Various geological phenomena on the surface and in the interior of the Earth, as well as their associated physical and chemical processes, are closely correlated with the action of in situ rock stress [1], [2], [3], [4], [5]. Understanding the rock stress state at great depths is not only an indispensable foundation for solving scientific problems associated with geology, geophysics, and geodynamics—such as plate-driving mechanisms, the earth’s energy equilibrium, earthquake mechanisms, and tectonic activities—but also a necessary prerequisite for the evaluation, exploitation, and disposal of deep energy and resources, such as coal and metal minerals. Due to the complexity and uncertainty of the origin of in situ rock stress, it is a difficult quantity to evaluate, in comparison with other rock properties. Currently, reliable information on the stress state in a region can only be determined through field stress measurement. Therefore, a variety of stress measurement techniques have been developed and applied worldwide to provide information on crustal contemporary stress at specific depth ranges [6].

Among the numerous stress measurement techniques that have been developed, the overcoring (OC) method developed in 1963 is a relatively mature and rational measurement technique for determining the 3D stress state at a certain point within a rock mass. However, it requires better geological conditions than other methods. The OC method isolates the rock core from subsurface stresses through stress relief, and the strain or deformation on the released core is measured using an instrumented device near the points where the stress condition needs to be evaluated (Fig. 1). This method stems from the recovery principle by which the elastic strain or deformation recorded using an instrumented device (e.g., a CSIR triaxial strain gauge, CSIRO hollow inclusion gauge, or USBM cell [7]; Fig. 2) in the process of stress relief is utilized to compute the complete stress tensor, using relevant formulas with known core deformability parameters [8].

Fig. 3 presents the main procedures typically involved in measuring in situ stress using the OC technique with an instrumented device. First, a large-diameter borehole is drilled in the volume of the rock to the required depth to measure the in situ stress. Subsequently, a concentric small-diameter pilot borehole is drilled at the end of the large-diameter borehole. Drilling this pilot borehole releases some elastic stress in the surrounding rock of the pilot borehole. In the next step, an instrumented device is inserted into the pilot borehole and firmly bonded to the wall of the pilot borehole. Finally, a hollow cylinder of rock with an inner diameter determined by the pilot borehole and an outer diameter determined by the inner diameter of the drilling bit is extracted through the OC, and the changes in strain or displacement within the instrumented device related to this stress relief are synchronously recorded. This technique can estimate a 3D stress state with a single measurement. According to statistics, approximately 80% of stress data worldwide are determined using the OC technique [9], so it has received widespread attention from scientists and engineers.

Although the OC technique possesses many advantages, its limitations in measuring stress in deep rock are also evident. The determined rock stress is a point tensor, and the measured stress has uncertainty related to the field’s geological and environmental factors (e.g., geological structures, anisotropy and heterogeneity of rock, rock temperature, and pore water pressure), measurement instrument precision, and data analysis and interpretation methods. In particular, the rock at great depths is often characterized by high in situ stress, high temperature, and high water pressure; it also exhibits strong time-dependent features, which may result in strong randomness and high variability in the measured stress data. Thus, there are major challenges in precisely measuring deep rock stress, and it is crucially important to understand and control all known local or regional influencing factors and error sources to increase the reliability and precision of stress measurements. In this study, the major challenges presented by the current OC technique for precise stress measurement at great depths are highlighted, and targeted potential solutions are proposed to improve the accuracy and reliability of OC stress measurement, particularly in environments with complex rock formations and extreme conditions. This paper also provides recommendations for future research and several fresh perspectives on the exploration of cutting-edge scientific problems related to deep in situ stresses and the safe and efficient exploitation of deep energy and other deep resources.

It is well-known that the reliability of rock measurements depends on the measurement technique and equipment and on the nature of the rock in question. Multiple endogenous or exogenous factors can affect the measurement accuracy of the OC technique. Major challenges in the precise measurement of stress at great depths using the OC technique largely originate from the following factors.

