Surface directional drilling method for crossing underground caves

Abstract

The application discloses a surface directional drilling construction method for crossing underground karst caves, and relates to the technical field of surface directional drilling construction, and aims to solve the problems of easy hole collapse, slurry leakage and uneconomic treatment when drilling through the karst caves in karst areas. The technical scheme comprises the following steps: detecting the distribution of the karst caves; designing a step-by-step casing sequence containing a risk response section and a final hole construction section; in the drilling, according to the filling state, scale and nature of the karst caves, a casing isolation or local reinforcement process is dynamically selected for treatment until the final hole is formed. The application realizes safe, economic and efficient drilling construction in the karst stratum.

Description

Technical Field

[0001] This invention relates to the field of surface directional drilling technology. More specifically, this invention relates to a method for surface directional drilling through underground karst caves. Background Technology

[0002] When conducting surface directional drilling projects in karst areas, underground caverns are the most significant geological obstacle. These caverns exhibit a high degree of uncertainty in size, shape, filling state, and number of layers, often leading to serious grout leakage, borehole wall collapse, drill string jamming, and even borehole abandonment during the drilling process, posing a significant threat to construction safety, cost, and schedule.

[0003] Currently, the commonly used treatment methods in engineering practice each have their limitations. While the full-casing approach is relatively safe, when encountering multi-layered karst caves, the excessive number of casing layers often results in an insufficient final borehole diameter, failing to meet engineering requirements, and incurring huge material consumption and poor economic efficiency. Pre-grouting reinforcement methods struggle to control the flow and setting effect of grout in complex karst cave networks, often offering limited reinforcement for large cavities, and are costly and time-consuming. Relying on mud slurry forcibly penetrating karst caves leads to massive mud loss when encountering unfilled caves, resulting in a loss of borehole support and extremely high risks. A more common problem is that existing construction techniques are mostly based on fixed plans developed during preliminary exploration, lacking the ability to flexibly adjust according to real-time geological conditions revealed during drilling. This makes it impossible to implement differentiated treatment measures for karst caves of different risk levels (e.g., whether they are fully filled or empty, and their size), easily leading to undertreatment or overtreatment.

[0004] Furthermore, existing technologies often focus only on the immediate treatment of a single exposed karst cave, neglecting the dynamic and temporal risks present during construction. For example, when dealing with multi-layered karst caves, cavities that have been traversed but not yet filled may become unstable under subsequent drilling disturbances, threatening the safety of the casing already installed above; simultaneously, the weak rock strata separating the upper and lower karst caves also pose a risk of collapse during construction. In large karst caves, ensuring the accuracy and stability of subsequent borehole enlargement operations, and ensuring the compactness of the filling of the huge cavities outside the casing, are problems that existing processes have failed to systematically solve.

[0005] Therefore, the industry urgently needs a systematic construction method that can integrate advanced detection, dynamic design, real-time decision-making, and risk control throughout the entire process to overcome the above-mentioned drawbacks and achieve safe, economical, and efficient drilling operations in karst formations. Summary of the Invention

[0006] One object of the present invention is to solve at least the above-mentioned problems and to provide at least the advantages that will be described later.

[0007] Another objective of this invention is to provide a surface directional drilling method for traversing underground karst caves. This method overcomes the problems of rigidity and poor targeting in existing karst cave drilling methods, and provides a systematic construction method that can dynamically and accurately select and implement differentiated treatment measures based on the actual geological conditions of the karst cave revealed in real time, so as to ensure the safety, stability and economy of borehole formation.

[0008] To achieve these objectives and other advantages according to the present invention, a method for surface directional drilling through underground karst caves is provided, comprising the following steps:

[0009] S1. Conduct exploration along the designed borehole trajectory to identify and record the distribution, size, and number of layers of the karst caves;

[0010] S2. Based on the detection results of S1 and the required final borehole inner diameter for the project, design a casing sequence with a progressively decreasing diameter from the surface to the bottom of the borehole. The sequence is divided into three segments: a) Risk response segment: including at least one level of surface casing and several levels of cave isolation casing; b) Borehole construction segment: including at least one level of technical casing and the final level casing; wherein, the preset number of levels of the cave isolation casing is not less than the number of cave layers detected in S1.

[0011] S3. Drill a hole to the stable topsoil layer on the ground surface, and lower and solidify the topsoil casing.

[0012] S4. Drill downwards within the solidified casing. If a karst cavern is exposed in real time, proceed according to the following rules: 1) If the karst cavern is an open cavity that is unfilled or partially filled, execute the casing isolation procedure; 2) If the karst cavern is fully filled and its vertical height is greater than or equal to the first preset threshold, or the filling material is a weak or easily leaking medium, execute the casing isolation procedure; 3) If the karst cavern is fully filled and its vertical height is less than the first preset threshold, and the filling material is not a weak or easily leaking medium, execute the local reinforcement procedure.

[0013] The casing isolation process is as follows: control the drill bit to penetrate the karst cave and drill into the bedrock below its bottom plate to a predetermined depth, enlarge the hole section containing the karst cave, and from the risk response section, lower the first-level karst cave isolation casing and consolidate its upper and lower ends;

[0014] S5. Repeat step S4 until drilling reaches the preset depth of the complete rock stratum. Then, continue drilling within the complete rock stratum according to the designed trajectory and borehole diameter, and sequentially lower and consolidate the various levels of technical casing and the final stage casing of the final borehole construction section to finally form the final borehole.

[0015] S6. Fill the cavity outside the casing of the karst cave that has been traversed through the casing isolation process.

[0016] Preferably, in step S4, if the number of caves exposed in real time exceeds the preset number of cave isolation sleeve levels in the risk response section, the caves exceeding the preset number of levels are treated using a local reinforcement process.

[0017] Preferably, the first preset threshold is 2 meters to 3 meters.

[0018] Preferably, the karst cave isolation casing lowered in the casing isolation process has a grout-stopping ring or is wrapped with permeable geotextile on its outer side at the position corresponding to the karst cave section.

[0019] Preferably, the drill bit penetrates to a depth of at least 5 meters into the intact bedrock beneath the karst cave floor.

[0020] Preferably, in step S6, a differentiated filling scheme is adopted according to the vertical height of the karst cave being traversed, as follows:

[0021] For karst caves with a vertical height of less than 2 meters, backfill with a mixture of rubble and clay;

[0022] For karst caves with a vertical height of 2 to 5 meters, backfill in layers and inject cement mortar;

[0023] For karst caves with a vertical height greater than 5 meters, geotextile bags are used for sealing, combined with compensating grouting.

[0024] Preferably, in the casing isolation process of step S4, after the karst cave isolation casing is placed and its upper and lower ends are solidified, temporary support grout is injected into the karst cave cavity outside the casing through the grouting channel pre-set on the casing and solidified to form a temporary stable body; the temporary support grout is prepared by the following components in the following weight ratio: 100 parts of sulfoaluminate cement, 2-5 parts of quick-setting agent, 0.5-2 parts of foaming agent, and 40-60 parts of water;

[0025] After the temporary stabilizer reaches the predetermined strength, drilling continues downward; in step S6, the temporary stabilizer is retained or replaced with a permanent filler according to engineering requirements.

[0026] Preferably, in the casing isolation process of step S4, when the cavity is an unfilled or partially filled cavity with a vertical height greater than 5 meters, an intelligent reaming system with rotary guiding function is used to perform reaming, specifically including the following steps:

[0027] S4a. Connect the rotary guide reamer to the lower end of the drill string and install a measurement while drilling unit near its end.

[0028] S4b. After the reaming begins, the drilling measurement unit measures the well inclination angle, azimuth angle and tool face angle of the reamer in real time and uploads them to the ground control system.

[0029] S4c, The ground control system compares the received real-time trajectory data with the designed reaming axis and calculates the trajectory deviation; with the designed axis as the reference, a virtual cylindrical allowable deviation channel is established; when the real-time measured position of the reamer deviates from the center line of the channel, the system calculates the tool face angle and guide force adjustment amount to make the reamer vector direction point to the center line of the channel according to the direction and distance of the deviation, and generates a correction command;

[0030] S4d. The correction command is transmitted to the rotary guide reamer via drilling fluid pulse signal, controlling its internal guide mechanism to generate a corresponding bias force, adjusting the drilling direction of the reamer, and returning it to the preset trajectory.

[0031] The present invention has at least the following beneficial effects:

[0032] First, by integrating advanced geological exploration, functional zoning casing sequence design, and a dynamic decision-making process based on real-time exposure of karst cave characteristics, a significant improvement in the safety, economy, and reliability of drilling operations in complex karst formations was achieved. Its core effect lies in transforming the previously experience-dependent, reactive construction process into a systematic engineering approach that can be planned in advance, responded to in real time, and implemented with precision.

