Crude Oil is a complex mixture consisting of up to 200 or more different organic compounds, mostly hydrocarbons. Different crude contain different combinations and concentrations of these various compounds. The API (American petroleum institute) gravity of a particular crude is merely a measure of its specific gravity, or density.
The higher the API number, expressed as degrees API, the less dense (lighter, thinner) the crude. Conversely, the lower the degrees API, the more dense (heavier, thicker) the crude. Crude from different fields and from different formations within a field can be similar in composition or be significantly different. In addition to API grade and hydrocarbons, crude is characterized for other nonwanted elements like sulfur which is regulated and needs to be removed. Crude oil API gravities typically range from 7 to 52 corresponding to about 970 kg/m3 to 750 kg/m3, but most fall in the 20 to 45 API gravity range. Although light crude (i.e., 40-45 degree API) is good, lighter crude (i.e., 46 degree API and above) is not necessarily better for a typical refinery. Looking at the chemical composition of crude, as the crude gets lighter than 40-45 degrees API, it contains shorter molecules, or less of the desired compounds useful as high octane gasoline and diesel fuel, the production of which most refiners try to maximize. Likewise, as crude gets heavier than 35 degrees API, it contains longer and bigger molecules that are not useful as high octane gasoline and diesel fuel without further processing. 18 For crude that have undergone detailed physical and chemical property analysis, the API gravity can be used as a rough index of the quality of the crude of similar composition as they naturally occur (that is, without adulteration, mixing, blending, etc.). When crude of different type and quality are mixed, or when different petroleum components are mixed, API gravity cannot be used meaningfully for anything other than a measure of the density of the fluid. For example, consider a barrel of tar that is dissolved in 3 barrels of naphtha (lighter fluid) to produce 4 barrels of a 40 degree API mixture. When this 4-barrel mixture is fed to a distillation column at the inlet to a refinery, one barrel of tar plus 3 barrels of lighter fluid is all that will come out of the still. On the other hand, 4 barrels of a naturally occurring 40 degree API South Louisiana Sweet crude when fed to the distillation column at the refinery could come out of the still as 1.4 barrels of gasoline and naphtha, 0.6 barrels of kerosene (jet fuel), 0.7 barrels of diesel fuel, 0.5 barrels of heavy distillate, 0.3 barrels of lubricating stock, and 0.5 barrels of residuum (tar).
The higher the API number, expressed as degrees API, the less dense (lighter, thinner) the crude. Conversely, the lower the degrees API, the more dense (heavier, thicker) the crude. Crude from different fields and from different formations within a field can be similar in composition or be significantly different. In addition to API grade and hydrocarbons, crude is characterized for other nonwanted elements like sulfur which is regulated and needs to be removed. Crude oil API gravities typically range from 7 to 52 corresponding to about 970 kg/m3 to 750 kg/m3, but most fall in the 20 to 45 API gravity range. Although light crude (i.e., 40-45 degree API) is good, lighter crude (i.e., 46 degree API and above) is not necessarily better for a typical refinery. Looking at the chemical composition of crude, as the crude gets lighter than 40-45 degrees API, it contains shorter molecules, or less of the desired compounds useful as high octane gasoline and diesel fuel, the production of which most refiners try to maximize. Likewise, as crude gets heavier than 35 degrees API, it contains longer and bigger molecules that are not useful as high octane gasoline and diesel fuel without further processing. 18 For crude that have undergone detailed physical and chemical property analysis, the API gravity can be used as a rough index of the quality of the crude of similar composition as they naturally occur (that is, without adulteration, mixing, blending, etc.). When crude of different type and quality are mixed, or when different petroleum components are mixed, API gravity cannot be used meaningfully for anything other than a measure of the density of the fluid. For example, consider a barrel of tar that is dissolved in 3 barrels of naphtha (lighter fluid) to produce 4 barrels of a 40 degree API mixture. When this 4-barrel mixture is fed to a distillation column at the inlet to a refinery, one barrel of tar plus 3 barrels of lighter fluid is all that will come out of the still. On the other hand, 4 barrels of a naturally occurring 40 degree API South Louisiana Sweet crude when fed to the distillation column at the refinery could come out of the still as 1.4 barrels of gasoline and naphtha, 0.6 barrels of kerosene (jet fuel), 0.7 barrels of diesel fuel, 0.5 barrels of heavy distillate, 0.3 barrels of lubricating stock, and 0.5 barrels of residuum (tar).