The approach recommended by the International Society for Rock Mechanics for rock stress evaluation via the OC technique [7] emphasizes that the deformability parameters should be obtained by conducting confining pressure calibration experiments on cored cores still connected to probes. The rock is assumed to be continuous, homogeneous, and isotropic and to exhibit linear–elastic behavior. As a result, from the measurement principle to the formula for calculating the stress magnitude and direction, the OC technique is always based on the theory that rocks are isotropic. However, due to the presence of numerous pores, cracks, joints, and uneven mineral composition and grain structure within rock masses, the mechanical properties of actual rock are anisotropic. Anisotropy can be observed in rock masses at different scales, from intact samples to whole rock masses. When calculating the magnitude and direction of stress, if the anisotropy of the rock is not considered, the errors in the calculated stress magnitude and direction can be as high as 50%–80% and 90°–100°, respectively, in some cases [10]. Such a large error is unacceptable in practical applications. Although rock anisotropy can be explained using a variety of analytical and numerical approaches, challenges remain in exploring and characterizing anisotropic rocks today. The existing constitutive laws used for laboratory and on-site experimental analyses of anisotropic rocks greatly depend on simplified orthotropic and transversely isotropic linear elastic models.

Strain gauges (e.g., triaxial strain gauges and hollow inclusion strain gauges) are used in the OC technique to measure the small-borehole deformation or strain. Like many other strain measurement devices, resistance strain gauges are used as measurement components, and strain changes are converted into resistance changes based on the Wheatstone bridge principle; they are then converted into voltage changes and recorded. Because resistance strain gauges are sensitive to temperature, their resistance shifts with changes in temperature, producing a corresponding output voltage in the bridge that results in false additional strain values being calculated. For example, a conventional 120-Ω resistance strain gauge with a temperature coefficient of 100 part per million (ppm)·(°C)⁻¹ can cause an additional strain of up to 50 με due to a temperature change of 1 °C. To eliminate the additional strain caused by temperature changes, corresponding temperature-compensation measures must be taken. However, the traditional temperature-compensation method is not applicable to cemented strain gauges, because the strain values sensed by the compensation and the working strain gauges are inconsistent when the temperature changes. The impact of temperature changes on the lead wires in conventional wired stress gauges is another issue that has not been recognized. Calculations suggest that the two lead wires for each strain gauge can add dozens of virtual microstrains when the temperature changes by 1 °C.

On the other hand, for deep high-temperature rock (e.g., the rock temperature usually reaches 70 °C or even higher at a depth of several thousand meters because of the geothermal gradient), the circulation of colder drilling fluid (typically room-temperature water) will inevitably cool the rock around the borehole during the necessary drilling and OC processes. A decrease in temperature will produce stress changes. For example, given a thermal expansion coefficient α = 9 × 10⁶ K⁻¹, an elastic modulus E = 45.5 GPa, and a Poisson’s ratio v = 0.26, a stress change Δσ = αEΔT/(1 – v) = 5.53 MPa will be induced for a temperature variation ΔT = 10 °C. It is therefore clear that thermally induced stress changes cannot be ignored. However, in practice, the effect of temperature changes on deformation and stress redistribution around the borehole has not received widespread attention, and there is a lack of effective analytical solutions that include this effect. As a result, traditional OC analysis methods have to be adopted, limiting the acceptance of the calculation results.

The deformability parameters of the rock—that is, the elastic modulus and Poisson’s ratio—are required when computing the stress tensor using the measured deformation or strain. In particular, the elastic modulus has an important influence on the precision of the calculated stresses. Any inaccuracy in determining the elastic modulus will increase the inaccuracy of the stress values obtained, regardless of any inaccuracies in the on-site testing instruments. If the error in the elastic modulus is too large, the stress calculation results may be completely invalid. However, the traditional calculation theory assumes that rocks are linearly elastic and their deformability parameters are constant, ignoring the impact of the stress level on the elastic modulus. In fact, most rock—especially deep rock—is not linearly elastic, and its elastic modulus changes with the stress level, showing a general positive correlation [11]. As such, using the elastic modulus at high stress levels to calculate low stress or using the elastic modulus at low stress levels to calculate high stress will cause considerable errors in the calculation results. In addition, it is recommended that the deformability parameters be determined through confining pressure calibration tests; this is the most scientific and economical testing method because the deformability parameters of the rock must truly represent the rock where the strain gauges are located to ensure the accuracy of the stress calculation results. However, most of the confining pressure calibration equipment currently available on the market can only provide lower confining pressures (generally less than 60 MPa), making it difficult to simulate the high-stress condition of deep rock. This is likely to result in inconsistency between the obtained deformability parameters and the actual stress level.

