High pressure salt gypsum bed and low pressure clastic rock target bed are widely developed in the piedmont of Tarim Basin. The pressure difference between the two sets of formation systems is higher than 20MPa. During the drilling process, because of the inaccuracy of geological layer clamping, it is very easy to drill through the salt gypsum bed and enter the target bed, and then a vicious lost circulation accident occurs.To solve the problem of malignant well leakage in the salt bottom of Tarim Mountain, based on the known mechanism of malignant leakage caused by drilling through high and low pressure differential formations and the principle of "bottle plug" plugging, the optimal conditions for the formation (speed and quality) of sedimentation plugging plugs in oil-based drilling fluid and water-based drilling fluid were screened by evaluating the pressure bearing strength and settling velocity of plugging materials at high temperatures. Finally, a set of high-density, high pressure differential oil-based and water-based drilling fluid specific sedimentation plugging technology was developed.Among them, when the oil-water ratio of oil-based drilling fluid is 50:50, the demulsification voltage (ES) is in the range of 200-300V, and 0.2% XC, 0.2% demulsifier, and plugging agent are added to water-based drilling fluid, the sedimentation plug formation effect is the best. The sedimentation plugging technology has been successfully tested in three wells in front of the Tarim Mountains, among which the pressure bearing capacity of the plugging plug in DB1302 well reached 30MPa, indicating that the salt bottom malignant leakage plugging technology has achieved significant application effects.

Deep, ultra deep, and super ultra deep layers have become the main battlefield for major oil and gas discoveries in China, with a resource volume of 671×10⁸t oil equivalent, accounting for 34% of China's total oil and gas resources, of which more than two-thirds are distributed in basins such as Tarim, Sichuan, and Junggar.China Petroleum's deep, ultra deep, and super ultra deep natural gas production accounts for one-third of the total domestic production, with an average yield of over 30%, which is 2.3 times that of medium and shallow gas fields. Currently, the exploration rate is only 13%.Taking Tarim Oilfield as an example, the production of oil and gas equivalent exceeding 1×10⁶t per day and buried within 6000m accounts for over 52% of the total.A single ultra deep gas well can achieve a daily production capacity of 1×10⁶m³, equivalent to the output of dozens or even hundreds of shallow gas wells. Ultra deep and super ultra deep layers are important growth poles for oil and gas fields to increase reserves and production. Advancing into the deep parts of the Earth is a strategic technological problem that we must solve.The exploration and development of ultra deep and super ultra deep oil and gas resources is currently an important component of deep Earth exploration. Accelerating the exploration and development of ultra deep and super ultra deep oil and gas resources has become an inevitable choice to ensure national energy security.

The salt gypsum layer in front of the Tarim Oilfield is generally buried at a depth of ≥ 5000 m and a thickness of ≥ 500 m. At the same time, it is affected by structural geological stress, resulting in a high pressure coefficient of the salt gypsum layer (generally above 2.0, up to 2.7).Due to the fact that the target oil and gas reservoir in this area is a sandstone reservoir, the formation pressure coefficient is relatively low (generally below 2.0).During the actual drilling process, the bottom mudstone (barrier layer) between the high-pressure salt paste layer and the low-pressure sandstone reservoir is very thin. When the geological blockage is not timely, the low-pressure target layer is often uncovered in advance, resulting in malignant wellbore leakage in an instant. If the drilling is not started in time, it is easy to cause complex faults such as blockage and lateral drilling. In order to restore the wellbore fluid column pressure in the future, safe and efficient plugging operations need to be carried out.For example, when the DB1302 well experiences severe salt bottom leakage, the drilling fluid level with a density of 2.35g/cm³ in the wellbore instantly drops to 1300 m (pressure difference of 29.97 MPa), which poses a great challenge to the plugging construction operation.In the past two years, more than 5 wells in Tarim Oilfield have been stuck or sidetracked due to this type of well leakage, resulting in economic losses exceeding 50 million yuan. At the same time, secondary complex events underground often occur due to plugging construction, such as poor cementing quality that prevents the establishment of circulation.Therefore, in order to address the problem of malignant salt bottom well leakage in front of the Tarim Mountains, it is necessary to conduct in-depth technical research to form safe and efficient plugging technology countermeasures.

1. Research on the Characteristics of Malignant Leakage in Salt Bottom Wells and Analysis of Countermeasures

The fundamental reason for the occurrence of malignant leakage in salt bottom wells is that the high and low pressure difference (≥20 MPa) formations are drilled through and connected, resulting in malignant leakage.The drilling fluid density required for drilling high-pressure salt paste layers in front of the Tarim Mountains is generally ≥2.2g/cm³, while the drilling fluid density required for drilling sandstone oil and gas target layers is ≤ 2.0g/cm³.For example, the maximum pressure difference between the high-pressure salt paste layer and the sandstone target layer in DB1302 well reaches 35 MPa. The high pressure difference causes rapid fracturing loss and wellbore leakage in the low-pressure layer of the sandstone target layer. This type of leakage has the following characteristics.

(1)The fracturing leakage caused by high and low pressure layers (pressure difference ≥ 20MPa) can result in cracks that extend deep into the low-pressure target layer, and the crack opening can be very large, even reaching the diameter of the drill bit.

