Quality defects and prevention of oil well pipes

Quality defects of oil well pipes mainly come from three aspects:
First: quality defects of the oil well pipe body itself, such as mechanical properties, internal pass, weighing, etc. of the pipe body that do not meet the requirements;
Second: quality defects of oil well pipes during processing, such as thread parameters (taper, pitch, tooth height, Ken shape, concentricity, and close distance of threads at both ends of couplings) exceeding the standard, black skin buckle, broken buckle, thread deviation, screw torque exceeding the standard, leakage, thread damage (scratches, bumps), drill pipe weld quality not meeting the requirements, etc.;
Third: The performance of oil well pipes, including anti-squeezing performance, anti-corrosion performance, perforation performance, and anti-sticking performance, fail to meet the requirements.

1. Quality defects and prevention of oil well pipe thread processing
During the thread processing of oil well pipes, the threads may have quality defects such as black skin buckle, thread deviation, broken buckle, thread scratch (bump) damage, and thread parameters exceeding the standard.

(1) Black buckle of thread: Black buckle of thread is manifested as an “unsmooth” appearance due to too little local processing of thread, which is related to the outer diameter and wall thickness accuracy, ovality, and straightness of pipe end of steel pipe. The black buckle of the pipe body is often caused by the outer diameter of the pipe body being too small, the pipe end is not straight enough or the ovality being too large. The black buckle of coupling is generally caused by the outer diameter of the steel pipe exceeding positive tolerance the wall exceeding negative tolerance or the ovality being too large.

(2) Thread deviation: Thread deviation is the uneven wall thickness of steel pipe with one side thin and the other side thick after threading. The reason for thread deviation is similar to that for the black buckle of thread, which is caused by uneven wall thickness, bending, or too much ovality of the pipe end. Sometimes, when thread deviation or unreasonable processing control occurs, the wall thickness of the thread bottom may exceed negative tolerance, which will seriously affect the connection strength of the oil well pipe.

(3) Thread breakage: When the thread comb cutter is cutting the thread at high speed and forcefully, once the thread is broken or the thread is “lost”, it will cause the thread to break. Generally speaking, thread breakage is mainly caused by large non-metallic inclusions in the steel and is also related to the quality of the thread comb cutter and the stability of the threading process.

(4) Thread damage: The thread damage of the oil well pipe includes bruises and abrasions. It is caused during the production, transportation, and storage of finished products. To prevent the exposed threads of the oil well pipe from being bruised, crushed, and rusted, in addition to ensuring that the threads do not collide with hard objects (such as transport rollers, inclined grate bars, etc.) during production, an external protective ring with internal threads should be screwed on the threads of the oil well pipe body, and an internal protective ring with external threads should be screwed on the coupling threads.

API Spec 5CT standard stipulates:
① The thread processing plant should screw on internal and external thread protection rings. The design, material, and mechanical strength of the thread protection ring are required to protect the threads and pipe ends to avoid damage during normal loading and unloading, and transportation;
② During the transportation and normal storage of oil and casing, the design and material of the thread protection ring are required to isolate the threads from dirt and water. The normal storage period is about 1 year;
③ The material selection of the thread protection ring should not contain material components that may cause thread corrosion or cause the thread protection ring to adhere to the thread, and can be suitable for a service temperature of -46℃ to +66℃:
④ Bare steel thread protection rings shall not be used on L80 steel grade 9Cr and 13Cr pipe bodies.

(5) Thread parameters exceed the standard
Thread processing is the most important process in the production of oil well pipes and is also a key process that determines the thread quality of oil well pipes. At present, most oil well pipes are threaded using special CNC machine tools. When processing threads, the workpiece is automatically centered and floating clamped. The tool for processing threads uses a carbide tool, and the spindle rotation is stepless. There are two ways of thread processing: one is that the workpiece rotates and the tool performs plane feed motion; the other is that the workpiece does not move and the tool rotates and performs feed motion. These two types of machine tools have their characteristics. The former is flexible to use. It not only has a high productivity when processing general tapered threads, but can also process direct-connected and special-connected threads with good airtightness (special buckles); the latter has a higher productivity in processing general tapered threads than the former, but special buckles require pre-processing machine tools. The various parameters of the thread (mid-diameter, tooth height, taper, pitch, tooth angle, close distance, etc.) will affect the connection strength and sealing performance of the thread. The thread close distance is the comprehensive value of the fluctuation of each single parameter of the thread. Even if the single parameters of the thread are qualified, its close distance may not be qualified. The accuracy of the various parameters of the thread is not only related to the quality of the tube blank but also related to the thread processing method, machine tool type, and stability of the processing process, as well as the dimensional accuracy and wear resistance of the thread comb. When other conditions are the same, the dimensional accuracy of the thread comb determines the accuracy of the thread size. Generally, the dimensional tolerance of the thread comb is required to be only 1/3 to 1/4 of the product tolerance, or even higher.

