Borehole in SITU stress measurement system

Abstract

Displacement measurement device (1) for borehole in situ stress measurement comprising a displacement sensor (11) including one or more measurement probes (12) a housing, and electronics (17) mounted in the housing and connected to the one or more measurement probes. The housing is an annular housing (10) comprising an outer radial wall (13a) and an inner radial wall (13b), the inner radial wall surrounding a central passage configured to allow a coring tool (4) and a pressure tool (2) to pass therethrough, the one or more measurement probes (12) configured to measure a position and displacement of a wall of a measurement hole (202) surrounding the annular housing (10).

Description

[0001] P2992PC00

[0002] BOREHOLE IN SITU STRESS MEASUREMENT SYSTEM

[0003] The present invention relates to a system and method for measuring in situ stress in geological formations, in particular in deep ground accessed by a borehole.

[0004] Compared to conventional geotechnical applications, which are usually at or near the surface (e.g., foundations, retaining structures), understanding the in situ stress state for geo-energy projects — such as oil and gas reservoirs, geothermal energy, underground hydrogen storage, carbon capture and storage (CCS), and nuclear waste disposal — poses additional complexities. These arise due to the extreme depths (often reaching kilometers), complex stress history (including current tectonic stresses, residual stresses, erosive and diagenetic processes), and frequently anisotropic stress conditions.

[0005] Incomplete or unreliable knowledge of underground stress significantly heightens the risk of project failure. Accurate stress assessment is crucial for ensuring safety, optimizing performance, and securing economic success. Across geo-energy applications, poor understanding of the in situ stress state can lead to wellbore instability and inefficient extraction (oil and gas), compromised long-term containment (nuclear waste), underutilized reservoirs and potential leakage (CCS and hydrogen storage), and increased seismic risks.

[0006] In a principal reference system, in situ stress can be described by three principal stresses. Most often, one of these main stresses acts vertically (<JV), making the other two components acting in a horizontal plane, namely the maximum horizontal stress (<JH) and the minimum horizontal stress (o / .

[0007] In US 2023 / 0272712 a tool and method for measuring in situ stress in deep wells using a flatjack technique to measure in situ stress is described. Disadvantages of the system described in US 2023 / 0272712 include: Difficulty in accurately placing the flatjack: Accurately inserting the flatjack into the narrow slit created by the disc cutting process is challenging due to the limited space and precision required. This can lead to improper placement, which may affect the results or the function of the flatjack. Challenge in retracting the flatjack: Once the flatjack is inserted into the slit and pressurized, it becomes difficult to retract or recover it. This is because the flatjack expands P2992PC00 when pressurized, making removal from the narrow slit complicated and potentially damaging. Risk of tool blockage and operational consequences: Using a disc blade to cut into the host rock can be risky, as it may lead to the instrument becoming stuck in the borehole. If this happens, it could result in significant operational delays and increased costs. Uncertainty in stress distribution: In typical flatjacking procedures, the flatjack is grouted to ensure proper contact and stress distribution between the tool and the slit walls. In this case, without grouting, it's impossible to know the exact stress distribution between the flatjack and the rock. Since there is no pressure mapping system in place, this leads to uncertainty in calculating the actual stress, which can reduce the accuracy of the measurements. Measurement tool displacement: The measurement of displacement before and after formation of the slot cut vertically and radially into the bore sidewall is not performed with a measurement device that remains in place and therefore leads to inaccuracies due to the measurement tool displacement.

[0008] In view of the foregoing, it is an object of the invention to provide a device, system and method for borehole in situ stress measurement that is accurate.

[0009] It is advantageous to provide a device, system and method for borehole in situ stress measurement that allow accurate measurement of stress in a borehole in different directions.

[0010] It is advantageous to provide a device, system and method for borehole in situ stress measurement that can be used at large depths exceeding 500m depth, for instance depths of around 1000m or more.

[0011] It is advantageous to provide a device, system and method for borehole in situ stress measurement that is economical to implement.

[0012] It is advantageous to provide a device, system and method for borehole in situ stress measurement that is reliable.

[0013] It is advantageous to provide a device, system and method for borehole in situ stress measurement that is easy to implement in varying geological conditions. P2992PC00

[0014] Objects of the invention have been achieved by providing a device, system and method according to the independent claims.

[0015] Dependent claims and the summary of embodiments below set out various advantageous features of embodiments of the invention.

[0016] Disclosed herein is a displacement measurement device for borehole in situ stress measurement comprising a displacement sensor including one or more measurement probes, a housing, and electronics mounted in the housing and connected to the one or more measurement probes, characterized in that the housing is an annular housing comprising an outer radial wall and an inner radial wall, the inner radial wall surrounding a central passage configured to allow a coring tool and a pressure tool to pass therethrough, the one or more measurement probes configured to measure a position and displacement of a wall of a measurement hole at a bottom of a borehole surrounding the annular housing when the displacement measurement device is inserted in the measurement hole.

[0017] In an advantageous embodiment, the one or more measurement probes are disposed adjacent a bottom end of the annular housing.

[0018] In an advantageous embodiment, the measurement probe comprises an engagement tip extending outwardly beyond the outer radial wall configured for mechanically engaging said wall surrounding the annular housing.

[0019] In an advantageous embodiment, the measurement probe comprises a feeler from which the engagement tip extends, and a strain gauge mounted on the feeler.

[0020] In an advantageous embodiment, the feeler comprises a cantilever arm extending from a center fixing portion fixed to the outer radial wall of the annular housing and extending from the center fixing portion to the engagement tip.

[0021] In an advantageous embodiment, the measurement probe comprises a pair of feelers extending in opposite directions from a center fixing portion fixed to the annular housing outer radial wall.

[0022] In an advantageous embodiment, the at least one pair of feelers are integrally formed, P2992PC00 preferably from stamped and formed sheet metal.

[0023] In an advantageous embodiment, there are a plurality of measurement probes mounted in a spaced apart relationship circumferentially around the annular housing.

[0024] In an advantageous embodiment, there are a plurality of measurement probes positioned in spaced apart manner in an axial direction, said axial direction corresponding to a direction of a centre axis of the annular housing.

[0025] In an advantageous embodiment, there are at least three measurements probes or at least three pairs of measurement probes, preferably four or more measurement probes or four or more pairs of measurement probes.

[0026] In an advantageous embodiment, the electronics comprise a battery and a microprocessor configured to record measurement data output by the measurement probes.

[0027] In an advantageous embodiment, the device further includes a temperature sensor and optionally a mud sensor connected to the electronics.