[0033] Secondly, by establishing clear decision-making rules (based on whether the karst cave is unfilled / partially filled or fully filled, and its size and nature), precise matching of treatment measures was achieved. This effectively avoids two drawbacks of the traditional "one-size-fits-all" approach: firstly, unnecessary casing isolation is implemented for small-scale, stable karst caves, thus saving expensive material and construction costs; secondly, insufficient measures such as local reinforcement are only taken for high-risk large cavities or weakly filled karst caves, thereby fundamentally preventing major in-hole accidents caused by improper treatment and ensuring construction safety.

[0034] Third, by designing the casing functional zones of the "risk response section" and the "final borehole construction section," a flexible space is reserved to cope with the uncertain number of karst cave layers while ensuring the rigid target of the final borehole diameter. This design allows for the optimal allocation of construction resources, enabling the response to the most unfavorable geological conditions while avoiding waste when the actual situation is better than expected. The entire method forms a complete closed loop from macro-design to micro-execution, from risk warning to real-time processing, significantly improving the borehole success rate, project controllability, and overall economic benefits of drilling construction in karst cave areas.

[0035] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0036] Figure 1This is a schematic diagram of the five-stage casing tunneling through the karst cave in Embodiment 1 of the present invention.

[0037] Attached diagram descriptions: 1-Surface borehole, 2-Cave, 3-Grouting area, 4-First-level casing, 5-Second-level casing, 6-Third-level casing, 7-Fourth-level casing, 8-Fifth-level casing. Detailed Implementation

[0038] The present invention will be further described in detail below with reference to embodiments, so that those skilled in the art can implement it based on the description.

[0039] This invention discloses a method for surface directional drilling through underground karst caves, comprising the following steps:

[0040] S1. Conduct exploration along the designed borehole trajectory to identify and record the distribution, size, and number of layers of the karst caves;

[0041] S2. Based on the detection results of S1 and the required final borehole inner diameter for the project, design a casing sequence with a progressively decreasing diameter from the surface to the bottom of the borehole. The sequence is divided into three segments: a) Risk response segment: including at least one level of surface casing and several levels of cave isolation casing; b) Borehole construction segment: including at least one level of technical casing and the final level casing; wherein, the preset number of levels of the cave isolation casing is not less than the number of cave layers detected in S1.

[0042] S3. Drill a hole to the stable topsoil layer on the ground surface, and lower and solidify the topsoil casing.

[0043] S4. Drill downwards within the solidified casing. If a karst cavern is exposed in real time, proceed according to the following rules: 1) If the karst cavern is an open cavity that is unfilled or partially filled, execute the casing isolation procedure; 2) If the karst cavern is fully filled and its vertical height is greater than or equal to the first preset threshold, or the filling material is a weak or easily leaking medium, execute the casing isolation procedure; 3) If the karst cavern is fully filled and its vertical height is less than the first preset threshold, and the filling material is not a weak or easily leaking medium, execute the local reinforcement procedure.

[0044] The casing isolation process is as follows: control the drill bit to penetrate the karst cave and drill into the bedrock below its bottom plate to a predetermined depth, enlarge the hole section containing the karst cave, and from the risk response section, lower the first-level karst cave isolation casing and consolidate its upper and lower ends;

[0045] S5. Repeat step S4 until drilling reaches the preset depth of the complete rock stratum. Then, continue drilling within the complete rock stratum according to the designed trajectory and borehole diameter, and sequentially lower and consolidate the various levels of technical casing and the final stage casing of the final borehole construction section to finally form the final borehole.

[0046] S6. Fill the cavity outside the casing of the karst cave that has been traversed through the casing isolation process.

[0047] In the above technical solution, during step S1, when conducting detection along the designed borehole trajectory, a combination of ground-based ground-penetrating radar (GPR) and in-bore electromagnetic CT (CT) can be used. The GPR can be deployed along both sides of the borehole axis, with its antenna center frequency selectable between 80MHz and 100MHz to detect shallow karst caves. For deeper or more detailed detection, CT scanning can be performed in the pilot borehole, placing the transmitting and receiving probes at different depths in different boreholes or the same borehole. By measuring the electromagnetic field strength attenuation and phase change, the spatial location of the karst cave is identified using a tomographic inversion algorithm. The detection requires recording the depth of the cave's top and bottom plates, and calculating its vertical height, which may range from less than 1 meter to over 20 meters. Simultaneously, the length of the karst cave along the borehole axis and the nature of its filling material are recorded, such as unfilled, partially filled (filling material ratio less than 70%), or fully filled. A preliminary qualitative description of the fully filled material is also provided. In step S2, based on the above detection results and the required final borehole inner diameter (e.g., 0.219 meters), a casing sequence with progressively decreasing diameter from the surface to the bottom of the borehole is designed. The sequence design is based on meeting the final borehole diameter and is carried out using a backward calculation method. First, the outer diameter of the last-stage casing is determined, and then the inner diameter of the upper-stage casing is increased progressively. Sufficient annular space must be reserved between every two stages of casing for cement grout consolidation; the annular space is typically 30 mm to 45 mm. This casing sequence is divided into two sections: a risk response section and a final borehole construction section. The risk response section includes at least one surface casing that extends to a stable topsoil layer (such as a dense clay layer or a strongly weathered rock layer at least 3 meters below the top surface), and several stages of cave isolation casing for traversing caves. The preset number of stages of cave isolation casing should not be less than the maximum number of cave layers detected in S1; for example, if four cave layers are detected, at least four stages should be preset. Each casing unit should be long enough to cover the corresponding karst cave section, with at least 2 meters of intact rock strata anchoring at both its upper and lower ends. The final borehole construction section includes at least one level of technical casing for protecting the upper borehole section, and the last level of wall protection or filter casing. The casing material can be seamless steel pipe of J55 or N80 grade according to the Petroleum API standard, and its strength must be verified by professional software to ensure its resistance to external extrusion and internal pressure safety factors.

[0048] Step S3: When drilling the surface, a rotary drilling rig or a hydraulic top-drive drilling rig can be used. The drilling rig base is installed at the leveled and compacted borehole location, and leveled and aligned using a spirit level and theodolite. A tricone drill bit or a PDC full-face drill bit is used for drilling, with the borehole diameter approximately 50 mm larger than the outer diameter of the surface casing. After drilling through the loose overburden to the stable topsoil layer, the drill string is lifted, and the surface casing string is lowered to the bottom of the hole using the drilling rig's traveling block system. Cementing is performed by pumping cement slurry. The cement slurry (a neat slurry prepared with G-grade oil well cement, with a water-cement ratio of 0.44) is pumped through the drill pipe to the casing shoe, allowing it to return from the outer annulus of the casing to the surface. After the cement slurry initially sets, it solidifies. Step S4: Inside the solidified casing, a smaller drill bit is used to continue drilling downwards. During drilling, parameters such as cuttings logging (observing changes in the composition and shape of cuttings), drilling time logging (recording the time required to advance a unit of footage), and measurement-while-drilling (MWD) data such as well inclination and torque are used to determine in real time whether a cavern has been exposed. A cavern can be identified as exposed when drilling time suddenly increases, torque fluctuates drastically, a feeling of suspension is present, the amount of cuttings returned decreases sharply, or cavern filling material appears. Based on the exposure, the following rules apply: First, determine the cavern filling status. If it is an open cavity that is unfilled or partially filled (filling material volume fraction less than 70%), then execute the casing isolation procedure. If it is fully filled, then its vertical height and filling material properties need to be assessed. The vertical height can be estimated by the change in drill string length before and after exposure, the length of the abrupt change in resistivity during drilling, or the length of the abrupt change segment in the natural gamma ray logging curve. The first preset threshold can be set to 2.5 meters. Whether the filling material is a "weak, easily lost medium" can be determined through simple on-site testing. For example, a standard penetration test (SPT) under saturation can be performed on the extracted filling material. If the blow count (N) is less than 10, or if it is observed to be fluid and easily crumbled by hand, it is considered weak and easily lost. If the vertical height of the fully filled cavern is greater than or equal to 2.5 meters, or if its filling material is a weak, easily lost medium, then a casing isolation procedure should be executed. If its vertical height is less than 2.5 meters and the filling material is not weak and easily lost (such as dense cemented breccia), then a local reinforcement procedure should be executed. The specific operation of the casing isolation procedure is as follows: control the drill bit to quickly pass through the cavern cavity or filling body, and continue drilling until drilling into the intact bedrock below the cavern floor. The drilling depth can be set to 5 meters. The criteria for determining whether the intact bedrock has been reached can be that the drilling speed recovers to a stable state, the torque is stable, and the returned cuttings are fresh and intact bedrock cuttings. Afterwards, the drill string is lifted, and a reaming drill bit (such as a hydraulic reamer) is used to enlarge the borehole section containing the karst cave. The enlarged diameter must match the outer diameter of the karst cave isolation casing to be lowered. After the enlargement is completed, the first-level karst cave isolation casing is lowered from the preset sequence. A steel grout stop ring or highly permeable non-woven geotextile can be pre-welded to the outside of the casing at the corresponding karst cave section location. After the casing is lowered into place, cement grout is injected into its upper and lower ends (i.e., the annulus of the complete rock strata above the top slab and below the bottom slab of the karst cave) for consolidation.The specific operation of the local reinforcement process is as follows: Instead of enlarging the borehole or lowering the isolation casing, pressure grouting is performed through the drill pipe into the exposed karst section's borehole wall and surrounding filling material. Ordinary silicate cement grout can be used as the grouting material, with a water-cement ratio of 0.8:1 to 1:1. Appropriate amounts of bentonite or water glass can be added as modifiers. The grouting pressure is controlled between 0.5 MPa and 1.5 MPa, and the process ends when the grout has diffused to a range of at least 0.5 meters around the borehole wall or when there is slight grout return at the borehole opening. After the grout has solidified to form a reinforced shell (curing time is typically 12 to 24 hours), drilling can continue downwards.