Thus far, the influence of drilling has not been considered in OC stress measurements. When drilling in deep rock, the high-stress environment that deep rock is subjected to requires strong power from the drilling rig equipment to successfully drill boreholes. This inevitably causes strong mechanical vibration and high-load impact of the drill bit and drill rod during drilling, resulting in numerous artificially induced micro-fractures in the surrounding rock within a range of several centimeters or even tens of centimeters from the borehole wall. In a phenomenon that has been observed in practical measurements, the closer to the borehole wall, the more micro-fractures there are. These micro-fractures significantly decrease the rock deformation and strength parameters, leading to an increase in the rock anisotropy. As a result, the mechanical properties of the borehole wall are not uniformly distributed, which may lead to severe distortion of the stress measurement results. Thus, the influence of drilling is evidently not negligible.

Deep rock is often located in high-temperature and high-pressure environments and therefore exhibits a strong time effect and remarkable creep behavior. The creep behavior of deep rock may cause the correlation between stress and strain to no longer follow elastic laws, such that no one-to-one correspondence exists between the stress and strain. In other words, a strain value can correspond to multiple stress values under such circumstances; similarly, a stress value can correspond to multiple strain values. Drilling and OC in deep rock usually take several hours or even longer, depending on the rock conditions and drilling rig performance. Due to the time effect of deep rock, the surrounding rock of the borehole wall can experience significant creep strain (10⁻⁴–10^–1) after a few minutes, in some cases. If all the creep strains are mistakenly classified as elastic strains, the resulting stress calculation error can reach several megapascals or even tens of megapascals, which is likely to make the stress measurement results meaningless.

Groundwater is one of the main occurrence environments for rock. The influence of water on rock properties is closely related to the rock structure and tends to manifests in two forms: physical and chemical effects (i.e., changing the composition and structure of the rock) and mechanical effects (e.g., hydrostatic pressure and seepage (hydrodynamic pressure)). Numerous experimental studies have shown that the existence of water significantly deteriorates the mechanical properties of rock. In particular, high water pressure in deep rock can alter the mechanical behavior of the rock and the stress environment in which the rock is located. When using the OC technique, the necessary drilling operations in deep rock will not only lead to the release (unloading) of rock stress but also cause the loss of pore water in the rock, resulting in changes in the local high water pressure distribution and subsequent changes in the rock’s mechanical parameters (e.g., strength and elastic modulus), pore stress, seepage field, and stress field, thereby affecting the stress measurement results. In particular, changes in the pore pressure within rock formations lead to significant changes in the in situ stresses, especially in the two horizontal principal stresses [12], [13]. However, the effects of groundwater on stress and pore pressure changes have not been fully studied.

At present, the OC technique is usually used to measure the stress of hard or moderately hard rock; however, its measurement effect in soft rock is not ideal. When using this technique in soft rock, two main problems arise [14]: one associated with stress redistribution and the other related to stress concentration because of stress relief. Soft rock, especially deep soft rock, exhibits prominent creep characteristics, which are more pronounced in soft rocks such as coal and salt. The rock constitutive equation involved in the existing OC measurement theory is not applicable to soft rock and cannot characterize its creep behavior. Drilling in creeping soft rock that was originally in equilibrium can generate stress gradients, which are probably affected by the nonelastic strain that occurs during drilling. The stress gradient will sequentially cause a time-dependent redistribution of stress around the drilling until a new equilibrium state is reached. Because OC stress measurement is conducted in holes, it is inevitably influenced by such redistribution. Additionally, the radial displacement (i.e., convergence) of the borehole is caused by the movement of the stress front generated by the combination of the stress around the pilot borehole and the boundary conditions limited by the cutting tool channel around the pilot borehole [14]. The loading of the pilot borehole through the stress front is completed at a stress level exceeding the previous level—that is, in some cases in soft rock, beyond the yield point—resulting in significant inelastic strain that will not recover after the OC is completed. The stress front will cause the calculated stress to be significantly underestimated, as the recovered displacement is computed by subtracting the initial drilling diameter from the ultimate diameter. Therefore, the traditional OC technique cannot be directly used for stress measurement in soft rock without modifications to the technology or calculation models.

