Facilities for steam generation

Steam generation for the purposes of thermal recovery includes facilities to treat the water (produced water or fresh water), generate the steam, and transport it to the injection wells. This article discusses these key components.

Steam production

A steamflood uses high-quality steam injected into an oil reservoir. The quality of steam is defined as the weight percent of steam in the vapor phase to the total weight of steam. The higher the steam quality, the more heat is carried by this steam. High-quality steam provides heat to reduce oil viscosity, which mobilizes and sweeps the crude to the producing wells. As the heat is absorbed by the crude and formation, the steam:

1. Condenses
2. Mixes with the crude and formation water
3. Is produced with the crude

This hot produced water is then

1. Separated from the crude
2. Treated
3. Injected as steam to complete an entire steamflood cycle

Water treating processes for steam production

A typical water-treatment process for steam production is shown in Fig. 1. Separators or tanks remove the bulk water from oil. The oily water is then treated with flotation cells to remove most of the dispersed oil (95%). The treated water contains < 10 mg/L of oil and suspended solids. Final water polishing is done by filtration, such as sand, multimedia, or walnut-shell filters, to reduce the oil and suspended solids to < 1 mg/L. The clean water is treated by water softeners to reduce the total hardness to < 0.5 mg/L so that it can be fed into the steam generators.

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  Fig. 1—A typical water-treatment process for steam generation.

If the produced water has high silica content, a warm-lime or hot-lime process should be used to remove silica to the desirable level. If the produced water has high TDS, either RO or weak acid softeners should be used to soften it for steam generation. Recently, an RO process was used to treat produced water to meet drinking and irrigation quality. A similar process has been used for generating electricity after polishing with a demineralization treatment.

Chemical treatment is generally required to maintain the integrity of equipment and improve the quality of water for steam generation. Depending upon the needs, this treatment could include:

• Corrosion and scale inhibitor
• Oxygen scavenger
• Water clarifier
• Coagulant
• Biocide treatment

Quality of water for steam production

High-quality water is necessary for maintaining a reliable, continuous steam-injection program. Although there is no specification for heavy metal ions, these materials should be kept low to minimize poisoning of ion-exchange resins. The quality of water required for steam generators operating at a steam quality of 70 to 80% is shown here:

• Total hardness: < 0.01 mg/L
• Oil content: < 0.5 mg/L
• Total suspended solids: < 0.5 mg/L
• Total iron: < 0.5 mg/L
• Oxygen content: < 0.02 mg/L
• Silica concentration: < 200 mg/L

Steam generators

Steam generators are used to produce steam for the steamflood. The most popular steam generators are the 25- and 50-MMBtu/hr units. The 25-MMBtu/hr units are used as mobile units and provide steam for cyclic-steaming or remote-injection wells. The 50-MMBtu/hr units provide steam from a central banked location, which simplifies the water- and fuel-treatment plants and the steam-distribution system. Moreover, if exhaust-gas scrubbing is required, the scrubber system can also be centralized. Significant savings can be realized by centralizing these units.

A typical steam-generator and system flow schematic is shown in Figs. 2 and 3. It consists of a convection section and a radiant section. The convection section is designed to preheat the softened feed water, and the radiant section further heats the steam pipe for generating steam. A steam generator produces 60 to 80% quality steam, depending on the reservoir requirements. Higher-quality steam can be generated if good-quality water and fuel gas are used. When the required steam quality reaches 90%, control becomes difficult, and the chance of overheated pipe increases because of the relatively low liquid phase near the pipe effluent.

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  Fig. 2—A typical steam generator consists of a convection and a radiant section.

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  Fig. 3—Flow schematic of a steam-generation system.

