Thursday, 16 July 2015

Heavy oil and Tar sands

Heavy oil

Heavy oil seeping out on the surface has been known for a long time and this was easy to exploit for use in small quantities. Even after drilling for oil became successful (in 1857 in Pennsylvania), mining for heavy oil continued in many parts of the world including Germany. In southern California (Ventura and Los Angeles basins) oil was mined from the early 1860s to the 1890s because the heavy oil would not flow to the wells. Tar sands are sandstone reservoirs which have been filled with oil at shallow depth <2 km (<70–80◦C) so that the oil has become biodegraded. Reservoir rocks which have been buried more deeply and then uplifted before the oil migration may be sterilised at higher temperatures and are less likely to be biodegraded. When the sandstone has not been buried to more than about 2 km depth the sand will remain mostly uncemented as loose sand because of a lack of quartz cement. Tar sand contains asphaltic oil rich in asphaltenes and resins. It has a high content of aromatics and naphthenes compared to paraffins, and a high content of nitrogen, sulphur and oxygen (NSO). Most of the hydrocarbon molecules have more that 60 carbon atoms and the boiling point and viscosity are therefore very high. The viscosity of the biodegraded oil is very low and the oil must be heated so that the viscosity is reduced before it can be produced by drilling wells. There are transitions between reservoirs with heavy oil and nearly solid bitumen. Heating can be achieved by soaking the reservoir with injected steam so that the heat of condensation to water helps to heat the oil. This is called cyclic steam injection when the steam is injected in the production well and left to soak for a few weeks before production starts when the oil is warmer. The steam may also be injected in a nearby well and driven towards the production well. Much of the energy is used to heat the water in the reservoir and it is important to reduce the inflow of water from adjacent rocks. One method includes freezing the ground at a distance from the well to avoid the flow of water towards the well. It is also possible to burn some of the oil in the subsurface to provide heat to heat the oil. It has also been proposed to heat the oil electrically, possibly powered by a nuclear reactor to reduce the CO2 emissions from burning oil to produce heat.

Albertan Tar sands
The tar sands in Alberta, Canada (Athabasca) are of Middle Cretaceous age (Aptian, 100 million years). The main reservoir rock is the McMurray Formation, a sandstone representing fluvial to tidal environments, and it is important to find thick sequences of sand with few clay layers. The oil was generated from older source rocks during the Laramide folding of the Rocky Mountains to the west and migrated into the Athabasca sands during the late Cretaceous and early Tertiary. Starting in the Eocene (50 million years) the sand was uplifted and most of the overburden eroded. The oil was then biodegraded by bacteria. These tar sands contain 1.7 trillion bbl (270 × 109 m3) of bitumen in-place, comparable in magnitude to the world’s total proven reserves of conventional petroleum. There are currently large mining operations at Athabasca. At surface temperatures, which are low in northern Canada, this tar sand is rather hard because of the high viscosity of the oil. The oil must be separated from the sand using hot water. The oil (tar) is very viscous and may be denser than water (API <10). Only about 20% is close enough to the surface to be economically mined and the rest must be heated in place. A cubic metre of oil, mined from the tar sands, needs 2–4.5 m3 of water. Oil may be extracted by steam-assisted gravity drainage (SAGD). Two parallel horizontal wells are drilled so that one is about 5 m below the other. Steam is injected in the shallowest well and the heated, less viscous, oil is drained into the lower well where it is produced. The steam will remain near the top due to its low density but will sink when condensed to water. The shales will reduce the vertical permeability and the drainage, but the heating will cause some fracturing which may allow more vertical flow. This method requires rather thick and homogeneous sand. Steam can also be injected into one well for a few weeks until the oil is heated and then produced from the same well. This is called cyclic steam stimulation (CSS). Large amounts of energy and water are required to produce the steam for this method. If petroleum is burned to produce the heat, high emissions of CO2 will result not only from when the oil is burnt as fuel but also from producing it. The steam may be mixed with solvents (gas) and this may reduce the amount of heat (steam) required. Much of the water used to produce steam can be recovered and used again. For each 3 bbl of oil produced one barrel is burned to produce the steam required for the production. This heavy oil should also be mixed with gas during the refining and a supply of gas is therefore important. Heating the oil with electricity has also been proposed and hydroelectric or nuclear power would reduce the CO2 emissions. 173 billion bbl (27.5×109 m3) of crude bitumen, which is 10% of the total bitumen in place, are economically recoverable using current technology from the three Alberta oil sand areas based on benchmark WTI market prices of $62/bbl in 2006, rising to a projected $69/bbl in 2016. The environmental problems associated with oil production from tar sand are considerable and very large amounts of water are required both for the production of steam for subsurface operations and for the processing of loose sand excavated from surface pits. As a result the release of CO2 is higher than with normal petroleum production if the CO2 is not captured and sequestrated. In Venezuela there are also very large reserves of heavy oil and tar sand such as the Orinoco tar sand. While the average surface temperature in Northern Alberta is only slightly above 0◦C the surface temperature in Venezuela is much higher (>20◦C) and the oil therefore needs less heating to reduce its viscosity. Much of the oil in Venezuela is heavy oil and it occurs in a foreland basin in front of the Cordilleran Mountains in a similar plate tectonic position to the heavy oil in Alberta. Also the oil in the Middle East is located in a similar foreland basin but here the reservoir shave not been uplifted so close to the surface and are therefore in most cases not so biodegraded. It is possible to make an emulsion between bitumen and water which has low viscosity and which can flow in pipelines and be burned directly to produce electric power. The heavy oil and bitumen is rather rich in sulphur which should be removed to avoid pollution. It can also be mixed with gas so that it can be used to produce normal oil products. There are other important tar sand deposits in the USA (Utah) and Africa (Congo and Madagascar). Tar sand has only recently been included in data on world oil reserves and as a result Canada has become one of the nations with the highest oil reserves.

0 comments:

Post a Comment