[0049] Step S5 repeats the drilling, exposure, judgment, and corresponding process handling procedures of Step S4. Here, "repeated" means that for each exposed cave, a casing isolation or local reinforcement process is dynamically selected and executed based on its geological conditions, and drilling continues until the drill bit reaches a preset "complete rock stratum depth" that serves as the starting point for final hole construction. This depth is a pre-determined elevation based on regional geological data and engineering objectives (such as entering stable, thick, slightly weathered or unweathered bedrock), and it is typically located at least 10 meters below the deepest floor of all known caves. Upon reaching this depth, the borehole has essentially escaped the risk strata. Subsequently, within this complete rock stratum, directional drilling is performed using a directional drilling tool assembly (such as a screw drill with a bend joint) along the designed trajectory, gradually enlarging to the final hole design diameter. During this open-hole section or reaming process, the designed levels of technical casing and the final casing within the final hole construction section are sequentially lowered and consolidated, ultimately forming a final hole that meets the size and trajectory requirements. Step S6: After the final borehole construction is completed and all casing cementing slurry has fully set, the annular cavity between the casing and the borehole wall in the karst section traversed by the casing isolation process is permanently filled. Filling is typically performed through a grouting valve pre-embedded in the karst isolation casing or through a pre-reserved channel between the casing string and the borehole wall. Different filling materials and processes can be used depending on the vertical height of the karst cavity. For example, for cavities less than 2 meters high, a mixture of graded crushed stone and clay can be backfilled through a guide pipe; for cavities 2 to 5 meters high, M20 cement mortar can be injected in sections; for large cavities greater than 5 meters high, a support structure can be formed by lowering formwork bags in sections and injecting quick-setting cement slurry, followed by pressure-compensated grouting to fill the remaining voids. The grouting pressure and rate must be controlled during the filling process, and the compaction of the filling must be monitored by the grouting volume.

[0050] Compared to the closest existing technologies, such as the full-casing follow-up method, this technical solution, through preliminary exploration and dynamic decision-making based on real-time geological criteria, implements casing isolation only for high-risk karst caves, significantly reducing the number of casing layers and material consumption, and is more economical while ensuring borehole stability. Compared to the full-hole pre-grouting reinforcement method, this solution adopts differentiated treatment measures for karst caves of different risks, avoiding ineffective grouting in stable strata and uncontrollable diffusion of grout in large cavities. It is more targeted in construction, has a more controllable construction period, and is more cost-effective. Compared to the method of simply relying on high-performance mud slurry to forcibly penetrate, this solution provides a reliable means of physical isolation or pre-reinforcement for karst caves with instability risks, fundamentally reducing the probability of serious grout leakage, borehole collapse, and stuck drill accidents during construction, and improving the overall safety and success rate of drilling in complex karst strata.

[0051] In another technical solution, in step S4, if the number of caves exposed in real time exceeds the preset number of cave isolation sleeve levels in the risk response section, the caves exceeding the preset number of levels are treated with a local reinforcement process.

[0052] In the above technical solution, during the drilling process in step S4, if the total number of karst caves revealed in real time exceeds the number of karst cave isolation casing levels preset based on the previous detection results in step S2, this contingency plan will be activated. For example, if the previous detection showed the existence of 3 karst caves, then 3 levels of karst cave isolation casings were preset in the casing sequence. However, during the actual drilling process, a fourth or more previously undetected hidden karst caves may be revealed. In this case, for the fourth and subsequent revealed karst caves, the casing isolation process will no longer be used; instead, a unified local reinforcement process will be adopted. The decision is based on real-time recording and comparison at the construction site, checking the cumulative number of revealed karst caves against the number of physical karst cave isolation casings reserved and deployable in the risk response section. When the latter has been exhausted, this plan will be triggered.

[0053] For local reinforcement procedures performed on karst caves exceeding the preset level, the core principle is to establish borehole wall stability using grouting as the core reinforcement method under the constraint that it is impossible to lower a dedicated isolation casing. In specific implementation, a geological assessment of the karst cave is first required. If the karst cave is fully filled and the filling material is relatively dense, reinforcement grout can be directly injected through the drill pipe. Cement-water glass dual-liquid grout can be used as a rapid consolidation material. During operation, liquid A and liquid B are mixed at the bottom of the borehole and then injected using a dual-liquid grouting pump. The grouting pressure can be controlled between 0.3 MPa and 1.0 MPa. If the karst cave is an unfilled, partially filled, or large cavity filled with a soft and easily leaking medium (e.g., with a vertical height greater than 3 meters), an enhanced reinforcement step is required: First, a quantitatively graded crushed stone or dry-mixed quick-setting concrete aggregate is injected into the karst cave cavity at the bottom of the borehole through the drill pipe or a dedicated guide pipe. The aggregate particle size can be selected between 5 mm and 40 mm to form a primary filling skeleton and a filter layer. Subsequently, quick-setting grout is immediately injected into the voids filled with aggregate through the grouting pipe. The grout can be sulfoaluminate cement grout or the aforementioned two-component grout. The grouting pressure should start low, and after the grout has initially penetrated and coated the aggregate, the pressure can be gradually increased to 0.5 MPa to 1.5 MPa to form a more stable aggregate-grout binder. Throughout the reinforcement process, the grouting volume and pressure changes must be monitored. The process is considered complete when the grout returns to the top of the karst cave or when the pressure reaches the design upper limit and stabilizes for a certain period.

[0054] The core advantage of this approach for handling excess karst caves lies in providing a systematic strategy for construction under limited resources. Compared to the traditional full-casing follow-up method, which may face stagnation or forced borehole reduction when encountering unplanned karst caves, this method provides a feasible path for continuous drilling through standardized grouting reinforcement procedures (especially aggregate filling reinforcement for large cavities). Although the long-term stability and disturbance resistance of its reinforced body may not be as good as physical casing isolation, its process combination significantly improves the effectiveness of simple grouting in adverse geological sections, thus achieving an acceptable balance between cost, schedule, and safety risks, and enhancing the overall construction method's adaptability to geological uncertainties.

[0055] In another technical solution, the first preset threshold is 2 to 3 meters. Specifically, in the decision rule of step S4, the first preset threshold is used to scientifically define the lower limit of the scale of "fully filled karst caves with significant instability risk," and its specific numerical range is set to 2 to 3 meters. The determination of this threshold integrates engineering experience, the mechanical properties of common karst cave filling materials, and statistical analysis of construction risks. For example, in engineering practice in karst areas, fully filled karst caves with a vertical height of less than 2 meters, if the filling material is well cemented, have a higher probability of maintaining short-term stability under the pressure of overlying strata and borehole disturbance; however, when the height exceeds 3 meters, the risk of borehole wall instability and overall collapse of the filling material increases significantly. Therefore, setting the threshold in this range aims to establish a clear and operable quantitative dividing point. In specific implementation, the threshold can be selected as a specific value, such as 2.5 meters, as a unified implementation standard. The operator estimates the vertical height of the exposed cavern based on the real-time survey method described in step S4 (such as combining the drill string suspension length and the length of the abrupt change segment of the logging-while-drilling curve), and directly compares it with the selected threshold, thereby quickly making a decision on whether to adopt the casing isolation process or the local reinforcement process.

[0056] When applying this threshold for decision-making, a comprehensive judgment must be made in conjunction with the properties of the karst cave's filling material. If the measured vertical height of the karst cave is 2.8 meters, exceeding the selected 2.5-meter threshold, the casing isolation process will be triggered regardless of the initial assessment of the filling material's properties, as its size already constitutes a significant risk. If the measured height is 2.2 meters, below the 2.5-meter threshold, further assessment is needed to determine whether the filling material is a "weak, easily leaking medium." If not, a local reinforcement process will be implemented; if so, due to its severe properties, the casing isolation process must be upgraded. The existence of this threshold transforms the handling of fully filled karst caves from relying solely on qualitative experience to a hierarchical decision-making process that prioritizes quantitative judgment of scale and supplements it with qualitative judgment of properties, improving the standardization and consistency of construction response. For situations near the threshold boundary (such as height measurements between 2.4 and 2.6 meters), the on-site technical supervisor can make minor adjustments within the threshold framework based on more detailed drilling parameters (such as the severity of torque fluctuations) and adjacent formation conditions, but the core of the decision-making logic still revolves around this preset numerical benchmark.

