What are the core categories and functions of OCTG?
OCTG (special tubing for petroleum) is not a single tubing category, but a set of special tubing system covering the whole process of oil and gas exploration and development, and its core categories are accurately divided according to functional positioning, which are suitable for key links such as drilling, cementing, oil production and gathering and transportation. Each category has been scene-designed, optimized in material, structure and performance, which together form the "blood vessel" and "skeleton" of the oil and gas industry, directly determining the construction safety, service stability and production efficiency of oil and gas wells. In this paper, the four core categories, subdivision types and core functions of OCTG are systematically dismantled with the subtitle of question sentence, and the application boundaries and values of each category are clarified.
1.Casing: the core support of cementing link. How to stratify its functions?
Casing is one of the categories with the largest consumption and the most critical function in OCTG. Its core function is to support the borehole wall and isolate formation fluid during cementing, prevent well collapse and oil and gas leakage, and lay a solid safety foundation for subsequent drilling and oil production operations. According to the difference of running depth, position and function, casing can be divided into three types: surface casing, technical casing and oil casing, which form a protective system step by step to adapt to different formation conditions.
The surface casing is mainly run into the shallow stratum, and the depth is usually tens to hundreds of meters. Its core function is to fix the wellhead, isolate the loose stratum on the surface from the shallow groundwater, avoid the shallow collapse and block the wellbore during drilling, and provide guidance for subsequent casing running. Technical casing is mostly used in deep and complex strata, and it is used when the running depth exceeds 2000m and it encounters high-pressure fluid layer, easy-to-collapse rock layer or salt-gypsum layer, which can isolate the erosion and extrusion of the wellbore caused by complex strata, protect the drilling fluid circulation channel and ensure the continuous progress of drilling operation. The oil layer casing directly runs into the oil and gas interval and is in close contact with the oil and gas layer. Its core function is to isolate the oil and gas layer from other formations, prevent fluids from different formations from mixing, and provide a channel for oil and gas to be transported from the formation to the ground. It needs high sealing, corrosion resistance and high pressure resistance.
Working condition adaptability is the core difference between OCTG and ordinary pipes. The former needs to deal with the extremely complex environment of oil and gas development, while the latter only adapts to the simple scene of normal temperature and pressure and no strong corrosion. In oil and gas wells, especially in deep and ultra-deep wells, OCTG needs to withstand the continuous high temperature of 150℃ to 350℃, and the temperature of some ultra-deep wells even exceeds 400℃, and at the same time bears the high pressure of 20MPa to 140MPa, which is equivalent to bearing the pressure of 200 to 1400 kg per square centimeter, far exceeding the tolerance limit of ordinary pipes.
In addition, the connection mode of casing should be matched with steel grade as required. Corrosion-resistant steel-grade casing is selected for high-acid gas fields, and high-strength casings such as P110 and Q125 are selected for ultra-deep wells. Special threads with excellent sealing performance are used for the connection parts to ensure no leakage and deformation under extreme working conditions, and the service life can reach more than 20 years.
2. Tubing: the "channel" for oil and gas transportation. What are its core characteristics?
The core function of tubing is to transport crude oil and natural gas in oil and gas reservoirs to the surface, and at the same time, it is suitable for auxiliary oil production operations such as water injection, gas injection, chemical addition and workover, and it is the key link connecting underground oil and gas reservoirs with the surface gathering and transportation system. Compared with casing, tubing is frequently subjected to oil-gas medium erosion, pressure fluctuation and operation wear, and the requirements for sealing, corrosion resistance and fatigue resistance are more stringent.
According to the connection mode, tubing is mainly divided into coupling tubing and non-coupling tubing. The coupling tubing is connected with a single tubing through a special coupling, which is simple in structure and convenient to assemble and disassemble, and is suitable for conventional oil and gas wells. The coupling-free tubing adopts integral threaded connection, eliminating the coupling structure, which not only reduces the leakage risk, but also reduces the space occupied by the wellbore, and is suitable for oil and gas wells with high pressure, high yield and large displacement, with better sealing performance and structural strength. According to the classification of materials, carbon steel tubing is selected for conventional oil and gas wells, 13Cr stainless steel tubing or nickel-based alloy tubing is selected for high-corrosion scenes, and high-strength alloy tubing is selected for ultra-deep wells, which accurately matches the requirements of different working conditions.
In the development of unconventional oil and gas such as shale gas and coalbed methane, the tubing needs to adapt to hydraulic fracturing operation and bear the impact and erosion of high-pressure fracturing fluid. Therefore, it is necessary to optimize the thickness and toughness of the pipe wall to ensure the smooth development of fracturing operation and the stability of subsequent oil and gas transportation.