The figure to the right illustrates weight percent distributions of three different hypothetical petroleum stocks that could be fed to a refinery with catalytic cracking capacity. The chemical composition is generalized by the carbon number which is the number of carbon atoms in each molecule. The medium blend is desired because it has the composition that will yield the highest output of high octane gasoline and diesel fuel in the cracking refinery. Though the heavy stock and the light stock could be mixed to produce a blend with the same API gravity as the medium stock, the composition of the blend would be far different from the medium stock, as the figure indicates. Heavy crude can be processed in a refinery by cracking and reforming that reduces the carbon number to increase the high value fuel yield.
2. Natural Gas
The natural gas used by consumers is composed almost entirely of methane. However, natural gas found at the wellhead, although still composed primarily of 19 methane, is by no means as pure. Raw natural gas comes from three types of wells: oil wells, gas wells, and condensate wells. Natural gas that comes from oil wells is typically termed 'associated gas'. This gas can exist separate from oil in the formation (free gas), or dissolved in the crude oil (dissolved gas). Natural gas from gas and condensate wells, in which there is little or no crude oil, is termed 'non associated
gas'. Gas wells typically produce raw natural gas by itself, while condensate wells produce free natural gas along with a semi-liquid hydrocarbon condensate. Whatever the source of the natural gas, once separated from crude oil (if present) it commonly exists in mixtures with other hydrocarbons; principally ethane, propane, butane, and pentanes. In addition, raw natural gas contains water vapor, hydrogen sulfide (H2S), carbon dioxide, helium, nitrogen, and other compounds. Natural gas processing consists of separating all of the various hydrocarbons and fluids from the pure natural gas, to produce what is known as 'pipeline quality' dry natural gas. Major transportation pipelines usually impose restrictions on the makeup of the natural gas that is allowed into the pipeline and measure energy content in kJ/kg (also called calorific value or wobbe index).
3. Condensates
While the ethane, propane, butane, and pentanes must be removed from natural gas, this does not mean that they are all 'waste products. In fact, associated hydrocarbons, known as 'natural gas liquids' (NGL) can be very valuable by-products of natural gas processing. NGL include ethane, propane, butane, iso-butane, and natural gasoline. These NGLs are sold separately and have a variety of different uses; raw materials for oil refineries or petrochemical plants, as sources of energy, and for enhancing oil recovery in oil wells,. Condensates are also useful as diluent for heavy crude, see below.
4. The Reservoir
The oil and gas bearing structure is typically a porous rock such as sandstone or washed out limestone. The sand might have been laid down as desert sand dunes or seafloor. Oil and gas deposits form as organic material (tiny plants and animals) deposited in earlier geological periods, typically 100 to 200 million years ago, under ,over or with the sand or silt, is transformed by high temperature and pressure into hydrocarbons. For an oil reservoir to form, porous rock needs to be covered by a non porous layer such as salt, shale, chalk or mud rock that can prevent the hydrocarbons from leaking out of the structure. As rock structures become folded and uplifted as a result of tectonic movements, the hydrocarbons migrates out of the deposits and upward in porous rocks and collects in crests under the non permeable rock, with gas at the top, then oil and fossil water at the bottom. . Ill: UKOOA
This process goes on continuously, even today. However, an oil reservoir matures in the sense that a too young formation may not yet have allowed the hydrocarbons to form and collect. A young reservoir (e.g. 60 million years) often has heavy crude, less than 20 API. In some areas, strong uplift and erosion and cracking of rock above have allowed the hydrocarbons to leak out, leaving heavy oil reservoirs or tar pools. Some of the world’s largest oil deposits are tar sands where the volatile compounds have evaporated from shallow sandy formations leaving huge volumes of bitumen soaked sands. These are often exposed at the surface, and could be strip mined, but must be separated from the sand with hot water, steam and diluents and further processed with cracking and reforming in a refinery) to improve its fuel yield.
The oil and gas is pressurized in the pores of the porous formation rock.
Ill: UKOOA When a well is drilled into the reservoir structure, the hydrostatic formation pressure drives the hydrocarbons out of the rock and up into the well. When the well flows, gas, oil and water is extracted, and the levels will shift as the reservoir is depleted. The challenge is to plan the drilling so that the reservoir utilization can be maximized.
Seismic data and advanced visualization 3D models are used to plan the extraction. Still the average recovery rate is 40%, leaving 60% of the hydrocarbons trapped in the reservoir. The best reservoirs with advanced Enhanced Oil Recovery (EOR) allow up to 70%. Reservoirs can be quite complex, with many folds and several layers of hydrocarbon bearing rock above each other (in some areas more than 10). Modern wells are drilled with large horizontal offsets to reach different parts of the structure and with multiple completions so that one well can produce from several locations. Ill: UKOOA
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