Overview of Lost Circulation

With the rapid development of the industrial economy, its demand for oil is increasing day by day. Many new drilling technologies in the oil extraction process have also been applied to the oil exploration and development industry. However, while improving drilling efficiency and industrial production, the unresolved problem of oil drilling plugging has become a major challenge in the oil drilling industry, seriously restricting the in-depth development of China's oil drilling industry. If scientific measures are not taken to prevent leakage during the oil drilling process, it is likely to lead to further safety accidents, thereby threatening the safety of drilling personnel. This is a major hidden danger in the oil drilling and extraction process.

Lost circulation is a complex downhole situation in which various working fluids (including drilling fluid, completion fluid, cement slurry, workover fluid, etc.) flow into the formation under the pressure difference during drilling, completion, and other downhole operations. In drilling construction, well leakage often occurs. If severe leakage is not detected and dealt with in a timely manner, it not only loses a large amount of production time, but also may cause malignant accidents such as well collapse, blowout, and sticking, and even lead to the scrapping of some or all sections of the well. So the problem of well leakage is one of the most common technical difficulties in drilling engineering.

There are three main reasons for the occurrence of well leakage: firstly, the formation of original leakage channels during geological sedimentation, groundwater dissolution, or tectonic activities. Once the leakage channels are drilled, leakage will occur. The second is that the equivalent circulating pressure of drilling fluid during drilling construction is greater than the formation's rupture resistance (i.e. the structural strength formed by compaction and cementation of the formation), or is greater than the pressure of the fluid in the pores, cracks, and karst caves of the formation, and the opening size of the leakage channel is larger than the particle size of the solid phase in the drilling fluid, causing the drilling fluid to enter the leakage channel and cause leakage. The third reason is that during the construction process, human factors such as excessive pumping and rapid drilling caused new leakage channels, resulting in leakage. The artificial leakage channel mainly refers to induced fractures, most of which are vertical fractures, with lengths ranging from tens to hundreds of meters, which have a significant impact on well leakage.

1. Principle of Leak Plugging Construction

The essence of oil drilling plugging is to select a location close to the location of the leakage accident and establish a partition wall. By using plugging materials to block the flow channel of the leaked liquid, the accurate location of the leakage can be found, the cause can be identified, and the leakage layer can be drilled according to the actual severity. When drilling through the leakage layer, if there is a secondary leakage, it is strictly prohibited to immediately lift the drill. The leakage slurry should be readjusted and a certain proportion of sawdust should be added for pressure suppression treatment to ensure the sealing effect. Meanwhile, controllable expansion plugging agent is a cement slurry added with inert substances, which can improve the sealing ability and success rate.

2. Sealing Method

2.1 Plugging while Drilling

Drilling and sealing should be carried out simultaneously. The plugging material with bridging function can be introduced into the drilling fluid to complete the plugging without stopping drilling. This not only saves time, but also achieves the purpose of plugging the leak. If the effect is not significant, drilling must be stopped and oil drilling plugging must be stopped.

2.2 Bridge Slurry Plugging

If there is porous leakage in the sandstone formation and the opening size is not large, the bridge slurry sealing method can be used. Mix the base slurry, well slurry, and plugging materials appropriately to meet the requirements of plugging slurry, and then inject the drilling pump into the accurate position of the lost layer for plugging. Observe the effect after completion until the plugging is maintained for a period of time, proving the success of oil drilling plugging.

2.3 Cement Slurry Sealing

The fluidity of cement can be well applied to various leakage channels. When cement flows in, it begins to solidify, forming a certain degree of pressure and compression resistance, which plays a role in plugging the leak. But it is important to pay attention to the principles of balance and economy, and choose the appropriate cement density and dosage.

2.4 Controllable Expansion Plugging Agent

Controllable expansion plugging agent has the advantages of bridging, swelling when encountering water, controllable cementing and plugging, and firm structure. Mainly used in areas with poor lithological cementation and weak fracture zone structure for leakage. In fact, in oil drilling plugging work, rapid and safe plugging can also reduce work intensity.

3. Lost Circulation Material

3.1 Plugging with Mixed Inert Materials

①. Adaptability: suitable for well losses with a plugging loss of 30m³/h-60m³/h and cracks less than 5mm.

②. Sealing principle: bridging with large particle inert materials, filling with small and medium-sized particle inert materials, settling with bentonite, and increasing the shear force of the mud to seal the leakage layer.