(6) Torque and J values exceed the standard: The torque of the oil and casing refers to the make-up torque generated when the coupling and the pipe body are screwed together. The purpose of controlling the torque is to ensure the connection strength between the coupling and the pipe body and the contact pressure stress on the thread side and to cooperate with the corresponding thread sealing grease to achieve the anti-leakage of the oil and casing. For API standard threads, the J value represents the distance from the pipe end to the center of the coupling after the coupling and the pipe body are tightened, which is one of the important parameters that determine the quality of the threaded connection.

(7) Leakage: To avoid oil and casing leakage due to insufficient contact pressure between the oil and casing pipe body and the coupling thread, the oil and casing with coupling are subjected to a hydrostatic pressure test according to the standard. The leakage of the thread connecting the pipe body and the coupling is related to the type and quality of the thread, the screwing of the oil and casing, and the quality of the thread sealing grease. In terms of thread type, the sealing performance of round threads is better than that of trapezoidal threads, and special threads are even better. High-precision thread shape and reasonable oil and casing screwing torque are conducive to improving the sealing performance of threads. Thread sealing grease can play a role in lubrication, filling thread gaps (sealing), and anti-corrosion during the screwing of couplings and the use of oil and casing.

2. Performance of oil well pipes
The performance of oil well pipes includes anti-sticking performance, anti-collapse performance, anti-corrosion performance, and perforation performance.

(1) Anti-sticking performance: According to standard requirements, threaded joints of oil and casing need to be made and unmade. It is stipulated that each joint must be made and unmade 6 times each. Make up to the maximum torque recommended by the manufacturer, then unmake, and check the sticking of the internal and external threads of the oil and casing. The sticking of oil and casing threads is related to factors such as thread quality, thread surface hardness, make-up speed, surface friction coefficient, and contact stress (coupling screwing torque). To improve the anti-sticking performance of oil and casing threads, the finish of the threads, the hardness and uniformity of the threads should be improved, the threading speed should be reduced and the screwing torque should be controlled. At the same time, a softer metal or non-metal film layer should be plated on the inner thread surface of the coupling to separate the pipe body of the oil and casing from the coupling to prevent the metal surface of the two threads from bonding and to avoid tearing or even tearing of the threads. Before the coupling is screwed, the thread surface needs to be coated with thread grease to prevent the threads from sticking after the coupling is screwed and to improve the sealing performance of the threads. There are many coating methods for the surface of the coupling thread: such as galvanizing and phosphating processes; for some special materials and special connection threads, copper plating is often required. Factory-related sticking factors: thread parameters (pitch, tooth height, taper, tight moment, tooth profile half angle, etc.), matching of internal and external threads (surface treatment, surface finish, phosphating, galvanizing, copper plating, etc.), thread grease (function: lubrication, filling and sealing, etc., composed of metal powder and grease), thread control (threading torque, threading speed, etc.), material factors, etc. Factors related to the use and operation of oil fields include: lifting without thread protectors, skewed threading (the pipe swings in the air and is not concentric with the well thread), no threading or fewer threading, thread grease (not meeting standard requirements, sand and other debris), threading speed and threading torque, and clamping force of large tongs.

(2) Anti-collapse (crush) performance: As the drilling depth increases, the pressure on the oil and casing in the oil and gas wells increases, especially in deep wells, ultra-deep wells, or oil and gas wells in complex formations such as rock salt, salt paste, shale, and soft rock formations that need to isolate plastic flow. When the external pressure exceeds a certain limit, the oil well pipe body will produce a groove or elliptical deformation, which is called oil well pipe collapse.

(3) Corrosion resistance: Some oil and gas fields contain a large amount of corrosive media such as hydrogen sulfide, carbon dioxide, or chloride ions, which puts forward corrosion resistance requirements for oil and casing, including resistance to sulfide stress corrosion, resistance to CO2 and Cl- corrosion, etc. The corrosion resistance of oil and casing is mainly related to factors such as the chemical composition of steel and the residual stress value of steel pipe. Reducing the content of non-metallic inclusions and harmful elements in steel, increasing the corrosion-resistant elements such as Cr and Ni, reducing the residual stress in steel pipes, and improving the yield strength ratio of steel pipes are all conducive to improving the corrosion resistance of oil and casing.

(4) Perforation performance: The oil production part of the oil layer casing (layered oil production in multi-oil layer oil wells) requires perforation to allow crude oil to flow into the casing from the designated oil-bearing oil sand layer. For this reason, the oil layer casing is required to have good perforation performance, especially when using gunless perforation operations, the perforation performance of the casing is required to be higher. The perforation performance of the casing is obtained through perforation tests. That is, the casing to be tested is suspended in a simulated well, and a certain number of shaped perforating bullets with a certain distance and different directions are hung in the casing. Then perforation is carried out. After perforating, if there are no cracks around the holes of the test casing, the perforating performance is good; if there are a few small cracks around the holes, but their number and length do not exceed the requirements of the technical conditions, then the perforating performance is qualified; and if the number or length of cracks around the holes exceeds the requirements, especially if the cracks between two adjacent holes are connected, the perforating performance is unqualified. The oil field also has clear requirements on the amount of expansion of the casing after perforation and the height of the inner and outer burrs around the holes.