A method for drilling and cementing a well

Abstract

The present application belongs to the technical field of oil drilling, and particularly relates to a cementing plugging and plug leaving method for oil drilling, which comprises the following steps: throwing gravel into the well above 80-100 m of the leakage layer through the water eye of the drill pipe to plug the cracks, filling with plugging slurry, and finally performing cementing operation to leave a cement plug in the well, thereby realizing plugging and plug leaving. The cementing plugging and plug leaving method for oil drilling is suitable for the case that the existing plugging is invalid after well leakage occurs, and the cement plug cannot be left. The method has simple construction process, high operability, and high success rate of plug leaving, and has strong guidance for the cementing plugging and plug leaving in the drilling site, and is very worthy of popularization and application in the large lost circulation filling well sidetracking site.

Description

Technical Field

[0001] This invention belongs to the field of oil drilling technology, specifically relating to a method for cementing and plugging leaks in oil drilling. Background Technology

[0002] In oil drilling operations, well leakage is one of the most common downhole complications. If the leakage rate is less than 20 cubic meters per hour, it can be plugged by injecting cement slurry, and the plugging effect is relatively good. However, if the leakage rate is greater than 20 cubic meters per hour, or if there is a loss of return leakage, it is often difficult to retain the cement plug when injecting cement slurry, resulting in a low success rate of plugging.

[0003] Currently, for cases where large-scale leakage plugging is ineffective, the common practice is to use cement filling and side-drilling. However, a cement plug must be left in the well before side-drilling. For wells less than 1000 meters deep, a wooden plug can be made to seal the leaking layer. However, for wells deeper than 1000 meters, due to the varying stability of different formations during drilling, the wellbore cannot be of a regular size and will have a "skewer" shape of varying sizes. In small wellbore areas, the wooden plug may be crushed by the drill string. In larger wellbore areas, the wooden plug may not be able to move vertically downwards and may be squeezed into the enlarged pit of the wellbore, unable to descend. This makes it difficult to accurately send the wooden plug to the intended well depth, resulting in a low success rate of cement filling and plug placement, and increased construction time and cost. Summary of the Invention

[0004] The purpose of this invention is to provide a method for cementing and plugging leaks in oil drilling, overcoming the aforementioned technical problems in the prior art.

[0005] Therefore, the technical solution provided by the present invention is as follows:

[0006] A method for sealing leaks in oil drilling involves injecting crushed stone into the well through the drill pipe water hole at a depth of 80-100m above the leaking layer to block the cracks, then filling the cracks with sealing slurry, and finally injecting cement to leave a cement plug in the well, thus achieving the purpose of sealing the leak.

[0007] A method for sealing leaks in oil drilling using cement, comprising the following steps:

[0008] Step 1) Before dropping stones, calculate the amount of stones to drop per meter based on the wellbore size;

[0009] Step 2) Lower the drill rod to 10m above the leaking layer and start dropping stones in stages from the water inlet of the drill bit. After dropping stones in each stage, turn on the pump to check if the water inlet is unobstructed. After ensuring that the water inlet is unobstructed, lower the drill bit to press down. Then repeat the above process to drop stones until 80-100m has been dropped.

[0010] Use crushed stone to fill the large cracks at the bottom of the well first, so as to reduce the cracks and decrease the leakage rate when injecting grout later, making it easier for cement to remain in place.

[0011] Step 3) After the stones are dropped, start the pump to circulate the well fluid so that the well leakage returns. After the drilling fluid returns, pump 8-10 cubic meters of plugging slurry above the stones. After shutting the well and squeezing out 4-5 cubic meters, fill the gaps between the stones. Finally, carry out the cementing operation.

[0012] After filling the wellbore with crushed stone, there are still gaps between the crushed stone and the drilling fluid still leaks after the pump is turned on. It is necessary to use a plugging slurry to fill the gaps between the crushed stone.

[0013] The crushed stone has a particle size of 5-10 mm.

