The world's first large-scale offshore thermal development oilfield for ultra-heavy oil has been put into production
Pry the seabed "black amber"
Our reporter Wang Zhen
Energy is an important basic resource for the development of human society.
Northeastern Bohai Sea, Liaodong Bay waters.
The production platform of Lvda 5-2 North Oilfield, the world's first large-scale development of offshore super-heavy oil reserves, stands proudly here.
9 tons, 60 tons, 109 tons... Nearly a month after it was put into production, the daily oil production of the first ultra-heavy oil steam huff and puff well in the oilfield has gradually increased, basically reaching the peak output designed in the development plan.
The successful application of this model will stimulate the development of 100 million tons of ultra-heavy oil resources like "black amber" in the Bohai Bay Basin, which is of great significance to safeguarding national energy security.
What is heavy oil and super heavy oil?
What is the status of heavy oil development in my country?
Where is the difficulty in the development of super-heavy oil that makes people "worry on top of worry"?
What are the technological breakthroughs behind the commissioning of the world's first large-scale offshore large-scale ultra-heavy oil thermal development oilfield?
The reporter recently interviewed the responsible comrades of China National Offshore Oil Corporation.
Awaken the sleeping "undersea gold"
Converting scale reserves to production
Lubricating oil, lubricating grease, various types of asphalt used in the transportation and construction industry, fuel oil for industrial and power plants... In the future, many of these products sold in the market may be derived from the super-heavy oil in the Bohai Bay Basin.
According to the relevant person in charge of CNOOC, the Lvda 5-2 North Oilfield project, which was put into operation on April 23, is expected to produce over 6.5 million tons of crude oil after it is fully put into operation, and all of it will be refined into asphalt and can be laid over 4,000 kilometers. The expressway is equivalent to the straight-line round-trip distance from Beijing to Sanya.
Different from other offshore oilfields in my country, Lvda 5-2 North Oilfield belongs to unconventional heavy oil, and the viscosity of crude oil in the formation is mostly over 50,000 mPas, which is more than 20 times that of the thickest crude oil developed in the Bohai Sea.
What concept is this?
"It is close to solid bitumen at room temperature and cannot flow at all." Liu Yingxian, chief reservoir engineer of Bohai Petroleum Research Institute of CNOOC Tianjin Branch, gave two examples to reporters, "The viscosity of honey we usually eat is usually about 3000 mPa. The viscosity of the cream used is usually around 10,000 mPas.”
The reporter learned that the oil that has not been processed after exploitation is called crude oil, which is a complex mixture, mainly including various hydrocarbons, colloids, asphaltenes, etc.
In different oilfields, there are differences in the color, composition, viscosity or density of the produced crude oil due to factors such as the burial depth of the oil layer, the condition of the caprock and the migration distance.
"Crude oil with a viscosity of more than 50 mPas is heavy oil and is regarded as 'underground gold'." Liu Yingxian explained that 50 mPas is equivalent to the viscosity of butter at room temperature.
On this basis, taking 10000 mPas and 50000 mPas as the dividing points, heavy oil can be divided into ordinary heavy oil, extra heavy oil and super heavy oil.
Statistics show that about 70% of the world's remaining oil resources are heavy oil.
Sustained, effective and economical development of heavy oil has become the main direction for countries to increase oil production.
As one of the four major heavy oil producers in the world, my country has about 19.87 billion tons of heavy oil resources, of which 3.55 billion tons have been proven, and the development potential is huge.
In the Bohai Sea alone, heavy oil reserves account for nearly half of the total proven crude oil reserves.
It is self-evident that it is of great importance to my country's energy security to wake up this part of the dormant "subsea gold" and change the scale of reserves into production.
However, heavy oil extraction is not easy.
According to experts, heavy oil like "black amber" also has a name in the industry - "sorrowful oil".
Because of its high viscosity and poor fluidity, its mining is difficult and costly, and it has been a black treasure that is "unreachable" by human beings for many years.
In particular, offshore oilfields cannot arrange dense well patterns like onshore oilfields. If heavy oil is to be lifted from the subsea formation to the surface and then transported, extremely high requirements are placed on oil production technology, process processing, and offshore pipeline transportation. It is a recognized world-class mining problem.
The exploitation of ultra-heavy oil at sea is even more worrying.
