As oil field exploration continues to expand and drilling depths gradually increase, the number of deep and ultra-deep wells is also increasing. As the depth of the well increases, the stress and deformation experienced by the well casing pipebecome more complex. Therefore, in terms of well casing strength design, the factors that must be considered become more comprehensive and complex. Well casing columns are subjected to a variety of complex loads during oil and gas drilling and production, including axial load, external extrusion force and internal pressure, etc. The changes and effects of these loads are critical to the strength design of well casing pipe columns. The load analysis and strength design of well casing pipe will be analyzed in detail below.
Axial load refers to the force acting along the axis of the well casing pipe column, which is mainly caused by the following factors:
Self-gravity: The weight of the well casing itself is one of the most basic axial loads. Especially when the well is long and deep, the self-weight will produce greater pressure on the well casing pipe.
Fluid pressure in the well: The oil and gas produced in the well or the injected water, gas and other fluids will produce a certain pressure on the well casing string. Especially during cement injection and production, the pressure in the well will change with different operating stages.
External squeezing force refers to the squeezing force from the formation, which is mainly affected by factors such as formation pressure, formation lithology, and well depth. Under different formations and geological conditions, the external extrusion force experienced by the casing string will also be different. For example, in a salt rock layer, due to the rheological properties of the salt rock, the casing string bears a large external extrusion force, and the pressure gradient calculation of the salt rock layer needs to be considered.
Internal pressure refers to the pressure generated inside the well casing string, usually due to the presence of fluids in the well. During operations such as acidizing fracturing, the interior of the casing string may be subjected to large internal pressures, and the impact of these operations on the strength of the well casing string needs to be considered.
The strength design of the well casing pipe string needs to comprehensively consider the effects of the above-mentioned various loads to ensure that the casing string can operate safely and reliably under various working conditions. The specific analysis is as follows.
First, select appropriate materials and strength levels based on the working environment and conditions of the casing string. Materials should have good properties such as tensile strength, yield strength and hardness to cope with the effects of different loads.
The material selected for the casing string is critical to its strength design. Commonly used casing materials include carbon steel, alloy steel, etc. When conducting material property analysis, the following aspects need to be considered:
Tensile Strength and Yield Strength: This is a key indicator of the tensile and yield strength of a material. Tensile strength represents the maximum resistance of a material under tensile stress, while yield strength represents the stress level at which plastic deformation begins.
Hardness: Hardness refers to a material’s resistance to external hard objects and is usually used to evaluate a material’s wear resistance and pressure resistance.
Chemical composition: The chemical composition of a material has an important impact on its mechanical properties and corrosion resistance. Appropriate materials need to be selected according to the actual working environment.
Combined with the influence of axial load, external extrusion force and internal pressure, a reasonable casing string structure is designed, including diameter, wall thickness, joint design, etc. Load changes under different working stages and geological conditions must be considered to ensure the stability and safety of the casing string.
Load calculations and strength assessment: Calculations and assessments of various loads, including static and dynamic loads, as well as possible sudden load conditions. Conduct strength assessment and verification in accordance with API 5CT and other relevant standards and specifications to ensure that the well casing pipe string meets the strength design requirements.
Safety factor and monitoring: Consider the safety factor during the design process and make adequate safety reserves for the carrying capacity. At the same time, a real-time monitoring system for casing strings is established to promptly discover and deal with potential safety issues to ensure the safe operation of underground operations.
Well casing pipe diameter and wall thickness: Diameter and wall thickness are key parameters that determine the load-bearing capacity of the casing string. They need to be reasonably selected based on factors such as well depth, well pressure, and formation properties.
Well casing pipe joint design: The joint of the well casing string is its weakest part. It is necessary to design a suitable joint structure and connection method to ensure that the joint can withstand the expected load and pressure.
Thread design: The thread design between the casing string and the joint is also very important. The quality of the threaded connection directly affects the overall stability and sealing of the well casing string.
To sum up, the strength design analysis of well casing needs to consider many aspects such as material properties, structural design, load analysis, thermal expansion and temperature effects, and compliance with standard specifications. Through reasonable design and analysis, the performance of well casing pipe in drilling operations can be ensured. Safety and reliability.