Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. More information
Cyclic steam stimulation
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.
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).
Viscosity reduction by heating
A remarkable characteristic of heavy oil is that its viscosity 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 migrates towards 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 heated zone 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 heated belt decreases from thousands mPa*S to around ten mPa*S. In this way, the oil flow resistance is greatly reduced, the flow coefficient (kh/μ) increases by orders of magnitude and the oil well production rate increases dramatically.
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 solution gas drive. 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.
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 heated 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 becomes more significant in heavy oil reservoirs with large single layer thickness.
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 dead-end 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 weight of the overlying layers compresses the reservoir layer to squeeze out the oil.
In the process of steam injection, 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 water-in-oil 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 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.
The cyclic steam stimulation (CSS) method, also known as “huff-and-puff” or “steam soak,” consists of three stages:
Steam injection stage (steam intake), Shut-in stage (steam soak) and production stage (steam ejection).
- 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 process 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
- Shut-in stage: after the steam injection is completed, the steam injection is stopped and the well is shut-in (also called steam soak). The time of steam soak is generally 2-7 days.
- Production stage: The well operation enters the production stage after steam injection and shut-in reach the designed soak time. In the recovery stage, due to the high reservoir pressure, the well can flow without any artificial lift and requires the installation of a nozzle to prevent the reservoir from producing sand due to excessively high rate. In the first few days of well production, the water cut is usually high, but soon the peak oil production appears, and the oil production is dozens of times of the conventional production. When the well cannot gush, pump is installed for production.
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.
- 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.
- 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 significant.
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.
- Sheng, J. (2013). Enhanced oil recovery field case studies. Waltham, Mass.: Elsevier.
- Canadian Association of Petroleum Producers. 2019. Cyclic Steam Stimulation(CSS) - How does it work? https://www.youtube.com/watch?v=XBfY-lkuXpM
- United States Department of Energy, Cylic steam stimulation production process, USDOE, Washington, DC (accessed 2021).
- 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.