How to effectively leverage these heavy oil reserves and find a "good recipe" suitable for efficient offshore development has become an important part of my country's scientific research.
From scheme design to mechanism characterization, from effect evaluation to replacement technology... A series of research and technological breakthroughs have made important breakthroughs in my country's offshore heavy oil development.
In 2010, the first-phase comprehensive adjustment project of Suizhong 36-1 oilfield was put into operation, breaking the prediction that the recovery rate of conventional heavy oil water flooding development would not exceed 20%; The 21-2 platform was successfully put into production, filling the technical gap of large-scale thermal recovery of heavy oil in my country's offshore oilfields.
In April this year, the commissioning of Luda 5-2 Beichao heavy oil oilfield marked a new stage in my country's offshore heavy oil development.
"Steam Sauna" for Subsea Oil Formation
Accurately determine the inflection point temperature to make heavy oil thinner
Although heavy oil is stubborn, it also has a weakness - fear of heat.
Test data show that the viscosity of heavy oil will decrease by about half for every 10 degrees Celsius increase in temperature.
Therefore, different from the "cold recovery" development mode of conventional crude oil, the "thermal recovery" mode of heavy oil is usually adopted.
That is, inject hot water or high-temperature and high-pressure steam into the formation to increase the temperature of the crude oil in the formation, thereby reducing the viscosity of the crude oil, turning it into "thin oil" with good fluidity, and then exploiting these "thin oil" while it is hot.
According to industry experts, there are two commonly used "thermal recovery" modes for heavy oil.
One is the "steam flooding" mode, which is to continuously inject high-temperature and high-pressure steam into the well, heat the underground crude oil through the steam, and then "push me" to drive the adjacent thinned crude oil to the production well for production.
The other is the "steam huff and puff" mode, which is to inject high-temperature and high-pressure steam into the oil layer first, heat the oil layer around the oil well to reduce viscosity, and then heat the well and then open the well to produce oil.
Lvda 5-2 North Oilfield adopts the "steam huff and puff" mode.
Injecting steam, simmering wells, mining... Liu Yingxian vividly gave the reporter an analogy, "It's like 'steaming a sauna' for the submarine oil layer."
However, "steaming a sauna" for heavy oil is not a simple project.
How high the temperature must be reached to thin the heavy oil, how can the steam injection be more uniform, how can the thinned crude oil be produced smoothly... These are all technical difficulties to be overcome.
Accurately determining the inflection point temperature of heavy oil thinning is the key to the smooth development of Lvda 5-2 North Oilfield.
"Only after the heat injection is ensured, the temperature of the crude oil in the formation, the wellbore and the surface pipeline is higher than this inflection point temperature, and the heavy oil can be produced smoothly." Liu Yingxian told reporters, and this is also a problem that has plagued the industry for many years.
"It's not that the higher the viscosity of the heavy oil, the higher the inflection point temperature. Even if the viscosity is similar, the inflection point temperature of crude oil in different oil fields may be very different." According to the relevant researchers, through a lot of experiments, they have worked out a set of quantitative calculation methods for the inflection point temperature. , and found that although the viscosity of Luda 5-2 North Oilfield crude oil exceeds 50,000 mPas, the inflection point temperature is similar to that of ordinary heavy oil in developed oilfields.
"I'm very happy, this shows that it is feasible to develop the ultra-heavy oil in Lvda 5-2 North Oilfield by 'steam huff and puff'!" Liu Yingxian said, in order to verify the reliability of the calculation results, the researchers also carefully designed a verification plan, and finally passed Hundreds of sets of experimental data have confirmed that the inflection point temperature of crude oil in Lvda 5-2 North Oilfield is about 90℃.
"The water used in the 'steam sauna' is very particular, and strictly treated pure water should be used." Liu Yigang, deputy chief engineer of CNOOC Tianjin Branch, told reporters.
However, for offshore operating platforms, freshwater resources are scarce, and they can only choose from seawater or formation water.
According to experts, seawater and formation water have their own advantages and disadvantages: seawater desalination can provide a larger amount of water, but the treatment process is longer and more complicated; the formation water treatment process is relatively simple, but if it contains oil, it will cause membrane damage in the water treatment process Component failure, unable to produce a qualified boiler water supply.
How to avoid risks on the premise of reducing costs and increasing efficiency?
After repeated demonstrations, the researchers finally developed the first offshore dual-source boiler water treatment system.
