Heavy oil cannot easily be pumped for a variety of reasons relating to its viscosity, density, and sometimes to its level of contamination with rock and other solid debris. A multitude of different methods have been devised that aid in extraction of heavy oil.
Cold Heavy Oil Production with Sand (CHOPS)
CHOPS is a technique applied to both tertiary recovery of conventional oil as well as to “quasi primary” production of oil sands and oil shale. CHOPS is really nothing more than the idea that if sand filters are removed from pumping equipment and sand is produced with oil, then the two can be separated above ground. The technique has the advantage that removing sand from the well can also result in enough space being created for oil to form small liquid pockets that are easily produced. The system is called “cold” because not heat is injected to help liquefy the petroleum, thereby saving on energy investment and improving EROEI.
CHOPS only recovers 5 -6% of the oil in a given reservoir, but it is cheap to implement. Disposal of the sand, which is contaminated with petroleum, is a serious drawback to this method. Some locations use oily sand in road construction, but this poses problems as well. Currently, most sand is disposed of in underground salt caverns.
Steam Assisted Gravity Drainage (SAGD)
The use of steam in oil recovery has been going on since the 1950s, with steam assisted gravity drainage being developed in the 1980s to aid in the removal of oil from the Alberta Oil Sands in Canada. SAGD relies on the advent of horizontal drilling, which allows two horizontal wells to be drilled parallel to one another, with one well sitting approximately 5 meters above the other. Steam is injected into the upper well, sometimes along with chemical solvents, and heats the crude to increase its liquidity. The liquid crude then flows with the assistance of gravity to the lower well where it is collected and pumped to the surface. The upper well is often referred to as the “steam chamber.”
There are several benefits to SAGD, the first of which is that it is highly efficient, allowing for up to 60% of total reserve to be processed on average and as much as 70 to 80% in some cases. Beyond production efficiency, SAGD also helps in contaminant removal. Because contaminants like hydrogen sulfide, methane, and carbon dioxide are light, they rise with the steam while the oil falls due to its greater density. The result is underground removal of a large portion of contaminants that are common in heavy oil deposits.
The major disadvantage to SAGD is the high cost of steam generation. Water supply is also an issue in this technique as it must be recycled and processed in order to remove contaminants and prevent pollution. Opponents of this method of oil recovery worry that its reliance on water from nearby streams and lakes will result in depletion and that leaking my contaminate aquifers.
Cyclic Steam Stimulation (CSS)
CSS is an older technique and the precursor to SAGD. In this method, steam is injected into a vertical well for a period of weeks or even months. The well is then allowed to sit while the petroleum is liquefied from the heat. Finally, the hot oil is pumped out of the well for as long as possible before the procedure is repeated again. This process, sometimes called “huff-and-puff,” will be repeated until the oil returned is not longer enough to offset the investment in energy to create steam. Roughly 25% of total reserve can be processed with this technique.
Vapor Extraction (VAPEX)
VAPEX is like SAGD, but solvents are used instead of steam. It is more efficient in terms of energy used than is SAGD. It is often used in conjunction with SAGD and/or CSS.
Toe-to-Heel Air Injection (THAI)
This is the newest heavy oil extraction process on the market and is used in horizontal wells. In THAI, air is injected into a well at the “toe.” The toe is the farthest end from the surface bore hole. With air now introduced into the reservoir, a fire can be ignited, which will burn the heavier oil components and even upgrade some bitumen into lighter oil. Because the air was injected at the toe, the fire burns toward the open end of the well, hence the name toe-to-heel. Because the amount of oxygen in the well is limited to that which is introduced, the fire is self-limiting, burning out when all the injected oxygen as been used.
THAI is purported to use less freshwater than other extraction methods like SAGD and also produces less greenhouse gas emissions (mostly methane and carbon dioxide). The reduction in greenhouse gases is on the order of 50%. The process is also said to have less surface impact due to a smaller “footprint,” which basically means less equipment is need and thus less land must be utilized.
Open-pit mining is the least popular and most expensive form of petroleum recovery. It is only used in extremely sandy, tar-rich deposits. This form of mining is exceptionally damaging to the environment because it has the largest footprint, creates the most greenhouse gases, leads to water pollution, and requires that a great deal of energy be invested for the return. Open-pit mining is traditionally used in the recovery of metal ore and diamonds.
This technique is somewhat new and is treated in a separate section as it is used primarily in the removal of solid petroleum, such as oil shale, in lieu of surface mining. The process basically consists of horizontal well drilling followed by water and chemical injection to break apart and dissolve sandstone in an effort to release petroleum. It often combines components from SAGD, VAPEX, and THAI.