[0028] In an advantageous embodiment, the outer radial wall and inner radial wall are made of metal, for instance invar.

[0029] In an advantageous embodiment, the device further comprises positioning protrusions mechanically coupled to the annular housing and projecting outwardly beyond the outer radial wall configured for engaging said wall of the measurement hole for centering the displacement measurement device within the measurement hole.

[0030] Also disclosed herein is a borehole in situ stress measurement system configured for measuring stress in a borehole, comprising a displacement measurement device according to any preceding embodiment.

[0031] In an advantageous embodiment, the borehole in situ stress measurement system comprises a wireline device including a wireline, a tubular shaft gripper, and a tool coupling, the tubular shaft gripper configured to clamp the wireline device to a tubular transmission shaft, the tool coupling configured for coupling to a plurality of different tools. P2992PC00

[0032] In an advantageous embodiment, the borehole in situ stress measurement system further comprises a DMD placement tool including a gripper configured to grip the displacement measurement device, for instance the gripper is configured to grip against the inner radial wall of the annular housing.

[0033] In an advantageous embodiment, the DMD placement tool comprises an angular orientation sensor.

[0034] In an advantageous embodiment, the angular orientation sensor comprises a gyroscope and a magnetometer.

[0035] In an advantageous embodiment, the borehole in situ stress measurement system further comprises a coring tool including a coring bit and an outer sleeve movably mounted around the coring bit, biased by a spring towards an extended position in which the outer sleeve surrounds an end of the coring bit.

[0036] In an advantageous embodiment, the outer sleeve comprises a sealing ring at an end configured to abut against a bottom of said measurement hole.

[0037] In an advantageous embodiment, the borehole in situ stress measurement system further comprises a pressure tool including pressure pads and a hydraulic system configured to move the pressure pads outwardly to exert pressure on walls of a pilot hole.

[0038] In an advantageous embodiment, the pressure tool comprises a pressure mapping sensor on an outer surface of the pressure pads configured to measure a distribution of pressure on the pressure pads when they are biased against the wall of the pilot hole.

[0039] In an advantageous embodiment, the pressure tool comprises a position sensor and the displacement measurement device comprises a complementary reference point detectable by the position sensor to orient the pressure tool angularly relative to the displacement measurement device.

[0040] In an advantageous embodiment, the borehole in situ stress measurement system further comprises a measurement hole grinding tool. P2992PC00

[0041] Also disclosed herein is a method of measuring stress in situ in a borehole using the borehole in situ stress measurement system of any preceding embodiment, comprising the steps of: -drilling a measurement hole at a bottom of a borehole,

[0042] - inserting the displacement measurement device in the measurement hole and measuring an initial position of the wall of the measurement hole by the displacement sensor of the displacement measurement device,

[0043] - inserting the coring tool through the central passage of the annular housing and coring a pilot hole, and measuring a displacement of the wall of the measurement hole by the displacement sensor,

[0044] - inserting the pressure tool in the pilot hole and applying pressure on the pilot hole wall by the pressure pads of the pressure tool by actuating the hydraulic system, and measuring a displacement of the wall of the measurement hole by the displacement sensor.

[0045] In an advantageous embodiment, the displacement measurement device remains in the same position for said measuring an initial position and said measuring a displacement of the measurement wall.

[0046] In an advantageous embodiment, the displacement sensor continuously measures the position of the wall of the measurement hole during the step of applying pressure on the pilot hole wall with the pressure tool, and detects a position of the wall of the measurement hole corresponding to said initial position prior to coring the pilot hole.

[0047] In an advantageous embodiment, the pressure tool is rotated after a previous measurement to a different angular orientation and actuated to apply pressure against the wall of the pilot hole in a different angular orientation than the preceding angular orientation.

[0048] In an advantageous embodiment, the pressure pads of the pressure tool are aligned with opposite pairs of measurement probes of the displacement sensor in one or more angular orientations.

[0049] In an advantageous embodiment, the pressure tool is successively rotated to all different angular orientations corresponding to said opposite pairs of measurement probes. P2992PC00

[0050] Embodiments of the present invention enables the determination of the maximum and minimum horizontal stresses with great accuracy and reliability despite the extreme conditions encountered in deep boreholes and overcomes the limitations encountered with current in situ stress measurement methods. Accurate and reliable in situ stress estimations enable safe geoenergy applications and maximize their exploitation potential.

[0051] The displacement measurement device inserted at the bottom of a borehole is configured for measuring displacement with a resolution of 1 micrometer. The peripheral cantilever feeler arms enable precise micrometric measurement of borehole wall deformations The annular shape of the displacement measurement device allows a coring tool to pass therethrough, so that the position of the surrounding rock can be recorded before and after coring a pilot hole without displacing the displacement measurement device. Thus the baseline position of the surrounding rock can be measured accurately and subsequently a pilot hole is drilled without moving the displacement measurement device. As rock is extracted, the borehole walls slightly deform in response to in situ stresses, causing inward deformation which can be measured by the displacement measurement device. The annular displacement measurement device then allows a pressure cell tool, configured for applying stress, for instance with an accuracy of + / - O. IMPa, to be introduced within the pilot hole. By applying pressure on the pilot hole wall with the pressure cell tool until the originally measured baseline position is restored, as measured by the displacement measurement device, the in situ stress can be deduced based on the stress imposed by the pressure cell tool. This comprehensive method offers a systematic approach to reliably and accurately assess in situ stress conditions.

[0052] Further advantageous features of the invention will be apparent from the following detailed description of embodiments of the invention and the accompanying illustrations.

