How to reduce working fatigue of coiled tubing

Introduction

During operation, fatigue of the tubing string is the main reason affecting the service life of the coiled tubing. This article discusses methods for controlling pipe string fatigue from aspects such as improving construction methods, configuring equipment reasonably, and tracking product usage, in order to extend the service life of coiled tubing, eliminate safety hazards during operation, and ultimately improve economic benefits.

Method

1. Use software to track fatigue life

The software tracking system inputs job parameters into a computer, monitors parameters such as depth, speed, direction, weight, wellhead pressure, and pump pressure in real-time, detects cumulative fatigue, and transmits the current fatigue limit information of the coiled tubing string to the operator to prevent unnecessary reciprocating operation in the fatigue section of the string. For example, using Cerberus coiled tubing software, after inputting the tubing string data, the tubing string can be set to a segment length of at least 3m. During operation, the fatigue of the tubing string can be recorded through a data acquisition system for analyzing its lifespan, in order to achieve the best use effect of the tubing string.

2.Cut off the end of the oil pipe to disperse fatigue

When using the same coil of coiled tubing for a batch of operations with the same well depth, the accumulation of fatigue is particularly concentrated, resulting in significant on-site fracture hazards. In response to this situation, the “conventional truncation method” can be used, which involves cutting a section from the end of the coiled tubing after each operation. The recommended truncation length is 6-15m, which disperses the cumulative fatigue of the coiled tubing in a wider area and avoids the occurrence of fatigue peaks in repeated operation sections. This method is often used to control the fatigue of the tubing string in more than 20 years of coiled tubing operations in the Sichuan Chongqing region.

Another method for fatigue control is the “actual truncation method”, which evaluates the life curve of the tubing string after each operation and determines whether to truncate the tubing string or how long to truncate it based on the parameters of the next operation to minimize the cumulative fatigue of the tubing string and avoid the interruption and fracture of the coiled tubing during operation.

When purchasing coiled tubing, it should be noted that each coil of coiled tubing should have a certain margin to cut off a small amount of tubing string and increase the number of times the entire coil of coiled tubing is used.

3. Adopting variable diameter coiled tubing

The wellhead section of the tubing string in the well has the maximum load and cumulative fatigue corresponding to it. For conventional oil pipes and drill rods, they have high tensile strength and are not subject to bending deformation. They are often used interchangeably, and can be replaced even if they do not meet the requirements. However, coiled tubing is different. If the coiled tubing uses a variable diameter pipe string that changes thickness from the inner layer to the outer layer, it will overcome the weakness caused by the continuity that distinguishes coiled tubing from conventional tubing and drill pipe.

When injecting working fluid through a coiled tubing pump, due to the small diameter of the inner coiled tubing on the drum and the high pump pressure it bears, the pressure inside the tubing has a more serious impact on the inner tube string. Southwest Pipeline Co., Ltd. in the United States has designed a simulation test device that uses a continuous oil pipe material A-606 with a wall thickness of 2.21mm and a diameter of 31.75mm. After cold rolling and tempering treatment, internal pressures of 17.22MPa and 34.45MPa are applied respectively. The test results show that when the internal pressure is 34.45MPa, the continuous oil pipe reaches the fatigue limit after 150 trips; When the pressure inside the pipe is 17.22MPa, the continuous tubing reaches the fatigue limit after 500 trips. When using variable diameter continuous tubing with a constant internal pressure, an increase in wall thickness can reduce the circumferential stress on the pipe wall.

When using variable diameter coiled tubing, when the coiled tubing is released from the drum and enters the well through the injection head, due to the thick walls of the inner layer of coiled tubing on the drum and the wellhead section, it will greatly improve its fatigue resistance, extend the service life of the coiled tubing, avoid accidents, and ensure the success and effectiveness of the operation.

4. Reverse using coiled tubing

Due to the fact that the closer the coiled tubing on the drum is to the inner layer of the drum, the smaller the coil diameter, the higher the internal pressure it bears, and the greater the load it carries into the well. Therefore, the fatigue accumulation of the inner coiled tubing is faster than that of the outer coiled tubing. After using the tubing string for a period of time, the most fatigued end of the inner layer is reversed to the outer layer. As the coil diameter increases, yield is released, the internal pressure it bears is reduced, and the effective reduction of the depth it enters the well is increased, Therefore, the fatigue accumulation rate of the entire coil of coiled tubing is reduced.

5. Reduce corrosion

Due to long-term exposure to corrosive working fluids, sulfur-containing oil and gas, and exposure to the atmosphere, coiled tubing is prone to corrosion. An appropriate amount of corrosion inhibitor can be added to the working fluid. After pumping the working fluid, immediately pump at least one column volume of clean water to clean the inner wall of the coiled tubing. When using the coiled tubing, the outer wall should be washed and coated with oil to reduce atmospheric corrosion on the outer wall of the coiled tubing. After the operation is completed, use inert gas to blow out the liquid inside the pipe and seal it to protect the inner wall of the coiled tubing.

Summarize

The most commonly used method for controlling string fatigue is the actual job truncation method, which uses the continuous tubing software sequence to calculate the fatigue life of the string, track the fatigue of the string, and evaluate it in conjunction with the string life curve to minimize the cumulative fatigue of the string. By using variable diameter tubing and reversing the use of continuous tubing to control string fatigue, the working string can be more effectively used by preventing mechanical and corrosive wear of the string.

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