The system is based on the use of formation water as the water source. During the production process, once it is detected that the oil in the formation water is no longer suitable as the boiler water supply source, it can be switched to seawater water source through simple equipment replacement and pipeline connection.
As a result, the investment cost can be reduced by about 30% compared with the traditional desalination system.
At the same time, due to the limited reach of the steam and the further away from the steam injection well, the thermal energy will continue to decrease.
In order to improve the heat utilization rate and economic benefits of injected steam, the researchers also innovatively formed a quantitative prediction technology for the sweep range of steam injection, and established a prediction model for the sweep range of steam injection in thick-layer heavy oil reservoirs, which can be used for different crude oil viscosity and oil layer thickness. The vertical sweep coefficient of the injected steam is evaluated, and the parameters are dynamically and timely adjusted according to the evaluation results to control the steam injection speed and injection volume.
Innovate and implement the integration of injection and production
Efficient connection between heat injection and production
Oil production, water content, equipment pressure... On May 12, when the reporter contacted Wang Hongyu, engineer of the Oil Production Technology Research Institute of Bohai Petroleum Research Institute, CNOOC Tianjin Branch, she was working according to the Luda 5-2 North Oilfield. The operating parameters of the first ultra-heavy oil well fed back by the platform, analyze the production and operation of the jet pump, and provide technical support for the subsequent thermal recovery wells to be put into production.
The jet pump injection-production integration process is the core process for lifting crude oil from the formation economically and efficiently, and it is also a major innovation in the field of heavy oil thermal recovery.
"The jet pump injection-production integration process we adopted is that a set of wellbore string is used together in the heat injection process and the production process, which realizes the efficient connection between heat injection and production." Liu Yigang introduced.
Due to the limited area of the offshore oilfield operating platform and the complicated oil well trajectory, the rod-type pumping unit commonly used in the production of super-heavy oil on land cannot be used, and the conventional two-pass injection and production string is injected into the heat injection string during steam injection. It takes a lot of time and operating costs to propose that the heat injection string is then run into the production string.
After repeated comparison and demonstration, the scientific researchers innovatively adopted the jet pump injection-production integrated pipe string technology, which combined the heat injection pipe string and the production pipe string into one, which greatly reduced the workover time and saved the workover operation cost by more than 30%. .
According to estimates, 8 rounds of operations in 26 wells can save more than 200 million yuan in operating costs, greatly improving the economics of large-scale thermal recovery of ultra-heavy oil.
"The Lvda 5-2 North Oilfield operating platform is divided into a wellhead platform and a production support platform. The two platforms are connected by a trestle, and the projected area is not as large as a standard football field. For thermal recovery platforms, whether it is process, The process and supporting facilities are more complicated." Liu Yigang told reporters.
Take injector as an example.
For conventional cold mining platforms, supporting water injection equipment is sufficient.
The thermal recovery platform not only needs to add special water treatment equipment, but also needs to add large-displacement boilers and related supporting facilities suitable for large-scale thermal recovery.
Therefore, at the beginning of the design, the researchers took these factors into consideration, and the related supporting facilities were as small and light as possible.
It is understood that the large-scale thermal recovery and large-displacement steam boiler equipment formed by the innovation of the project not only realizes the simultaneous injection of 4 wells, but also has the function of superheating, and greatly reduces the floor space.
At the same time, the electrostatic gathering separator + cyclone desander separator is used to realize the efficient separation of super-heavy oil, water and sand in the production stage.
Not only that, the equipment's ability to resist high pressure and high temperature, thermal insulation effect, and heat injection safety are also important considerations for researchers.
On the one hand, researchers have carried out in-depth research on the heat injection pipe string, and formed a fully enclosed high dry steam injection pipe string process without hot spots, which greatly reduces the heat loss of the wellbore and ensures the heat injection effect.
On the other hand, it has innovated and formed a high-temperature downhole safety control system including key tools such as high-temperature downhole safety valve and high-temperature thermal recovery packer, which has reached the international leading level and built a solid safety barrier for the development of Lvda 5-2 North Oilfield.
High-temperature downhole safety control, jet pump injection-production integration, offshore small-scale multi-functional thermal recovery equipment, physical-modulo-digital-model integration design... A series of technical breakthroughs have enabled my country to achieve breakthroughs in the large-scale and economic development of ultra-heavy oil.