[0014] If the particle size is too large, it will easily accumulate in the water hole and will not sink easily, thus clogging the water hole; if the particle size is too small, it will not be able to stay in the crack, because the maximum particle size of the sealing material is 5mm, and it still cannot effectively penetrate the crack after entering the formation.

[0015] When the leaking layer is at the bottom of the well, the drill pipe is lowered and the rock is directly filled; when the leaking layer is not at the bottom of the well, a plug is made and lowered to 10m below the leaking layer before filling with rock.

[0016] When the leaky layer is at the bottom of the well, the crushed stones can be directly inserted into the leaky layer. When the leaky layer is not at the bottom of the well, the amount of crushed stones used for filling is too large, which is inconvenient for operation. A plug can be made at the bottom of the leaky layer to reduce the amount of stones to be inserted and the time required for insertion.

[0017] The phased stone-dropping process described in step 2) is as follows: after dropping 30m of stone, stop for 20-30 minutes; during this process, fill 1m every 6-8 minutes, and after filling 10m, lift the drill string 10m to continue dropping stone.

[0018] Throw stones for 30 meters and then pause for 30 minutes to allow the stones to sink into the water hole. Continuous throwing of stones can easily clog the water hole. Every 10 meters of filling, lift the drill bit to prevent excessive accumulation of stones from clogging the water hole at the bottom of the well. Always leave some space at the bottom of the well to allow the stones to sink and accumulate there.

[0019] In step 2), the pump displacement is 8-12 L / S.

[0020] The normal drilling flow rate is 18-20 L / S. When the rock is dropped, the water hole contains gravel. A large flow rate can easily cause the gravel to accumulate and block the water hole, making it impossible to carry out construction later. Through multiple on-site simulation experiments, a flow rate of 8-12 L / S is found to be the best.

[0021] The sealing grout is composed of the following materials by weight percentage: 20% fine sealant, 15% coarse sealant, 15% limestone, 10-15% bentonite, and the balance being water;

[0022] The fine plugging agent has a particle size of no more than 1 mm, and the coarse plugging agent has a particle size of more than 1 mm. The fine plugging agent is one or more of the following: magnesium hydroxide fiber, cotton lint, mica powder, wood fiber, and polyethylene fiber.

[0023] After the wellbore is filled with crushed stone, there are gaps between the crushed stones. The gap width is 1-5mm. The fine plugging agent has a particle size of 0.1-1mm, the coarse plugging agent has a particle size of 1-5mm, the limestone particle size is less than 1mm, and bentonite is used to thicken the gaps between the crushed stones.

[0024] Step 2) involves lowering the drill string to press down: Lower the drill string to probe the rock surface, and after reaching the rock surface, apply light pressure of 1-2 tons. The purpose is to compact the rock that has sunk to the bottom of the well and reduce the porosity between the rocks.

[0025] The plug is made of wood, 1.5-2 meters in length, with an outer diameter 15-20 mm smaller than the wellbore. It is wrapped with felt on the outside and then secured with wire. When the leaking layer is not at the bottom of the well, in order to reduce the amount of rock thrown and improve construction efficiency, it is not necessary to fill the entire wellbore below the leaking layer with gravel. It is only necessary to fill the area near the leaking layer with gravel so that cement can be poured to form a plug.

[0026] The crushed stone is road construction stone, and the cement used in the cement-making operation is Grade G cement.

[0027] The beneficial effects of this invention are:

[0028] The method for sealing leaks in oil wells by applying cement provided by this invention is applicable to situations where existing sealing methods are ineffective after well leakage occurs and cement cannot be used to seal the leak. Compared with simply applying cement, this method involves first filling the cracks with stones, then filling the gaps between the stones with sealing slurry, and finally applying cement to seal the leak.

[0029] The invention features a simple construction process, strong operability, and a high success rate in plugging. It provides strong guidance for drilling site operations where cement plugging fails to retain plugs, and is highly worthy of promotion and application in large-scale lost-loop well filling and side-drilling operations.

