With the development of the petroleum industry, oil reserves are declining year by year, and oil exploitation is becoming more and more difficult. However, nearly two-thirds of the crude oil is still blocked underground because it has not been harvested by the first exploitation. Nowadays, new methods and technologies have been explored, which is injecting nitrogen into oil layer. This is a technology that uses the characteristics of nitrogen to maintain oil layer pressure, mixed phase and immiscible phase drive and gravity drainage, which can greatly improve oil recovery, and has great significance for stable production and high yield of petroleum industry.
From the perspective of multiple reservoirs, the injection of nitrogen in oil layer mainly has the following effects:
Maintain reservoir pressure
The pressure of the oil and gas layer is maintained at or above its dew point pressure or bubble point pressure, or maintained at the current pressure level, so that the reservoir fluid can flow smoothly.
Gravity drainage and immiscible flooding
According to characteristics of low nitrogen density, it is injected into the top of the structure or allowed to migrate to the top of the structure, enhancing the role of downward displacement reservoir fluid or gravity and stabilizing the mixed phase slug, and increasing the production of the reservoir fluid.
The multiple-contact miscible phase of nitrogen is used to displace oil and gas in the oil and gas reservoir.
Driving carbon dioxide slug
Nitrogen is used as a displacement fluid to ignite a mixed phase slug formed by a solvent such as carbon dioxide in the oil and gas layer.
Attic oil extraction
High pressure nitrogen can displace trapped crude oil from the attic layer at the top of the construction into the production well.
Gas cap displacement
Injecting nitrogen into the best part of the reservoir maintains or increases the reservoir pressure while simultaneously displacing and producing the gas capping gas.
High pressure nitrogen application
Fracturing and acidizing operation is one of the effective measures to stabilize production and increase production in oil fields. After fracturing and acidification, the fracture channel of the oil layer is lengthened, the permeability is improved, and the oil and gas recovery is improved. According to statistics, with proper acidification operation of the oil and gas wells, the output can be increased several times to several tens of times.
Gas lift drainage
After the fracturing acidification is completed, the necessary draining and squirting operations are carried out. In recent years, oil field has adopted a new technology of gas lift induced spraying, and the effect is good. Gas lift drainage and gas lift inducement are directed to inject a certain amount of high-pressure gas into the wellbore. The high-pressure gas smashes the liquid in the oil pipe to the lowermost part of the oil pipe, and the high-pressure gas enters the casing space from the lower part of the oil pipe into a positive lifting method, so in the annular space, the liquid and gas are thoroughly mixed and ejected together from the wellhead. At this time, the bottom hole pressure is drastically reduced, and the oil and gas is sprayed to the bottom of the well in the low pressure zone, which penetrates and causes self-spraying. For oil well stopped by water filled bottom, after drained with nitrogen, the well can be pumped into a self-draining well. The high-pressure gas injected into the oil well must have characteristics of flame retardant and anti-explosion, but oxygen in the air is easy to cause combustion and explosion under high pressure and high temperature. In the past, liquid nitrogen and liquid carbon dioxide were commonly used in oil fields. Liquid nitrogen and liquid carbon dioxide are expensive, require long-distance transportation with special equipment, they are not suitable for oilfield processes. In terms of thermal energy, it is also a great waste. As on-site nitrogen generation system can directly separate nitrogen from the air, consumes less energy, and is easy to move. It has become the most important nitrogen supply method at wild oil field.
Steam, foam, nitrogen injection
Practice has proved that steam, foaming agent and nitrogen injection are the most effective means for heavy oil exploitation and secondary mining of oil and gas wells. In heavy heavy oil formations, heavy oils have high viscosity, poor permeability and fluidity, and cannot be effective with traditional harvesting techniques. Injecting steam alone will cause a steam train to reduce the oil displacement efficiency of the steam. Using the steam foam injection process, high-temperature steam foam is injected into the well, the steam foam is greatly damped, and the original crack passage in the formation increases the steam resistance, thereby reducing the negative effect of the steam string and increasing the temperature of the oil-bearing formation under the action of high-pressure nitrogen. Heavy oil wells can become artesian wells.
