How to reduce the stress of the casing in the thermal recovery wellhead to maintain the stability of the wellhead？
Heavy oil thermal flooding refers to a more effective method for heavy oil oilfields to increase the temperature of the formation crude oil and reduce the viscosity of the crude oil, thereby increasing the mobility ratio of the formation oil and increasing the oil recovery factor.
Heavy Oil features
- The high content of colloidal asphaltenes and less light fractions. High viscosity and high relative density are the most important characteristics of heavy oil;
- The content of heteroatoms such as sulfur, oxygen, and nitrogen is high. For example, the sulfur content of heavy oil in the United States, Canada, and Venezuela is as high as 3% to 5%;
- Heavy oil contains more rare metals, such as Ni, V, Fe, Mo, etc.;
- The paraffin content in heavy oil is generally low, but there are also very few “double high crude oils”;
- In the same heavy oil reservoir, the properties of crude oil are often very different in different sections of the vertical oil layer and between wells on the plane; in the same oil field or oil area, the properties of crude oil are even more different.
- Visco-temperature characteristics of heavy oil (the theoretical basis for thermal recovery of heavy oil);
- Distillation characteristics of heavy oil (one of the mechanisms of steam flooding and fire flooding); when the temperature rises to the bubble point (the lowest temperature when crude oil begins to vaporize), the light components in the crude oil will be separated into the gas phase and recombined. The fraction remains in the liquid phase;
- Thermal cracking characteristics of heavy oil (more prominent in the process of burning oil layers than in the process of steam flooding); thermal cracking of heavy oil means that when the temperature rises to a certain extent, the heavy components in heavy oil will be cracked into coke and light components;
- Thermal expansion characteristics of heavy oil;
According to the driving mode, heavy oil steam injection can be divided into two stages: steam huff and puff and steam flooding, which is a common practice abroad.
Steam huff and puff: The method is simple, and the economic risk is small. Each well can perform 5 to 8 cycles of huff and puff, and the oil recovery rate is as high as 3% to 8%, but the crude oil recovery rate is only 10% to 20%, and a large number of recoverable reserves are lost. The effective heating radius of steam huff and puff is less than 30m. (analog result)
Steam Drive: High technology, high investment, high speed, high energy consumption, whether high profit or high level depends on the advanced reservoir geological conditions and process technology.
With the increase of high temperature (150℃), high pressure (100 MPa), and high H2S (2%~70%) gas wells. Once the tubing and casing in this type of gas well burst and fail, it will cause the tubing and casing to collude, and even the sulfur-containing gas will overflow the ground, causing great harm.
In the design of oil casing. Although API 5CT gives the minimum internal pressure resistance strength of tubing and casing, it does not give the maximum burst failure strength, and it is very important to accurately predict the internal pressure resistance ultimate strength of tubing and casing, that is, burst strength.
The device for suspending the casing on the wellhead, according to the formal requirements, after each layer of the casing is laid and cementing is completed, the top joint should be removed, and the casing head should be connected to the top of the casing to seal the two layers. The annular space between the casings suspends the second casing string and bears part of its weight.
For example, a standard wellhead device should include: a casing head connected to the surface casing on the surface casing, in which the casing hanger is connected to the technical casing; the oil casing head installed on the surface casing head, in which the casing The oil layer casing is connected to the hanger.
Install the tubing head on the oil layer casing head, and the tubing is connected to the tubing hanger in it. According to the different connection methods, the casing head is divided into two categories: thread type and flange type. In recent years, automatic sealing casing heads have been produced abroad, which are suitable for high-pressure gas wells.
With the widespread application of heavy oil thermal recovery, the demand and role of thermal recovery wellheads have become increasingly prominent, and thermal recovery wellheads have become an important part of steam injection oil recovery. At present, high-temperature and high-pressure thermal wellheads in heavy oilfields generally connect the bottom flange of the thermal wellhead directly to the casing. When steam is injected down to the well through the wellhead, the casing rises and grows as the temperature rises, and the wellhead moves upward; after braising well and blowout, the casing temperature returns to the normal, and the wellhead moves downward, so the stability of the wellhead is not good.
Therefore, the bottom flange of the thermal recovery wellhead device developed by Sanjack Company is directly connected to the upper flange of the casing head, and the outer diameter sealing ring of the upper part of the hanger and the inner hole of the bottom of the upper flange form a metal seal, which ensures a reliable seal performance and effectively eliminate the stress caused by thermal expansion and contraction of the casing.
The thermal recovery wellhead consists of upper joint (1), adapter flange (2), inner hole (3), annular groove (4), sealing ring (5), hanger (6), casing head housing (7), casing (8) and other components.
Working principle: An adapter flange is added to the upper part of the casing head housing, and there is an upper joint on the adapter flange. The upper joint is connected with the bottom flange of the thermal wellhead device. The lower part of the adapter flange is a cylindrical structure inserted into the casing head housing. There are a number of sealing rings on the outer diameter of the upper end of the hanger. The sealing ring cooperates with the inner hole of the adapter flange to form a metal seal.
When the steam is injected into the well and then the casing extends or contracts due to thermal expansion and contraction, the lower part of the hanger then moves up and down in the casing head housing, so that the thermal wellhead device installed on the top of the adapter flange will not move up and down due to the rise and fall of the casing, ensuring the stability of the wellhead.
This kind of metal-sealed thermal casing head can effectively eliminate the stress formed by thermal expansion and contraction of the casing and ensure the stability of the wellhead device.