis results (Fig. 17), the connection
strength of the pressure probe tube gradually decreases with the
increase of the pitch, and the maximum drop rate is reduced to
90.9 %. In practical applications, the smaller the pitch, the
greater the number of thread buckles needed, and the labor
intensity of the shackle buckle is increased. Therefore, the
pitch should be 8
In summary, from the pressure field simulation analysis of the pressure probe, the optimized parameters for the pressure probe and its thread are shown in Table 4.
Optimized parameters of the pressure probe and its thread.
Due to the in borehole limitations, the metal vacuum flask is designed with a cylindrical structure. The protected measuring probe is loaded into the flask. As shown in Fig. 18, the metal vacuum flask is mainly composed of a vacuum-insulated bottle, heat absorbers, and a heat insulator.
Structural diagram of the metal vacuum flask.
The metal vacuum flask consists of a gland, a plug,
a heat-insulating tube, an upper heat absorber, a bottle body
(vacuum), a medium heat absorber, and a lower heat absorber. These
components are mainly composed of aluminum alloy, 1Cr18Ni9Ti,
titanium alloy, and 45 steel. In the process of temperature
simulation of the complete fiber-optic gyroscope, the parameters of
the relevant materials should be determined (Fang, 2002). The
bottle body uses 1Cr18Ni9Ti as the inner shell material, similar to
the heat preservation cup, and the material adopts vacuum
structure. The heat absorber is mainly made of paraffin, and the
heat absorption process is realized by a solid- to liquid-phase
transformation. Accounting for the heat from the fiber-optic
gyroscope movement and the external temperature, the length of the
phase change body is designed; the insulator is made of
Material property parameters of various materials of the metal vacuum flask.
The heat calculation in the flask consists of the following three
parts: heat power calculation of internal measuring components
(including FOGs, accelerometers, acquisition board, navigation
solution board, data storage output board, and high-temperature
battery); calculation of the leakage heat of the flask; and
calculation of the heat storage (endothermic). In the trajectory
measurement process of the DTMI, the total heat calculation
formula is shown in Eq. (
When the DTMI is energized, the entire measurement system (three-axis FOG and three-axis accelerometer) is in a moving state. The orientation of each FOG and accelerometer changes under different moving states. We assume that one hot surface is up, one is down, and one is vertical during the convection process. When the inertial measurement unit is working, the related components will generate heat and compensate for the internal and external temperature difference, so the internal and external temperature differences are not considered. The power consumptions of the main heat sources are shown in Table 6.
Among them, the battery conversion efficiency is assumed to be
80 %, and the rest is converted into heat;
The heat leakage of the metal flask is mainly the heat transfer
from the outside to the inside, which includes two aspects:
(1) axial heat transfer through the flask mouth, including
solid heat conduction of the inner tube wall and the heat-insulating plug; (2) radiation leakage between the inner
and outer tubes, heat conduction of residual gas, and solid heat
transfer between the vacuum layers. The total leakage heat flow
rate is set to
The heat storage material uses the sensible heat and latent heat of
paraffin (phase transition temperature is 60
The heat consumption table of main heat source.
In summary, the theoretical calculation of the heat in the flask
proves that when paraffin wax is used as the heat-absorbing
material and its length is designed to be 400
The initial temperature of the internal measuring component was set
to 25
Figure 19a is a temperature distribution nephogram of the metal
flask after 4 h of working. It can be seen that the overall trend
of temperature change in the thermos bottle is gradually reduced
from the bottle head to the bottom of the bottle. At the mouth of
the bottle, the maximum temperature reaches
113.23
In this section, the temperature field–pressure field coupling effect is taken into consideration. Through finite element analysis, a stress and strain distribution, a cloud diagram for the whole measuring instrument is obtained. The deformation state of the outer tube, under the coupling of the temperature field and the pressure field, is analyzed and used to verify the resistance to high temperatures and compression.