[0053] Brief description of the figures

[0054] Figures 1 to 14 are schematic cross-sectional views illustrating successive steps in measurement of in situ stress conditions at the bottom of a borehole, implementing a borehole in situ stress measurement system according to an embodiment of the invention, whereby

[0055] - figure 2a is an enlarged view of portion Ila of figure 2,

[0056] - figure 3a is an enlarged view of portion Illa of figure 3,

[0057] - figure 4a is an enlarged view of portion IV of figure 4,

[0058] - figure 4b is a cross-section through line F-F of figure 4,

[0059] - figure 5a is a cross-section through line F-F of figure 5, P2992PC00

[0060] - figure 6a is a cross-section through line F-F of figure 6,

[0061] - figure 8a is an enlarged view of portion VIII of figure 8,

[0062] - figure 9a is an enlarged view of portion IX of figure 9,

[0063] - figure 10a is an enlarged view of portion X of figure 10,

[0064] - figure 1 la is an enlarged view of portion XI of figure 11,

[0065] - figure 1 lb is a cross-section through line J-J of figure 11;

[0066] - figure 11c is a schematic vertical cross-section and figure l id horizontal cross sections of a pressure tool and measurement displacement tool of a borehole in situ stress measurement system according to an embodiment of the invention, inserted in a borehole, to illustrate the rotation of the pressure tool to apply pressure at different rotational angles in a pilot hole;

[0067] - figure l ie is a plot of displacement of a measurement wall due to pressure applied on a pilot wall by a pressure tool, over time;

[0068] Figure 15a is a perspective view of a coring tool of a borehole in situ stress measurement system according to an embodiment of the invention;

[0069] Figure 15b is a cross-sectional view of the coring tool of figure 15a;

[0070] Figure 15c is an enlarged view of portion XV of figure 15b;

[0071] Figure 16a is a perspective view of a displacement measurement device of a borehole in situ stress measurement system according to an embodiment of the invention;

[0072] Figure 16b is a cross-sectional view of the displacement measurement device of figure 16a;

[0073] Figure 16c is a top view of the displacement measurement device of figure 16a;

[0074] Figure 16d is a perspective view of a variant of the displacement measurement device of figure 16a provided with a protective cover;

[0075] Figure 16e is a perspective view of the embodiment of figure 16d with the protective cover in a retracted position;

[0076] Figure 17 is a perspective view of a measurement hole grinding tool of a borehole in situ stress measurement system according to an embodiment of the invention;

[0077] Figure 18 is a perspective view of a DMD placement tool of a borehole in situ stress measurement system according to an embodiment of the invention;

[0078] Figure 19 is a perspective view of a pressure cell tool of a borehole in situ stress measurement system according to an embodiment of the invention;

[0079] Figure 19a is a perspective view of the pressure cell tool of figure 19;

[0080] Figure 19b is a side view of the pressure cell tool of figure 19; P2992PC00

[0081] Figure 19c is an end view of the pressure cell tool of figure 19;

[0082] Figure 19d is a cross-sectional view through line 19d-19d of figure 19c.

[0083] Referring to the figures, a borehole in situ stress measurement system 100 according to an embodiment of the invention is configured to operate with a borehole drill rig comprising a tubular transmission shaft 102 and a wireline device 7. Such tubular transmission shaft, (often referred to as “outer barrel” or “outer tube” in the industry) and wireline device drilling systems are per se well known and typically employed for boreholes ranging from 200m to over 1000m, offering optimal efficiency in extracting core samples from the rock mass without requiring the retrieval of the entire tubular transmission shaft 102. The rock sample is extracted from the bottom of the borehole within an inner tube assembly (not shown) using an overshot attached to a wireline cable 8. The stress measurement method according to embodiments of the invention can be employed once the inner tube assembly containing the rock core (not shown) is removed from the drill rig. Once the measurement activities are achieved, the wireline coring activities may resume seamlessly.

[0084] A borehole in situ stress measurement system 100 according to embodiments of the invention comprises a displacement measurement device 1 (also referred to herein as a DMD), a pressure tool 2, a DMD placement tool 3, a coring tool 4, and a measurement hole grinding tool 5. The borehole in situ stress measurement system 100 further comprises a control system 6 for controlling the measurement operations and processing the measurement results.

[0085] The placement and removal of the displacement measurement device 1, pressure tool 2, DMD placement tool 3, coring tool 4, and measurement hole grinding tool 5 may be effected by the wireline device 7 which is adapted to couple to each of these tools. The wireline device 7 comprises a wireline 8 and a tubular shaft gripper 9. The tubular shaft gripper comprises a tool coupling 40 for removably mounting different tools to the wireline, and clamp platens 38 with an actuator mechanism 36 to move the clamp patens from a retracted position in which the clamp platens are disengaged from the tubular transmission shaft 102, to an engaged position in which the clamp platens press and grip against the inner side of the tubular transmission shaft 102. In the engaged position, the tool is rigidly coupled to the tubular transmission shaft 102 can be driven in rotation and translation up or down by the tubular transmission shaft 102. P2992PC00

[0086] The displacement measurement device 1 may be inserted into the borehole to measure wall displacement during stress relief from pilot hole coring and when recovery stress is applied by the pressure tool 2 onto the pilot hole walls.

[0087] The displacement measurement device 1 comprises an annular housing 10, a displacement sensor supported by the annular housing, and electronics 17 mounted in the housing connected to the displacement sensor to process measurement signals and supply power. The annular housing comprises an outer radial wall 13a and an inner radial wall 13b forming a space therebetween for housing the electronics 17 which may comprise an autonomous power source such as a battery, one or more circuit boards 19 and one or more microprocessors and optionally other electronic components. The housing may further comprise a plurality of positioning protrusions 14 projecting outwardly from the outer radial wall 13a to position and / or anchor the displacement measurement device to the borehole wall.

[0088] In an embodiment, the displacement sensor comprises a plurality of measurement probes 12 arranged around the annular housing 10.

[0089] In a preferred embodiment, the measurement probes 12 of the displacement measurement device 1 are advantageously disposed at a bottom end of the displacement measurement device, such that they may be positioned adjacent a bottom end of a measurement hole wall where wall displacement amplitudes due to the coring of a pilot hole are the largest.

[0090] In an advantageous embodiment, the measurement probe comprises one or more feelers 15 on which strain gauges 16 are mounted, the feelers configured to engage the borehole wall such that a displacement of the borehole wall in the measurement plane can be measured by the strain gauges. In the illustrated embodiment, each feeler comprises a centre fixing portion 15a fixed to the annular housing outer radial wall 13a and at least one cantilever arm 15b extending from the centre fixing portion. The cantilever arm 15b comprises an engagement tip 15c projecting outwardly to engage the borehole wall. The strain gauge 16 may be mounted on the cantilever arm, whereby the bending of the cantilever arm varies the strain applied on the strain gauge. In a preferred embodiment, the strain gauges can be sealed with a special resin or material to protect the electronic contacts from the mud or water-filled environment. Epoxy may for instance be used for this purpose. P2992PC00

[0091] In the illustrated embodiment, each feeler comprises a pair of cantilever arms 15b extending from the centre fixing portion 15a in opposite directions, which advantageously allows to provide more measurement accuracy and greater stability of the feeler in a simple, robust and compact configuration.