(2) The crack extends towards the lower low-pressure target layer, in the same direction as the minimum ground stress, with a large leakage channel and space. The maximum leakage of a single well exceeds 5000m³.

(3) Conventional bridging and plugging methods are difficult to solve such leakage problems. The success rate of bridge plugging for hydraulic fracturing induced lost circulation in the salt bottom of the Tarim Mountains is basically zero, and there may be situations where all the plugging bridge slurry enters the low-pressure leakage layer of the salt bottom. The plugging material cannot effectively seal the leakage layer, and the difficulty of plugging is high. The actual on-site construction process has also verified this.

(4) Unlike conventional wellbore leakage, the direction of malignant leakage at the salt floor in front of the mountain is towards the low-pressure target layer downwards. Conventional wellbore leakage is mainly caused by drilling into low-pressure layers, weak layers, cracks, etc. The length, width, and number of cracks are generally fixed and can be solved through conventional plugging measures.

In response to the basic determination of the "length, width, and quantity" of leakage cracks in weak layers of conventional wellbore walls, the "stress cage" filling theory can be applied, and conventional bridge plugging methods can be used for plugging operations; Regarding the fracturing fracture leakage caused by the low pressure target layer in the salt bottom, according to the analysis of its characteristics mentioned above, it can be concluded that due to the uncertainty of the "length, width, and quantity" of cracks, a high-strength and high-density sealing agent can be used based on the "bottle stopper" sealing principle to form a high-strength settling sealing layer on the fracture surface at the bottom of the well, that is, to form a certain height of artificial settling at the bottom of the well, effectively separating the high and low pressure layers and achieving the purpose of sealing.After successful plugging, the wellbore can be restored to normal circulation. According to the requirements of the intermediate completion, a settlement plugging plug can be left underground to provide safe construction conditions for subsequent intermediate completion operations, ensuring that there will be no more leakage during cementing construction after the casing is in place.

2. Indoor Research on Settlement Plugging Technology

2.1 Oil based Drilling Fluid Settling and Plugging Technology

Based on the characteristics of oil-based drilling fluid, performance evaluations were conducted on different parameters such as the strength (≥ 30 MPa), settling velocity (≤ 24 hours), oil-water ratio, and temperature of the plugging material.

2.1.1 Strength Evaluation of Sealing Materials

The experimental plugging materials selected are commonly used rigid plugging agents, fruit shell plugging agents, and resin plugging agents in Tarim Oilfield. The oil-based drilling fluid formula is: diesel+25% CaCl₂+3% main emulsion+3% auxiliary emulsion+1% wetting agent+3% CaO+2% fluid loss agent+weighting agent (ρ=2.0 g/cm³).Different plugging materials were added to oil-based drilling fluid for high-temperature rolling aging at 160℃ for 12 hours, followed by screening and drying treatment. The 30MPa compressive strength was evaluated using a cylinder press, and the experimental results are shown in Table 1.

Note: 1. Crushing rate=(1-sieve residue/total mass) × 100%; 2. Select a compressive strength of 30MPa; 3. Due to the temperature range of 140-160 ℃ at the bottom of the salt bed in front of the mountain, the experimental temperature for the compressive strength of the plugging material was chosen to be 160 ℃.

From Table 1, it can be seen that after high-temperature aging at 160 ℃, the crushing rate of both rigid plugging agent and synthetic resin plugging agent is less than 9% at 30MPa, indicating good compressive strength; The crushing rate of the fruit shell plugging agent is greater than 30%, indicating that the compressive strength of the fruit shell sealing material is lower after high-temperature aging.Therefore, oil-based drilling fluid sedimentation plugging materials should choose rigid plugging agents with high compressive strength and synthetic resin plugging materials.

2.1.2 Evaluation of Pressure bearing Capacity of Plugging Slurry

By using a self-made settlement plugging simulation device (Figure 1), the pressure bearing capacity of the plugging slurry settlement plug was tested.The mature oil-based drilling fluid sedimentation plugging formula on site is: 3% SQD-98 fine (0.20mm)+3% SQD-98 medium (0.45mm)+10% rigid plugging agent-2 (3mm)+20% rigid plugging agent-3 (5mm)+20% rigid plugging agent-4 (10mm)+5% synthetic resin plugging agent-3 (2mm)+5% synthetic resin plugging agent-4 (6mm).

A 10mm drain cap is installed at the bottom of the self-made settlement plugging simulation device. The above-mentioned plugging material is added to the oil-based drilling fluid with a formula ratio of ρ=2.0g/cm ³. After settling and aging at 160 ℃ for 24 hours in the mud tank of the device, the leakage amount of the plugging slurry is tested under pressure differentials of 5MPa, 10MPa, 20MPa, and 30MPa. The results are shown in Table 2.From Table 2, it can be seen that the leakage of the plugging slurry under a pressure difference of 30MPa is only 4.2 mL. This indicates that the plugging slurry has a good plugging effect on the bottom leakage disc, proving that the pressure bearing capacity of the mature oil-based drilling fluid settling plugging formula on site can reach 30MPa. That is, the combination of rigid plugging materials and synthetic resin plugging materials can meet the requirement of 30MPa pressure bearing for settling plugging.