Firstly, we simplify the mechanical model of the DTMI; then the
coupling of the temperature field and the pressure field is solved
by the following three steps: (1) apply temperature
boundary conditions, setting the temperature to
270
The stress nephogram of the coupling between the temperature field and the pressure field: the upper left is the total deformation nephogram; the upper right is the equivalent strain nephogram; the lower left is the minimum principal stress nephogram; the lower right is the Mohr–Coulomb stress displacement safety factor nephogram.
The total deformation nephogram comparison: the left picture shows the pressure field total deformation nephogram; the right picture shows the pressure field and temperature field coupling total deformation nephogram.
Analysis of the finite element results shows that under the action
of high temperature and high pressure, the total deformation of the
DTMI increases and the equivalent strain increases, when compared
with the action of the pressure field alone. The maximum
deformation of the DTMI is calculated to be 3.47
Due to thermal field distribution and heat transfer under the thermal transient state, the physical properties, geometrical features, and thermal transfer of the FOG are dynamically changing over time, leading to a Shupe error in the FOG inclinometer. The Shupe error is negatively correlated to the accuracy of the inclinometer. To enhance the accuracy of the IMU, the finite element method (FEM) was adopted to analyze the heat conduction features of the inclinometer in a thermal field. This method can overcome the limits of traditional analytical techniques. For instance, it can handle the complex boundary conditions of the FOG. According to the differential control equations for heat conduction, a FOG error compensation formula in thermal field was derived through Shupe error analysis, laying the basis for FOG thermal field modeling and error compensation.
In the thermal transient state, the Shupe error can be calculated
based on the phase delay
Under the joint action of Shupe error
Due to the temperature field and the thermal stress in the FOG,
the fiber loop length
The CCSD project in the Songliao Basin aimed to investigate deep geothermal energy, establish a deep
stratigraphic structure profile, seek geological evidence of
Cretaceous climate change, and develop deep detection
techniques. This was the third international CCSD project funded by the China Mainland Scientific Drilling
Program (ICDP) (Wang et al., 2013, 2017). As the main
borehole of this project, the SK-2 east borehole was designed to
reach a depth of 6400
Geophysical logs played an important role in the subsequent
geoscience research because very few core samples were recovered
over the Upper Cretaceous intervals (i.e., Spud 1 and Spud 2). After the borehole was drilled, just two uncased and cased
borehole logging operations (using Beck Atlas' ECLIPS-5700 and
Halliburton's EXCEL 2000) were carried out in the Upper Cretaceous
intervals using advanced imaging logging tools (Zou et al.,
2016). Therefore, DTMI's engineering application was selected for
the SK-2 east borehole to test the performance in the high
temperature and high pressure of the scientific detection wells,
and to test the startup performance and overall performance of the
power supply in the low-temperature field environment (the ambient
surface temperature is about
The engineering application time was on 15–16 October 2017. At that time,
the construction depth of the SK-2 east borehole had exceeded 6200
Photo of DTMI which applies to the SK-2 east borehole.
The main contents of the engineering application were three
methods: a closed water test, downward trajectory measurement, and
upward trajectory measurement. The purpose was to test the sealing,
pressure, thermal insulation performance, and reliability of the
measurement data of the DTMI.
The closed water test is a pilot test to verify the sealing
performance of the pressure probe and to investigate the inside of
the hole. In order to prevent the core measuring component, based
on the FOG, from being damaged due to seal failure, when the water
shutoff test was carried out, the FOG movement was not placed in
the pressure probe tube. Instead, a humidity test paper and
a boiling point thermometer were placed in the tube. After the
water shutoff test was completed, the measuring device was taken
out and the pressure and sealing ring of the pressure-reducing
probe were verified to be non-destructive, the inside of the
pressure-exploring probe was dry, and the humidity test paper showed
no obvious changes. This indicated that the pressure-exploring tube
had good sealing performance and could meet the design
requirements. When preparing for measurements in the borehole, raw tape was
wrapped on the connecting threaded joint and thread oil was applied
to protect the thread and further improve the sealing performance;
then the downward measurement was started, and the DTMI was lowered
through the wire rope, or cable, of the ground gauge winch; after
reaching the bottom of the hole, the upward measurement was
started, and the measured data were stored in the memory of the
probe; after the trajectory measurement process was finished, the
internal measurement probe was taken out of the confining tube and
metal flask; finally, the movement probe was connected to the PC
using a data line and a conversion joint, and the trajectory
measurement data stored in the movement were read. Then, data
acquisition and processing were performed, thereby obtaining data
on the drilling trajectory.