[0092] In advantageous embodiments, each feeler arm may be integrally formed as a single piece. In an advantageous embodiment, the feeler cantilever arms, engagement tips and center fixing portion may be integrally stamped and formed as a single piece from sheet metal. In a variant, the feeler cantilever arms, engagement tips and center fixing portion may be integrally formed by molding of a polymer material.

[0093] In variants, instead of elastic cantilever arms 15b, the feeler may comprise other elastically displaceable supports, for instance an elastic closed shape support such as a ring or oval shaped elastic beam, a compressible post or pad, or an arched beam connected at both ends to the housing outer radial wall.

[0094] Within the scope of the invention, it may further be noted that the displacement sensor may comprise other measurement probes instead of strain gauges on elastic supports such as cantilever beams, for instance using per se known capacitive, inductive or optical distance measurement techniques for measuring a position and displacement of the borehole wall relative to the annular housing of the displacement measurement device.

[0095] The annular housing 10 advantageously allows drilling tools and instruments to pass through the displacement measurement device to perform operations while the displacement measurement device remains in a set measurement position within the borehole. Measurements can thus be made before coring, after coring and before applying pressure, and after coring and applying pressure, without removal of the displacement measurement device such that an accurate reference position can be maintained. The displacement measurement device however can be removed and repositioned if desired or needed, whereby the benefits of the annular housing remain in that the tools can pass through the displacement measurement device.

[0096] The displacement measurement device may advantageously comprise several measurement probes 12 positioned around its circumference. For instance, two opposite measurement probes measure diameter deformation differentially, ensuring accurate readings even if the P2992PC00 displacement measurement device moves. The displacement measurement device preferably has three or more measurement probes distributed in a spaced apart manner around the outer radial wall 13a to measure borehole wall displacement within multiple directions.

[0097] In an advantageous embodiment, multiple measurement probes 12, located at different heights, can be arranged in the same orientation, offering redundant measurement points in the event of failure of one or more measurement probes 12.

[0098] In the illustrated embodiment with feelers 15, the engagement tip 15c of each cantilever arm 15b makes contact with the borehole wall, causing the cantilever arm to bend. The strain gauge 16 mounted at the base of the cantilever arm 15b is connected to an electronic data acquisition controller of the electronics 17 within the annular housing 10 of the displacement measurement device 1. This setup allows precise measurement of cantilever bending with micrometric resolution.

[0099] In an advantageous embodiment, each cantilever arm may be equipped with four strain gauges, two on each side, configured in a Wheatstone bridge. This configuration offers advantages such as accurate measurement of bending strain, effective compensation for temperature effects, and a strong output signal.

[0100] The displacement measurement device has a sealed annular housing and in conjunction with the robust displacement probes can function in both mud / water-filled boreholes, enduring high pressure, and in dry boreholes.

[0101] The displacement measurement device 1 is advantageously autonomously powered and comprises electronics not wired to the surface for power supply or data transmission. The electronics 17 may be powered by a rechargeable battery integrated in the device. The electronic data acquisition controller may comprise a built-in memory for storing recorded data and amplification electronics for the strain gauges. Additionally, the electronic data acquisition controller incorporates a wireless communication system to link up with the pressure tool 2, which may be connected electrically to the surface. This enables seamless transmission of data from the displacement measurement device 1 to the surface whenever the pressure tool 2 is in proximity to the displacement measurement device 1. P2992PC00

[0102] The displacement measurement device 1 may be equipped with additional sensors for comprehensive monitoring. In particular, the displacement measurement device 1 may comprise a pressure sensor to track water-filled environment pressure and a temperature sensor. Moreover, to ascertain its orientation within the borehole, the displacement measurement device 1 may include a gyroscope and / or a magnetometer to determine its alignment relative to Earth's magnetic field.

[0103] Extra strain gauges can be affixed to the base of the displacement measurement device 1 to monitor any strain, particularly deformation induced by temperature variations, which could impact the displacement of the measurement probes 12. In advantageous embodiments, the displacement measurement device 1 housing 10 comprises a material with minimal thermal expansion, such as invar steel.

[0104] The displacement measurement device 1 may include a holding or locking mechanism, for instance in the form of spring biased positioning protrusions 14, configured to engage the measurement hole wall to ensure a fixed reference with respect to the measurement hole 15. Positioned at least 100mm above the borehole surface, the position of the holding or locking mechanism minimizes the influence of measurement hole wall deformation caused by pilot hole coring. Instead of spring biased positioning protrusions 14, the locking mechanism may include a mechanical clamping mechanism triggered upon contacting the measurement hole bottom 203, or activated using an electric motor or hydraulic system. Alternatively, it may feature multiple spring elements with a higher spring constant compared to the measurement probes, effectively centering the displacement measurement device 1 within the measurement hole.

[0105] The displacement measurement device 1 may include a protective cover 11 for the measurement probes, safeguarding them from damage during insertion into the borehole and retrieval. This cover is designed to slide, allowing the measurement probes to be exposed and make contact with the measurement hole wall when the displacement measurement device 1 is installed in the measurement hole 202.

[0106] The protective cover 11 safeguards the measurement probes during both insertion into and extraction from the borehole. This protective cover is configured as a tubular structure that surrounds the DMD during insertion and extraction, offering complete protection. When the P2992PC00 displacement measurement device 1 is positioned for measurement, the protective cover is shifted upwards to allow the engagement tip to contact the measurement hole walls.

[0107] A pre-loaded spring 39 is positioned between the upper portion of the protective cover 11 and the DMD body. Openings 41a are incorporated into the protective cover, allowing positioning protrusions 14 to extend through these openings. Additionally, openings 41b allow for the deployment of the engagement tips of the feelers 15, enabling them to press against the inner walls of the measurement hole when the DMD is correctly positioned inside the hole.

[0108] During insertion, the lower portion of the protective cover makes initial contact with the bottom surface of the measurement hole. As the DMD continues to advance further into the hole the protective cover slides upward relative to the DMD, against the pre-loaded spring force. This vertical displacement aligns the feelers 15 with their respective openings 41b in the protective cover, allowing the feelers to extend outward and engage the walls of the measurement hole for measurements.

[0109] Upon retrieval of the DMD, the protective cover moves downward in the opposite direction due to the pre-loaded spring. Due to the inclined surface of the engagement tip, the feelers retract smoothly under the protective cover, ensuring they are fully shielded during extraction. This design ensures secure retraction of the feelers 15, preventing any potential damage during insertion or transportation.