[0030] Further details will be provided below. Detailed Implementation

[0031] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0032] Exemplary embodiments of the present invention are now described; however, the present invention may be embodied in many different forms and is not limited to the embodiments described herein. These embodiments are provided to fully and completely disclose the invention and to fully convey its scope to those skilled in the art. The terminology used in the exemplary embodiments is not intended to limit the invention.

[0033] Unless otherwise stated, the terms used herein (including technical terms) have their common meaning as understood by one of ordinary skill in the art. Furthermore, it is understood that terms defined in commonly used dictionaries should be understood to have a meaning consistent with the context of their relevant field, and not to be interpreted as having an idealized or overly formal meaning.

[0034] Example 1:

[0035] This embodiment provides a method for sealing leaks in oil drilling by applying cement. Crushed stone is dropped into the well 80-100m above the leaking layer through the drill pipe water hole to block the cracks. Then, sealing slurry is used to fill the cracks, and finally, cement is applied to leave a cement plug in the well, thus sealing the leak.

[0036] To solve the problem of existing cement filling methods failing to retain the plug, this invention employs a method of first filling cracks with stones, then filling the gaps between the stones with sealing grout, and finally filling with cement to retain the plug. This is a fast, reliable, and low-cost method for filling well leaks with cement for sealing.

[0037] Example 2:

[0038] Based on Example 1, this example provides a method for cementing and plugging leaks in oil drilling, the specific process of which includes the following steps:

[0039] Step 1) Before dropping stones, calculate the amount of stones to drop per meter based on the wellbore size;

[0040] Step 2) Lower the drill rod to 10m above the leaking layer and start dropping stones in stages from the water inlet of the drill bit. After dropping stones in each stage, turn on the pump to check if the water inlet is unobstructed. After ensuring that the water inlet is unobstructed, lower the drill bit to press down. Then repeat the above process to drop stones until 80-100m has been dropped.

[0041] The purpose of this step is to first fill the large cracks at the bottom of the well with crushed stone, so as to reduce the cracks and decrease the leakage rate when injecting grout later, making it easier for cement to remain in place.

[0042] Step 3) After the stones are dropped, start the pump to circulate the well fluid so that the well leakage returns. After the drilling fluid returns, pump 8-10 cubic meters of plugging slurry above the stones. After shutting the well and squeezing out 4-5 cubic meters, fill the gaps between the stones. Finally, carry out the cementing operation.

[0043] Step 3) aims to fill the gaps between the gravel after it has been filled into the wellbore. After the pump is turned on, the drilling fluid will still leak. Therefore, it is necessary to use a plugging slurry to fill the gaps between the gravel.

[0044] The method for cementing and plugging leaks in oil drilling provided by this invention is applicable to situations where existing plugging methods are ineffective after well leakage has occurred, and cement cannot be retained as a plug. This method is simple to implement, highly operable, and has a high success rate in plug retention. It provides strong guidance for drilling site operations where cementing and plugging fail to retain a plug, and is highly worthy of widespread application in large-scale, non-returnable wells and sidetracking drilling.

[0045] Example 3:

[0046] Based on Example 1, this example provides a method for cementing and plugging leaks in oil drilling, wherein the crushed stone has a particle size of 5-10mm.

[0047] Excessively large particles tend to accumulate in water pores, making them difficult to settle and easily clogging the pores; conversely, small particles cannot remain in the cracks because the maximum particle size of the sealing material is 5mm, which is insufficient to effectively seal the cracks even after entering the formation. This method first uses crushed stone to plug the cracks, forming a rigid framework, then fills the gaps with sealing grout, and finally injects cement to form a cement plug, completing the sealing work.

[0048] Example 4:

[0049] Based on Example 2, this example provides a method for cementing and plugging leaks in oil drilling. When the leaking layer is at the bottom of the well, the drill pipe is lowered and the rock is directly filled. When the leaking layer is not at the bottom of the well, a plug is made and lowered to 10m below the leaking layer before filling with rock.