[0057] By adopting a specific threshold range of 2 to 3 meters, this approach offers a clear and repeatable basis for technical judgment compared to the closest existing technology. In existing technologies, the assessment of "larger" or "risky" karst caves often relies on the subjective experience of construction personnel, lacking a unified standard and easily leading to inconsistent treatment measures—either overly conservative and wasteful, or overly risky and causing accidents. This solution, by introducing this quantitative threshold, standardizes and objectifies the crucial risk assessment process. It provides a common and clear dimensional benchmark for pre-construction design (such as casing level estimation) and real-time decision-making during construction, reducing the arbitrariness and uncertainty of human decision-making. This not only helps to more accurately estimate material costs before construction but also enables refined control between safety risks and economic costs during construction. Thus, while ensuring borehole stability, it minimizes the over-treatment of small and medium-sized stable karst caves, improving the efficiency and economic benefits of the construction process.

[0058] In another technical solution, the karst cave isolation casing lowered in the casing isolation process has a grout-stopping ring or is wrapped with permeable geotextile on its outer side at the position corresponding to the karst cave section.

[0059] In the above technical solution, during the casing isolation process in step S4, when a grout-stopping ring needs to be installed on the outside of the casing to be lowered into the corresponding karst section, the specific implementation method is as follows. The grout-stopping ring can be made of a Q235B or higher strength grade annular steel plate of the same material as the casing, with its inner diameter matching the outer diameter of the casing, and fixed by interference fit or welding. For example, for a casing with an outer diameter of 273 mm, an annular steel plate with a thickness of 8 to 12 mm and a width of 150 to 200 mm can be used as the grout-stopping ring. During factory prefabrication or on-site installation, the grout-stopping ring is precisely positioned and welded to the outer wall of the casing column. The welding position is determined based on the detection results of step S1 and the real-time exposure data of step S4, aiming to ensure that the grout-stopping ring, after being finally lowered into place, is located precisely near the top and bottom plates of the karst section to be treated. Typically, one grout-stopping ring is installed at each of the upper and lower interfaces of a karst section, and the casing section between the two grout-stopping rings is the part that passes through the karst. Welding must be continuous full welding, and non-destructive testing must be performed to ensure sealing. After the casing is lowered into the hole, the outer edge of the grout stop ring will be in close contact with the borehole wall or embedded in the shallow layer of the borehole wall. Its main function is to effectively seal the cement slurry within the annular space defined by the two grout stop rings during subsequent cementing and grouting, preventing the slurry from flowing upwards or downwards indefinitely into other parts of the karst cavity or other formations.

[0060] As an alternative or supplement to the grout-stopping ring, a permeable geotextile can be wrapped around the corresponding section of the karst cave isolation casing. This geotextile can be a long-filament needle-punched nonwoven geotextile with a unit area mass controlled between 300 g / m² and 500 g / m², possessing appropriate permeability and soil retention. Before lowering the casing, the geotextile should be cut to a size sufficient to wrap the karst cave section of the casing, allowing for overlap. During wrapping, the geotextile should be tightly wrapped around the outer wall of the casing, with an overlap width of no less than 200 mm, and secured circumferentially with corrosion-resistant soft straps (such as nylon cable ties or stainless steel wire) at intervals of 300 mm to 500 mm to ensure it does not slip or fall off during casing lifting and lowering. The total length of the geotextile-wrapped section should exceed the estimated vertical height of the karst cave by at least 1 meter to provide sufficient coverage. Its function is as a permeable filter medium: during cementing, some water in the cement slurry can seep out, which helps the slurry thicken early, while cement particles are effectively retained inside the geotextile, thereby quickly forming a locally dense cement stone filter cake between the casing and the borehole wall, accelerating the annular plugging and stabilization process. This method is particularly suitable for working conditions where the borehole wall is irregular or the grout-stopping ring is difficult to completely seal.

[0061] Whether using a grout-stop ring or wrapping with permeable geotextile, the core purpose is to optimize the flow and solidification behavior of the cementing slurry during the casing isolation process, thereby improving the sealing effect on the karst cavity. The specific process is as follows: after the karst isolation casing is lowered to the designed depth, cement slurry is injected into the outer annulus of the casing using cementing equipment. If a grout-stop ring is used, the slurry is confined within the cavity between the two rings, solidifying under pressure into a complete cement-stone ring bonded to the upper and lower rock strata, thus firmly anchoring the casing and completely isolating the karst cavity from the borehole passage. If geotextile is wrapped, the slurry rapidly loses water and solidifies under infiltration, forming a relatively dense early support layer around the casing.

[0062] In existing technologies, grouting is performed directly after casing is installed. This allows the grout to easily leak out over a large area in the unconstrained karst cavities, leading to insufficient grout return to the bottom of the casing, poor cementing quality, or even drilling failure. This solution, by adding a simple structure such as a grout-stop ring or geotextile, actively guides and controls the grout's retention location and setting process. This significantly improves the utilization rate of cement grout and the reliability of sealing karst sections, providing a more solid guarantee for subsequent safe drilling. It also reduces material waste and construction delays caused by grout loss.

[0063] In another technical solution, the drill bit penetrates to a depth of no less than 5 meters into the intact bedrock beneath the karst cave floor.

[0064] In the above technical solution, during the casing isolation process in step S4, after the drill bit rapidly passes through the karst cave, it needs to continue drilling into the intact bedrock below its base. Specifically, the operator needs to comprehensively monitor multiple parameters to determine whether the drill bit has entered the intact bedrock. Judgment criteria may include: the returned rock cuttings changing from karst cave filling material (such as clay or breccia) to fresh, hard bedrock fragments; the drilling speed (drilling time) returning from the abnormally accelerated state when passing through cavities or weak layers to a relatively stable normal drilling speed matching the lithology of the formation; and simultaneously, the drastic fluctuations in the drilling rig torque significantly decreasing and stabilizing. Once it is confirmed that the drill bit has entered the intact bedrock interface, drilling continues into that rock layer. From the determined interface, the required drilling depth should not be less than 5 meters. This depth refers to the footage length measured along the borehole trajectory. During drilling, the footage can be monitored and accumulated in real time using the footage counter on the drilling rig's instrument panel or the depth measurement module in the measurement-while-drilling system to ensure that the preset depth requirement is met.

[0065] The minimum depth requirement of 5 meters is primarily based on mechanical stability considerations in engineering practice. The rock mass at this depth will serve as a critical anchoring section for the subsequently run-in cavern isolation casing. Through cementing operations, cement slurry will seal the annulus between this section of casing and the borehole wall, forming a cement sheath at least 5 meters in length. This sufficiently long cement sheath provides reliable gripping force and end support for the casing, effectively resisting possible casing uplift, sinking, or axial displacement during and after construction. During implementation, if localized fracturing or low strength of this rock mass is found during drilling, the drilling depth can be appropriately increased, for example, to 6 or 8 meters, to seek a more stable anchoring formation. After drilling to this depth and running the casing for cementing, the bonding quality and continuity of the cement sheath in this anchoring section can be checked using electrical logging or sonic logging methods to verify whether effective sealing has been achieved.

[0066] The explicit requirement to drill at least 5 meters into intact bedrock, compared to the shallow anchoring or reliance on experience-based judgment common in existing technologies, has the advantage of establishing the casing anchoring safety on a quantifiable and verifiable technical indicator. In current construction practices, sometimes drilling is stopped only after penetrating the karst cave, or the drilling depth in intact rock strata is insufficient. This can lead to inadequate anchoring length at the lower end of the casing, making it prone to instability under disturbance. This solution, by setting and enforcing this explicit lower depth limit, ensures that the casing obtains a sufficiently long and reliable composite anchor of rock strata-cement sheath-casing at both ends of the critical risk section. This significantly enhances the overall longitudinal stability of the casing string traversing the karst cave, reduces the risk of in-hole accidents caused by anchoring failure, and thus provides a fundamental guarantee for the long-term safety of subsequent drilling operations and the final borehole.

[0067] In another technical solution, in step S6, a differentiated filling scheme is adopted according to the vertical height of the karst cave being traversed, as follows:

[0068] For karst caves with a vertical height of less than 2 meters, backfill with a mixture of rubble and clay;

[0069] For karst caves with a vertical height of 2 to 5 meters, backfill in layers and inject cement mortar;

[0070] For karst caves with a vertical height greater than 5 meters, geotextile bags are used for sealing, combined with compensating grouting.

[0071] In the above technical solution, in step S6, when permanently filling the cavity outside the casing of a karst cave traversed through the casing isolation process, a differentiated filling scheme should be implemented based on the actual vertical height of the karst cave (which can be determined through construction records or borehole television measurements). For karst caves with a vertical height of less than 2 meters, the cavity volume is relatively small, and a mixture of rubble and clay can be used for backfilling. The rubble can be hard limestone or granite crushed stone with a particle size of 30 mm to 100 mm, and the clay can be remolded clay with a plasticity index greater than 15. During construction, the rubble and clay can be mixed near the borehole opening using a small excavator or manually at a volume ratio of approximately 7:3. Then, the mixture is slowly poured into the annulus between the casing and the borehole wall through a guide pipe or a special chute. The pouring process should be carried out in layers, with each layer not exceeding 0.5 meters in thickness, and moderately compacted with a tamping rod to reduce the porosity until the top of the karst cave is filled.