3. Drill pipe: the "transmitter" of drilling power. What is its core function?
Drill pipe is the core power transmission carrier of drilling operation, and it also undertakes the function of transporting drilling fluid. It needs to transfer the torque, tension and pressure of surface drilling equipment to the downhole drill bit to drive the drill bit to break the rock stratum, and at the same time, it transports drilling fluid from the ground to the drill bit to cool the drill bit and bring cuttings back to the ground, which directly affects drilling efficiency and drilling safety.
Drill pipes are usually made of seamless steel pipes, with special joints welded at both ends and high-strength threaded connections at the joints to ensure efficient power transmission and sealing of drilling fluid. Because the drill pipe is subject to alternating torque, tension, pressure and drilling fluid erosion during drilling, it is prone to fatigue fracture and corrosion damage. Therefore, the drill pipe is made of high-strength alloy carbon steel, which is tempered and heat treated to improve its toughness and fatigue resistance, and the joint will be treated with wear resistance and corrosion resistance to prolong its service life.
According to the length and use, drill pipes can be divided into conventional drill pipes, weighted drill pipes and transition drill pipes. Conventional drill pipe undertakes the main tasks of power transmission and drilling fluid transportation; Weighted drill pipe is used to increase drilling weight, improve the ability of bit to break rock strata, and adapt to drilling in hard strata; Transition drill pipe is used to connect drill pipes with different specifications with drill bits, adjust torque transmission efficiency and avoid stress concentration at joints. Before leaving the factory, each drill pipe must undergo strict nondestructive testing to ensure that there are no defects such as cracks and blowholes, so as to prevent drilling accidents during drilling.
4. Pipeline: the "link" of oil and gas gathering and transportation, which scenarios are suitable for?
Pipeline pipe is mainly used in oil and gas gathering and transportation network, connecting oil and gas wells, metering stations, treatment plants, storage facilities and transportation trunk lines. It is the core component of oil and gas ground transportation system, undertaking the task of long-distance transportation of large-scale oil and gas media, and needs to adapt to complex laying environments such as outdoor, underground and ocean, taking into account the ability of compression resistance, corrosion resistance, soil stress resistance and environmental impact resistance.
According to the classification of manufacturing process, pipeline can be divided into seamless pipeline and welded pipeline, and welded pipeline includes LSAW pipeline and SSAW pipeline. Seamless pipes and conduits have no weak points of welds, and are suitable for high pressure and high stress scenes; LSAW pipeline has high weld strength and uniform stress distribution, which is suitable for large-diameter and long-distance transportation. SSAW pipeline has high production efficiency and low cost, and is suitable for medium and low pressure gathering and transportation pipeline network. According to the classification of steel grade, the steel grade of pipeline pipe varies from X42 to X100. The higher the steel grade, the better the strength and toughness, which can meet the transportation needs of deeper distance and higher pressure.
In order to improve corrosion resistance, buried pipelines are usually equipped with FBE fused epoxy coating and 3PE composite coating, and offshore pipelines are also equipped with cathodic protection technology. At the same time, the pipe structure is optimized to adapt to environmental factors such as soil settlement and wind and wave impact, so as to ensure stable, safe and efficient oil and gas long-distance transportation and reduce leakage risk and maintenance cost.
5. Category collaboration: How to support the whole process of oil and gas development?
The four core categories of OCTG do not exist in isolation, but cooperate with each other according to the oil and gas development process to form a complete protection and transportation system. In the drilling stage, the drill pipe transmits power and drilling fluid, and the surface casing and technical casing are gradually lowered into the fixed borehole wall to escort the drilling operation; In the completion stage, the oil layer casing is run and cemented to isolate the oil and gas layer from other formations and build oil and gas transmission channels; In the oil production stage, tubing transports oil and gas to the surface, which is suitable for various auxiliary operations; In the gathering and transportation stage, the pipeline will transport the medium of dispersed oil and gas wells to the treatment plant to complete the last link of oil and gas exploitation.
The materials, steel grades and specifications of various categories should be accurately matched according to well types, formation conditions and working conditions. For example, full-process corrosion-resistant OCTG should be selected for high-acid gas fields, high-strength OCTG should be selected for ultra-deep wells, and special OCTG for seawater corrosion resistance should be selected for offshore oil and gas fields. This ability of category coordination and scene adaptation makes OCTG an indispensable core equipment for oil and gas exploration and development, which supports the efficient and safe exploitation of conventional and unconventional oil and gas resources.