To address the challenges outlined above, new measures involving theoretical innovation, technological change, and operational level improvement must be implemented to increase the precision of the OC technique in deep measurements. The following countermeasures have been proposed to address the influence of each factor mentioned above:

(1) Rock anisotropy. The results of OC measurement in anisotropic rock largely depend on the anisotropic characteristics of the rock, the degree of rock anisotropy, and the direction of the rock anisotropy plane relative to the borehole where the OC is implemented. Thus, a primary prerequisite is to fully understand the anisotropic characteristics of the rock and their impact on OC stress measurements. Before conducting OC stress measurements, detailed rock mechanics tests should be conducted to provide basic information about the mechanical properties of the rock in the intended measurement area, which can help identify the anisotropic nature of the rock. A notable practical issue with anisotropic rock is that there are currently no specific standards or recommended methods for sampling, preparing, and testing it. Therefore, new standards and procedures are urgently needed for the laboratory and field exploration and testing of anisotropic rock. For rock with a simple anisotropic nature, such as transverse and orthogonal anisotropy, a theoretical analytical solution or numerical simulation for anisotropy can be utilized to calculate and analyze the measurements. However, for most rocks, the combination of anisotropy with discontinuity and heterogeneity is extremely complicated and cannot be characterized using simple models. Under these circumstances, based on the existing OC stress calculation theory, a feasible solution is to determine and correct the influence of anisotropy by using indoor confining pressure calibration experiment results after OC in the field. More specifically, first, the measured strains in a similar direction from the experiment are averaged to obtain three average strains—namely, the circumferential average strain, axial average strain, and average strain in the ±45° angle direction. Then, individual strains in these directions are divided by the corresponding average value to determine the respective anisotropic correction coefficients for the strain gauges. The corrected stress relief strain is obtained by dividing each strain obtained through OC by its corresponding anisotropic correction coefficient. Finally, stresses can be calculated more accurately using the corrected strain, considering the rock anisotropy. According to the above idea, a corresponding calculation program for correcting rock anisotropy has been compiled by our research group. In addition, to reduce the influence of rock anisotropy, appropriate measuring instruments and equipment must be selected according to the specific rock properties. Laboratory simulation tests for OC stress measurement using a CSIRO hollow inclusion gauge and a USBM cell showed similar results; thus, when these devices are used in combination, the average of their measured stresses is closer to the actual stress. In addition, a new type of cementing agent has been developed, which can be injected into the pores, cracks, and defects in the rock mass around the strain gauge to integrate the rock mass. This not only makes it possible to easily obtain a complete hollow cylinder of rock but also weakens the anisotropy of the rock. The influence of rock anisotropy on the results of OC measurement can also be reduced by increasing the number and density of measurement points. Nevertheless, the strategies described above are preliminary. In the future, measurements should be based on the anisotropy of the rock’s mechanical properties, and the necessary anisotropic mechanical parameters of the rock should be measured simultaneously in situ during the stress measurement process. Moreover, it is necessary to establish an OC stress analysis and calculation model suitable for the nonlinear or elastic–plastic nature of deep rock; this is the fundamental strategy to solve the problem of rock anisotropy.

(2) Rock temperature. To eliminate the impact of temperature changes on the strain gauge record, a series of improvements have been made in the design and structure of conventional strain gauges to realize temperature compensation. More specifically, a high-precision temperature self-compensation strain gauge is adopted to reduce the temperature sensitivity of the strain gauge, and a thermistor is placed near the strain rosette to measure the temperature changes at the measurement location during the OC. After each OC test, the overcored cell, while still in the rock annulus, is placed in a constant thermostat with adjustable and stable temperature and humidity for temperature calibration to determine the relationship between the induced strain and the temperature change for each strain gauge. According to the calibration result and the temperature change recorded during stress relief, the true strain caused by the OC can be obtained and used as the correct data for calculating stress. Moreover, to address the influence of temperature changes on the lead wires, a successful practice is to connect lead wires of the same length and type to the adjacent arms of the strain gauge in a Wheatstone bridge to offset the resistance changes caused by temperature changes. When using a new wireless digital strain gauge that integrates the measurement and acquisition systems, it is necessary to eliminate the influence of temperature changes on both the measurement circuit and the acquisition circuit. For the influence of temperature on the acquisition circuit, significant improvements need to be made to the traditional acquisition circuit to achieve direct connection of high-sensitivity thermistor and ensure consistency and stability in the correspondence between the thermistor channel indication and the temperature, minimizing the acquisition error caused by the heating and current changes in the acquisition system. A dual temperature compensation electric circuit designed for this purpose is illustrated in Fig. 4. Additionally, to assess the thermally induced stress change, it is necessary to study the influence of the thermal effect on the strain change around the borehole, especially at the borehole wall [15]. The focus should be on establishing a mathematical relation between the stress and the measured strain on the borehole wall and the rock core before and after OC, while considering the temperature and pore pressure. To do so, it is necessary to obtain the mechanical response of the borehole and rock core, especially the analytical solution for the strain, throughout the entire OC process. This will provide an effective and accurate new model that can couple rock deformation, temperature changes, and pore pressure changes for stress calculation.