[0072] For medium-sized karst caves with a vertical height of 2 to 5 meters, a layered backfilling combined with cement mortar injection is required to ensure long-term compaction and stability. First, the aggregate layer is backfilled. Graded crushed stone can be used, with a maximum particle size not exceeding 25 mm. The aggregate is filled into the annulus in sections through a guide pipe, with each section's filling height controlled between 1.0 and 1.5 meters. After each section of aggregate is filled, cement mortar is immediately injected into that section of aggregate layer through a pre-embedded grouting pipe (which can be pre-fixed to the outside of the casing during casing installation). The mortar can be of M15 or M20 strength grade, and its mix ratio can be prepared according to cement:sand:water = 1:3:0.45 (by weight), and mixed using a mobile mortar mixer. A piston-type grouting pump can be used for grouting, with the grouting pressure controlled between 0.2 MPa and 0.5 MPa. The end of grouting for that section is marked by the overflow of grout from the adjacent upstream grouting pipe or observation hole. This process is repeated, with aggregate backfilling and mortar injection completed in sections from bottom to top until the entire cavity is filled and compacted.

[0073] For large karst cavities with a vertical height greater than 5 meters, a combination of geotextile bag sealing and compensating grouting is adopted. The geotextile bags can be cylindrical geotextile bags woven from high-strength polyester or polypropylene filaments, with a radial tensile strength of not less than 50 kN / m and an equivalent pore size O90 of less than 0.1 mm. During construction, the geotextile bag material is cut and sewn into cylindrical bags of appropriate size according to the height of the karst section and the borehole diameter. Suspension straps and grouting ports for grouting are pre-installed at the top and bottom. The folded and bundled geotextile bags are then lowered to the predetermined bottom position of the karst section outside the casing using ropes. Then, through the grouting pipe connected to the injection port, highly fluid, quick-setting cement grout (such as sulfoaluminate cement grout with a water-cement ratio of 0.6) is injected into the formwork bag. The grouting pressure needs to be increased slowly, usually controlled between 0.1 MPa and 0.3 MPa, so that the formwork bag expands uniformly under constraint until it tightly fits the borehole wall and the casing, forming a solid "bag-like" support. After it has solidified and reached a certain strength, the remaining cavity from the top of the formwork bag to the top of the karst cave is backfilled with aggregate and grouted in sections (refer to the second section of the scheme), or a one-time low-pressure compensation grouting is performed in the final stage to fill all the remaining voids. Compared with the existing technology that often uses a single material (such as pure cement grout) to fill all karst caves, this scheme optimizes technical reliability and engineering economy by matching different complexities and costs according to the size of the karst cave. For small karst caves, a simple and economical mixed backfill method is used; for medium-sized karst caves, layered grouting ensures that the filling material is uniform and dense; for large karst caves, the formwork technology first forms a stable boundary, preventing the loss of filling material, which significantly improves the success rate of treating large cavities and the integrity of the final filling material.

[0074] In another technical solution, during the casing isolation process in step S4, after the karst cave isolation casing is placed and its upper and lower ends are solidified, temporary support grout is injected into the karst cave cavity outside the casing through the grouting channel pre-set on the casing and allowed to solidify to form a temporary stable body; the temporary support grout is made of the following components in the following weight ratio: 100 parts of sulfoaluminate cement, 2-5 parts of quick-setting agent, 0.5-2 parts of foaming agent, and 40-60 parts of water;

[0075] After the temporary stabilizer reaches the predetermined strength, drilling continues downward; in step S6, the temporary stabilizer is retained or replaced with a permanent filler according to engineering requirements.

[0076] In the above technical solution, during the casing isolation process in step S4, after the karst cave isolation casing is lowered to the designed depth, temporary support grout is injected first, followed by the final consolidation of the casing. The temporary support grout is injected through an independent grouting channel pre-installed on the casing. This grouting channel can be a small-diameter grouting pipe fixed axially along the outer wall of the casing, with its lower end opening at the height corresponding to the middle of the karst cave cavity section on the casing column, and its upper end extending to the ground surface and connecting to the grouting equipment. The pipe opening is equipped with an openable and closable valve. The grout is prepared on-site according to the following weight ratio: 100 parts of rapid-hardening sulfoaluminate cement (e.g., grade 42.5R), 3 parts of quick-setting agent (sodium aluminate can be selected), 1 part of foaming agent (hydrolyzed animal protein foaming agent can be selected), and 50 parts of clean water. During preparation, the foaming agent and water are first mixed in a specialized foaming device to generate uniform and stable foam. Then, cement and an accelerator are dry-mixed and added together with the foam to a high-speed mixer for thorough mixing, forming a fluid slurry with a density of approximately 1.3 g / cm³. After preparation, the slurry is immediately pumped into the cavity outside the casing using a grouting pump and the aforementioned grouting pipe. The grouting pressure can be controlled between 0.3 MPa and 0.8 MPa, and the grouting process continues until the pressure stabilizes and the grout volume reaches more than 80% of the estimated cavity volume.

[0077] The injected grout solidifies within the karst cavity, typically setting within a few hours and forming a "temporary stabilizer" with a certain supporting strength within 24 hours. This stabilizer encapsulates the casing traversing the karst section, providing temporary lateral restraint. Once its unconfined compressive strength reaches at least 1.0 MPa (which can be determined by testing with concurrently cured test blocks), permanent consolidation of the upper and lower ends of the casing can be performed. At this point, using conventional cementing techniques, permanent cementing grout is injected into the annulus outside the casing (i.e., the section above the top and below the bottom of the temporary stabilizer). The cement grout can be prepared using G-grade oil well cement. The permanent cement grout encapsulates and anchors the temporary stabilizer within the intact rock strata above and below, completing the final fixation of the casing. Afterward, proceed to step S5. In the final step S6, the treatment of the existing "temporary stabilizer" is determined based on the long-term performance requirements of the borehole structure: for boreholes with low requirements, it can be retained as part of the permanent filling structure; for projects with higher durability or impermeability requirements, in step S6, the temporary stabilizer can be broken and partially removed by lowering a high-pressure jet drill bit or a small mechanical crushing tool into the casing, and then the designed permanent filling material (such as micro-expansion cement grout) can be re-injected through the original grouting channel or other channels.

[0078] Before the casing is finally solidified, a special temporary support grout is injected to form a stable body. Compared with the direct process of "running casing-cementing-continuing drilling" in existing technologies, this step has the advantage of proactively and quickly stabilizing the most vulnerable link in the construction. In existing processes, the large unfilled cavity on the outer side of the casing after cementing is a risk point in subsequent long-term construction. This method significantly improves the overall rigidity and disturbance resistance of the casing string during construction by constructing a temporary support body tightly integrated with the casing at key nodes, which is particularly beneficial for dealing with multi-layered karst caves and complex formations. At the same time, the temporary grout has the characteristics of fast hardening and lightweight, which not only meets the time requirement of rapid support but also does not impose excessive additional loads on the casing and formation. In addition, the replaceable design of the temporary body provides process flexibility, allowing the same core method to adapt to different levels of engineering end requirements. This enhances the safety and adaptability of the entire construction method in complex karst formations.

[0079] In another technical solution, during the casing isolation process in step S4, when the cavity is an unfilled or partially filled cavity with a vertical height greater than 5 meters, an intelligent reaming system with rotary guiding function is used to perform reaming, specifically including the following steps:

[0080] S4a. Connect the rotary guide reamer to the lower end of the drill string and install a measurement while drilling unit near its end.

[0081] S4b. After the reaming begins, the drilling measurement unit measures the well inclination angle, azimuth angle and tool face angle of the reamer in real time and uploads them to the ground control system.

[0082] S4c, The ground control system compares the received real-time trajectory data with the designed reaming axis and calculates the trajectory deviation; with the designed axis as the reference, a virtual cylindrical allowable deviation channel is established; when the real-time measured position of the reamer deviates from the center line of the channel, the system calculates the tool face angle and guide force adjustment amount to make the reamer vector direction point to the center line of the channel according to the direction and distance of the deviation, and generates a correction command;

[0083] S4d. The correction command is transmitted to the rotary guide reamer via drilling fluid pulse signal, controlling its internal guide mechanism to generate a corresponding bias force, adjusting the drilling direction of the reamer, and returning it to the preset trajectory.

