Steam assisted gravity drainage
Encyclopedia
Steam Assisted Gravity Drainage (SAGD) is an enhanced oil recovery
technology for producing heavy crude oil
and bitumen. It is an advanced form of steam stimulation
in which a pair of horizontal well
s are drilled into the oil reservoir, one a few metres above the other. Low pressure steam is continuously injected into the upper wellbore
to heat the oil and reduce its viscosity
, causing the heated oil to drain into the lower wellbore, where it is pumped out.
s are drilled in the formation
, one about 4 to 6 metres above the other. The upper well injects steam
, possibly mixed with solvents, and the lower one collects the heated crude oil
or bitumen that flows out of the formation, along with any water from the condensation of injected steam. The basis of the process is that the injected steam forms a "steam chamber" that grows vertically and horizontally in the formation. The heat from the steam reduces the viscosity
of the heavy crude oil
or bitumen which allows it to flow down into the lower wellbore. The steam and gases rise because of their low density
compared to the heavy crude oil
below, ensuring that steam
is not produced at the lower production well. The gases released, which include methane
, carbon dioxide
, and usually some hydrogen sulfide
, tend to rise in the steam chamber, filling the void space left by the oil and, to a certain extent, forming an insulating heat blanket above the steam. Oil and water flow is by a countercurrent, gravity driven drainage into the lower well bore. The condensed water and crude oil
or bitumen is recovered to the surface by pumps such as progressive cavity pump
s that work well for moving high-viscosity fluids with suspended solids.
Usually sub-cool is the term used to describe the difference between the saturation temperature (boiling point) of water at the producer pressure and the actual temperature at the same place where the pressure is measured. The higher the liquid level above the producer the lower the temperature and higher is the sub-cool. However real life reservoirs are invariably heterogeneous therefore it becomes extremely difficult to achieve a uniform sub-cool along the entire horizontal length of a well. As a consequence many operators, when faced with uneven stunted steam chamber development, allow small quantity of steam to enter into the producer to keep the bitumen in the entire wellbore hot hence keeping its viscosity low with the added benefit of transferring heat to colder parts of the reservoir along the wellbore. Another variation sometimes called Partial SAGD is used when operators deliberately circulate steam in the producer following a long shut-in period or as a startup procedure. Though a high value of sub-cool is desirable from a thermal efficiency standpoint as it generally includes reduction of steam injection rates but it also results in slightly reduced production due to a corresponding higher viscosity and lower mobility of bitumen caused by lower temperature. Another drawback of very high sub-cool is the possibility of steam pressure eventually not being enough to sustain steam chamber development above the injector, sometimes resulting in collapsed steam chambers where condensed steam floods the injector and precludes further development of the chamber.
Operating the injection and production wells at approximately reservoir pressure eliminates the instability problems that plague all high-pressure steam processes and SAGD produces a smooth, even production that can be as high as 70% to 80% of oil in place
in suitable reservoirs. The process is relatively insensitive to shale streaks and other vertical barriers to steam and fluid flow because, as the rock is heated, differential thermal expansion causes fractures in it, allowing steam and fluids to flow through. This allows recovery rates of 60% to 70% of oil in place, even in formations with many thin shale barriers. Thermally, SAGD is twice as efficient as the older cyclic steam stimulation (CSS) process, and it results in far fewer wells being damaged by high pressure. Combined with the higher oil recovery rates achieved, this means that SAGD is much more economic than pressure-driven steam process where the reservoir is reasonably thick.
for Imperial Oil
around 1969. But it wasn't until 1975 when Imperial Oil moved him from Sarnia, Ontario
to Calgary, Alberta
to head their heavy oil research effort that he pursued the concept. He tested the concept with Imperial Oil in 1980, in a pilot at Cold Lake, Alberta
which featured one of the first horizontal wells in the industry, with vertical injectors. The latter were established to be inefficient by research at the Alberta Oil Sands Technology and Research Authority
(AOSTRA) in the early '80s. This resulted in the first test of twin (horizontal) well SAGD, at their Underground Test Facility (UTF) in the Athabasca Oil Sands
, which proved the feasibility of the concept, briefly achieving positive cash flow in 1992 at a production rate of about 2000 bbl/day from 3 well pairs.