[0050] When the leaky layer is at the bottom of the well, the crushed stones can be directly inserted into the leaky layer. When the leaky layer is not at the bottom of the well, the amount of crushed stones used for filling is too large, which is inconvenient for operation. A plug can be made at the bottom of the leaky layer to reduce the amount of stones to be inserted and the time required for insertion.

[0051] Example 5:

[0052] Based on Example 2, this example provides a method for cement plugging in oil well drilling. The plugging slurry is composed of the following materials by mass percentage: 20% fine plugging agent, 15% coarse plugging agent, 15% limestone, 10-15% bentonite, and the balance is water.

[0053] Among them, the particle size of the fine plugging agent is no greater than 1 mm, and the particle size of the coarse plugging agent is greater than 1 mm. The fine plugging agent is one or more of the following: magnesium hydroxide fiber, cotton lint, mica powder, wood fiber, and polyethylene fiber.

[0054] By using sealing grout to fill the gaps between cracks and gravel, the success rate of sequential sealing is significantly improved, and the pressure-bearing capacity of the leaking layer is greatly enhanced.

[0055] Example 6:

[0056] Based on Example 1, this example provides a method for cementing and plugging leaks in oil drilling, the specific process of which includes the following steps:

[0057] Step 1) Calculate the amount of rock to be thrown per meter based on the wellbore size;

[0058] Step 2) Determine the location of the leaking layer. If the leaking layer is not at the bottom of the well, proceed to Step 3). If the leaking layer is at the bottom of the well, skip to Step 4).

[0059] Step 3) Make a wooden plug and lower it to 10 meters below the wellbore. The wooden plug should be 1.5-2 meters long and 15-20 mm smaller in outer diameter than the wellbore. Wrap the outside with felt and secure it with wire.

[0060] Step 4) Lower the drill string to 10 meters above the leaking layer and begin dropping rocks into the water hole of the drill string. Fill 1 meter every 6-8 minutes. After filling 10 meters, raise the drill string 10 meters and continue dropping rocks. After dropping rocks 30 meters, stop for 20-30 minutes and start the pump at a low displacement (8-12 L / S) to ensure the water hole is unobstructed. Then slowly lower the drill string to probe the gravel surface. After probing the gravel surface, apply light pressure of 1-2 tons. Then continue the above steps to drop rocks until 80-100 meters have been dropped.

[0061] During this process, stones are dropped for 30 meters and then paused for 20-30 minutes to allow the stones to sink into the water hole. Continuous dropping of stones can easily clog the water hole. Every 10 meters of filling, the stones are lifted up to prevent excessive accumulation of stones from clogging the water hole at the bottom of the well. A section is always left empty at the bottom of the well to allow the stones to sink and accumulate.

[0062] The normal drilling flow rate is 18-20 L / S. When the rock is dropped, the water hole contains gravel. A large flow rate can easily cause the gravel to accumulate and block the water hole, making it impossible to carry out construction later. Through multiple on-site simulation experiments, a flow rate of 8-12 L / S is found to be the best.

[0063] Step 5) After the stones are dropped, start the pump to circulate and ensure that the drilling fluid returns from the well. After the drilling fluid returns, pump 8-10 cubic meters of plugging slurry above the stones. After shutting off the well and squeezing out 4-5 cubic meters of plugging slurry, fill the gaps between the stones and then carry out cementing operations.

[0064] The sealing slurry is composed of the following materials by mass percentage: 20% fine sealant, 15% coarse sealant, 15% limestone, 15% bentonite, and the balance being water;

[0065] The fine plugging agent has a particle size of no more than 1 mm, and the coarse plugging agent has a particle size of more than 1 mm. The fine plugging agent is one or more of the following: magnesium hydroxide fiber, cotton lint, mica powder, wood fiber, and polyethylene fiber.