[0084] In the above technical solution, during the casing isolation process in step S4, when the exposed cavity is an unfilled or partially filled cavity with a vertical height greater than 5 meters, an intelligent reaming system with rotary steering function is used to perform reaming. The downhole part of this system mainly includes a rotary steerable reamer and a measurement-while-drilling (MWD) unit. The rotary steerable reamer can be a hydraulically driven downhole tool, which internally includes a reaming mechanism that can radially extend the cutting arm and a steering unit that receives surface signals and drives the biasing mechanism; the two are integrated into one unit. During tool string assembly, the rotary steerable reamer is connected to the lower end of the drill string. At its proximal end (usually located 5 to 10 meters above the reamer), it is connected in series to the MWD unit via a male-female connector. This unit can be a sensor sub that includes a triaxial accelerometer, a triaxial magnetometer, and a gyroscope, capable of measuring and calculating parameters such as well inclination angle, azimuth angle, and tool face angle in real time. The measurement data is encoded into drilling fluid pulse signals through its internal transmitter module and transmitted to the surface via the drilling fluid channel within the drill string.

[0085] After reaming begins, the measurement-while-drilling unit (MWD) acquires and uploads data several times per second in real time. The surface control system can be an industrial computer running dedicated drilling trajectory monitoring and guidance software. The software interface pre-inputs the "designed reaming axis," which is determined as follows: using the spatial distribution of the karst cave detected in step S1 and the actual trajectory of the drill bit traversing the cave in step S4 as references, a starting point is selected in the intact rock strata above the cave's top plate, and an ending point is selected in the intact rock strata below the cave's bottom plate. The straight line connecting these two points, or a gentle curve designed according to the strata's attitude, is the target axis. The software compares the received real-time trajectory data (three-dimensional coordinates converted from well depth, inclination, and azimuth) with this designed axis. Simultaneously, a virtual cylindrical "allowable deviation channel" is established in the software, centered on the designed axis. Its radius can be set according to the casing centering requirements, for example, 0.3 meters. When the real-time measured coordinates of the reamer's center point deviate from the centerline of this channel, the system automatically calculates correction parameters based on a geometric algorithm. For example, if the measuring point is offset 0.4 meters north of the design axis, the system calculates that the tool needs to be adjusted to face south at a specific angle, and instructs the guiding mechanism to generate a corresponding lateral force to the south. These parameters are encapsulated to generate a specific correction command.

[0086] The generated correction command is encoded into a specific pressure wave sequence by a drilling fluid pulse generator on the surface and superimposed on the normal drilling fluid circulation. This pressure wave signal is transmitted through the drilling fluid medium to the rotary reamer downhole. The reamer's internal guiding unit decodes this signal and drives its biasing mechanism (such as an independently controllable hydraulic pusher) to generate the calculated lateral force. This force acts on the drill string, changing the direction of the resultant force, thereby adjusting the reamer's actual drilling trajectory and gradually bringing it back within the allowable deviation range. This is a dynamic, closed-loop, continuous control process.

[0087] Compared to existing technologies that rely on conventional reamers in large cavities, intermittent drilling and inclination measurements, and repeated adjustments based on manual experience, this solution integrates closed-loop control with drilling-while-drilling measurement, real-time transmission, automatic calculation, and guided execution. This achieves continuous monitoring and active correction of the reaming trajectory. This effectively solves the technical challenges of trajectory loss of control and irregular borehole formation during reaming in cavities without lateral constraints. It significantly improves the axial accuracy and wellbore quality of the reamed section, laying a solid foundation for the successful one-time casing placement and ensuring its good centering. This enhances the reliability, success rate, and overall efficiency of construction in complex, large karst formations.

[0088] In another technical solution, when step S1 detects the existence of adjacent multi-layered karst caves with a vertical spacing less than a preset safety distance, a pre-reinforcement process is added in step S4: after drilling through and treating the upper karst cave, drilling continues until the safe grouting thickness is reserved at the top plate of the lower karst cave; high-pressure curtain grouting is performed on the top plate of the lower karst cave and the interlayer rock strata through the drill rod to form a columnar reinforcement zone; after the reinforcement zone reaches the predetermined strength, drilling continues and the lower karst cave is treated.

[0089] In the above technical solution, when the advance detection results (such as cross-hole CT or high-density electrical resistivity tomography) in step S1 indicate that there are two or more layers of karst caves on the designed drilling trajectory, and the vertical distance between adjacent karst caves (i.e., the thickness of the intact or relatively intact rock strata between the bottom plate of the upper karst cave and the top plate of the lower karst cave) is less than the preset safety distance, a pre-reinforcement process is added in step S4. This preset safety distance can be determined based on the integrity of the regional rock mass and construction experience; for example, it can be set to 3 meters. This means that if the detection shows that the thickness of the rock strata separating the upper and lower karst caves is less than 3 meters, it is considered that there is a risk of collapse and connection under subsequent drilling disturbance, and pre-treatment is required. In specific implementation, after drilling through and treating the upper karst cave according to the established process (casing isolation or local reinforcement), drilling does not immediately continue downward to expose the lower karst cave. Instead, the drill bit is controlled to continue drilling downward until the distance between the drill bit tip and the pre-determined elevation of the lower karst cave top plate is only one "grouting safety thickness". This safe grouting thickness is a buffer layer set up to prevent grout leakage or loss of control of construction caused by the drill bit accidentally penetrating the roof of the lower karst cave. Its thickness can be set according to the strength of the rock strata, for example, 1.5 meters to 2 meters. At this time, the drill bit stays in the rock strata between the upper and lower karst caves.

[0090] After the drill bit stops drilling, the drill string is removed, leaving the drill rod inside the hole as a grouting pipe. High-pressure curtain grouting is then performed through this drill rod to the top strata of the lower cave and the entire interlayer rock between the upper and lower caves. A high-pressure grouting pump can be used as the grouting equipment. Cement-water glass dual-liquid grout can be used as the grouting material to control the grout diffusion range and setting time. During grouting, a segmented lifting method is used to inject grout into the interlayer rock mass from bottom to top. The grouting pressure must exceed the hydrostatic pressure and rock fracture strength at that depth, for example, controlled within the range of 2.0 MPa to 4.0 MPa, to allow the grout to fracture and penetrate the fracture network in the rock strata. The goal of grouting is to form a "columnar reinforcement zone" with a certain radius surrounding the future drilling trajectory above the top strata of the lower cave and within the interlayer rock. The grouting process requires monitoring of the grouting pressure and volume, with the pressure reaching the design upper limit and stabilizing for a certain period as the end standard. After grouting is completed, curing is required. Subsequent work can only proceed after the reinforced area reaches the predetermined strength (which can be verified by core sampling or sonic testing later).

[0091] After the columnar reinforced zone reaches the predetermined strength, drilling continues, penetrating the reinforced interlayer rock mass and the top slab of the lower karst cave, and the lower karst cave is treated according to the rules in step S4. Compared with the closest existing technology, current construction methods typically treat each layer independently when encountering closely adjacent multi-layered karst caves, neglecting the risk that the thin interlayer rock slabs may become unstable and collapse under construction disturbance, leading to the connection of upper and lower karst caves and causing large-scale borehole accidents. This solution, by adding a pre-reinforcement process, proactively reinforces the weakest interlayer rock mass with high-pressure grouting, substantially improving its overall strength and stability, and transforming the construction risks of the two karst caves from "serial superposition" to "isolated treatment." This effectively prevents serious accidents such as stuck drill and buried drill due to the collapse of interlayer rock mass, ensuring the safety of construction personnel and equipment, and creating a more stable working environment for subsequent treatment of the lower karst cave, thereby improving the overall controllability and success rate of construction through complex multi-layered karst strata.

[0092] Example 1

[0093] This embodiment combines Figure 1 This paper provides a specific application example of a surface directional drilling method under geological conditions where a single large underground karst cave exists.

[0094] 1. Project Overview and Advanced Detection

[0095] The construction site has a topsoil layer beneath a layer of karst limestone. The designed borehole is a directional borehole, requiring vertical penetration through a suspected large underground cavern before entering the lower stable rock strata and completing directional drilling and horizontal sections. First, advanced geological exploration is conducted. This exploration utilizes a common-center frequency domain ground-penetrating radar system, primarily composed of a control unit, a 100 MHz shielded antenna, and a measuring wheel. The specific implementation steps are as follows: Using the center of the designed borehole opening as a reference point, a main survey line of at least 50 meters in length is laid out on the surface along the azimuth of the designed borehole trajectory. The antenna is connected to the control unit and the measuring wheel, and the operator moves the antenna along the survey line at a constant speed, while the measuring wheel synchronously records the mileage position. The radar control unit continuously transmits electromagnetic pulses within a specific time window (e.g., 200 nanoseconds) and receives reflected signals from the underground medium; signal strength and two-way travel time are recorded in real time.

