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Cyclic steam stimulation

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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 (huff-n-puff, huff ‘n’ puff) are also used to describe CSS.[1]

Video demonstration

[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. When high temperature and high-pressure steam and hot water at 250 ~ 350℃ are injected into the reservoir, the reservoir and crude oil in a certain distance near the well zone are heated. This reduces the viscosity of the oil in the heating zone from a few thousand to tens of thousands of Mpps to a few Mpps, reducing the drag of the oil to the bottom of the wellbore. When the flow coefficient (Kh/μ) increases by tens of times, the oil well production will surely increase by many times.

The release of elastic energy in the reservoir after heating

For the oil layer with high reservoir pressure, the elastic energy of the oil layer is fully released after heating the oil layer and becomes the oil displacement energy. Moreover, the heated crude oil expands by about 10% at 200℃. If there is a small amount of free gas in the original oil layer, it will also be dissolved in the hot crude oil.

Gravity drive

For thick reservoir, hot oil flowing to bottom hole is driven by gravity as well as reservoir pressure.

Absorbs waste heat during production

When the oil well is recovered after gas injection, a large amount of heat energy is taken away during the recovery process of the oil heated by steam and the steam condensed water under the larger production pressure difference, but the cold oil near the heating belt flows to the area near the well at a very low flow rate and is added to the depressurized heating belt.

Compaction of formation

Formation compaction is a mechanism of oil displacement that cannot be ignored.

Steam expansion drive

The steam injected into reservoir plays a driving role in recovery. The steam distributed in the steam heating zone will expand greatly during the process of stopping to reduce the bottom hole pressure, and part of the high-pressure condensed hot water will turn into steam due to the sudden depressurization flash. These all have a driving effect.

Solvent extraction

The crude oil in the reservoir cracked under high temperature steam to some extent, which increased the light fraction of crude oil and played a role in solvent extraction.

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


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.

References

  1. Sheng, J. (2013). Enhanced oil recovery field case studies. Waltham, Mass.: Elsevier.
  2. 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).