[0110] The displacement measurement device 1 may further comprise a permanent magnet or other detectable element mounted near a bottom of the annular housing 10, serving as a reference point 33. This reference point 33 can be detected by a position sensor 31 located on the pressure tool 2. This reference point 33 aids the pressure tool in determining its angular position relative to the displacement measurement device 1 when lowered down the pilot hole 204. The reference point and associated position sensor can take various forms, such as optical, inductive, mechanical, magnetic, or any other type of sensor.

[0111] The displacement measurement device 1 is fully recoverable and may be used multiple times.

[0112] The pressure tool 2 comprises movable pressure pads 22 actuated by a hydraulic system 24. The pressure pads 22 may for instance include two curved platens which can expand and P2992PC00 contract through the actuation of one or several hydraulic pistons of the hydraulic system mounted between the pressure pads 22.

[0113] The pressure tool is shaped to be inserted within a pilot hole 204 and apply stress onto the pilot hole wall.

[0114] The pressure tool 2 is linked to the surface via a wireline connection, which incorporates hydraulic tubing for hydraulic fluid injection into the pressure tool hydraulic pistons, and electric wiring for data communication and power supply.

[0115] A precise understanding of the stress distribution between the pressure tool and the host rock is important for accurately determining in situ stresses from the pressure tool piston pressure. The pressure tool may advantageously comprise a pressure mapping sensor 23 positioned on an external side of the pressure pads 22 to assess the stress distribution exerted on the host rock by the pressure pads. The pressure mapping sensor 23 comprises a grid of measurement pixels with measurable electrical resistance. The electrical resistance of each measurement pixel of the pressure mapping sensor 23 varies in response to the compression stress applied to it. The pressure tool 2 may further comprise electronics, a controller and power components to facilitate data acquisition for pressure mapping.

[0116] As schematically illustrated in figures 11c to l ie, the pressure tool may further include an electric motor 29 to enable rotation of the pressure tool 2 to align the pressure pads 22 with a desired pair of measurement probes 12 as shown in figure l ie. To maintain precise control, the electric motor 29 may include a rotary encoder to continuously monitor the motor's absolute angular position. The pressure tool may thus be rotated with respect to the displacement measurement device in order to be aligned with each of the different measurement probes where the pressure tool pressure pads 22 expansion is parallel to the direction of the selected measurement probe. Once a measurement has been performed in one direction, the pressure tool may be rotated to the next measurement probe. By measuring the stresses along multiple directions, it is possible to estimate the maximum and minimum horizontal stresses and to recover the horizontal stress state c11c11of the surrounding rock.

[0117] The pressure tool may further include a position sensor 31 to detect a reference point 33 of the displacement measurement device 1. This position sensor may be a hall sensor, an optical sensor, a magnetic sensor, or an inductive sensor and the reference point may comprise a P2992PC00 complementary element such as a magnet for the magnetic sensor, Hall sensor or inductive sensor, and a marking of the optical sensor.

[0118] When the pressure tool 2 is being retrieved by the wireline device 7 after the measurement has been completed, the pressure tool may advantageously also be configured, by actuating the hydraulic system 24 acting upon the pressure pads 22, to grip onto the annular housing 10 inner radial wall 13b and recuperate the displacement measurement device 1 as well.

[0119] The specialized coring tool 4 comprises a coring bit 28 tailored to core a pilot hole at the borehole's bottom while the displacement measurement device 1 monitors wall deformation. It passes through the annular housing 10 of the displacement measurement device 1.

[0120] Lowered via the wireline 8, the coring bit 28 is coupled to the tubular shaft gripper 9 of the wireline device 7 and can latch onto the tubular transmission shaft 102 by actuating gripper pads with the actuator mechanism 36, enabling torque and motion transmission. During coring, the tubular transmission shaft 102 rotates and descends, allowing the coring bit to penetrate the borehole's base.

[0121] The coring tool 4 comprises a movable outer sleeve 30 biased by a spring 32 and configured to telescopically surround the coring bit 28 in its fully extended position. The outer sleeve abuts against the bottom 203 of the measurement hole and retracts relative to the coring bit as the coring bit drills into the pilot hole being formed. The outer sleeve 30 may include a sealing ring 35 in rubber or soft material at the base of the outer sleeve for sealing against the bottom 203 of the measurement hole 202. The outer sleeve 30 shields the displacement measurement device 1 from debris, mud flow, or temperature fluctuations caused by pilot hole coring, preserving instrument integrity and measurement accuracy.

[0122] The DMD placement tool 3 comprises a gripper 26 to fix to the displacement measurement device 1 to safely lower the displacement measurement device 1 using the wireline device 7 from the surface to the borehole bottom. Once the displacement measurement device 1 is installed in situ, the DMD placement tool 3 is retrieved to the surface via the wireline device 7.

[0123] In an embodiment, the DMD placement tool 3 can employ an electric motor to rotate the displacement measurement device 1, orienting it at a desired angle within the borehole. The P2992PC00

[0124] DMD placement tool 3 may comprise embedded electronics and angular orientation sensor, such as a gyroscope or magnetometer, configured to enable the DMD placement tool 3 to ascertain its angular orientation.

[0125] Upon the displacement measurement device 1 making contact with the borehole bottom, the DMD placement tool 3 releases the gripper 26 securing the displacement measurement device 1 in place.

[0126] The DMD placement tool 3 may also comprise a wireless communication system to establish communication between the displacement measurement device 1 and the control unit at the surface during placement.

[0127] The measurement hole grinding tool 5 is deployed down the tubular transmission shaft 102 via the wireline device 7. Upon reaching the bottom of the tubular transmission shaft 102, it is actuated to grip onto the tubular transmission shaft 102 to transmit torque and motion.

[0128] The measurement hole grinding tool 5 is employed to create a measurement hole for inserting the displacement measurement device 1, the grinding tool drills a hole with a diameter closely matching the inner diameter of the tubular transmission shaft 102. As the tubular transmission shaft 102 is activated by the drill rig installed at the surface, the measurement grinding tool 5 rotates and applies pressure against the borehole bottom to remove material.

[0129] The depth of the measurement hole typically ranges from 100 to 500mm. Subsequently, fluid is flushed through to clear away debris.

[0130] To oversee all activities and data gathered by the various sensors from the stress measurement devices, a control unit computer equipped with a screen display and adequate data acquisition software is situated at the surface. This setup enables the operator to have complete visibility throughout the stress measurement process.