[0096] After on-site detection, the collected raw data was transmitted to a computer and analyzed using the accompanying radar data processing software. The processing flow included: first, time zero-point correction and removal of direct wave interference; then, gain adjustment to enhance the deep signal and bandpass filtering to suppress high-frequency noise and low-frequency drift. Geological interpretation of the processed radar profile revealed a typical hyperbolic diffraction wave phase axis observed at depths of approximately 12 to 18 meters. Below this axis, areas exhibited chaotic signal reflections, significant energy attenuation, and even blank (no reflection) regions. This characteristic indicates the presence of an interface with significantly different electrical properties from the surrounding rock and a cavity or loose filling beneath it. By measuring the two-way travel time of the reflected waves from the top and bottom plates of this anomalous area, and based on the previously calibrated electromagnetic wave velocity of the site limestone (0.12 m / ns), the top burial depth was calculated to be approximately 12 meters, the bottom burial depth approximately 18 meters, and the vertical height approximately 6 meters. Based on its reflection characteristics (signal attenuation or interruption at strong reflection interfaces) and regional geological knowledge, the anomaly was interpreted as a single large karst cave, designated D1, and its filling condition was preliminarily determined to be poor (unfilled or partially filled). All interpreted location, depth, height, and characteristic information were recorded in detail and marked on the borehole trajectory profile as a key basis for subsequent design.

[0097] 2. Casing sequence design and surface construction

[0098] Based on the detection of a large karst cave (D1) and according to the engineering requirements for the final borehole diameter (133 mm), a five-stage casing sequence was designed. This sequence is divided into a risk response section and a final borehole construction section.

[0099] Risk Response Section: Primary Casing 4 (Surface Casing): Outer diameter 377 mm, wall thickness 9 mm, steel grade J55. Designed to stabilize the borehole opening and loose topsoil layer, with a designed depth to the stable bedrock surface (approximately 10 meters). Secondary Casing 5 (Cave Isolation Casing): Outer diameter 325 mm, wall thickness 9 mm, steel grade N80. Designed specifically to isolate and traverse the D1 cave, it is a pre-designed cave isolation casing.

[0100] Final borehole construction section: Third-stage casing 6 (first-stage technical casing): outer diameter 245 mm, wall thickness 9 mm, steel grade N80. Designed to be lowered after the karst cave treatment is completed, used to stabilize the borehole wall of the upper straight borehole section, with a designed depth of approximately 40 meters. Fourth-stage casing 7 (second-stage technical casing): outer diameter 177.8 mm, wall thickness 8 mm, steel grade P110. Used to construct the borehole wall of the inclined section. Fifth-stage casing 8 (production casing): outer diameter 133 mm, wall thickness 7 mm, steel grade P110. The final stage casing, forming the final borehole.

[0101] After construction begins, step S3 is performed first. Using an XY-4000 drilling rig equipped with a Φ445 mm drill bit, a vertical borehole is drilled to a depth of 10 meters into stable bedrock, forming surface borehole 1. Subsequently, the first-stage casing 4 is lowered to this depth, and the entire well is cemented using PO 42.5 cement grout with a water-cement ratio of 0.9:1 through the casing string grouting system, followed by 72 hours of curing.

[0102] 3. Casing isolation treatment for large karst caves

[0103] Inside casing 4, drilling continued using a Φ216 mm drill bit (corresponding to step S4). When drilling reached approximately 12 meters, the drilling rate suddenly increased dramatically, resulting in significant drilling fluid loss and minimal cuttings return. Based on the drilling-while-drilling data, it was confirmed that the roof of the D1 karst cave had been exposed. According to the decision-making rules, for this large, unfilled / partially filled open cavity, a casing isolation procedure was executed.

[0104] Maintain drilling parameters by controlling the drilling rig to forcefully penetrate the entire cavern space and continue drilling 8 meters (approximately 26 meters) into the intact, hard limestone layer below the cavern floor to ensure a sufficiently stable section. After hoisting the drill string, use a Φ350 mm reamer to enlarge the borehole section (approximately 12 to 26 meters) encompassing the entire cavern to create a borehole diameter capable of accommodating the secondary casing 5.

[0105] After the borehole was enlarged, the secondary casing 5, serving as an isolation casing for the karst cave, was lowered. The lower end of this casing rests in intact bedrock at a depth of 26 meters, while the upper end extends to a depth of 10 meters (located inside the primary casing). Using the cementing device on the secondary casing, quick-setting cement grout was injected into the annulus at both its lower end (approximately 24-26 meters) and upper end (approximately 10-12 meters) for sealing, thereby anchoring both ends of the casing in stable rock strata. This made the casing itself a supporting structure, completely isolating the karst cave 2 cavity from the borehole. Before lowering the secondary casing 5, a grout stop ring was pre-installed on its outer wall at the location corresponding to the karst cave section.

[0106] 4. Subsequent final hole construction

[0107] Inside the secondary casing 5, switch to a Φ216 mm drill bit to continue drilling downwards. After confirming that there are no other karst caves at a depth of 40 meters, enlarge the hole and lower the tertiary casing 6 to a depth of 40 meters, and cement the well (corresponding to the construction after entering the complete rock strata in step S5).

[0108] Subsequently, directional drilling was carried out according to the designed trajectory: A Φ216 mm drill bit with a curved casing screw drill string was used within the third-stage casing 6 for directional drilling to the designed depth of 60 meters (end of the directional drilling section). After hoisting the drill string, the fourth-stage casing 7 was lowered to this depth and cemented throughout. Next, a Φ152 mm drill bit was used within the fourth-stage casing 7 for horizontal drilling to the target depth of 80 meters. Finally, the fifth-stage casing 8 was lowered to the end and cemented, forming the final borehole.

[0109] 5. Subsequent filling of karst cavities

[0110] After all drilling and casing work is completed, proceed to step S6. Through the grouting holes pre-drilled on the wall of secondary casing 5, inject filling material into the annular cavity between its outer side and the wall of cave 2. Given that the vertical height of cave D1 is greater than 5 meters, this example adopts a combination of formwork bag sealing and compensating grouting: First, a formwork bag with an internal grouting pipe is lowered to the lower part of the cavity, and cement mortar is injected to cause the formwork bag to expand and initially seal the lower space; then, low-shrinkage cement mortar is injected into the cavity above the formwork bag through other grouting pipes until the entire cavity is filled, forming grouting zone 3.

[0111] 6. Effects

[0112] This embodiment of the method successfully and safely traversed a single large underground karst cave. During construction, a dedicated karst cave isolation casing (secondary casing) was lowered to effectively isolate the unstable large cavity from the drilling work space, ensuring the safety of subsequent drilling operations and the stability of the borehole wall. The final borehole maintained the design-required final diameter, meeting the engineering requirements. Compared to the process of attempting to completely fill the large cavity with grouting before drilling, this method significantly shortened the construction period for karst cave treatment; compared to the conservative approach of blindly using small-diameter casing throughout the hole, this method, while ensuring safety, maximized the final borehole size, demonstrating the economy and reliability of dynamic design and precise implementation.

[0113] Example 2

[0114] A horizontal directional drilling project was being carried out in a certain location, requiring the laying of a 219 mm diameter pipe. The designed borehole trajectory needed to traverse a limestone stratum known to have developed karst caves. This embodiment utilizes the method of the present invention for construction.

[0115] First, step S1 was performed, using ground-based ground-penetrating radar and pre-drilling cross-hole electromagnetic CT for comprehensive exploration along the designed trajectory. The exploration results indicated the existence of three layers of karst caves within the predetermined depth range: the first layer, located at a depth of 15-16.5 meters, is a fully filled cave with a vertical height of 1.5 meters, filled with dense clay mixed with gravel; the second layer, located at a depth of 28-34 meters, is a large, unfilled cavity with a vertical height of 6.0 meters; and the third layer, located at a depth of 38-39 meters, is a fully filled cave with a vertical height of 1.0 meter. Simultaneously, the exploration revealed that the rock strata thickness between the second and third layers of caves was only 2.8 meters, less than the predetermined safety distance of 3 meters.

[0116] Based on the detection results of step S1 and the final borehole diameter requirements, the casing sequence design for step S2 is carried out. A casing string with a progressively decreasing diameter from the surface to the bottom of the borehole is designed. The risk response section includes: a first-stage Φ508 mm surface casing (reaching a depth of 5 meters below the stable clay layer), and three pre-designed karst cave isolation casings (corresponding to three karst caves, with specifications of Φ406 mm, Φ325 mm, and Φ273 mm respectively). The final borehole construction section includes: a first-stage Φ244.5 mm technical casing and a final-stage Φ219 mm casing.

[0117] Construction begins. Step S3: A rotary drilling rig is used to create a borehole, drilling to a depth of 20 meters to reach a stable rock stratum. A Φ508 mm surface casing is then lowered and secured. Step S4: Inside the casing, a Φ406 mm drill bit is used to continue drilling. At a depth of approximately 15.5 meters, the first layer of karst cave is exposed. Based on real-time analysis of the rock cuttings (dense clay) and drilling time, it is determined to be fully filled, with a vertical height of 1.5 meters (less than the first preset threshold of 2.5 meters), and the filling material is not a soft or easily lost medium. According to regulations, a local reinforcement procedure is implemented for this karst cave. Cement grout with a water-cement ratio of 0.8:1 is injected through the drill pipe at a pressure of 0.8 MPa and a volume of 3.5 cubic meters. After curing for 24 hours, drilling continues.