③. Preparation of base slurry: Add 200 kilograms of bentonite to 1m³ of clean water, and then add 3-5 kilograms of sodium carboxymethyl cellulose to prepare a base slurry with a specific gravity of 1:15 and a leakage viscosity of over 80 seconds.

3.2 Plugging the Leak with Mixed Inert Materials and Expansion Agents

①. Adaptability: Suitable for plugging lost circulation wells.

②. Sealing principle: Mix inert materials and expansion agents to build a bridge at the narrow area of the leakage layer, and then expand to fill the leakage layer. Some small pores are then sealed by the solid phase in the mud.

③. Sealing material: 1 ton of mixed inert material; Expansion and plugging king 0.5 tons; 2.5 tons of bentonite; 4 bags of high viscosity plugging agent.

④. Preparation of plugging mud: Add 10 m³ of clean water and 2.5 tons of bentonite to the mud pit, and hydrate for 2 hours.

3.3 Plugging with Calcium Chloride Limestone Slurry

①. Plugging principle: the calcium chloride aqueous solution contacts with the limestone slurry and has chemical reaction. Within 1 minute, it will become gel state and lose liquidity, thus sealing the plugging layer.

②. Sealing material: 1 ton of calcium chloride; 1.5 tons of bentonite; 0.2 tons of broad-spectrum wall protector type III; 3 tons of limestone powder.

③. Slurry preparation:

•  Dig a 1.5 m³ small pool, add 1 m³ of clean water, add 1 ton of calcium chloride, and rinse for 1 hour.
•  Clean the mud pit, add 8 m³ of clean water to the pit, and add 1.5 tons of bentonite through a mixing funnel.
•  Add 0.2 tons of broad-spectrum wall protector type III to 2 m³ of clean water and stir for 2 hours.

3.4 Using Limestone Powder and Cement to Plug the Leak

①. Adaptability: Lost circulation, non flowing water in the lost layer, and a depth of no more than 2000 meters.

②. Sealing principle: The slurry still has good fluidity within 1 hour, starts to set in 1 and a half hours, and completely solidifies in 7 hours, thereby completely sealing the leakage layer.

③. Sealing material: 5 tons of cement; 5 tons of limestone powder; 250 kilograms of calcium chloride; 4m³ of clean water.

3.5 Using Instantaneous Plugging Agent and Oil Well Cement for Plugging

①. Adaptability: Lost circulation, strong water activity in the lost layer, and no more than 1000 meters deep in the lost layer.

②. Sealing principle: After the slurry enters the leakage layer, it quickly solidifies.

③. Sealing material: 2 tons of instantaneous sealing agent; 2 tons of oil well cement; 6 bags of high viscosity plugging agent.

3.6 Using Asbestos Fiber, Cotton Wool, Walnut Shell, Expansion Agent, and Polymer Dry Powder to Plug the Leak

①. Adaptability: Large leakage and unclear leakage layer.

②. Sealing principle: asbestos fiber, cotton wool, walnut shell, and expansion agent act as a framework and bridge, and the polymer forms a strong gel, which together seals the leakage layer.

③. Sealing material: 250 kilograms of asbestos fiber; Cotton velvet 250 kilograms; 500 kilograms of walnut shells; 250 kilograms of expansion agent; 500 kilograms of polymer dry powder.

3.7 Sealing the Leak with Chemical Adhesive

①. Adaptability: The formation is under negative pressure, and the mud weight is less than 1.10.

②. Plugging principle: Guar gum contacts with borax to form gel, which is bonded with the wall of the leakage layer to prevent mud leakage.

③. Sealing materials: 150 kilograms of guar gum and 15 kilograms of borax.

3.8 Using Instantaneous Plugging Agent ,Oil Well Cement with Retarder for Plugging

①. Sealing principle: After the sealing slurry enters the leakage layer for 3-5 minutes, it can quickly solidify into a hard solid and completely seal the leakage channel.

②. Sealing material: 1 ton of instantaneous sealing agent; 1 ton of oil well cement; 4 kilograms of retarder.

Due to the complexity of different oil and gas reservoirs and formations, oil drilling and plugging is a highly technical and extremely complex task. Therefore, how to choose plugging agents, how to determine the location of the leakage layer, how to effectively complete the plugging work, how to prevent leakage accidents, and how to avoid the harm caused by them are research directions that we have been striving to explore.

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