[0131] A stress measurement method according to an embodiment of the invention may comprise the following steps:

[0132] Step 1 (see figure 1): A borehole is drilled with a conventional wireline drilling method to the depth at which the measurement is to be taken. P2992PC00

[0133] Step 2 (see figure 2, 2a): The tubular transmission shaft 102 is lifted up a few centimetres and the measurement hole grinding tool 5 is lowered down the tubular transmission shaft 102 by the wireline device 7. The measurement hole grinding tool 5 is then coupled, via actuation of the tubular shaft gripper 9, to the tubular transmission shaft 102 to transmit the tubular transmission shaft torque onto the measurement hole grinding tool 5.

[0134] Step 3 (see figure 3, 3a): The drill rig rotates the tubular transmission shaft 102 and therewith the measurement hole grinding tool 5 to grind at the bottom of the borehole a measurement hole at a smaller diameter a few centimetres below the bottom of the borehole. The tubular shaft gripper 9 is then actuated to uncouple the measurement hole grinding tool 5 from the drill rig tubular transmission shaft 102. The measurement hole grinding tool 5 is then recovered at the surface using the wireline device 7. Water is injected into the borehole to flush out debris remaining in the measurement hole.

[0135] Step 4 (see figure 4, 4a, 4b): The displacement measurement device 1 is lowered by the wireline device 7 down the tubular transmission shaft 102 using the DMD placement tool 3.

[0136] Step 5 (see figure 5, 5a): The gripper 26 of the DMD placement tool 3 is actuated to couple the DMD placement tool to the tubular transmission shaft 102.

[0137] Step 6 (see figure 6, 6a): The displacement measurement device's angular position relative to the borehole can be adjusted using the DMD placement tool 3 before insertion into the measurement hole.

[0138] When inserted within the measurement hole, the displacement measurement device 1 measurement probes 12 contact the wall of the measurement hole. In an embodiment, positioning protrusions 14 of the displacement measurement device 1, for instance in the form of spring-loaded headless screw balls, press against the wall of the measurement hole, thereby positioning and centering the displacement measurement device 1 within the measurement hole.

[0139] Step 7 (see figure 7): After inserting the displacement measurement device 1 down the measurement hole, the DMD placement tool 3 is retrieved to the surface using the wireline device 7. P2992PC00

[0140] The displacement measurement device's electronics 17 records the initial position of the measurement hole wall measured by the displacement sensor. The electronics 17 may further monitor and record temperature and mud pressure. Data from all sensors may be recorded throughout the entirety of the measurement process.

[0141] Step 8 (see figure 8, 8a): The tubular transmission shaft 102 is raised a few centimetres upwards. Then, the coring tool 4 is lowered down within the tubular transmission shaft using the wireline device 7. The tubular shaft gripper 9 of the wireline device 7 is actuated to couple the coring tool 4 to the tubular transmission shaft 102 to transmit torque and motion to the coring bit 28.

[0142] Step 9 (see figure 9, 9a): The drill rig rotates the tubular transmission shaft 102 and cores a pilot hole concentric to the displacement measurement device 1. The coring bit 28 passes through the central passage of the displacement measurement device 1 annular housing 10. The telescopic outer sleeve 30 prevents rock debris and fluid to interfere with the measurement by the displacement measurement device 1. Rubber sealing at the bottom of the spring-loaded telescopic outer sleeve 30 prevents fluid or debris to interfere with the displacement measurement device 1.

[0143] Step 10 (see figure 10, 10a): Once the pilot hole coring is completed, a core catcher firmly grips the rock core. Lifting the tubular transmission shaft 102 upwards causes the core to break. The rock core and coring tool 4 are retrieved at the surface through the wireline device 7.

[0144] The excavation of rock during pilot hole coring induces local stress relief, leading to inward deformation of the measurement hole wall. This diametrical deformation is captured by the measurement probes 12 in contact with the measurement hole wall. Since the displacement measurement device 1 does not communicate with the control unit located at the surface during pilot hole coring, data is stored within a memory of the electronics 17 for later retrieval.

[0145] The measurement probes 12 of the displacement measurement device 1 are advantageously disposed at a bottom end of the displacement measurement device, which is positioned at a bottom end of the borehole, such that the measurement probes are located very close to the bottom surface of the measurement hole. This way, when the pilot hole is cored, the wall deformation, which is largest at the measurement probe location, provides a highest amplitude P2992PC00 reading and therefore most accurate overall measurement for a given sensor noise-free resolution. In a conventional configuration such as described in US4149409 where the borehole diameter measurement part is located higher up in the borehole, the deformation would be less pronounced, leading to less precise results.

[0146] Step 11 (see figure 11, 1 la, 1 lb, 11c, 1 Id, l ie): The pressure tool 2 is lowered down by the wireline device 7 and mechanically coupled to the tubular transmission shaft 102 via the tubular shaft gripper 9 of the wireline device 7. The pressure tool pressure pads 22 are then inserted into the pilot hole with the tubular transmission shaft 102 being lowered downwards. The pressure tool 2 is angularly aligned relative to the displacement measurement device 1 reference point 33. The angular alignment may be actuated either by rotation of the tubular transmission shaft 102, of by means of an electric motor 29 comprised in the pressure tool 2. Once the relative angular position between the pressure tool 2 and the displacement measurement device 1 is determined, the pressure pads 22 are rotated to align with a selected measurement probe 12 of the displacement measurement device 1.

[0147] With the pressure tool 2 positioned near the displacement measurement device 1, communication between the displacement measurement device 1 and the control unit at the surface is established. At this point, the operator gains access to information collected by the displacement measurement device 1 during the pilot hole coring procedure, including data on measurement probe displacement, temperature, pressure, and strain.

[0148] Once the pressure tool's pressure pads 22 are aligned with the selected measurement probe 12, a hydraulic pump is activated from the surface to pressurize the pressure tool 2 through connected hydraulic tubing. This increase in pressure within the pressure tool's hydraulic pistons causes the pressure pads 22 to expand, applying recovery stress to the pilot hole wall. As a result, the measurement hole wall expands outwards, the deformation being recorded by the displacement measurement device 1.

[0149] The pressure pads 22 incrementally increase stress until the initial position recorded by the displacement measurement device 1, aligned with the pressure tool 2 prior to pilot hole coring, is reached as best seen in figure I lf. Additionally, in another embodiment, it is possible to apply stress beyond the recovery stress for data interpolation purposes. Throughout this procedure, the pressure distribution between the pressure pads 22 and the pilot hole wall is recorded using the pressure mapping sensor 23 and corresponding data acquisition. P2992PC00

[0150] Multiple cycles of loading and unloading the stress applied by the pressure tool onto the pilot hole wall can be executed to acquire redundant data points as best illustrated in figure I lf.