Pressure

Steam-injection pressures vary from field to field, depending on the:

• Reservoir requirement
• Steam-injection rate
• Tightness of the formation

A steam generator can be operated at a pressure of approximately 2,700 to 3,000 psig and can be designed specially for a higher operating pressure if required. The operating temperature changes with the corresponding pressure. The correlation can be seen in a typical steam chart found in many mechanical engineering textbooks. This chart contains thermodynamic information about the steam. According to the steam chart, the steam delivers a specific amount of heat to the reservoir at a certain:

• Steam quality
• Pressure
• Temperature

Water quality

Poor-quality feed water will scale the steam generators, causing poor heat transfer or plug-up. High-hardness feed water will cause:

• Calcium carbonate scales
• Magnesium carbonate scales
• Sulfate scales

A high iron concentration in the feed water will cause iron carbonate or oxide scales in either the convection or radiant section. A high silica concentration will cause silica scale in the radiant section. Other types of scale are also present, such as:

• Complex compounds
• Scales induced by produced fines, silt, or injected chemicals

Downtime

Because of various operating and maintenance conditions, a steam-generator bank usually has a finite percentage of downtime, which is generally factored into the design so that a constant steam flow can be assured. A well-operated field can have steam-generator downtime as low as 5%.

Steam distribution

After steam leaves the generators, it is transported and distributed by pipelines to steam injectors. This pipeline network is generally insulated to reduce heat loss and to provide safety for the people working in the area. Because the steam generators are not generating 100% steam, the pipeline flow consists of a vapor phase and a liquid phase. The steam may flow with different flow patterns, depending on:

• Steam-flow rate
• Pipe size
• Temperature
• Pressure

A phenomenon known as phase splitting is known to occur at piping junctions, branches, and tees, resulting in widely varying steam qualities at the steam injectors in any large steamflood project. Inconsistent steam delivery results in inconsistent heat delivery to the reservoir, interfering with the optimization of steam-injection rates, oil recovery, project economics.

Various designs exist in the literature for handling the phase-splitting problems. One company has developed a patented steam-splitting device to control steam distribution, as shown in Fig. 3. This device controls the steam quality delivered to the “branch” (side flow) of a junction, while the “run” quality (straight-through flow) is uncontrolled. As a result, this device’s control of the branch steam quality is almost independent of changes in the flow rate through the branch. Although the device itself is not used to control either the branch or run flow rate (chokes or control valves at the injection wells are necessary for rate control), the uniformity of steam quality provided by this device makes rate control more reliable.

Steam injectors

Steam injectors are used to inject steam into the formation. There is a concentric pipe design, made of an inner pipe for transporting steam down to the reservoir, which uses the casing as the outer pipe. The casing is cemented to the injection well with a special blend of cement, including silica flour. This cement can withstand large amounts of heat with minimal expansion. The well casing design prevents heat loss to the surroundings. The inner injection tubing holds heat from steam, and the air/steam gap between the inner pipe and casing acts as an insulator to reduce heat loss. Deep injection wells are equipped with insulation tubing around the outside of the steam pipe. This insulation tubing is specially designed with a vacuum in the jacket to provide additional heat insulation.

Noteworthy papers in OnePetro

Garbutt, C. F. 1997. Innovative Treating Processes Allow Steamflooding With Poor Quality Oilfield Water. Presented at the Society of Petroleum Engineers Annual Technical Conference and Exhibition, 5-8 October, San Antonio, Texas, USA. SPE-38799-MS. http://dx.doi.org/10.2118/38799-MS

Al Bahlani, A. M. M., Babadagli, T. 2008, January 1. A Critical Review of the Status of SAGD: Where Are We and What Is Next? Presented at the Society of Petroleum Engineers Western Regional and Pacific Section AAPG Joint Meeting, 29 March-4 April, Bakersfield, California, USA. SPE-113283-MS. http://dx.doi.org/10.2118/113283-MS

Barge, D. L., Carreras, P. E., Uphold, D. D., Al-Yami, F. M., Deemer, A. R., Al-Anezi, T. 2009. Steamflood Piloting the Wafra Field Eocene Reservoir in the Partitioned Neutral Zone, Between Saudi Arabia and Kuwait. Presented at the Society of Petroleum Engineers Middle East Oil and Gas Show and Conference, 15-18 March, Bahrain, Bahrain. SPE-120205-MS. http://dx.doi.org/10.2118/120205-MS

External links

Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro

See also

Thermal recovery by steam injection

Formation damage in injection wells

Removing solids from water

Removing hydrocarbons from water

Page champions

Hank Rawlins, PhD, PE