The maximum well depth for this measurement was 5800
Measured zenith angle of the CCSD SK-2 east borehole.
Measured azimuth angle of the CCSD SK-2 east borehole.
Measured azimuth angle of the CCSD SK-2 east borehole.
The zenith between the downward and upward measurements of one DTMI and
between the two sets of DTMIs were in good agreement with each other. The
result had good repeatability, with an error less than 0.15 When the zenith was less than 3 In the process of downward and upward measurements, the temperature-measured data of the two sets of instruments were consistent with each
other. The two sets of the DTMI-measured data for zenith and azimuth were
compared with previous logging data. We found that the data were roughly
consistent with each other, although there were certain deviations in
individual points. The main reason for these deviations in the data may be
that the previous logging data were measured in the open well, while this
measurement was done after the casing was placed in the well. The data
deviation was normal, and its accuracy is acceptable for deep exploration
engineering and underground resources and energy engineering. There was a deviation in the temperature data between the downward and
upward measurements, mainly due to the short time of the measurement at each
point (except for 5 min at the well bottom, at 5800 The deviation between the measured temperature value at 5800
Figure 23a shows a curve of measured zenith against borehole depth, Fig. 23b shows a curve of measured azimuth against borehole depth, and Fig. 23c shows a curve of measured temperature against borehole depth.
The Xingreguan-2 well is a geothermal well within the dolomite
formation of the Wuyishan Formation, in Jixian county. It is
located in the Xingming Lake Resort in the Daxing district,
Beijing. The well location coordinates are latitude 39
The maximum well depth for this measurement was 1750
Measured zenith angle of the Xingreguan-2 well, Beijing.
Measured azimuth angle of the Xingreguan-2 well, Beijing.
Measured temperature of the Xingreguan-2 well, Beijing.
The zenith angles between downward and upward measurements were in good
agreement with each other, and the result showed good repeatability, with an
error of less than 0.15 In the process of downward and upward measurements, the temperature-measured data of the two sets of instruments were consistent with each
other. The DTMI-measured data for zenith and azimuth were compared with
previous logging data. We found that the data were roughly consistent with
each other, although there were certain deviations at individual points. The
main reason for these deviations in the data may be that the previous
logging data were measured in the open well and these measurements were done
after the casing was placed in the well.
Figure 24a shows a curve of measured zenith against borehole depth, Fig. 24b shows a curve of measured azimuth against borehole depth, and Fig. 24c shows a curve of measured temperature against borehole depth.
The FOG has the advantages of high measurement accuracy, small size, high sensitivity, and strong anti-interference ability. This is especially important for the magnetic material with the greatest influence on the trajectory measurement technology in geological exploration and external magnetic field interference. The DTMI, based on the FOG, can be used for borehole trajectory measurements in any formation conditions. The FOG, used in geological and oil drilling fields, generally requires a wide operating temperature range. However, the measurement accuracy of the FOG is very sensitive to changes in ambient temperature, especially in deep borehole testing. The high external temperature and high-pressure environment, along with internal self-heating, will have an impact on the performance of the FOG. This mostly manifests as noise and drift. The noise directly affects the working accuracy of the fiber-optic gyroscope, that is the minimum detectable phase shift, while the drift reflects the degree of change in the output signal. Therefore, the thermal non-reciprocal error (Shupe error) caused by the temperature field will seriously affect the performance of the FOG inclinometer. This is characterized by poor data repeatability, short working time, and low precision (Shupe, 1980; Kurbatov, 2013).