(3) Determination of rock deformability parameters. Because the deformability parameters of nonlinear elastic rocks are related to the stress level, the deformability parameters corresponding to the actual stress level should be adopted when calculating the stress. In the confining pressure calibration test, the stress–strain curve of the core can be obtained by pressurization to determine the deformability parameters of the rock at the locations of the measurement points. That is, the strain values involved in the calculation of the rock deformability parameters can be derived from the confining pressure calibration experiment. The rock elastic modulus is calculated according to the thick-walled cylinder formula for elasticity, and the Poisson’s ratio is the ratio of the average axial strain to the average circumferential or radial strain in the same stress segment. It should be noted that the loading and unloading paths of nonlinear elastic rock differ, and the elastic modulus used to calculate stress must be determined by the unloading path because the OC is an unloading process. Furthermore, the elastic modulus is required when calculating stress, and determining the elastic modulus requires knowledge of the stress level. Hence, considering the coupling relation between the calculations for stress and the elastic constant, we have developed a double iteration algorithm to ensure the correctness of the calculation process. In addition, a high-pressure confining pressure calibration testing system has been successfully developed by our team that can simulate a deep high-stress level and achieve both a high confining pressure loading (with a confining pressure reaching 160 MPa) and long-term loading (stable loading with a confining pressure of 70 MPa for more than two months). This provides advanced and reliable equipment for determining the deformability parameters of cored rock at deep stress measurement points.

(4) Influence of drilling. To reduce or avoid disturbances to the surrounding rock during drilling, in addition to using a stress calculation theory that considers rock anisotropy, it is necessary to ensure stable performance of the drilling rig and proficient operation by experienced workers. Moreover, measures such as chemical grouting and preloading should be attempted to partially restore the mechanical properties of the rock surrounding the borehole. Considering the on-site drilling situation, efforts to improve the strain gauge function are underway; we plan to add ultrasonic detection components to the strain gauge, which will allow the measurement system to simultaneously detect and evaluate damage to the rock surrounding the borehole. An alternative promising direction is to abandon the traditional mechanical drilling method and adopt new drilling techniques with less disturbance to the surrounding rock; examples include plasma, particle jet, electron beam, laser, and ultrasound drilling.

(5) Time effect. When conducting OC stress measurements in deep rock, OC operations should be carried out in the newly excavated accessible space with an intact (or as complete as possible) rock mass to minimize the influence of the rock time effect on the measurement results and to avoid significant creep deformation of the surrounding rock due to prolonged exposure. The drilling speed has a direct influence on stress measurement, and the creep characteristics of deep rock may lead to a redistribution of stress during drilling, thereby affecting the measurement results. Therefore, it is necessary to strictly control the drilling speed to reduce the disturbance to the surrounding rock and thus reduce the influence of creep effects on the measurement results. If conditions permit, the drilling and OC speeds should be increased as much as possible during the measurement, and the entire measurement time should be shortened. Considering the time effect of deep rocks, measurements should be taken during the stable stage after stress relief. This requires measurement personnel to have rich experience and professional knowledge and to be able to accurately determine when the surrounding rock stress has been fully released and has reached a stable state. Strengthening on-site monitoring and data recording is crucial during stress measurement, as it helps to promptly detect and handle potential abnormal situations. Effective processing and analysis of the measured data is also crucial to eliminate possible abnormal data caused by rock creep and ensure the accuracy and reliability of the data. Moreover, considering the time effect of deep rock during stress measurement, it is necessary to study the time characteristics of rock creep parameters and then establish a nonlinear creep model that considers the timescale effects based on the damage mechanics to solve the stresses. Once established, this model can be used to validate and correct the measurement results to more accurately reflect the true stress state in deep rock. Numerical simulation and theoretical analysis can also help us to deeply understand the influence mechanism of the time effect and creep characteristics of deep rock on the stress measurement results. This will assist in formulating more effective measurement strategies, optimizing the implementation process for OC stress measurement, and further improving the accuracy and reliability of the measurement results. It should be noted that time-dependent deformation is very complex and may vary widely depending on rock types and environmental conditions, requiring further research.

