Introduction Shale gas, as an important unconventional energy source, is of great significance for energy supply in terms of the development of its extraction technology. In shale gas extraction, the work of drilling plugs after drilling in severely cased - deformed horizontal wells is both crucial and challenging. Factors such as casing deformation make traditional drilling and milling methods face problems such as difficulty in cuttings return and increased risk of sticking, which seriously affect the operation efficiency and safety. Therefore, in - depth research on the technology for drilling and milling bridge plugs in severely cased - deformed horizontal shale gas wells and its field application has important practical significance.
Research Background When drilling plugs after fracturing in severely cased - deformed horizontal shale gas wells, if the milling shoe size is less than 75% of the inner diameter of the casing, the size of cuttings will increase. At the same level of annular flow velocity, it is more difficult for cuttings to return. Especially at the deformation point, cuttings are likely to accumulate due to the change of flow pattern, and then lead to sticking. Meanwhile, factors such as the length of the horizontal section, the deviation angle of the well, the cleanliness of the wellbore, and the perforation holes will increase the frictional force in the wellbore and the difficulty of running the coiled tubing to the artificial bottom of the well, bringing great difficulties and risks to the operation of drilling and milling bridge plugs.
Research Ideas and Methods Based on the positive - displacement motor theory and the actual working characteristic model, the parameter control technology for drilling and milling bridge plugs in severely cased - deformed horizontal wells is studied. The influence of coiled tubing diameter and wall thickness on the buckling load in the horizontal section is analyzed, and technical requirements such as the selection of drilling plug timing, the extension of running - in in the horizontal section, the control of drilling plug parameters, and real - time on - site analysis are discussed. In view of the casing deformation during the fracturing process, a strategy of giving priority to drilling and milling bridge plugs below the deformation point is proposed to prevent the casing deformation from intensifying, which may lead to increased drilling and milling difficulties or inability to drill and mill, and affect the subsequent production.
Research Results Importance of Wellbore Caliper Testing The vast majority of casing deformations occur during the fracturing stimulation process. If the bridge - perforation combined string pumping is blocked and casing deformation is suspected, wellbore caliper testing should be carried out first to confirm the degree of casing deformation and provide a basis for the selection of milling shoe size. At the same time, the bridge plugs below the deformation point should be drilled and milled immediately. If fracturing continues, the casing deformation will intensify, and it will be impossible to deal with it later. The cuttings after being drilled and milled by a small - size milling shoe are larger, and the rubber element is not easy to be drilled and broken, which is likely to cause the risk of sticking. Influence of Coiled Tubing Parameters The larger the outer diameter and the thicker the wall thickness of the coiled tubing work string are, the larger the critical buckling load values of sine and helix are. When the outer diameter and wall thickness of the coiled tubing are the same, the larger the inner diameter of the wellbore is, the smaller the critical buckling load value is. In order to improve the treatment efficiency and operation safety of cased - deformed wells, coiled tubing with a large outer diameter, thick wall thickness, and high steel grade should be selected as far as possible. This can not only increase the running - in depth in the horizontal section, but also the larger the outer diameter is, the smaller the friction resistance is and the faster the annular flow velocity is at the same displacement, which is beneficial to the cuttings return; the thicker the wall thickness and the higher the steel grade are, the stronger the lifting capacity is in case of abnormal and complex situations. At the same time, the use of metal drag - reducing agents and hydraulic oscillators as auxiliaries can reduce the frictional force between metals and increase the running - in depth in the horizontal section. Working Characteristics of Screw Motor The screw motor is a typical positive - displacement drilling tool. Without considering losses, according to the principle of conservation of energy, parameters such as WOB and displacement have a great influence on its working characteristics. When the WOB increases, the resistance torque of the milling shoe increases, the pressure drop of the motor increases, and the rotational speed decreases; when the pressure drop reaches the critical value, the rotational speed of the milling shoe is zero, and braking occurs. The braking condition is harmful to the tool and is likely to cause cuttings to settle, increasing the risk of sticking. In cased - deformed wells, the WOB should be reduced as much as possible, and the speed of running - in and - out of the coiled tubing through the deformation section should not be too fast. At the same time, the pump injection displacement should be increased as much as possible within the allowable range to increase the annular return velocity. On - site Construction Plan For drilling and milling bridge plugs in severely cased - deformed wells, a targeted plan should be formulated according to the actual situation of the well and the degree of deformation, achieving "one - well - one - strategy". At the same time, the construction plan should be adjusted in real - time on - site according to the cuttings return situation. Low - viscosity slick water is suitable for the horizontal section, and high - viscosity liquid is suitable for the vertical section, and the short - trip frequency should be increased; for soluble cuttings, a cosolvent should be pumped to dissolve the cuttings and reduce the risk of sticking; the construction frequencies of strong - magnetic fishing and high - displacement well flushing should be increased, and special fishing tools can be customized for other cuttings to improve the treatment efficiency.
Field Application Case In the field application of drilling and milling bridge plugs in a severely cased - deformed horizontal shale gas well, wellbore caliper testing was carried out first to determine the degree of casing deformation. According to the test results, a milling shoe of an appropriate size was selected, and coiled tubing with a large outer diameter and thick wall thickness was used. During drilling and milling, the WOB and the running - in and - out speed of the tubing were strictly controlled, and at the same time, the pump injection displacement was increased to increase the annular return velocity. By real - time monitoring the cuttings return situation, the construction plan was adjusted in a timely manner, and the frequencies of strong - magnetic fishing and high - displacement well flushing were increased. Eventually, the operation of drilling and milling bridge plugs was successfully completed, improving the operation efficiency and safety and laying a good foundation for the subsequent production.
Conclusions Through the research and field application of the technology for drilling and milling bridge plugs in severely cased - deformed horizontal shale gas wells, we are fully aware of the importance and complexity of this technology. In practical applications, various factors should be fully considered, and a scientific and reasonable construction plan should be formulated to achieve "one - well - one - strategy". At the same time, close attention should be paid to the parameter changes during the drilling and milling process, and the construction strategy should be adjusted in a timely manner to ensure the safe and efficient progress of the operation. In the future, with the progress of technology, the technology for drilling and milling bridge plugs in severely cased - deformed horizontal shale gas wells will be continuously improved, providing more reliable technical support for shale gas extraction.