[0066] After the wellbore is filled with crushed stone, there are gaps between the crushed stone and the gap width is 1-5mm. The fine plugging agent has a particle size of 0.1-1mm, the coarse plugging agent has a particle size of 1-5mm, the limestone has a particle size of less than 1mm, and bentonite is used to thicken the gaps between the crushed stone. In order to completely fill the gaps between the crushed stone, in this embodiment, the mass ratio of various plugging agents is fine plugging agent: coarse plugging agent: limestone: bentonite = 4:3:3:3.

[0067] Crushed stone with a diameter of 5-10mm is selected. The crushed stone is filled to a depth of 80-100 meters above the leaking layer. The amount of crushed stone used is calculated based on the wellbore size. The crushed stone is the same type used for road construction. The cement used for cementing operations is Grade G cement.

[0068] Example 7:

[0069] Based on Example 2, this example uses the design well of the Sudong ** well as an example to further illustrate the method of the present invention in detail.

[0070] The Su Dong** well was designed to be 3250 meters deep with a borehole size of 165.1 mm. When the well reached 2307 meters, it experienced a loss of return and leakage. Subsequently, the well was plugged with a bridge plug slurry once, a high-water-loss slurry once, and cement injection twice, but all of these methods were ineffective. Moreover, there was no cement plug after the cement injection. It was decided to drop stones into the bottom of the well and then drill cement side-drilling.

[0071] The implementation steps of this method are as follows:

[0072] Using 3 cubic meters of 5-10mm diameter crushed stone, the drill pipe was lowered to a well depth of 2297 meters. Based on the wellbore dimensions, the volume per meter of wellbore was calculated to be 0.021 cubic meters.

[0073] Begin by dropping rocks into the water inlet of the drill string, controlling the dropping of rocks in 10-meter sections for 1 hour. After dropping rocks into each 10-meter section, raise the drill string 10 meters and continue dropping rocks. After dropping rocks into a 30-meter section, pause for 30 minutes, then start the pump at a flow rate of 8 L / s. Ensure the water inlet is clear, then slowly lower the drill string to probe the rock surface. Once the rock surface is reached, apply 1 ton of pressure to ensure the rock is compacted. Then continue raising the drill string to drop rocks, repeating the steps. The above operation continues until the stone-throwing section reaches 100 meters;

[0074] After the rock is dropped, the drilling fluid is pumped out of the wellhead, followed by pumping in 10 cubic meters of plugging slurry (plugging slurry formula: 20% fine plugging + 10% coarse plugging + 15% limestone + 10% bentonite), then the well sealer is closed, and 5 cubic meters are squeezed in.

[0075] ④ After the plugging grout was squeezed out, cement was pumped in. A total of 11 cubic meters of cement was pumped in. The expected plug section length was 500 meters, but the actual plug section length was 450 meters. The plug was successfully left in place, and then normal side drilling operations were carried out.

[0076] Comparative example:

[0077] Taking the design well of Sudong ** well as an example, this paper further explains the method of filling the well and side-drilling after the leakage plugging method of this invention has failed.

[0078] The Sudong No. 1 well was designed to be 3180 meters deep with a borehole size of 215.9 mm. When the well reached 2259 meters, it experienced a loss of return and leakage. Subsequently, it was plugged with bridge plug slurry three times, with high water loss slurry three times, and with cement injection twice, but all of these methods were ineffective. Moreover, after the cement injection, there was no cement plug. Finally, at a depth of 700 meters, a wooden plug was used to inject cement and leave a plug for side drilling.

[0079] The implementation steps of this method are as follows:

[0080] Three applications of bridge plug plugging slurry were prepared. The formula consisted of 20% fine plugging agent, 15% coarse plugging agent, 20% limestone, 15% bentonite, and the remainder being water. Each application used 45 cubic meters of slurry. The drill pipe was lowered to a depth of 2250 meters. After the plugging slurry emerged from the water hole in the drill pipe, the wellhead was sealed with the sealing device. The casing pressure was 0 in all three applications. After plugging was completed, the pump was turned on but the wellhead still failed to return to the source, indicating that the plugging had failed.