(6) Groundwater. In addition to constructing a stress calculation model that considers the rock pore pressure, as mentioned earlier, the water content of the rock core should theoretically be kept unchanged when measuring stress in deep rock. If conditions permit, it is recommended to conduct on-site in situ confining pressure calibration experiments to accurately determine the deformability parameters of the field rock masses. When conducting indoor confining pressure calibration experiments, it is necessary to ensure that the parameters of the core in the field environment—including the water content and humidity—remain unchanged during drilling, transportation, and storage. Xie et al. [16] proposed a coring and testing system that retains the in situ geological conditions for deep rock; this would make it possible to meet the above requirements, with the aim of obtaining authentic rock cores at different depths through drilling. Based on the principles of phase transformation and polymer cross-linking solidification, a layer of polymer film with barrier properties can be solidified on the rock core by using different special liquids, to achieve self-triggering water retention via a solid polymer sealing film during rock core extraction and thus maintain the water content of the rock core. A preliminary demonstration indicated good performance using a polymer film for this purpose. Nevertheless, it is necessary to be aware of the difficulty of controlling water loss during drilling—especially the complexity of groundwater management in deep-pressure environments, which deserves more attention.

(7) Measurement in soft rock. To expand the applicability of the OC technique to the stress measurement of deep soft rock, it is necessary to have a good understanding of the rock behavior at the measurement points in soft rocks because the stress path, constitutive relationship, and related parameters have a significant impact on the interpretation of stress measurements. Furthermore, there may be stress redistribution in the area where the stress gradient is detected, which must be taken into account when interpreting the stress measurements. In addition, due to the tendency of soft rock to exhibit inelastic or time-dependent behavior, a short measurement period is required, including the determination of rock deformability parameters. This not only helps to improve the operation efficiency but also minimizes the influence of the time-dependent behavior of soft rock on the stress measurements, including the influence caused by water content change. Moreover, because the stiffness of the measurement systems used in the current OC method is not suitable for the measurement of soft rocks with different stiffnesses, it is necessary to solve this stiffness matching problem by reasonably redesigning the structure and material parameters of the measurement system, in order to significantly improve the sensitivity and reliability of stress measurement in soft rock. Efforts are being made to develop a strain gauge with variable stiffness using flexible materials to adapt to different soft rock conditions, and relevant testing is currently underway. This development is expected to fill the gap in the field of in situ stress measurement in soft rock worldwide.

In the practice of deep rock stress measurement, it may not be a single factor that affects stress measurement but rather a combination of multiple influencing factors, which undoubtedly increases the difficulty of stress measurement and the potential measurement error. In such cases, it is necessary to integrate the solutions for each influencing factor described above into the entire process of OC stress measurement to form a complete technical process of stress measurement. In this stress measurement process, the changes in various influencing factors should be monitored in real time and dynamically adjusted to ensure the accuracy of the measurement results. It should be noted that this is a comprehensive solution that must be appropriately adjusted during implementation according to the site conditions and actual situation. After the measurement is completed, the reliability of the measured in situ stress can be examined using the World Stress Map quality ranking system, which is an internationally accepted stress data quality ranking system ranging from A to E [17]. The stress-induced failure position and characteristics of the surrounding rock or cavern in deep rock engineering can also be considered as actual proof for certifying the measured in situ stress.

In summary, given the severe challenges presented by the current OC technique in the precise measurement of rock stress at great depths, which involve rock anisotropy, rock temperature change, the determination of rock deformability parameters, the influence of drilling, the rock time effect, groundwater, and measurement in soft rock, this article proposed targeted solutions aimed at surpassing previous efforts to address these challenges. These targeted solutions include rock anisotropy correction methods, temperature-compensation methods, new stress calculation models, high-pressure confining pressure calibration test systems, new drilling techniques, methods to shorten measurement time, nonlinear creep models that consider timescale effects, rock core water content maintenance methods, and new OC measurement systems that match the stiffness of soft rock. They are forward-looking and predictable and indicate directions that are worthy of attention for future research in this field, with the potential to greatly improve the accuracy and reliability of OC stress measurement. When implementing these measures, it is necessary to take into account the complexity of the actual situation and formulate appropriate measurement strategies and methods based on the specific circumstances. The integration of fresh perspectives in the abovementioned solutions is expected to greatly promote the advancement of OC stress measurement theory and technique and lead to the innovative development of accurate in situ stress measurement instruments and equipment. Achieving this goal, however, requires the joint efforts of researchers in both OC measurement theories and techniques.

This work was financially supported by the National Key Research and Development Program of China (2022YFC3004601), the National Natural Science Foundation of China (52204084), and the Science, Technology and Innovation Project of Xiongan New Area (2023XAGG0061).