ABSTRACT
Petroleum is a complex mixture of a wide range of compounds of varying complexity mostly of the hydrocarbon type� Its composition can vary widely from one source to another and sometimes for the same source with production rates and time� However, through processing and refining, products derived from petroleum have well-defined properties� Petroleum can be found in reservoirs of varying sizes of heterogeneous composition under high pressures and moderate temperatures, often in association with natural gas and water� Many of the world’s largest petroleum reservoirs are very massive in scale and often are made up of carbonate rocks� The voids in these porous rocks are filled by hydrocarbons, water, and gases�
It is normally agreed that petroleum was formed mainly from marine and animal life deposits over many millions of years under the combined action of time, temperature, pressure, and mobility of components� These were aided by bacterial and mineral actions� Petroleum is generally found trapped in certain rock formations, usually over pockets of salt water and often in association with natural gas� The oil as it comes out of the ground also contains compounds of sulfur, oxygen, nitrogen, and a variety of salts and minerals�
Petroleum is made up largely of hydrocarbons with the gradual elimination of oxygen from the original deposits aided by bacterial and catalytic actions� It is suggested that at certain locations petroleum could have been formed as a distillate from accumulated deposits of vegetation where the oil was able to migrate long distances through porous rocks to another location, leaving largely coal behind� Oils with high proportions of light distillates and low concentrations of sulfur compounds are considered more desirable and economically more valuable than sulfurous and high-viscosity heavy oils� There can be significant differences in the availability and price of different crudes depending on their quality�
Geophysical exploration methods have been developed to ascertain underground rock formations and conditions� Seismic surveys are widely employed where signals in the form of shock waves are emitted after being generated by detonating relatively small explosive charges� A recording is
made of the shock waves bouncing back from the rock layers below the surface� The time taken for the sound waves to be reflected back and their direction from each of the successive rock layers below are recorded on receiving geophones spread over the test area surface� The differences in the reflection and refraction of these shock waves are then employed to indicate the differences between one surface and another, since the speed at which the shock wave passes through any rock structure depends on the density and elasticity of the rock� From the pattern of these recordings, the depth and undulations of the rock structure can be derived� However, despite great advances in geophysics, seismic survey, computerized data processing, and imaging, the risk of drilling dry wells cannot be eliminated� It is only through direct drilling that a sure way can be provided to establish whether gas or oil is present in any suitable formation� Offshore deep sea drilling, which has increased in recent years, is very challenging, and its costs are extremely high compared to those on land or shallow waters� Horizontal drilling and directional drilling are being increasingly applied, especially in hard-to-reach regions�
An oil reservoir is a porous sedimentary rock formation capped with a layer of impermeable rock through which liquids and gases cannot pass (Figure 10�1)� The shape of a successful reservoir must allow oil or gas to accumulate, topped with cap rock preventing its migration further outward� Because of the capillary forces within the formation, some of the water originally in the pores could not be displaced fully by the accumulating hydrocarbons� This immovable water is called connate or interstitial water�
The volume of all the pores in a reservoir rock is normally expressed as a percentage of the total rock volume ranging from 10% to 30% and is called porosity� The larger the porosity, the more oil can be stored through the cavities� Seismic data allow the interpretation of what the rocks deep underground look like� The mathematical simulation of a reservoir that is basically
a complex material balance requires much relevant and detailed input information such as reservoir geometry, rock and fluid properties, as well as relevant logs and drilling records� The geological model of the reservoir when established is broken up into grid blocks� Reservoir properties such as those of porosity, permeability, net pay-thickness, water saturation, and formation volume are then input for each of these grid blocks, creating a computational picture of the reservoir and its contents�
When drilling a new well, core samples can be extracted to determine key information such as the porosity and permeability� This latter term quantifies the capacity of a porous material to transit fluids under the application of a pressure difference� Permeable beds have a multitude of relatively large and connected pores that will permit the flow of fluids easily and require the application of moderate pressure differences�
Rapid advances have taken place in recent years that showed much improved control and accuracy of directional drilling� This has helped greatly in finding more reserves and enabling the exploitation of hard-to-get deposits, including through entirely horizontal long wells�
The consequences of uncontrolled well blowout whether during exploration, testing, or production stages can be disastrous and extremely expensive� Great care is taken to ensure safe operation throughout all activities� Some of the main consequences of blowout are the following:
• Safety risk due to the flow of potentially dangerous formation fluids of gas, oil, salt water, and/or hydrogen sulfide
• Loss of equipment and materials with reservoir hydrocarbon losses • Environmental pollution with some long-term damage • Water mobility at the bottom of the reservoir produces water while
losing oil and gas • Blowout control and remedial costs are high with the potential for
loss of human lives or injuries • Loss of operators and personnel credibility
Of course, the risks associated with deep blowout under water, as have been demonstrated in the Gulf of Mexico, are much more serious than those on land�
Production of oil reservoirs using conventional methods remains largely inefficient, leaving significant amounts of unrecovered oil underground� The displacement of the oil from the reservoir rock is governed largely by overcoming the local viscous and capillary forces� In recent years, various
methods have been developed and are being applied increasingly to enhance the recovery of oil and improve the overall yield (Figure 10�13)� These methods are of the types discussed next�
Primary production methods produce oil via the action of the natural high pressure of the reservoir� It is usually associated with very low recovery effectiveness, typically less than 20% (Figure 10�2)�
Secondary production methods include water flooding and/or natural gas injection, mainly to maintain the reservoir pressure as it gets depleted with production and time to help continued removal of the oil from the rock� The recovery rate in these methods can be quite high for light crude, but remains relatively low for heavy oil reservoirs�
Tertiary production methods involve thermal, miscible, and chemical approaches, which are usually applied for certain reservoirs after the exhaustive application of primary and secondary methods�
Enhanced oil recovery methods are mainly associated with tertiary-type methods� The object of such approaches is to increase the effectiveness of
resource displacement by injecting an external fluid into the reservoir� The most common approaches in this category are as follows:
• Miscible: Solvent flooding is applied such as hydrocarbons or carbon dioxide�
• Chemical: Water with suitable chemical additives is injected into the formation� These include polymer and alkaline flooding, surfactant, polymer injection, and utilizing caustic, polymer, or surfactants�
• Thermal: Steam injection is applied mainly to heat the reservoir to decrease the viscosity of the oil and make it more mobile� These methods include steam stimulation, steam flooding, in situ combustion, and steam-assisted gravity drainage�
The successful application of any one of these approaches will depend on the nature of the reservoir and the economic suitability of the approach� Thermal methods tend to be the most common� They are particularly effective with heavy oil, including oil sands, where through raising the temperature of the oil, its viscosity is reduced, rendering the oil more mobile� In comparison, miscible methods are usually less effective� In any case, the combined application of primary and enhanced secondary recovery methods still can extract on average only around half of the resource in place�
A variety of steam injection approaches are used� These include the following:
• Steam flooding, described sometimes as steam drive, where steam is injected continuously through the injection well with the fluids withdrawn to come out through other production wells� The injected steam helps lower the viscosity of the oil and provides its drive� Thermal losses to the immediate surroundings of the well and formation can be very substantial and costly� Typically, steam-to-oil ratios used are in the range of 2�0 to 5�0 by mass� A successful steam drive is not usually applied to deep or thin reservoirs, as the thermal losses of the steam on its way to the formation can be very excessive� Spacing between wells can range up to a few acres for not-so-deep reservoirs�
• Cyclic steam injection or steam stimulation is when steam is injected continuously into the reservoir for one to a few weeks� The steam supply is then shut off, and the well contents are allowed to soak for a while, such as around a week or two, so that the injected steam cools and condenses with the associated latent heat of condensation spreading into the reservoir� This lowers the oil viscosity and permits the oil to flow and be brought up to the surface through the same injection well� This procedure may be repeated over the active life of the well� This method is sometimes described loosely as the “huff and puff” method (Figure 10�3)�
• In situ combustion: Oil recovery may be enhanced through initiating underground combustion within the reservoir with compressed air or occasionally oxygen injection� During the in situ combustion process, many chemical reactions take place� The three major types of these reactions are thermal cracking, low-temperature oxidation, and high-temperature oxidation� The progress of these reactions will depend on the characteristics of the oil, the reservoir’s porous medium, its oil saturation, and other operating parameters such as air flux and injection pressure� In situ combustion can be of the forward, reverse, and wet combustion types� In forward combustion, the combustion front moves within the bed in the same direction as the injected air, whereas in reverse combustion the combustion front moves in the opposite direction to that of the injected air� Wet combustion is a modification of dry forward combustion, but includes, in addition to the injection of air, the simultaneous or alternative injection of water or steam (Figures 10�4 and 10�5)�
Forward combustion has been the preferred combustion process because it burns oil fractions that are less desirable to recover (Figure 10�6)� During this process, thermal cracking produces heavy residue described as coke, which is deposited on the core matrix� This residue acts as the main fuel source for the process� The main difficulties associated with recovering oil via in situ combustion are associated with controlling the combustion process that is proceeding largely unmonitored deep underground�
Oil or tar sands are mainly made up of sand with around 10% or less by mass bitumen and additionally water (Figures 10�7 through 10�9)� The quality of oil sands varies widely with location and depth� The reserves in some parts of the world, especially Western Canada and particularly Alberta, are exceedingly large�
Bitumen is a highly viscous fluid made up of complex huge molecules with enormously large molecular weights of largely hydrocarbon material� It contains also a wide range of elements, such as sulfur (e�g�, 4�5-5�5% by mass) and metallic compounds (e�g�, vanadium and nickel ~400-500 ppm)�
A schematic representation of an oil sand fragment is shown in Figure 10�8 and the extracted bitumen in Figure 10�9 Typically, oil sand deposits when found up to around 50 m below the surface are harvested by surface mining� Deposits below 100 m deep require thermal in situ methods, where steam injection is used to soften and render less viscous the bitumen that is later pumped with water upward�
The Steam Assisted Gravity Drainage (SAGD) process is being used increasingly in recent years for bitumen thermal recovery� Nearly up to 50% of recent Canadian oil output has been made via this method� Figures 10�10 and 10�11 show schematically the working of the SAGD process� They show two horizontally drilled small-diameter underground long channels with drilled slots pointing upward� Steam is injected continuously in the upper channel
to emerge through the slots into the surrounding bed� The steam condenses along the interface and releases its latent heat of condensation to its immediate oil sands surroundings� This will help to heat and reduce the viscosity of the bitumen in the neighborhood and make it mobile� The oil and the condensed water will then drain via the action of gravity into the lower channels containing similar slots but pointing downward, where they will be withdrawn to the surface� In time, the chamber grows upward and sideways, leaving mainly sand behind�
Other in situ methods involving combustion have been tried, but they still require further research and development to render them fully controllable and commercially and environmentally viable for extraction of the bitumen from the huge oil sand deposits�
The extraction and processing of oil sands are very energy intensive with a serious environmental impact� Much energy is lost in mining, processing, refining, and subsequent mobility� In addition, there are some negative issues that need continued attention and effort� These include excessive consumption of natural gas; very high water consumption and contamination;
production of high levels of air pollution, including much greenhouse gas emissions; soil degradation and contamination in case of bitumen seepage; and waste residuals disposal problems� Technology and environmental research and development into improving the recovery process and subsequent transport and refining are being actively pursued, especially as the price of conventional oil continues to escalate�
During the early stages of oil sand processing after its mining and early transport, the bitumen must be removed from the oil sand before upgrading� This separation is made usually with hot water and with the resulting bitumen-rich froth skimmed� Bitumen in its raw form, as indicated earlier, is a large and complex molecule consisting of many different species� It is a very heavy oil that contains a very large number of carbon atoms per molecule (e�g�, 2000), and the hydrogen content is quite low (e�g�, H/C ≤1�50)� There is a need to break such large molecules and to either remove relatively some of the carbon or add much hydrogen in the upgrading processes� This is accomplished by the coker that cracks the bitumen molecule into smaller chains� Hydrogen that is manufactured mainly through steam reforming of natural gas is added during upgrading under high temperature and pressure in the presence of catalysts in the hydrocracker to change the composition and make it suitable for refineries to turn it into useful refined petroleum products� Usually, conventional refineries cannot deal with bitumen readily because of its very high molecular weight� The partially processed oil from the sands is described as synthetic crude oil� The sulfur and nitrogen content of the oil sand bitumen is high relative to conventional crude petroleum� They have to be dealt with in an upgrader before the synthetic crude can be sold�
Bitumen has an extremely high viscosity, which makes it difficult to transport on its own and refine� The upgrading of bitumen is an important part of the production of synthetic crude and is difficult to refine and ship by pipeline� The upgrading process includes thermal and catalytic conversions,
hydrotreating, and distilling to produce various products, such as naphtha, gas oils, and kerosene� These can be blended later into the synthetic crude that is shipped to refineries� The synthetic crude then is somewhat similar in properties to a low-sulfur conventional crude oil�
Special refining methods for the extracted bitumen are needed� Large supplies of natural gas are consumed to produce the hydrogen for the upgrading and refining processes through hydrotreating of bitumen to produce synthetic crude with a solid mainly carbon as a by-product described as coke� Bitumen, when separated, is diluted with conventional lighter fuels to render the mixture more mobile�
The production of oil from the oil sands is economically attractive at present� However, there is increasing pressure to reduce the impact of its exploitation on the environment and resources, especially through water pollutants, excessive natural gas dependence, and greenhouse gas emissions� The price of the oil produced will no doubt reflect increasingly the costs of the development and implementation of the remedial measures necessary� Figure 10�12 shows an indication of the massive scale of equipment employed in the mining of oilsands�
Oil shale is a tar-like substance contained in laminated shale rock� When the rock is fractured, crushed, and heated, vapors are released that can be converted by condensation to raw shale oil that can be refined subsequently (Figure 10�13)� In some instances, the oil can also be removed rather expensively by the action of solvents�
There are massive reserves of shale oil in many parts of the world� However, mineral matter in the deposits typically constitutes more than 90% by mass� In general, the technology for extracting oil from the shale is simpler than that of the liquefaction of coal and less developed than that of the production of oil from oil sands� There are serious problems facing the commercial exploitation of oil shale deposits� Some of these relate to the high financial and energy costs of oil removal from such stocks and the serious environmental disruption associated with the extraction process of oil from shale� Typically, more than a ton of rock may need to be removed per barrel of oil produced� Thus, an extraction plant may have to deal with the disposal of millions of tons of spent rock� The plant may need to carry out necessary
remedial measures for preserving the environment while possibly producing nearly a million tons of spent rock to dispose of per day� Consequently, so far very little oil tends to be produced from this resource�
1� Explain the difference between the terms “porosity” and “permeability” as they apply to the bed of an oil reservoir�
2� List some of the main methods that are being used to maintain and increase the yield of oil from producing wells� Explain why enhanced oil recovery methods that depend on thermal means via in situ combustion have not been fully developed to the production stage in recent years�
3� Canadian oil sands are being considered increasingly as the major source of liquid fossil fuels for Canada and export� Outline briefly some of the efforts that need to be taken further to make this exploitation of the resource environmentally, technically, and economically more acceptable�
4� Contrast between the problems facing the wide exploitation of oil sands and shale oil deposits�
5� There is hardly any industrial application where oil sands can be burned directly in a furnace for the production of low-pressure steam� What key performance criteria would be considered to explain the reasons behind this lack of such applications?