[0151] All operations may be closely monitored by the operator through the control unit.

[0152] After measuring stress in one direction, the pressure tool pressure pads 22 can be rotated to align with another selected measurement probe 12. Stress is then applied onto the pilot hole wall again and deformation recorded by the displacement measurement device 1.

[0153] The operation may be repeated for all measurement probes 12.

[0154] Step 12 (see figure 12): The pressure tool is unlatched from the tubular transmission shaft 102 and recovered at the surface by the wireline device 7. The DMD placement tool 3 is then lowered down by the wireline device 7 and actuated to grip to the displacement measurement device 1.

[0155] Step 13 (see figure 13): The DMD placement tool 3 and displacement measurement device 1 are recovered to the surface by the wireline device 7.

[0156] Step 14 (see figure 14): Upon the completion of the stress measurement process, conventional wireline core drilling activities can resume.

[0157] Advantages of the proposed stress measurement method according to the invention include: The pressure tool can be effortlessly inserted into the pilot hole, as it is precisely guided and centered by the outer barrel. Additionally, the curved platens feature chamfered edges, further facilitating smooth insertion into the pilot hole. Unlike a flatjack, which consists of a metal sleeve that plastically deforms under pressure, the pressure tool utilizes hydraulic pistons to apply stress to the walls of the pilot hole. These pistons can retract after the measurement is completed, significantly simplifying the extraction of the pressure tool. In the proposed stress measurement method, stress relief is achieved by undercoring a pilot hole using a specialized coring tool. Coring is a well-established cutting technique with minimal risk of the drill rig becoming stuck, particularly when dealing with smaller diameter cores. P2992PC00 With the pressure mapping system positioned around the curved platens, the stress distribution can be precisely measured as the pressurized platens apply recovery stress to the walls of the pilot hole. This stress distribution plays a crucial role in the subsequent data analysis when evaluating in-situ stresses from the collected data. In the proposed stress measurement method, the DMD is inserted at the bottom of the measurement hole and the engagement tips are deployed. The engagement tips remain in the same position throughout the process. This includes the initial measurement before coring the pilot hole, the pilot core extraction, and the stress recovery phase using the pressure tool. By keeping the reference position constant throughout the procedure, the accuracy of the displacement measurements is improved. In addition to the advantages mentioned above, the proposed stress measurement method allows for the assessment of the full 2D stress state with only a single rock excavation phase. By rotating the pressure tool at various angles, stress can be evaluated in multiple directions.

[0158] P2992PC00

[0159] List o f references

[0160] Borehole 200

[0161] Measurement hole 202

[0162] Measurement hole bottom 203

[0163] Pilot hole 204

[0164] Pilot core 206

[0165] Borehole in situ stress measurement system 100

[0166] Tubular transmission shaft 102

[0167] Displacement measurement device 1

[0168] Annular housing 10

[0169] Outer radial wall 13a

[0170] Inner radial wall 13b

[0171] Positioning protrusions 14

[0172] Displacement sensor

[0173] Measurement probe 12

[0174] Feeler 15

[0175] Centre fixing portion 15a

[0176] Cantilever arms 15b

[0177] Engagement tip 15c

[0178] Strain gauge 16

[0179] Protective cover 11

[0180] Spring 39

[0181] Openings 41a, 41b

[0182] Electronics 17

[0183] Power source (battery)

[0184] Microprocessor 18

[0185] Circuit board 19

[0186] Reference point 33

[0187] Pressure tool 2

[0188] Pressure pads 22

[0189] Curved platens

[0190] Pressure mapping sensor 23

[0191] Hydraulic system 24

[0192] Hydraulic pistons 25 P2992PC00

[0193] Inlet 27a

[0194] Outlet 27b

[0195] Electronics

[0196] Controller

[0197] Power components

[0198] Electric motor 29

[0199] Position sensor 31

[0200] DMD Placement tool 3

[0201] Gripper 26

[0202] Electronics

[0203] Wireless communication system

[0204] Angular orientation sensor

[0205] Gyroscope

[0206] Magnetometer

[0207] Coring tool 4

[0208] Coring bit 28

[0209] Outer sleeve 30

[0210] Sealing ring 35

[0211] Spring 32

[0212] Core catcher 37

[0213] Gripper 34

[0214] Measurement hole grinding tool 5

[0215] Control system 6

[0216] Wireline device 7

[0217] Wireline 8

[0218] Tubular shaft gripper 9

[0219] Actuator mechanism 36

[0220] Clamp platens 38

[0221] Tool coupling 40

Claims

P2992PC00Claims1. Displacement measurement device (1) for borehole in situ stress measurement comprising a displacement sensor (11) including one or more measurement probes (12), a housing, and electronics (17) mounted in the housing and connected to the one or more measurement probes, characterized in that the housing is an annular housing (10) comprising an outer radial wall (13a) and an inner radial wall (13b), the inner radial wall surrounding a central passage configured to allow a coring tool (4) and a pressure tool (2) to pass therethrough, the one or more measurement probes (12) configured to measure a position and displacement of a wall of a measurement hole (202) at a bottom of a borehole (200) surrounding the annular housing (10) when the displacement measurement device (1) is inserted in the measurement hole (202).

2. The device according to claim 1 wherein the one or more measurement probes are disposed adjacent a bottom end of the annular housing.

3. The device according to claim 1 wherein each measurement probe comprises an engagement tip (15c) extending outwardly beyond the outer radial wall (13a) configured for mechanically engaging said wall surrounding the annular housing (10).

4. The device of the preceding claim wherein each measurement probe (12) comprises a feeler (15) from which the engagement tip (15c) extends, and a strain gauge (16) mounted on the feeler, the feeler preferably comprising a cantilever arm (15b) extending from a center fixing portion (15a) fixed to the outer radial wall (13a) of the annular housing and extending from the center fixing portion (15a) to the engagement tip (15c).

5. The device of the preceding claim wherein each measurement probe comprises a pair of feelers (15) extending in opposite directions from a center fixing portion (15a) fixed to the annular housing outer radial wall (13a).

6. The device of any preceding claim wherein there are a plurality of measurement probes (12) positioned in spaced apart manner in an axial direction, said axial direction corresponding to a direction of a centre axis of the annular housing.