The original UTF SAGD wells were drilled horizontally from a tunnel in the limestone underburden, accessed with vertical mine shafts
. The concept coincided with development of directional drilling
techniques that allowed companies to drill horizontal wells accurately, cheaply and efficiently, to the point that it became hard to justify drilling a conventional vertical well any more. With the low cost of drilling horizontal well pairs, and the very high recovery rates
of the SAGD process (up to 60% of the oil in place), SAGD is economically attractive to oil companies.
and Venezuela
have the world's largest deposits).
The SAGD process allowed the Alberta Energy Resources Conservation Board (ERCB)
to increase its proven oil reserves
to 179 billion barrels, which raised Canada's oil reserves to the second highest in the world after Saudi Arabia
and approximately quadrupled North American oil reserves. As of 2011, the oil sands reserves stand at around 169 billion barrels.
has been used as a fuel for Canadian oil sands projects, due to the presence of large stranded gas reserve
s in the oil sands area. However, with the building of natural gas pipelines to outside markets in Canada and the United States, the price of gas has become an important consideration. The fact that natural gas production in Canada has peaked and is now declining is also a problem. Other sources of generating heat are under consideration, notably gasification of the heavy fractions of the produced bitumen to produce syngas
, using the nearby (and massive) deposits of coal
, or even building nuclear reactors to produce the heat.
A source of large amounts of fresh and brackish water and large water re-cycling facilities are required in order to create the steam for the SAGD process. Water is a popular topic for debate in regards to water use and management. As of 2005 Oil and Gas (including drilling) uses approximately 4.91% of total surface water and approximately 20.11% of ground water. Note that these numbers do not account for industrial uses such as injection, though the figures are also exclusive of recycled, reused and/or water returned to the water grid. Keep in mind that there has been drastic oil sands expansion since 2005 and one should consider this when estimated usage to date. The concern of using large amounts of water has little to do with proportion of water used, rather the quality of the water. Traditionally close to 70 million meters cubed of the water volume that was used in the SAGD process was fresh, surface, water. There has been a significant reduction in fresh water use as of 2010, where only around 18 million meters are used. Though to offset the drastic reduction in fresh water use, industry has began to significantly increase the volume of saline, ground, water involved. Saline ground water, in Alberta, is obtained from wetlands and so by increasing the volume of water drained may leave cause for concern of marsh and wetland conditions as a result of the SAGD process.
Relying upon gravity drainage, SAGD also requires comparatively thick and homogeneous reservoirs, and so is not suitable for all heavy-oil production areas.
mechanisms include VAPEX (for Vapor Extraction), Electro-Thermal Dynamic Stripping Process (ET-DSP), and ISC (for In Situ Combustion). VAPEX uses solvent
s instead of steam
to displace oil and reduce its viscosity. ET-DSP is a patented process that uses electricity to heat oil sands deposits to mobilize bitumen allowing production using simple vertical wells. ISC uses oxygen
to generate heat that diminishes oil viscosity; alongside carbon dioxide
generated by heavy crude oil
displace oil toward production wells. One ISC approach is called THAI for Toe to Heel Air Injection.
Enhanced oil recovery
Enhanced Oil Recovery is a generic term for techniques for increasing the amount of crude oil that can be extracted from an oil field...
technology for producing heavy crude oil
Heavy crude oil
Heavy crude oil or extra heavy crude oil is any type of crude oil which does not flow easily. It is referred to as "heavy" because its density or specific gravity is higher than that of light crude oil. Heavy crude oil has been defined as any liquid petroleum with an API gravity less than 20°.Extra...
and bitumen. It is an advanced form of steam stimulation
Steam injection (oil industry)
Steam injection is an increasingly common method of extracting heavy oil. It is considered an enhanced oil recovery method and is the main type of thermal stimulation of oil reservoirs. There are several different forms of the technology, with the two main ones being Cyclic Steam Stimulation and...
in which a pair of horizontal well
Directional drilling
Directional drilling is the practice of drilling non-vertical wells. It can be broken down into three main groups: Oilfield Directional Drilling, Utility Installation Directional Drilling Directional drilling (or slant drilling) is the practice of drilling non-vertical wells. It can be broken down...
s are drilled into the oil reservoir, one a few metres above the other. Low pressure steam is continuously injected into the upper wellbore
Wellbore
A wellbore is any hole drilled for the purpose of exploration or extraction of natural resources such as water, gas or oil where a well may be produced and a resource is extracted for a protracted period of time....
to heat the oil and reduce its viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
, causing the heated oil to drain into the lower wellbore, where it is pumped out.