[0081] Prepare high-fluid-loss plugging slurry three times. The formula is: 15% fine plugging agent, 15% coarse plugging agent, 20% limestone, 10% bentonite, 10-15% high-fluid-loss plugging agent, and the balance is water. Each time, prepare 40 cubic meters. Drill to 2259 meters with the drill pipe. Pump in 30 cubic meters of plugging slurry. Drilling fluid can be returned from the wellhead. During the pumping of plugging slurry, the leakage rate is 50-60 cubic meters / hour. Pull out the drill to 1800 meters and shut in the wellhead for squeezing. The casing pressure is 0.5-1 MPa. After stopping the pump for 1 minute, the casing pressure is 0. When the well is opened for circulation, the leakage rate is 60-80 cubic meters / hour. The plugging fails.

[0082] Two attempts were made to plug the leak using cement. Each attempt used 15 cubic meters of cement. The drill pipe was lowered to 2230 meters, and 15 cubic meters of cement slurry was injected. There was no return flow from the wellhead during the cement injection. After the cement was injected, the drill string was pulled out to allow it to set. Sixteen hours after setting, the drill string was lowered again to drill for a cement plug, but no cement plug was found on either attempt. When the pump was started, drilling fluid did not return from the wellhead, and the leak plugging attempt failed.

[0083] ④ Pluging and Side-drilling: Select a 210mm diameter wooden plug, 2 meters long, wrapped with 15mm thick felt, and lower it into the well. Use the drill pipe to press the wooden plug down to a depth of 700 meters. Inject 10 cubic meters of high-concentration plugging slurry, with a formula of 20% fine plugging agent, 30% coarse plugging agent, 20% bentonite, and the remainder being water. After pumping in the plugging slurry, drilling fluid returns from the wellhead. Raise the drill pipe to 650 meters and inject 8 cubic meters of cement. After injecting the cement, pull out the drill and wait for it to set. After 24 hours of setting, lower the drill. The expected plug length is 210 meters, but the actual plug length is 182 meters. Drill the cement plug to 600 meters and then begin side-drilling until the well is completed.

[0084] Field implementation shows that although the above comparative example was successful in the end, it resulted in the loss of a large number of already constructed well sections and a long construction period, causing significant losses in manpower and resources. In contrast, this method can quickly construct in the leaking layer after multiple failed attempts to plug the leak, and can efficiently retain cement plugs in the upper part of the leaking layer to carry out the next step of side-drilling.

[0085] Example 8:

[0086] Based on Example 2, this example uses the Tao 2** well design well as an example to further illustrate the method of the present invention in detail.

[0087] The Tao 2** well was designed to be 4528 meters deep with a borehole size of 241.3 mm. When the well reached 1614 meters, it experienced a loss of return and leakage. Subsequently, eight attempts were made to plug the leak with bridge plug slurry and seven attempts were made to plug the leak with cement, but all of these were ineffective. Moreover, there was no cement plug after the cement was injected. It was decided to put stones at the bottom of the well and then drill cement side-drilling.

[0088] ① Select 5 cubic meters of 5-10mm diameter crushed stone, and run the drill pipe down to a well depth of 1604 meters. Based on the wellbore size calculation, the volume per meter of wellbore is 0.046 cubic meters.

[0089] ② Begin dropping stones from the water inlet of the drill string, controlling the stone dropping in 10-meter sections for 1 hour. After dropping stones in every 10-meter section, raise the drill string 10 meters and continue dropping stones. After dropping stones in a 30-meter section, pause for 30 minutes, start the pump, discharge at 12L / S, ensure the water inlet is clear, slowly lower the drill string to probe the gravel surface, press down 1 ton after probing to ensure the gravel is compacted, then continue raising the drill string to drop stones, repeating the above operation in step ② until the stone dropping section reaches 80 meters;

[0090] ③ After the stones are dropped, start the pump to return drilling fluid from the wellhead and observe for leakage. There is no leakage.

