3. On Site Application Situation

3.1 Application Cases of Oil-based Sedimentation Plugging Technology

In 2023, DB1302 well experienced wellbore leakage and backflow when drilling with a density of 2.35g/cm³ using a four hole drilling fluid to 5214m. The monitoring showed that the annular liquid level was 1200m and the liquid column pressure decreased by 27.6 MPa. It was determined on site that the high-pressure salt paste layer was connected to the low-pressure target layer at the bottom of the well, and the fracturing leakage was caused by excessive pressure in the upper part.To isolate the high-pressure salt paste layer from the low-pressure target layer and ensure the safety of intermediate operations, it has been decided to adopt oil-based drilling fluid settling plugging technology, and conduct indoor settling plug height evaluation experiments on the drilling fluid on site.The drilling fluid and base fluid properties used in DB1302 well are shown in Table 7, and the results of the indoor settling plug height evaluation experiment are shown in Figure 5. From Figure 5, it can be seen that the sealing slurry configured on site remains stationary for 5 minutes, and all sealing materials have settled. The sealing material of the oil-based sealing slurry configured settles quickly.

Note: 1. The plugging formula is base slurry+3% SQD-98 (fine)+3% SQD-98 (medium)+10% rigidity -2+20% rigidity -3+20% rigidity -4+5% resin -3+5% resin -4+20% JXD-4 (coarse); 2. The settlement height is measured after 24 hours of static aging at 100℃.

During the on-site implementation process, in order to avoid the blockage of the upper water pipeline and water hole by large particles and high concentration of leak sealing materials, and to prevent the occurrence of high viscosity and cutting of oil-based drilling fluid, which leads to low pumping efficiency, the mud pump and upper water pipeline were modified before the leak sealing construction operation (the filter screen and foam rubber matched with the mud pump were removed) to improve pumping efficiency, and a 44mm single water hole drill bit was used to prevent water hole blockage.After the leak sealing construction, the height evaluation of the settlement plug in the large sample is shown in Table 8. It can be seen that the oil-based plugging slurry provided has a hard settling plug and good quality.At the same time, the on-site working conditions reflected that when drilling down to 5171 meters, there was a 5t obstruction, forming a 42m settling plug at the bottom of the well. With drilling pressure, there was no leakage during the circulation of 5210m high displacement, indicating successful settling and plugging.The DB1302 well plugging operation achieved a significant effect of bottom hole pressure of 30MPa, providing safety assurance for subsequent intermediate operations.

Note: The settlement height is measured after 24 hours of static aging at 100℃.

3.2 Application Cases of Water-based Sedimentation Plugging Technology

In 2023, when drilling into the fourth open salt layer of BZ107 well using 2.35 g/cm³ saturated saline based drilling fluid to 7038m, a wellbore leak occurred, causing the liquid level to instantly drop to 1082m and the liquid column pressure to decrease by 24.9MPa.It was also determined on site that the high-pressure salt paste layer was connected to the low-pressure target layer at the bottom of the well, and the fracturing leakage was caused by excessive upper pressure.In order to isolate the high-pressure salt paste layer from the low-pressure target layer and ensure the safety of intermediate operations, it has been decided to carry out water-based drilling fluid static plugging technology on site.The drilling fluid and plugging slurry properties of BZ107 well are shown in Table 9. It can be seen that the Markovnikov funnel viscosity of the plugging slurry reaches 178s, with a dynamic shear force of 15Pa and initial and final static shear forces of 7Pa and 21Pa, respectively. The prepared plugging slurry meets the condition of being able to float on the ground.

Note: 1. The base slurry formula is well slurry+40% gel solution+0.2% XC; 2. The formula for settling plug is base slurry+3% SQD-98 (fine)+3% SQD-98 (medium)+10% rigidity -2+30% rigidity -3+10% rigidity -4+5% resin -3+5% resin -4.

After the leak sealing construction, the evaluation results of the settlement plug sample are shown in Table 10. It can be seen that the settlement plug is hard and sinking, and the quality is good. When drilling down to 6943m, a blockage of 2 tons was found on the plug surface. The pump was turned on to circulate and sweep the settling plug to 7013m below the artificial well bottom. After retaining 25m of settling plug, the high displacement circulation did not leak. There was no leakage during the later casing and cementing operations.

 Note: The settlement height is measured after 24 hours of static aging at 140℃.

4. Conclusions

1) The fundamental reason for the occurrence of leakage in the salt bottom well in front of the Tarim Mountains is the malignant leakage caused by the drilling connection between the high-pressure salt paste layer and the low-pressure target layer. Conventional bridging plugging is difficult to solve the leakage problem, and settlement plugging technology is needed to form a settlement plug at the bottom of the well to seal the leakage layer.

2) The rigid sealing agent and synthetic resin sealing agent used for settling plugs have good compressive strength, with a crushing rate of less than 10% at 30 MPa. These materials are preferred for settling plugs.

3) The settling plug velocity and quality of oil-based drilling fluid are optimal when the oil-water ratio is 50:50 and the demulsification voltage is 200-300V; Water based drilling fluid+0.2% XC+plugging agent+0.2% demulsifier can achieve optimal settling speed and quality of water-based plugs.

4) This technology has been successfully applied in wells such as BZ107 and DB1302 in front of Kuche Mountain, where DB1302 has a pressure of up to 30 MPa, making it a safe and efficient disposal method to solve the problem of salt bottom leakage