Description
In the SAGD process, two parallel horizontal oil wellOil well
An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...
s are drilled in the formation
Petroleum geology
Petroleum geology refers to the specific set of geological disciplines that are applied to the search for hydrocarbons .-Sedimentary basin analysis:...
, one about 4 to 6 metres above the other. The upper well injects steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...
, possibly mixed with solvents, and the lower one collects the heated crude oil
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
or bitumen that flows out of the formation, along with any water from the condensation of injected steam. The basis of the process is that the injected steam forms a "steam chamber" that grows vertically and horizontally in the formation. The heat from the steam reduces the viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
of the heavy crude oil
Heavy crude oil
Heavy crude oil or extra heavy crude oil is any type of crude oil which does not flow easily. It is referred to as "heavy" because its density or specific gravity is higher than that of light crude oil. Heavy crude oil has been defined as any liquid petroleum with an API gravity less than 20°.Extra...
or bitumen which allows it to flow down into the lower wellbore. The steam and gases rise because of their low density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
compared to the heavy crude oil
Heavy crude oil
Heavy crude oil or extra heavy crude oil is any type of crude oil which does not flow easily. It is referred to as "heavy" because its density or specific gravity is higher than that of light crude oil. Heavy crude oil has been defined as any liquid petroleum with an API gravity less than 20°.Extra...
below, ensuring that steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...
is not produced at the lower production well. The gases released, which include methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, and usually some hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
, tend to rise in the steam chamber, filling the void space left by the oil and, to a certain extent, forming an insulating heat blanket above the steam. Oil and water flow is by a countercurrent, gravity driven drainage into the lower well bore. The condensed water and crude oil
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
or bitumen is recovered to the surface by pumps such as progressive cavity pump
Progressive cavity pump
A progressive cavity pump is a type of positive displacement pump and is also known as a progressing cavity pump, eccentric screw pump or even just cavity pump. It transfers fluid by means of the progress, through the pump, of a sequence of small, fixed shape, discrete cavities, as its rotor is...
s that work well for moving high-viscosity fluids with suspended solids.
Usually sub-cool is the term used to describe the difference between the saturation temperature (boiling point) of water at the producer pressure and the actual temperature at the same place where the pressure is measured. The higher the liquid level above the producer the lower the temperature and higher is the sub-cool. However real life reservoirs are invariably heterogeneous therefore it becomes extremely difficult to achieve a uniform sub-cool along the entire horizontal length of a well. As a consequence many operators, when faced with uneven stunted steam chamber development, allow small quantity of steam to enter into the producer to keep the bitumen in the entire wellbore hot hence keeping its viscosity low with the added benefit of transferring heat to colder parts of the reservoir along the wellbore. Another variation sometimes called Partial SAGD is used when operators deliberately circulate steam in the producer following a long shut-in period or as a startup procedure. Though a high value of sub-cool is desirable from a thermal efficiency standpoint as it generally includes reduction of steam injection rates but it also results in slightly reduced production due to a corresponding higher viscosity and lower mobility of bitumen caused by lower temperature. Another drawback of very high sub-cool is the possibility of steam pressure eventually not being enough to sustain steam chamber development above the injector, sometimes resulting in collapsed steam chambers where condensed steam floods the injector and precludes further development of the chamber.
Operating the injection and production wells at approximately reservoir pressure eliminates the instability problems that plague all high-pressure steam processes and SAGD produces a smooth, even production that can be as high as 70% to 80% of oil in place
Oil In Place
Oil in place is the total hydrocarbon content of an oil reservoir and is often abbreviated STOOIP, which stands for Stock Tank Original Oil In Place, or STOIIP for Stock Tank Oil Initially In Place, referring to the oil in place before the commencement of production...
in suitable reservoirs. The process is relatively insensitive to shale streaks and other vertical barriers to steam and fluid flow because, as the rock is heated, differential thermal expansion causes fractures in it, allowing steam and fluids to flow through. This allows recovery rates of 60% to 70% of oil in place, even in formations with many thin shale barriers. Thermally, SAGD is twice as efficient as the older cyclic steam stimulation (CSS) process, and it results in far fewer wells being damaged by high pressure. Combined with the higher oil recovery rates achieved, this means that SAGD is much more economic than pressure-driven steam process where the reservoir is reasonably thick.