7. The device of any preceding claim wherein the electronics (17) comprise a battery andP2992PC00 a microprocessor (18) configured to record measurement data output by the measurement probes (12).

8. The device of any preceding claim further including a temperature sensor and optionally a mud sensor connected to the electronics.

9. The device of any preceding claim further comprising positioning protrusions (14) mechanically coupled to the annular housing (10) and projecting outwardly beyond the outer radial wall (13a) configured for engaging said wall of the measurement hole for centering the displacement measurement device within the measurement hole.

10. A borehole in situ stress measurement system (100) configured for measuring stress in a borehole (200), comprising a displacement measurement device according to any preceding claim.

11. The borehole in situ stress measurement system of the preceding claim comprising a wireline device (7) including a wireline (8), a tubular shaft gripper (9), and a tool coupling (40), the tubular shaft gripper configured to clamp the wireline device to a tubular transmission shaft (102), the tool coupling (40) configured for coupling to a plurality of different tools.

12. The borehole in situ stress measurement system of any preceding claim further comprising a coring tool (4) including a coring bit (28) and an outer sleeve (30) movably mounted around the coring bit, biased by a spring (72) towards an extended position in which the outer sleeve (70) surrounds an end of the coring bit.

13. The borehole in situ stress measurement system of any preceding claim further comprising a pressure tool (2) including pressure pads (22) and a hydraulic system configured to move the pressure pads (22) outwardly to exert pressure on walls of a pilot hole (204).

14. The borehole in situ stress measurement system of the preceding claim wherein the pressure tool comprises a pressure mapping sensor (23) on an outer surface of the pressure pads (22) configured to measure a distribution of pressure on the pressure pads (22) when they are biased against the wall of the pilot hole (204).P2992PC0015. A method of measuring stress in situ in a borehole (200) using the borehole in situ stress measurement system (100) of any preceding claim, comprising the steps of: -drilling a measurement hole (202) at a bottom of a borehole (200),- inserting the displacement measurement device (1) in the measurement hole (202) and measuring an initial position of the wall of the measurement hole by the displacement sensor (11) of the displacement measurement device,- inserting the coring tool (4) through the central passage of the annular housing (10) and coring a pilot hole (204), and measuring a displacement of the wall of the measurement hole by the displacement sensor (11),- inserting the pressure tool (2) in the pilot hole (204) and applying pressure on the pilot hole wall by the pressure pads (22) of the pressure tool by actuating the hydraulic system (24), and measuring a displacement of the wall of the measurement hole by the displacement sensor (H).

16. The method of the preceding claim wherein the pressure tool (2) is rotated after a previous measurement to a different angular orientation and actuated to apply pressure against the wall of the pilot hole in a different angular orientation than the preceding angular orientation.

17. A borehole in situ stress measurement system (100) configured for measuring stress in a borehole (200), comprising;- a coring tool (4),- a pressure tool (2) including pressure pads (22) configured to exert pressure on walls of a pilot hole (204), and- a displacement measurement device (1) for borehole in situ stress measurement, the displacement measurement device comprising a displacement sensor (11) including one or more measurement probes (12), a housing, and electronics (17) mounted in the housing and connected to the one or more measurement probes, wherein the housing is an annular housing (10) comprising an outer radial wall (13a) and an inner radial wall (13b), the inner radial wall surrounding a central passage configured to allow the coring tool (4) and the pressure tool (2) to pass therethrough, the one or more measurement probes (12) configured to measure a position and displacement of a wall of a measurement hole (202) at a bottom of a borehole (200) surrounding the annular housing (10) when the displacement measurement device (1) is inserted in the measurement hole (202).

18. The borehole in situ stress measurement system of claim 17 wherein the one or moreP2992PC00 measurement probes are disposed adjacent a bottom end of the annular housing.

19. The borehole in situ stress measurement system according to claim 17 or 18 wherein each measurement probe comprises an engagement tip (15c) extending outwardly beyond the outer radial wall (13a) configured for mechanically engaging said wall surrounding the annular housing (10).

20. The borehole in situ stress measurement system of the preceding claim wherein each measurement probe (12) comprises a feeler (15) from which the engagement tip (15c) extends, and a strain gauge (16) mounted on the feeler, the feeler preferably comprising a cantilever arm (15b) extending from a center fixing portion (15a) fixed to the outer radial wall (13 a) of the annular housing and extending from the center fixing portion (15a) to the engagement tip (15c).

21. The borehole in situ stress measurement system of the preceding claim wherein each measurement probe comprises a pair of feelers (15) extending in opposite directions from a center fixing portion (15a) fixed to the annular housing outer radial wall (13a).

22. The borehole in situ stress measurement system of any preceding claim 18-21 wherein there are a plurality of measurement probes (12) positioned in spaced apart manner in an axial direction, said axial direction corresponding to a direction of a centre axis of the annular housing.

23. The borehole in situ stress measurement system of any preceding claim 18-22 wherein the electronics (17) comprise a battery and a microprocessor (18) configured to record measurement data output by the measurement probes (12).

24. The borehole in situ stress measurement system of any preceding claim 18-23 further including a temperature sensor and optionally a mud sensor connected to the electronics.

25. The borehole in situ stress measurement system of any preceding claim 18-24 further comprising positioning protrusions (14) mechanically coupled to the annular housing (10) and projecting outwardly beyond the outer radial wall (13a) configured for engaging said wall of the measurement hole for centering the displacement measurement device within the measurement hole.P2992PC0026. The borehole in situ stress measurement system of the preceding claim comprising a wireline device (7) including a wireline (8), a tubular shaft gripper (9), and a tool coupling (40), the tubular shaft gripper configured to clamp the wireline device to a tubular transmission shaft (102), the tool coupling (40) configured for coupling to a plurality of different tools.

27. The borehole in situ stress measurement system of any preceding claim 18-26 wherein the coring tool (4) includes a coring bit (28) and an outer sleeve (30) movably mounted around the coring bit, biased by a spring (72) towards an extended position in which the outer sleeve (70) surrounds an end of the coring bit.

28. The borehole in situ stress measurement system of any preceding claim 18-27 wherein the pressure tool (2) includes pressure pads (22) and a hydraulic system configured to move the pressure pads (22) outwardly to exert pressure on walls of the pilot hole (204).

29. The borehole in situ stress measurement system of the preceding claim wherein the pressure tool comprises a pressure mapping sensor (23) on an outer surface of the pressure pads (22) configured to measure a distribution of pressure on the pressure pads (22) when they are biased against the wall of the pilot hole (204).

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