As conventional oil wells are starting to provide diminishing returns, new techniques are being developed to recover oil from unconventional sources. Oil shale (aka kerogen shale) is one such source that could prove to be particularly abundant.
A novel technique known as “In-Situ Retorting” is currently being developed that transforms kerogen shale to oil. However, the method requires special techniques to monitor the in-ground process. A key method called distributed temperature sensing (DTS) uses optical fiber cables, but DTS at temperatures exceeding 300°C is not possible with optical fibers using polymeric coatings. Optical fibers coated in gold produced by Fiberguide Industries have been used in a DTS system for a pilot project carried out by American Shale Oil, LLC.
Conventional vs. Unconventional Oil
A traditional way of acquiring oil involves drilling a well into a location underground, where oil flows freely from a pool of oil, called a reservoir. Along with diminishing numbers of conventional reservoirs, the oil prices have also recently increased. Thankfully, traditional oil sources are not the sole option.
Oil is present in other geological formations where petroleum is removed using methods other than traditional oil well techniques. Unconventional oils, such as oil shale, are becoming economically and technically accessible options due to elevated oil prices and novel technologies.
Confusion Over “Shale Oil” and “Oil Shale” – They Are Not the Same
“Shale Oil” is oil that is present in rock formations where the rock is not permeable enough for the free flow of oil through the rock to a well hole that is drilled into the rock formation. “Tight oil” is perhaps a more accurate term for shale oil.
Hydraulic fracturing, also known as “fracking,” is a modern method for generating fissures in the rock and injecting special fluids (e.g. water, sand, and chemical mixtures) at elevated pressures, to free up the oil and push it toward horizontal wells. Once it has been removed from the ground, the oil is ready to be delivered to refineries, where it is subsequently divided into numerous fuels and chemical intermediaries.
“Oil Shale” is different from shale oil. However, it can be employed as a source when creating petroleum products. Oil shale, or kerogen shale, is an organic-rich fine-grained sedentary rock that contains kerogen. Kerogen is a midway chemical state generated from organic matter that has decayed prior to transforming into petroleum in the earth. Retorting is a man-made process that is used to generate oil by heating kerogen to sufficiently high temperatures (pyrolysis).
Figure 1 shows an example of oil shale rock.
Figure 1: Oil Shale Rock.
Figure 2 shows the contribution of tight oil to past and future sources of energy that are supplying the current boost to US production. However, tight oil will soon experience a peak and then steadily decline. Along with a decline in tight oil, oil shale should turn into a common source of oil within the next ten years and come on stream when tight oil is diminishing.
Figure 2: Past and Future Petroleum Energy Sources.
DTS as Central Tool for Supervising Extraction of Unconventional Oil
Extracting unconventional oils has been made more efficient with sophisticated methods, such as the ability to measure the temperature of the whole well system. DTS allows users to monitor the temperature at several points along the well, where optical fibers are the temperature sensor element. In the majority of in-well DTS systems, special optical fibers equipped with polymeric coatings are typically used in cables that are rated for temperatures up to 300 °C. Above 300 °C, the life expectancy of polymeric coated fibers is limited.
In-Situ Retort Temperature of 350 °C or More Necessitates Metal Coated Fibers
Temperatures of 350 °C and above are needed to convert kerogen to oil. Fiberguide Industries’ gold coated optical fibers can endure temperatures as high as 700 °C, and thus easily handle the necessary temperatures. Identifying hot spots is crucial to regulating in-ground systems.
Trapped oil can be decomposed and released using chemical processes via mining and treating the shale, which is known as retorting. Otherwise, retortion of the shale oil can be done in the ground (“in-situ”) by heating the shale directly, propelling the oil to the surface, and refining the recovered shale oil.
Figure 3: Oil Shale Alternative Recover Schemes.
American Shale Oil (AMSO) Pilot Program
Fiberguide Industries’ gold coated optical fibers have been used in an RD&D test program of oil shale in-situ recovery run by AMSO, which published its first results at the 32nd Oil Shale Symposium in Golden Colorado on the 15th-17th of October 2012. This paper is available on the AMSO website.
DTS at 400 °C Facilitated by Fiberguide Gold Coated Optical Fibers
One of the main goals of AMSO’s pilot program was to measure the actual well temperatures. Continuously monitoring well temperatures is vital for accurately controlling the amount and rate of heat applied, as this ultimately decides the amount of oil that will be recovered during the in-situ retort process.
The DTS system employed by AMSO was provided by Petrospec, a frontrunner in applying fiber sensing methods to technologically challenging oil recovery applications. Xtreme-Duty™ DTS system by Petrospec is rated to 700 °C.
While Petrospec’s paper does not reveal any details regarding the performance of the DTS system and Fiberguide Industries’ gold coated fiber, both AMSO and Petrospec are reasonably content that the first DTS trials were successful in finding hot spots up to the maximum well temperatures, which climbed to almost 400 °C over many hundred meters of fiber.
This information has been sourced, reviewed and adapted from materials provided by Fiberguide Industries.
For more information on this source, please visit Fiberguide Industries.