Invention and development
The gravity drainage idea was originally conceived by Dr. Roger Butler, an engineerEngineer
An engineer is a professional practitioner of engineering, concerned with applying scientific knowledge, mathematics and ingenuity to develop solutions for technical problems. Engineers design materials, structures, machines and systems while considering the limitations imposed by practicality,...
for Imperial Oil
Imperial Oil
Imperial Oil Limited is Canada's largest petroleum company. The company is engaged in the exploration, production and sale of crude oil and natural gas. It is controlled by US based ExxonMobil, which owns 69.6% of its stock...
around 1969. But it wasn't until 1975 when Imperial Oil moved him from Sarnia, Ontario
Sarnia, Ontario
Sarnia is a city in Southern Ontario, Canada . It is the largest city on Lake Huron and is located where the upper Great Lakes empty into the St. Clair River....
to Calgary, Alberta
Calgary
Calgary is a city in the Province of Alberta, Canada. It is located in the south of the province, in an area of foothills and prairie, approximately east of the front ranges of the Canadian Rockies...
to head their heavy oil research effort that he pursued the concept. He tested the concept with Imperial Oil in 1980, in a pilot at Cold Lake, Alberta
Cold Lake
-Military Base:* CFB Cold Lake, also referred to as 4 Wing Cold Lake and formerly known as RCAF Station Cold Lake, is a Canadian air force base....
which featured one of the first horizontal wells in the industry, with vertical injectors. The latter were established to be inefficient by research at the Alberta Oil Sands Technology and Research Authority
Alberta Oil Sands Technology and Research Authority
The Alberta Oil Sands Technology and Research Authority was an Alberta crown corporation to promote the development and use of new technology for oil sands and heavy crude oil production, and enhanced recovery of conventional crude oil. It was funded by the Alberta Heritage Savings Trust Fund...
(AOSTRA) in the early '80s. This resulted in the first test of twin (horizontal) well SAGD, at their Underground Test Facility (UTF) in the Athabasca Oil Sands
Athabasca Oil Sands
The Athabasca oil sands are large deposits of bitumen, or extremely heavy crude oil, located in northeastern Alberta, Canada - roughly centred on the boomtown of Fort McMurray...
, which proved the feasibility of the concept, briefly achieving positive cash flow in 1992 at a production rate of about 2000 bbl/day from 3 well pairs.
The original UTF SAGD wells were drilled horizontally from a tunnel in the limestone underburden, accessed with vertical mine shafts
Shaft mining
Shaft mining or shaft sinking refers to the method of excavating a vertical or near-vertical tunnel from the top down, where there is initially no access to the bottom....
. The concept coincided with development of directional drilling
Directional drilling
Directional drilling is the practice of drilling non-vertical wells. It can be broken down into three main groups: Oilfield Directional Drilling, Utility Installation Directional Drilling Directional drilling (or slant drilling) is the practice of drilling non-vertical wells. It can be broken down...
techniques that allowed companies to drill horizontal wells accurately, cheaply and efficiently, to the point that it became hard to justify drilling a conventional vertical well any more. With the low cost of drilling horizontal well pairs, and the very high recovery rates
Peak oil
Peak oil is the point in time when the maximum rate of global petroleum extraction is reached, after which the rate of production enters terminal decline. This concept is based on the observed production rates of individual oil wells, projected reserves and the combined production rate of a field...
of the SAGD process (up to 60% of the oil in place), SAGD is economically attractive to oil companies.
Current applications
This technology is now being exploited due to increased oil prices. While traditional drilling methods were prevalent up until the 1990s, high crude prices of the 21st Century are encouraging more unconventional methods (such as SAGD) to extract crude oil. The Canadian oil sands have many SAGD projects in progress, since this region is home of one of the largest deposits of bitumen in the world (CanadaCanada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
and Venezuela
Venezuela
Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...
have the world's largest deposits).
