Steam-Oil Ratio Defined

What Is the Steam-Oil Ratio?

The steam-oil ratio is a metric used to assess the efficiency of thermal enhanced oil recovery (EOR) techniques. Specifically, it relates to EOR methods that utilize steam injections to retrieve oil from underground reservoirs. A low steam-oil ratio shows that relatively little steam is required to produce a barrel of oil, which indicates a more efficient extraction process.

How the Steam-Oil Ratio Works

Steam injection EOR methods are relatively costly to employ. As such, they are only used once the primary and secondary recovery techniques have been fully exhausted. Indeed, depending on factors such as the price of oil, it may not be economical to employ EOR techniques at all. In some cases, oil wells may even be abandoned because extracting the remaining reserves through EOR would be prohibitively costly.

When EOR is economically viable, oil-extraction companies will seek to maximize their return on investment by utilizing the most efficient methods possible. To measure thermal EOR efficiency, one metric they can monitor is the steam-oil ratio, which reflects the quantity of steam required to extract a given barrel of oil. For example, suppose the steam-oil ratio is 4.5. That indicates that 4.5 barrels of water — converted into steam and injected into the well — were required to extract a single barrel of oil. Therefore, lower steam-oil ratios would reflect more efficient extraction processes because they need less water to be converted into steam.

When possible, oil-extraction companies will seek to rely on primary recovery technologies, which are simpler and less costly than EOR. Primary recovery involves exploiting the existing disparity in pressure between the surface and the oil reservoir deep underground. By pumping water or gas into the well, companies can further increase the pressure inside the reservoir, causing the oil to rush toward the surface in search of lower pressure. If necessary, this process can also be further aided by using a rod pump to add additional pressure by mechanical means.

Steam injection may not be economical due to the potentially high cost of moving equipment into the area and fuel used to heat water into steam.

Benefits of the Steam-Oil Ratio

The steam-oil ratio can help companies make more efficient natural resource investments. As a company removes oil from a given well, it often becomes more challenging to extract. The steam-oil ratio can help firms determine the exact point when thermal EOR techniques are no longer worth the costs involved.

Disadvantages of the Steam-Oil Ratio

The steam-oil ratio only applies to thermal EOR techniques. It tells us nothing about the efficiency of thermal EOR compared to using carbon dioxide or polymers. Furthermore, the steam-oil ratio requires putting thermal EOR techniques in place before the ratio can be determined. That means it is not useful for deciding whether thermal EOR should be used in the first place.

Real-World Examples of the Steam-Oil Ratio

The particular steam-oil ratio will depend in part on the specific method of thermal EOR employed. For instance, the cyclic steam stimulation technique is generally associated with steam-oil ratios of 3 to 8. That means this technique requires the conversion of between three to eight barrels of water into steam for the recovery of one barrel of crude oil. This technique is generally reserved for wells with particularly heavy oil whose viscosity must be reduced by the steam in order to assist in extraction.

Meanwhile, the steam-assisted gravity drainage (SAGD) technique is generally more efficient, with steam-oil ratios in the 2 to 5 range. That means SAGD generally requires converting between two to five barrels of water into steam for the recovery of one barrel of crude oil. This technique consists of drilling two horizontal wells near the oil reservoir, one above it and the other below it. The force of gravity then causes the less viscous oil to fall into the lowermost well, where pumps are then used to lift it to the surface.