[0091] ④ Start pouring cement. A total of 10 cubic meters of cement was pumped in. The expected length of the plug section was 220 meters, but the actual length of the plug section was 200 meters. The plug was successfully placed, and normal side drilling operations were then carried out.

[0092] In summary, the cement plugging method for oil well drilling provided by this invention is suitable for situations where existing plugging methods are ineffective after well leakage has occurred, and cement cannot be retained as a plug. This method is simple to implement, highly operable, and has a high success rate in retaining the plug. It provides strong guidance for drilling site operations where cement plugging fails to retain the plug, and is highly worthy of widespread application in large-scale lost-loop wells and sidetracking drilling.

[0093] The above examples are merely illustrative of the present invention and do not constitute a limitation on the scope of protection of the present invention. All designs that are the same as or similar to the present invention are within the scope of protection of the present invention.

Claims

1. A method of cementing a lost circulation zone in a petroleum drilling operation, the method comprising: After the multiple plugging is invalid and the cement plug is not formed after the cement is injected, the gravel is injected into the well above the leakage layer by 80-100 m through the drill pipe water eye, the cracks are plugged, the plugging slurry is filled, and finally the cementing operation is carried out, so that the cement plug is left in the well, and the plugging and plug leaving are realized; The specific process comprises the following steps: Step 1) Before gravel injection, the gravel amount of 1 m is calculated according to the size of the wellbore; Step 2) The slick drill pipe is lowered to 10 m above the leakage layer, and the gravel is injected from the drill pipe water eye in stages. After the gravel is injected in each stage, the pump is opened to check whether the water eye is unobstructed. After ensuring that the water eye is unobstructed, the drill pipe is lowered and pressed, and then the above process is repeated to inject gravel until 80-100 m is injected; Step 3) After the gravel injection is completed, the pump is opened to circulate, and the drilling fluid is returned to the well. After the drilling fluid is returned, 8-10 The plugging slurry is pumped above the gravel, the well is closed, and 4-5 The plugging slurry is composed of the following materials in mass percentage: 20% fine plugging agent, 15% coarse plugging agent, 15% limestone, 10-15% bentonite, and the balance is water. The particle size of the fine plugging agent is not greater than 1 mm, the particle size of the coarse plugging agent is greater than 1 mm, and the fine plugging agent is one or more of brucite fiber, cotton flock, mica powder, wood fiber and polyethylene fiber.

2. The method of claim 1, wherein: The particle size of the gravel is 5-10 mm.

3. The method of claim 1, wherein: When the leakage layer is at the bottom of the well, the slick drill pipe is directly filled with gravel; when the leakage layer is not at the bottom of the well, the plug is lowered to 10 m below the leakage layer and then the gravel is filled.

4. The method of claim 1, wherein: The process of the gravel injection in stages in step 2) is: stop for 20-30 minutes after injecting 30 m of gravel; during this process, 1 m is filled every 6-8 minutes, and the drill pipe is lifted by 10 m to continue gravel injection after 10 m of filling.

5. The method of claim 1, wherein: The pump displacement in step 2) is 8-12 L / S.

6. The method of cementing a lost circulation zone in oil drilling according to claim 1, wherein: The process of lowering the drill pipe and pressing in step 2) is: the drill pipe is lowered to probe the gravel surface, and the drill pipe is pressed by 1-2 tons after the gravel surface is probed.

7. The method of claim 3, wherein: The plug is a wooden plug with a length of 1.5-2 meters and an outer diameter that is 15-20 mm smaller than the wellbore. The outside is wrapped with felt and then wrapped with iron wire.

8. The method of claim 1, wherein: The gravel is a road building stone, and the cement used in the cementing operation is G-grade cement.