The wellhead device is the hub of oil well production and the surface part of the entire oil well. Whether it is a self jetting wellhead device or a pumping well device, their main function is to suspend the tubing string and bear the weight of all tubing strings in the well; Seal the annular space of the oil pipe and casing to ensure the smooth progress of various underground operations, control and adjust production data and daily management. The performance of wellhead equipment is closely related to the oil well. As the development time of oil fields increases, wellhead equipment may be damaged to varying degrees due to natural conditions, external factors, and human factors. In order to maintain the normal production of oil wells, identifying the causes of damage to wellhead equipment, analyzing influencing factors, and repairing them in a timely manner is an important task in well repair.

1、 Types of wellhead faults

Due to the different types of wellheads and various types of wellhead equipment, each well has its own specific situation, resulting in many types of malfunctions in wellhead equipment. However, wellhead faults can be summarized in four words: "piercing," "leaking," "bad," and "dead." The main phenomena are as follows:

1. Thorn

The manifestation of this phenomenon is that the gate, flange, and connection points are punctured with oil or water outward. Generally, it is less common in oil wells and more common in water wells because the surface pump pressure is very high during water injection. Therefore, there are strict requirements for the performance and voltage resistance of the equipment. If some parts of the wellhead device are poorly sealed or the gasket is damaged, it is easy to cause wellhead puncture. Some also suffer from severe corrosion of equipment due to long-term high-pressure water injection, causing the sealing components to lose their function and resulting in punctures, making it impossible for oil and water wells to produce normally.

2. (Leakage)

This type of malfunction is caused by the leakage or dripping of oil (or water) from the components of the wellhead equipment. It is due to the deterioration of the equipment's long-term performance or damage to the threaded part during maintenance, such as not being tightly fastened, which leads to the leakage or seepage of high-pressure liquid from the well. This phenomenon not only pollutes the environment, is not easy to manage, affects the normal production of oil wells, but also is unsafe and prone to causing major accidents.

3. Bad

Damage to equipment or components can reduce the effectiveness of wellhead equipment and cause it to malfunction. The main causes of damage to wellhead equipment (or devices) are accidental collisions during normal production and maintenance, as well as damage during underground operations and well repairs. Improper management system or violation of operating procedures can also damage wellhead equipment.

4. Death

This type of malfunction is caused by equipment being inflexible or necrotic, which is due to collision, knocking, or loss of maintenance and lubrication leading to rusting. It can also be caused by contaminated and corrosive liquids not being completely removed. The operation of the wellhead equipment is not flexible, and even loses its function, affecting the smooth progress of production and construction operations. If the gate of the wellhead gate falls off, the thread bends and rusts, etc., it cannot be used normally.

2、 Reasons for wellhead malfunction

The reasons for wellhead equipment are multifaceted, with the main reasons being as follows:

(1) The equipment itself has poor quality.

(2) Failure to maintain and repair in a timely manner.

(3) Damage caused by violation of operating procedures.

(4) Measures shall not cause damage.

3、 General repair methods for wellhead malfunctions

1. Replace the oil tree

With the continuous improvement of process technology, there are more and more types of Christmas trees. Due to the different structural models of Christmas trees, the methods and steps for well repair are also different. In order to improve development efficiency and achieve the production policy of oil extraction under pressure, it is often carried out on the oilfield site to replace the untwisted flange Christmas tree with a Christmas tree equipped with a top threaded flange. According to the different types and methods of on-site replacement of Christmas trees, there are mainly two methods:

1) Top thread flange Christmas tree repair method

(1) Create construction design. Based on the specific situation of the construction well and the content that needs to be repaired, write the specific content, methods, and steps of the construction according to the purpose and requirements of the construction. At the same time, develop technical security measures.

(2) Lower the oil pipe plug. Firstly, close the main gate of the Christmas tree, use a relief nozzle to insert the plug into the well, and use a high-pressure flushing truck to hold the pressure. Only when it is confirmed that the oil pipe channel has been blocked (sealed), can the next process be carried out.

(3) Lift the wellhead. After confirming the sealing of the plug, observe the overflow with a trial release. When there is no spray at the wellhead, remove the wellhead screws of the Christmas tree as soon as possible and dismantle the Christmas tree.

(4) Repair or welding. Carry out repair work according to the construction requirements of the project.

(5) Install the Christmas tree. After the repair of the wellhead components and equipment, it is necessary to install the Christmas tree as soon as possible. (When the Christmas tree cannot be installed temporarily, the wellhead should also be covered.) Be sure to tighten the top thread on the top thread flange Prevent tubing damage caused by high pressure inside the well. Top up, allowing oil and gas to escape, etc.

(6) Salvage the plug. After the wellhead fault is repaired, the power equipment is used to salvage the blockage in the well and put it back into production.

2) Repair method of untwisted flange Christmas tree

The main difference between top threaded flange oil production and non top threaded flange oil production trees is that the former can be combined with non pressure well and non blowout wellhead equipment to form non pressure well facilities, which can be used for non pressure well operations, while the latter does not have non pressure well and non blowout wellhead equipment facilities, and cannot use non pressure well operations. Therefore, the two methods of repairing oil trees are different.

The repair and replacement methods for untwisted flange Christmas trees are determined on site based on the specific conditions of the well and the material conditions of the equipment. Generally, the following two methods are commonly used.

