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[[Glossary:CSS|Cyclic steam stimulation]] (CSS) is a method of producing heavy oil by injecting a certain amount of steam into a well and shutting it down for a period until the heat energy of steam diffuses to the reservoir. Cyclic steam injection is used extensively in heavy-oil reservoirs, tar sands, and in some cases to improve injectivity prior to steam flood or in situ combustion operations. Cyclic steam injection is also called steam soak or the huff `n puff (slang) method. The terms of steam soak and steam huff-and-puff are also used to describe CSS.<ref>Sheng, J. (2013). Enhanced oil recovery field case studies. Waltham, Mass.: Elsevier.</ref>
Cyclic steam stimulation (CSS) is a method of producing heavy oil by injecting a certain amount of steam into a well and shutting it in for a period until the heat energy of steam gets transferred to the reservoir and then putting it back on production. Cyclic steam injection is used extensively in heavy-oil reservoirs, tar sands, and in some cases to improve injectivity prior to steam flood or in situ combustion operations. Cyclic steam injection is also called steam soak or steam huff `n puff (slang) method.<ref>Sheng, J. (2013). Enhanced oil recovery field case studies. Waltham, Mass.: Elsevier.</ref>


===Video demonstration<ref name=":0" />===
===Video demonstration<ref name=":0" />===

Revision as of 00:39, 9 April 2021

Cyclic steam stimulation (CSS) is a method of producing heavy oil by injecting a certain amount of steam into a well and shutting it in for a period until the heat energy of steam gets transferred to the reservoir and then putting it back on production. Cyclic steam injection is used extensively in heavy-oil reservoirs, tar sands, and in some cases to improve injectivity prior to steam flood or in situ combustion operations. Cyclic steam injection is also called steam soak or steam huff `n puff (slang) method.[1]

Video demonstration[2]

[2]Canada’s oil sands are recovered using two main methods; drilling and mining. This video, which explains the Cyclic Steam Stimulation (CSS) drilling method was created by the Canadian Association of Petroleum Producers (CAPP).


Main mechanism

Viscosity reduction by heating

The outstanding characteristic of heavy oil is that it is very sensitive to temperature, which can be seen on the viscosity-temperature curve. By injecting high temperature and high-pressure steam into the reservoir, the formation temperature within a certain distance near the well zone increases and the reservoir and crude oil are heated. The steam injected into the reservoir first enters the high permeability zone. However, due to the low density of steam, the steam overlaps to the top of the reservoir under the action of gravity, and the reservoir is heated unevenly. However, due to the action of heat convection and heat conduction, when enough steam is injected, the heating range gradually expands, and the temperature in the steam zone remains at the bottom hole steam temperature (250 ~ 350℃). The temperature in the steam condensation zone, namely the hot water zone, is still very high although it drops to some extent. The viscosity of crude oil in the resulting heating belt decreases from thousands mPa*S to around ten mPa*S. In this way, the oil flow to the bottom of the well resistance is greatly reduced, the flow coefficient (Kh/μ) into dozens of times the increase, the oil well production is bound to increase.

Thermal expansion

The release of elastic energy in the reservoir after heating. When hot steam is injected into the reservoir, the heated crude oil expands. If there is a small amount of dissolved gas in the crude oil, it will also escape from the crude oil and produce the effect of dissolved gas flooding. At the same time, the fluid and rock skeleton in the reservoir have thermal expansion, the pore volume decreases, the fluid volume increases, and the elastic energy to maintain oil production increases. The thermal expansion degree of crude oil mainly depends on the composition of crude oil. Generally, the thermal expansion coefficient of light crude oil is greater than that of heavy crude oil.

Gravity drive

Because of the difference of vapor-liquid density, the overlap phenomenon is formed in the process of steam injection, and the longitudinal heating of the reservoir is uneven. However, the heating area of the reservoir increases and the non-displacement part of the reservoir is heated due to the heat conduction. Heated crude oil flows down the well by gravity. Gravity drainage mainly occurs in heavy oil reservoirs with large single layer thickness.

Steam distillation

In the process of steam injection, the vaporization pressure of oil and water increases with the increase of temperature. When the vaporization pressure of oil and water is equal to the current pressure of the reservoir, the light components in crude oil are vaporized into the gas phase, resulting in steam distillation. The effect of distillation on heavy oil recovery is mainly shown in the following aspects: low viscosity of gas phase, small flow resistance and solvent flooding at the displacement front, and the light components in the rock pores will be transferred to the connected pores, resulting in self - dilution and viscosity reduction.

Compaction of formation

Formation compaction is a mechanism of oil displacement that cannot be ignored. The effect of gravity on the overlying layers as they thicken. After compaction occurs, porosity decreases and volume decreases.

