How to solve the problems of easy corrosion and gas explosion of tubing head seals in high temperature and high-pressure oil and gas wells？
The tubing head is a device component that connects the uppermost casing head and the oil (gas) tree to suspend the tubing and seal the annular space between the tubing and the casing. The tubing head is located above the casing head, including the casing spool and the tubing hanger. Its function is to suspend the tubing, downhole tools and seal the annular space of the oil casing.
Ultra-high temperature and ultra-high pressure are common in deep and ultra-deep wells, and drilling instruments and tools, drilling fluids, and materials are facing severe challenges. In the Xujiaweizi area of Daqing, the bottom hole temperature of Well Gulong 1 is as high as 253°C, and the geothermal gradient is as high as 4.1°C/(100m); the formation pressure encountered in Well Shuntuo 1 is 170.0MPa.
The main problems caused by ultra-high temperature and ultra-high pressure are: casing and cement ring seal the formation failure, resulting in annular pressure; drilling and completion tools and downhole instruments have high requirements for temperature and pressure resistance, and the failure rate increases significantly. The failure rate of downhole instruments in the region used to be as high as 60%; drilling fluid treatment agents and materials are prone to failure, poor rheology and settlement stability, and difficulty in performance regulation, wellbore stability, and leakage prevention and plugging; cement slurry controls water loss and controls slurry Difficulties such as thickening time increase the difficulty and risk of cementing construction.
The world’s first deep, ultra-deep and ultra-deep wells were all born in the United States. Onshore deep and ultra-deep wells are mainly concentrated in Texas (accounting for more than half), and offshore deep and ultra-deep wells are mainly concentrated in the Gulf of Mexico. At present, more than 80 countries in the world can drill deep wells, and more than 30 countries can drill ultra-deep wells, indicating that deep and ultra-deep wells have become a major demand for global oil and gas resource exploration and development, and deep and ultra-deep well drilling and completion technologies have been mature. The international advanced level of deep and ultra-deep well drilling and completion technology has already exceeded 12,000m vertical depth, and major equipment such as drilling rigs initially have the ability to drill 15,000m deep and are developing towards automation and intelligence.
Judging from the drilling indicators of deep and ultra-deep wells, the average well depth of PetroChina’s deep wells is about 5540m, and the average well depth of ultra-deep wells is 6748m; the average drilling cycle is shortened year by year, deep wells are less than 105d, and ultra-deep wells are about 125d; the average ROP is increasing year by year, in 2019, the deep well reached 5.66m/h, and the ultra-deep well reached 4.64m/h. China’s onshore deep and ultra-deep wells have complex geological conditions, high drilling safety risks and long periods. In particular, problems such as ultra-high temperature and ultra-high pressure, multi-pressure systems, hard formations, poor drillability, and rich acid fluids coexist in the Tarim Basin and the Sichuan Basin. Drilling is the most challenging.
With the deepening of petroleum exploration and development, more and more high-temperature and high-pressure oil and gas wells are encountered, and the requirements for wellhead devices are also getting higher and higher. The tubing head is a key component in wellhead equipment, and its performance is of vital importance to safety and reliability. In the current technology, the seal of the hanger adopts a combined structure of rubber seal + metal seal, and the seal of the tiedown screw adopts a structure such as rubber seal or graphite. Rubber materials are easy to be corrosive and explode under high pressure; metal materials are prone to plastic deformation after repeated high and low-temperature changes and lose their sealing function; graphite materials are easy to react with certain elements in the medium to become loose and make the seal fail. Moreover, these types of sealing combinations require a certain pre-tighten to compress the seals to achieve the sealing function.
Therefore, Sanjack Company designed a special structure of PR2 tubing head, which can achieve pressure self-sealing without pre-tightening, and effectively solves the problem of easy corrosion and gas explosion of seals.
The tubing head structure consists of tubing head spool (1), pressure gauge (2), pressure gauge check valve (3), thread flange (4), valve (5), tiedown screw assembly (6), hanger assembly (7), adapter flange (8), plug (9), etc.
Working principle: The main and secondary seals of the hanger assembly adopt a U-shaped sealing structure, which can realize the medium pressure self-sealing without pre-tightening; the screw auxiliary seal of the tiedown screw assembly is a multi-piece combined V-shaped structure. The greater the pressure within the pressure-bearing capacity range, the better the sealing effect; the sealing material is made of PTFE, which effectively solves the problems of rubber seal corrosion and gas explosion, graphite seal looseness, and metal seal plastic deformation.
The structure developed by Sanjack is novel, the U-shaped sealing structure realizes the self-sealing of medium pressure, and the sealing material is PTFE, which effectively solves the problem of easy corrosion and gas explosion of the rubber seal, improves the reliability of the seal, and extends the service life of the seal.