The SAGD process allowed the Alberta Energy Resources Conservation Board (ERCB)
Ministry of Energy (Alberta)
The Ministry of Energy is a Cabinet-level agency of the government of the Canadian province of Alberta responsible for coordinating policy relating to the development of mineral and energy resources. It is also responsible for assessing and collecting non-renewable resource royalties, freehold...
to increase its proven oil reserves
Oil reserves
The total estimated amount of oil in an oil reservoir, including both producible and non-producible oil, is called oil in place. However, because of reservoir characteristics and limitations in petroleum extraction technologies, only a fraction of this oil can be brought to the surface, and it is...
to 179 billion barrels, which raised Canada's oil reserves to the second highest in the world after Saudi Arabia
Saudi Arabia
The Kingdom of Saudi Arabia , commonly known in British English as Saudi Arabia and in Arabic as as-Sa‘ūdiyyah , is the largest state in Western Asia by land area, constituting the bulk of the Arabian Peninsula, and the second-largest in the Arab World...
and approximately quadrupled North American oil reserves. As of 2011, the oil sands reserves stand at around 169 billion barrels.
Disadvantages
As in all thermal recovery processes, cost of steam generation is a major part of the cost of oil production. Historically, natural gasNatural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
has been used as a fuel for Canadian oil sands projects, due to the presence of large stranded gas reserve
Stranded gas reserve
A stranded gas reserve is found in a natural gas field which has been discovered, but remains unusable for either physical or economic reasons. Gas that is found within an oil well is conventionally regarded as associated gas and has historically been flared...
s in the oil sands area. However, with the building of natural gas pipelines to outside markets in Canada and the United States, the price of gas has become an important consideration. The fact that natural gas production in Canada has peaked and is now declining is also a problem. Other sources of generating heat are under consideration, notably gasification of the heavy fractions of the produced bitumen to produce syngas
Syngas
Syngas is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen. Examples of production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal, biomass, and in some types of waste-to-energy...
, using the nearby (and massive) deposits of coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
, or even building nuclear reactors to produce the heat.
A source of large amounts of fresh and brackish water and large water re-cycling facilities are required in order to create the steam for the SAGD process. Water is a popular topic for debate in regards to water use and management. As of 2005 Oil and Gas (including drilling) uses approximately 4.91% of total surface water and approximately 20.11% of ground water. Note that these numbers do not account for industrial uses such as injection, though the figures are also exclusive of recycled, reused and/or water returned to the water grid. Keep in mind that there has been drastic oil sands expansion since 2005 and one should consider this when estimated usage to date. The concern of using large amounts of water has little to do with proportion of water used, rather the quality of the water. Traditionally close to 70 million meters cubed of the water volume that was used in the SAGD process was fresh, surface, water. There has been a significant reduction in fresh water use as of 2010, where only around 18 million meters are used. Though to offset the drastic reduction in fresh water use, industry has began to significantly increase the volume of saline, ground, water involved. Saline ground water, in Alberta, is obtained from wetlands and so by increasing the volume of water drained may leave cause for concern of marsh and wetland conditions as a result of the SAGD process.
Relying upon gravity drainage, SAGD also requires comparatively thick and homogeneous reservoirs, and so is not suitable for all heavy-oil production areas.
Alternative methods
Alternative enhanced oil recoveryEnhanced oil recovery
Enhanced Oil Recovery is a generic term for techniques for increasing the amount of crude oil that can be extracted from an oil field...
mechanisms include VAPEX (for Vapor Extraction), Electro-Thermal Dynamic Stripping Process (ET-DSP), and ISC (for In Situ Combustion). VAPEX uses solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
s instead of steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...
to displace oil and reduce its viscosity. ET-DSP is a patented process that uses electricity to heat oil sands deposits to mobilize bitumen allowing production using simple vertical wells. ISC uses oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
to generate heat that diminishes oil viscosity; alongside carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
generated by heavy crude oil
Heavy crude oil
Heavy crude oil or extra heavy crude oil is any type of crude oil which does not flow easily. It is referred to as "heavy" because its density or specific gravity is higher than that of light crude oil. Heavy crude oil has been defined as any liquid petroleum with an API gravity less than 20°.Extra...
displace oil toward production wells. One ISC approach is called THAI for Toe to Heel Air Injection.