Emulsion displacement

In the process of steam flooding, the steam flow rate and specific volume in the steam chamber are large, and the steam in the leading edge of the steam chamber condenses and releases heat, leading to the disturbance effect. Therefore, emulsification occurs to form oil-in-water or oil-in-water emulsion. In heterogeneous reservoirs, these viscous emulsions clog the highly permeable bands, reducing steam pointing in the condensing zone and increasing sweep volume.

High temperature improves oil phase permeability

In the heterogeneous reservoir, the relative permeability of the reservoir to oil and water changes at high temperature after the reservoir is heated by wet steam injection, and the asphalt colloidal oil film on the surface of the sand grain is destroyed, and the wettability changes, from the original reservoir being oil-wet or strong oil-wet to water-wet or strong water-wet. At the same water saturation, the oil permeability increases, the water permeability decreases, and the bound water saturation increases. Moreover, the hot water is drawn into the low-permeability reservoir, and the replaced oil enters the percolation channels, increasing the flow of movable oil to the wellbore.


Production process

The cyclic steam stimulation (CSS) method, also known as “huff-and-puff” or “steam soak,” consists of three stages:

Steam injection stage (steam intake), soaking stage (steaming) and production stage (steam ejection).

  1. Steam injection stage: The steam injection stage is the process of steam intake into the oil layer, as shown in the figure above. According to the required construction parameters (injection pressure, injection speed, steam dryness, cycle steam injection volume), the high temperature and high-pressure saturated steam is injected into the reservoir. Injected steam preferentially enters the high-permeability zone and occupies the upper part of the reservoir due to the density difference between steam and reservoir fluids
  2. Shut-in stage: after the steam injection is completed, the steam injection is stopped and the well shut-in is also called well smoke-in. The time of well smoke-in is generally 2-7 days.
  3. Stop-up stage: well opening production enters the stop-up stage after steam injection and shut-in reaches the designed sock up time. In the recovery stage, due to the high reservoir pressure, the general well can spray production and install a large nozzle to prevent the reservoir from sand-producing. In the first few days of well production, the water cut is usually high, but soon the peak oil production will appear, and its production is dozens of times of the conventional production. When the well cannot gush, pump down immediately for production.


[3]Cyclic steam stimulation production process

Surface processes and equipment

  • Water supply: Seawater - Water tank - Seawater desalination equipment - Water treatment equipment - High pressure piston pump - Steam generator
  • Fuel supply: Fuel - Oil Tank - Oil pump room - Steam generator - Well
  • The nitrogen system: Air compressor - Membrane separation - Supercharger - Oil well annulus

Among them, the steam generator is the core equipment. After water treatment, the water enters the steam generator and produces a large amount of steam, which is continuously injected into the oil well.

Advantages

  • Low one-time investment, simple process technology, fast production increase and good economic benefits.
  • For ordinary and extra heavy oil reservoirs, steam stimulation almost has no technical and economic risks, so it has been widely used in heavy oil production and is also the best industrial application of thermal recovery method.

Disadvantages

  • Low recovery: As with conventional oil recovery methods, only 15-20% of oil is recovered from natural energy.
  • Because of the change of hot and cold period, the damage to the well is great.

Supercritical cyclic steam flooding

Compared with steam stimulation, supercritical cyclic steam is more injectable and more suitable for ultra-deep heavy oil. Supercritical cyclic steam has high injection pressure and excellent thermal conductivity, which can make up for the shortage of saturated steam. For oil reservoirs with extremely high crude oil viscosity, deep burial depth, high original formation pressure, low steam absorption capacity, and difficult steam injection, supercritical cyclic steam can meet their production requirements. The injection of supercritical CO2 coupled with superheated steam (SHS) for heavy oil recovery can improve development efficiency.[4]

References

  1. Sheng, J. (2013). Enhanced oil recovery field case studies. Waltham, Mass.: Elsevier.
  2. 2.0 2.1 Canadian Association of Petroleum Producers. 2019. Cyclic Steam Stimulation(CSS) - How does it work? https://www.youtube.com/watch?v=XBfY-lkuXpM
  3. United States Department of Energy, Cylic steam stimulation production process, USDOE, Washington, DC (accessed 2021).
  4. Sun, Fengrui, Yao, Yuedong, Li, Xiangfang, Li, Guozhen, Huang, Liang, Liu, Hao, . . . Han, Song. (2018). Exploitation of heavy oil by supercritical CO2: Effect analysis of supercritical CO2 on H2O at superheated state in integral joint tubing and annuli. Greenhouse Gases: Science and Technology, 8(3), 557-569.