Intuitive comparison between heavy oil and thin oil, you can see that thin oil flows like water, but heavy oil is difficult to flow, which is caused by the high viscosity of heavy oil, and some heavy oil viscosity is as high as several million mPa·s, like “black mud”, so it can be picked up with a shovel and hand. The fluidity of heavy oil is too poor, and such viscous oil is naturally difficult to be extracted from the ground. Ultra-heavy oil refers to oil with a viscosity of more than 50,000 (mPa.s).
Crude oil with high viscosity and large specific gravity is called heavy oil. Heavy oil has high flow resistance, and it is difficult to flow into the wellbore from the oil layer, or to lift from the wellbore to the surface. For the heavy oil that has flowed into the wellbore, the viscosity reduction method or the dilution method is adopted, and the thermal recovery method is adopted for the heavy oil in the oil layer.
Viscosity Reduction Method: Add a certain amount of water-soluble ethylene oxide, propylene oxide, lauryl ether, sodium alkyl benzene sulfonate and other active agents to the water to form an active aqueous solution. According to a certain proportion, it is injected into the well, and the active aqueous solution is mixed with the heavy oil in the well by mechanical action to form an unstable and low-viscosity oil-in-water emulsion, which is then exploited by conventional methods.
The dilution method: injects a certain amount of thin oil into the well and dissolves with the heavy oil in the wellbore to reduce the viscosity of the heavy oil, which can be exploited by conventional methods. Such a method is applicable to areas with rare oil resources. Both this method and the viscosity reduction method are only suitable for the production of heavy oil that can flow into the wellbore at ground temperature but cannot be lifted to the surface.
Thermal recovery method:
uses heating to reduce the viscosity of heavy oil in the reservoir. There are two approaches.
The steam huff and puff method: injects a certain amount of steam into the heavy oil layer to reduce the viscosity of the heavy oil, which is an effective method for the recovery of heavy oil. Steam is injected from the surface into the reservoir, requiring a high pressure steam generator. To prevent the heat from the steam from being lost around the wellbore in the upper part of the reservoir, an insulated string (VIT/VIC) is required. The wellbore and wellhead are subject to high temperature and high pressure, requiring special completion methods. Since the 1950s, the steam injection test has been developed to the stage of industrial application.
Use downhole electric furnace to heat the oil layer to reduce the viscosity of heavy oil. This method consumes too much electricity, and has a limited range of heating the strata around the wellbore, and has a complicated process. It can only be used for heavy oil testing or areas where other extraction methods are not available.
Thermal recovery refers to the method of artificial heating, for the underground oil reservoirs that are difficult to recover under normal circumstances, and the crude oil can be recovered by heating methods, thereby improving the recovery rate. Thermal recovery is also an emerging oil recovery method in oilfield development and production in recent years.
Thermal wellhead equipment is designed as the dedicated heavy oil thermal production equipment to meet the demand for heavy oil production. It can satisfy the need for steam huff and puff and steam-flooding heavy oil production process. The equipment adopts an upper suspension structure, which can facilitate large-load service when tripping in the pipe string. Composed of a tubing head, a high temperature resistant gate valve and an oil production tree, the equipment is compact-structured, convenient for operation and reliable in safety, reaching the world’s advanced technological level and satisfying the demand for thermal heavy oil production process.
Dual Tubing Wellhead & Christmas Tree is used for extracting two oil reservoirs at the same time and also independently. It allows to put down two tubing into the same casing. Can seal the different two oil reservoirs with dual packer and single packer.
Domestically produced dual wellhead has an intermediate flange connected to the upper part of the spool, and the tubing is hung on the intermediate flange, resulting in a higher overall height and larger volume of the wellhead device. Due to the increased sealing surface, there are more leakage points. In addition, the existing dual-tube wellhead device for thermal recovery, when one pipe is used for injection steam, and the liquid in the other pipe string expands due to heat, and the pressure cannot be released, causing damage to the downhole pipe string and the surface wellhead device.
Therefore, Shengji has developed a dual-tube wellhead device for steam injection and oil production that is compact in structure and can release pressure from the tubing cavity.
The dual wellhead structure: bottom flange (1), spool (2), casing valve (3), insulated pipe pup joint (VIT) (4), adapter flange (5), tubing pup joint (6), tubing hanger ( 7) Temperature measuring sub (8), steam injection valve (9), test valve (10), adapter (11), polished rod stuffing box (12), pressure relief pipe (13), production valve (14), The restrictor (15) and other components.
The adapter flange has two parallel eccentric steam injection through holes and oil production through holes. The upper part of the steam injection through hole is embedded with a test valve, and the lower part is connected with Insulation pipe pup joint, middle bypass connected to steam injection valve, and the upper part of oil production through hole connecting rod stuffing box, while the lower part connected to tubing hanger with middle bypass connected to production valve.
Oil production through hole of adapter flange connect with the inner bore of the spool through pressure relief pipe. When the well is filled with steam (braising), the pressure in the tubing cavity is released in the casing through the pressure relief pipe to reduce accidents。
The compact structure developed by Shengji allows the internal pressure of the tubing to release and reduces the occurrence of accidents.