[0118] When drilling reached a depth of approximately 28 meters, a second large, unfilled karst cave was revealed. Following protocol, a casing isolation procedure was executed, and an intelligent reaming system was activated. First, the drill bit forcefully penetrated the cavity, drilling 7 meters into the intact bedrock beneath its base. Then, the drill bit was withdrawn, and a rotary reamer and a measurement-while-drilling (MWD) unit were connected to perform reaming operations on the section containing the 6-meter-high karst cave. The ground control system used the designed straight line as its axis, with a 0.3-meter allowable deviation channel. Real-time monitoring and automatic correction were implemented, ultimately completing the reaming from Φ406 mm to Φ508 mm, with trajectory deviation controlled within 0.2 meters. After reaming, a Φ406 mm karst cave isolation casing was lowered. Two grout-stopping rings were pre-welded to the outside of the casing at the karst cave section location. Through the grouting sub on the casing, 12 cubic meters of temporary support grout (100 parts sulfoaluminate cement, 4 parts quick-setting agent, 1 part foaming agent, and 50 parts water) were injected according to the specified ratio to form a temporary stabilizer. Once the strength reaches 1.2 MPa, the upper and lower ends of the casing will be permanently cemented.

[0119] Because the distance between the second and third karst cave layers is only 2.8 meters, the pre-reinforcement process was initiated. Drilling continued to a depth of 36.2 meters (leaving a 1.8-meter safety grouting thickness from the top of the third karst cave layer) and then stopped. High-pressure curtain grouting was performed through the drill rod, using cement-water glass dual-liquid grout at a pressure of 3.0-3.5 MPa, injecting 8 cubic meters of grout to form a columnar reinforcement zone. After curing for 48 hours and testing to ensure the strength was satisfactory, drilling continued.

[0120] At a depth of approximately 38.5 meters, a third layer of karst cave was discovered, identified as a fully filled karst cave with a vertical height of 1.0 meter. Since the pre-designed Level 3 karst cave isolation casing in the risk response section had already used Level 1 (for the second karst cave), and given the small size and stable filling material of this cave, a local reinforcement process was again adopted according to regulations. At this point, the pre-designated complete rock stratum depth (45 meters) had been reached, completing step S4.

[0121] Step S5: In the intact rock strata below 45 meters, a directional drilling tool is used to drill directionally to the target point according to the designed trajectory, and then a Φ244.5 mm technical casing and a Φ219 mm final stage casing are sequentially installed and consolidated to form the final hole.

[0122] Step S6: Final filling. For the first layer of karst caves (locally reinforced), no additional filling is required. For the second layer of large karst caves, a differentiated filling scheme is adopted. Due to its vertical height exceeding 5 meters, a combination of formwork bag sealing and compensating grouting is used. High-strength polyester formwork bags are lowered through pre-placed channels to the bottom of the outer cavity of the casing. Quick-setting grout is injected to expand and seal the lower space. Then, low-pressure compensating grouting is performed on the upper cavity. The filling material is M20 cement mortar. For the third layer of karst caves, since it has already been locally reinforced, filling is complete.

[0123] This embodiment successfully traversed complex karst formations containing multiple layers of caves of varying sizes, filling states, and close proximity. Compared to the traditional full-casing follow-up method, it saved approximately two levels of casing material costs; compared to blind full-hole grouting, it improved efficiency and ensured treatment results; no accidents such as grout leakage or borehole collapse occurred during the entire construction process, verifying the comprehensive advantages of this method in ensuring safety, controlling costs, and adapting to geological uncertainties.

[0124] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and embodiments shown and described herein.

Claims

1. A method for directional drilling through underground karst caves, characterized in that, Includes the following steps: S1. Conduct exploration along the designed borehole trajectory to identify and record the distribution, size, and number of layers of the karst caves; S2. Based on the detection results of S1 and the required final borehole inner diameter for the project, design a casing sequence with a progressively decreasing diameter from the surface to the bottom of the borehole. The sequence is divided into two segments: a) Risk response segment: including at least one level of surface casing and several levels of cave isolation casing; b) Borehole construction segment: including at least one level of technical casing and the final level casing; wherein, the preset number of levels of the cave isolation casing is not less than the number of cave layers detected in S1. S3. Drill a hole to the stable topsoil layer on the ground surface, and lower and solidify the topsoil casing. S4. Drill downwards within the solidified casing. If a karst cavern is exposed in real time, proceed according to the following rules: 1) If the karst cavern is an open cavity that is unfilled or partially filled, execute the casing isolation procedure; 2) If the karst cavern is fully filled and its vertical height is greater than or equal to the first preset threshold, or the filling material is a weak or easily leaking medium, execute the casing isolation procedure; 3) If the karst cavern is fully filled and its vertical height is less than the first preset threshold, and the filling material is not a weak or easily leaking medium, execute the local reinforcement procedure. The casing isolation process is as follows: control the drill bit to penetrate the karst cave and drill into the bedrock below its bottom plate to a predetermined depth, enlarge the hole section containing the karst cave, and from the risk response section, lower the first-level karst cave isolation casing and consolidate its upper and lower ends; S5. Repeat step S4 until drilling reaches the preset depth of the complete rock stratum. Then, continue drilling within the complete rock stratum according to the designed trajectory and borehole diameter, and sequentially lower and consolidate the various levels of technical casing and the final stage casing of the final borehole construction section to finally form the final borehole. S6. Fill the cavity outside the casing of the karst cave that is traversed through the casing isolation process; In the casing isolation process described in step S4, when the cavity is an unfilled or partially filled cavity with a vertical height greater than 5 meters, an intelligent reaming system with rotary guiding function is used to perform reaming, specifically including the following steps: S4a. Connect the rotary guide reamer to the lower end of the drill string and install a measurement while drilling unit near its end. S4b. After the reaming begins, the drilling measurement unit measures the well inclination angle, azimuth angle and tool face angle of the reamer in real time and uploads them to the ground control system. S4c, The ground control system compares the received real-time trajectory data with the designed reaming axis and calculates the trajectory deviation; with the designed axis as the reference, a virtual cylindrical allowable deviation channel is established; when the real-time measured position of the reamer deviates from the center line of the channel, the system calculates the tool face angle and guide force adjustment amount to make the reamer vector direction point to the center line of the channel according to the direction and distance of the deviation, and generates a correction command; S4d. The correction command is transmitted to the rotary guide reamer via drilling fluid pulse signal, controlling its internal guide mechanism to generate a corresponding bias force, adjusting the drilling direction of the reamer, and returning it to the preset trajectory.

2. The surface directional drilling method for penetrating underground karst caves as described in claim 1, characterized in that, In step S4, if the number of caves exposed in real time exceeds the preset number of cave isolation sleeve levels in the risk response section, the caves exceeding the preset number of levels will be treated using a local reinforcement process.

3. The surface directional drilling method for penetrating underground karst caves as described in claim 1, characterized in that, The first preset threshold is 2 meters to 3 meters.

4. The surface directional drilling method for penetrating underground karst caves as described in claim 1, characterized in that, The karst isolation casing lowered in the casing isolation process has a grout-stopping ring or is wrapped with permeable geotextile on its outer side at the corresponding karst section.

5. The surface directional drilling method for penetrating underground karst caves as described in claim 1, characterized in that, The drill bit penetrates to a depth of no less than 5 meters into the intact bedrock beneath the karst cave floor.

6. The surface directional drilling method for penetrating underground karst caves as described in claim 1, characterized in that, In step S6, a differentiated filling scheme is adopted according to the vertical height of the karst cave, as follows: For karst caves with a vertical height of less than 2 meters, backfill with a mixture of rubble and clay; For karst caves with a vertical height of 2 to 5 meters, backfill in layers and inject cement mortar; For karst caves with a vertical height greater than 5 meters, geotextile bags are used for sealing, combined with compensating grouting.

7. The surface directional drilling method for penetrating underground karst caves as described in claim 1, characterized in that, In the casing isolation process in step S4, after the karst cave isolation casing is placed and its upper and lower ends are solidified, temporary support grout is injected into the karst cave cavity outside the casing through the grouting channel pre-set on the casing and solidified to form a temporary stable body; the temporary support grout is made of the following components in the following weight ratio: 100 parts of sulfoaluminate cement, 2-5 parts of quick-setting agent, 0.5-2 parts of foaming agent, and 40-60 parts of water; After the temporary stabilizer reaches the predetermined strength, drilling continues downward; in step S6, the temporary stabilizer is retained or replaced with a permanent filler according to engineering requirements.

8. The surface directional drilling method for penetrating underground karst caves as described in claim 1, characterized in that, When step S1 detects the existence of adjacent multi-layered karst caves with a vertical spacing less than the preset safety distance, a pre-reinforcement process is added in step S4: after drilling through and treating the upper karst cave, drilling continues until the safe grouting thickness is reserved at the top plate of the lower karst cave; high-pressure curtain grouting is performed on the top plate of the lower karst cave and the interlayer rock strata through the drill rod to form a columnar reinforcement zone; after the reinforcement zone reaches the predetermined strength, drilling continues and the lower karst cave is treated.

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