Due to the Shupe error caused by the temperature field of the FOG, it is difficult to carry out thermal simulation analysis of the whole structure and internal mechanism by a traditional thermal field analytical method. It is nearly impossible to determine the boundary range, control equation, and thermal characteristics of the main components (Yang et al., 2011). In this study, the finite element method was used to analyze the heat transfer characteristics of the borehole trajectory measuring instrument. In Sect. 3.3, the Shupe error of the fiber-optic gyroscope was calculated using the differential control equations for thermal conduction and the thermal boundary condition. The unified formula of the thermal error was derived. The external parameters that affect the Shupe error are mainly temperature change rate, the effective refractive index, and the angular velocity. Therefore, these formulas can be used as the basis for the temperature field model establishment and error compensation experimental scheme for the internal core measurement mechanism.
In order to reduce the influence of temperature changes on the output accuracy of the fiber-optic gyroscope, the FOG error compensation experiment was used to establish a temperature compensation model between the parameters (temperature change rate, the effective refractive index, and the angular velocity) and the FOG output value, and to explore the mathematical relationship between them. Coefficients of the model require the FOG measurement probe to be tested on a three-axis quadrature calibration bench with a high-temperature thermostat, thereby obtaining a series of experimental data (Liu et al., 2019); photos of the temperature compensation experiment are shown in Fig. 25.
Due to the higher temperature in boreholes, up to
250
Table of experimental factors and their levels.
The thermal error compensation experiments usually employ a network
structure of three to four layers (Meng et al., 2009). During these
experiments, we found that both full-scale design methods and
orthogonal design methods were time-consuming and low in
efficiency for FOG error compensation. For example, in the
full-scale design method, the number of experiments required is
A drilling hole trajectory measuring instrument, based on
interference FOG, is developed in this study. We examined the
mechanical design and strength. We carried out pressure field
simulation analysis for the pressure-bearing outer tube. The
structural design of the metal thermos was examined, and we carried
out temperature field simulation analysis, measuring with the FOG
in the probe. The study of Shupe error compensation in the field of
temperature field was carried out using finite element analysis,
for 4 h, in an environment where the maximum temperature does not
exceed 270
Aimed at the difficult working conditions in boreholes of hot dry
rock, deep geothermal investigation drilling, and ultra-deep
geological drilling (up to 5000
The main highlights of this paper are as follows:
A method for measuring the borehole trajectory based on a three-axis
interference-type fiber-optic gyroscope is proposed. The two-axis orthogonal
three-axis fiber gyroscope and three-axis accelerometer sensor are used to
form the strapdown inertial navigation system. The three-axis displacement
and the three-axis rotation angle can be calculated by the trajectory
calculation model described in the text. The DTMI can be applied in strong
magnetic interference mining areas and expands the scope of engineering
applications. Compared with existing drilling trajectory measuring equipment
using a fluxgate or single-axis and two-axis gyroscopes as sensitive
devices, it is a step forward in novelty and innovation. The DTMI pressure field–temperature field coupling analysis method is
proposed to guide the optimal design of the DTMI. Using a finite element
software platform, the DTMI thread parameters, the size and wall thickness
of the confined probe, optimization of the material and structure of the
metal vacuum flask was carried out, and the simulation analysis and optimization
design of the pressure field–temperature field coupling effect for the whole instrument were carried out. Finally, the reliability and stability of the
DTMI were verified by the engineering application in the CCSD SK-2 east
borehole.
The data used to support the findings of this study are available from the corresponding author upon request.
YL and GL designed and built the instrument with the help of CW and WJ. YL and CW prepared the paper with contributions from all authors.
The authors declare that they have no conflict of interest.
Ce Zhou and Wenjun Chen helped acquire the data of engineering application presented in this paper; Zhong Li processed the data in Tables 7–9, for which he is gratefully acknowledged.
This research has been supported by the National Natural Science Foundation of China (grant no. 41804089); the National Key Scientific Instrument and Equipment Development Project of China (grant no. 2013YQ050791); and the China Postdoctoral Science Foundation (grant no. 2019M650782).
This paper was edited by Luis Vazquez and reviewed by two anonymous referees.