Pipe and container pressure test
When a flammable or explosive gas or liquid remains in the pipe or container, high pressure nitrogen system can perform both purging and pressure testing.
Comparison of on-site nitrogen production methods in oil fields
On-site nitrogen production in oil fields is mainly achieved by air separation. There are two processes, namely pressure swing adsorption air separation method and membrane air separation method. These two methods are new air separation technologies developed in the 1970s and 1980s. They have the advantages of simple structure, simple process flow, short gas output time, few moving parts, good running stability and low price,etc.
Pressure swing adsorption air separation method (PSA)
Since oxygen and nitrogen diffuse at different speeds in the pores of the molecular sieve adsorbent, when air passes through molecular sieve, the smaller diameter oxygen is diffused into the micropores at a faster rate, and is preferentially adsorbed by the molecular sieve, leaving the larger diameter nitrogen be used as product gas, and the desorbed oxygen is released from molecular sieve with pressure drop and is regenerated for next time use.
The core material, carbon molecular sieve, is installed in adsorption towers A and B. When working, the compressed air is filtered and then enters the A tower and the B tower respectively. The two towers A and B alternately work to achieve oxygen and nitrogen separation with the process of nitrogen production with pressure rise and molecular sieve regeneration with pressure drop. In this way, continuous nitrogen production is realized.
Hollow fiber membrane air separation nitrogen method
The hollow fiber membrane is actually a microporous tube having the same inner and outer diameters, and its structure is similar to that of a tubular heat exchanger. The fiber bundles are independent of each other and are sealed with epoxy resin at both ends of the film set. Hundreds of thousands of fibers are bundled together to provide the required surface area. Under the action of pressure, the adsorption, diffusion and permeation rates of various gases in the hollow fiber membrane are different. We call the gas with big permeation rate “quick gas”, such as oxygen, and gas with small permeation rate as ” Slow gas”, such as nitrogen. After the mixed gas permeates through the membrane, “quick gas” is enriched outside the low pressure, and “slow gas” is enriched inside the high pressure, thereby achieving separation of the mixed gas and nitrogen is obtained.
The film air separation nitrogen generator is composed of air compressor, filters, heater, membrane separator and the like. The hollow fiber membrane separator is the core part, and the filtering and heating part is to ensure the raw material gas to be clean, improving nitrogen separation performance. The left end of the separator is the air inlet. After the gas enters the hollow fiber membrane, the oxygen quickly permeates, exits from the side port, and the nitrogen continues to advance out of the right end of the separator.
3 Selection of nitrogen production method in oilfield applications
For many years, in oil fields, compared with pressure swing adsorption nitrogen production, membrane nitrogen generation has been widely used because they are smaller in size and more suitable for field transportation. However, membrane nitrogen is suitable for applications where the purity of nitrogen is not more than 95%. If the purity is required to be 99% or higher, membrane nitrogen production process consumes a lot of compressed air, resulting in significant increase in operating costs and significant reduction in investment returns.
The practice of oil and gas exploitation has proved that when nitrogen purity reaches 99%, it can significantly reduce the corrosion of pipelines and prolong the service life.
Pressure swing adsorption nitrogen production process can easily achieve 99% purity, and with the improvement of performance of adsorbent and the progress of the nitrogen separation process, now the volume of pressure swing adsorption nitrogen production equipment is also greatly reduced, and can be designed and manufactured with vehicle-mounted structure. Moreover, at 99% purity, equipment costs is much lower than membrane nitrogen generator. Today, containerized high pressure pressure swing adsorption nitrogen generators have been accepted by more and more customers, becoming an alternative to membrane nitrogen generator, widely used in oilfield operations.