(1) Replacement method for well killing and blowout prevention.

This method utilizes the effect of well killing to obtain smooth construction conditions. The specific steps are: well killing, dismantling and replacing the wellhead and oil tree, replacing the spray well and washing it to be qualified, and resuming production.

The advantage of this method is that it uses a hydraulic well for well killing, and the high-pressure liquid inside the well is relatively stable, safe, and reliable during construction, making it less prone to blowout accidents. But its disadvantage is that it requires the use of well killing fluid (especially high-density well killing fluid), which not only causes a large amount of material consumption, but also easily injects pollutants into the oil reservoir, blocks the oil flow channel, and reduces the oil production (water injection) after the oil well (or water well) is repaired.

(2) Replacement method of packer card seal.

The principle of using this method to replace the Christmas tree is to utilize the sealing effect of the packer, in conjunction with the non pressure and non discharge operation device of the wellhead (wellhead controller), to temporarily prevent the oil and gas in the wellbore from spraying out, in order to replace the pressure and operation and replace the wellhead Christmas tree.

The advantage of this repair method is that it does not require well killing operations, which not only avoids the damage of well fluid to the oil reservoir during the well killing process, but also saves funds. At the same time, there is no need to carry out well control work, so there is no need for rework or missed work due to pressure or inability to control the well, which can shorten construction time and improve construction efficiency. However, this method has drawbacks. Firstly, if the packer or hydraulic anchor fails to work, it may cause a risk of blowout or personal injury. Secondly, if the hydraulic anchor gets stuck on the casing wall, it is easy to damage the casing. During construction, it is necessary to install and remove the wellhead device twice without pressure or discharge, and the process is complex.

2. Technology for handling four-way casing

The four-way casing of the oil well is located under the main gate of the Christmas tree. When it is damaged or needs to be replaced, it is impossible to control the oil and gas flow in the well solely through the wellhead device. Therefore, its treatment and repair methods are the same as those for the treatment of non top threaded flange wellhead equipment. There are two methods: one is to use the well killing and anti spray repair method; Another approach is to use the packer card sealing repair method. However, considering the potential safety and reliability drawbacks of using the second method, if this method is not handled properly when replacing the four-way valve, it will inevitably cause a major accident. So, the first method is usually used for repairing and replacing casing tees, which is the well killing and anti spray repair method. Especially for oil wells with high formation pressure and high production, this should be given special attention.

The specific construction steps for using the well killing and anti spray repair method to treat casing tees are:

(1) The well control, well control fluid, and well control method should be determined according to the specific situation of the well.

(2) Remove the flange screws of the wellhead controller and lift off the Christmas tree.

(3) Lift the oil pipe out of the well.

(4) Weld off the flange that was previously installed with a four-way casing and replace it with a new or top threaded flange for the installation of a new wellhead device.

(5) Install a four-way sleeve with a top screw flange on the newly welded sleeve flange.

(6) Insert the completion string (this completion string cannot replace the replacement nozzle string, and should be replaced with a new replacement nozzle string before), and replace the well pressure fluid completely. Then use a non pressure and non discharge wellhead device to lift the replacement nozzle column, and then lower the completion pipe column.

(7) Sit on the Christmas tree and tighten all screws, especially the top screw on the top screw flange, to prevent a blowout caused by high-pressure oil and gas flow in the wellbore after replacement.

(8) After using the completion string (or specialized replacement nozzle string) to replace the spray and wash the well, it can be put back into production.

3. Precautions for handling casing tees

(1) Electric welding at the wellhead must go through the procedures for using fire at the wellhead of the oil well, prepare fire-fighting equipment and tools, and welding and cutting must be carried out when the fluid flow in the well is stable and there is no oil or gas overflow or oil and gas display at the wellhead.

(2) After dismantling the Christmas tree, pay attention to the storage of steel rings and other components to prevent scratches, and be careful not to lose screws and small tools or drop them into the well. When installing the Christmas tree, attention should be paid to aligning and placing it gently to prevent breaking the ring or other components, causing leakage at the wellhead and rework.

(3) When lifting the Christmas tree, precautions should be taken to prevent it from falling and injuring personnel and wellhead equipment.

(4) If the part that needs to be replaced or cut for welding is too low, in order to facilitate the welding operation, a round pit should be excavated in advance to shorten the working time at the wellhead and ensure quality.

(5) Before cutting and welding the wellhead, the dimensions should be carefully measured, and after cutting and welding, the oil filling distance should be accurately corrected.

(6) For thick walled casing welding, butt welding should be adopted; For casing with thin wall thickness or severe corrosion, a casing with a diameter larger than the original casing should be welded firmly.

(7) The selected casing for replacing the wellhead should ideally have the same inner diameter as the original casing. When the rock is larger than the original casing inner diameter, a slope should be made on the top inner diameter of the original casing to prevent obstruction, drilling failure, and damage to the downhole string and tools, which may cause accidents. The inner diameter of the selected replacement casing shall not be smaller than the inner diameter of the original well casing.

(8) It is essential to ensure the quality of welding. Welding should be done at least twice at the weld joint, and some reinforcing ribs should be added around the weld joint to increase its strength. After welding, a well and pressure test should be conducted to ensure that the welding is straight and not skewed, and that it can withstand the rated pressure without piercing or leaking, which is considered qualified.