oil sand in-situ recovery techniques
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OIL SAND & IT·S IN-SITURECOVERY TECHNIQUES
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Oil Sand: Introduction
Definition: Contain naturally occurring mixtures of sand, clay, water and
bitumen (dense and extremely viscous form of petroleum) -
unconventional petroleum deposit.
Also called tar sands, crude bitumen or extra-heavy oil.
Global reserves: More than two trillion barrels of tar sands - largest
deposits being in Canada and Venezuela - tar sands represent about
40% of Canada's oil production.
API gravities range from 4° to 8°, easily the heaviest hydrocarbons
found ² Viscosity is so high that they do not flow at all at reservoirconditions; viscosity of Venezuela's Orinoco bitumen lies in the range
1000²5000 cP, while Canadian bitumen in the range 5000²10,000 cP.
World production rate of conventional crude will soon peak at an
estimated 27 billion bbl/year. Consequently, persistent demand will
encourage more permanent and economically stable development of oilsand production.
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Problem Definition
Extraction from oil sand reserves is very complex andexpensive. This is especially true in case of in-siturecoveries, since mining operation is relatively simpler
but can be carried out only in shallow reservoirs, usuallyat depths lesser than 50 -75 m. For deeper deposits,the only convenient option available is in-situ recovery.
Since the energy required and cost involved in thevarious in-situ recovery techniques are of enormous
proportions, it becomes highly imperative that the mostsuitable as well as economical production technique isselected for implementation in the given field.
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Production & Processing
Oil sand extraction done by: (i) mining (ii) in-situ recovery.
Methods for in-situ production include:
Cyclic steam stimulation (CSS)
Steam Assisted Gravity Drainage (SAGD)
In-situ combustion
Toe to Heel Air Injection (THAI)
Tar/oil sands processed to generate oil similar to conventional crudeoil - more complex and expensive.
Processing includes: (i) separation of bitumen from the otherconstituents like clay, sand and water (ii) bitumen upgrading beforerefining - being highly viscous, bitumen also requires dilution withlighter hydrocarbons to make it transportable via pipelines.
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In-situ Combustion
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In ² Situ Combustion
Involves generation of heat by combustion within the
reservoir.
Requires 2 vertical wells ² 1 injector & 1 producer.
Air or oxygen is injected into the injected wells to
sustain the combustion front.
As the combustion front advances through the
reservoir, the oil ahead of the front becomesheated and reduces the viscosity of native reservoir
by transferring the heat and mixing of flue gas.
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Mechanism
The mechanism of in-situ Combustion indecreasing order is as follows:
Flue gas sweeping
Field re-pressurization
Oil swelling
Viscosity reduction
Stripping of light oil components of oil
Thermal effect
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Mechanism
When air is injected into an oil reservoir, two simultaneousphenomena occur: displacement of oil and oxidation of oil.
According to the efficiency of displacement and the intensity ofoxidation, four main types of processes can occur.
1. Immiscible Airflooding (IAF) with High Temperature Oxidation(HTO)
2. IAF with Low Temperature Oxidation (LTO)
3. Miscible Airflooding (MAF) with HTO
4. MAF with LTO
For heavier crude oils or tar sands, heat and steam generationand subsequent viscosity reduction is the primary oil displacementmechanism.
For this reason, in-situ combustion in a heavy oil reservoir shouldoperate in the high temperature oxidation reaction regime.
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Reservoir Parameters suitable for in-situ
combustion
Oil Content : High oil saturation & low water saturation
required.
Pay thickness : between 5 ft. - 50 ft. Generally thickness
greater than 10 ft is desired to prevent excessive heat loss. Depth : > 200 ft.
Oil gravity & viscosity : between 100 - 400 API
Reservoir Permeability greater than 100md allows more flow
of viscous oil. Size of Reservoir : 100 acres(min.)
Reservoir Confinement : No gas cap or water zone.
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Advantages of in-situ combustion
Universal availability of air
Applicable in wide range of reservoir
High swept efficiency with thermal front
Economically and environmentally favorable
Reservoir pressurization
Improvement in viscosity and density contrast between theresidence oil and water.
It is the ideal process for producing oil from thin formation.
Formation permeability has minimal effect on this process Can be applied to a reservoir which has already undergone
water flooding or steam flooding
Can be applied in both heavy and light oil reservoirs.
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Limitations of in-situ Combustion
Oil saturation and porosity must be high enough so thatheat loss to the rock will minimize.
Not applicable in fracture reservoir because early
breakthrough may happen. Oil sweeping process takes place in upper part of the
layer hence, sweep efficiency is poor in thick reservoir.
Requires more controlled process of air injection rate or
flux.
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THAI� Process
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THAI� Process
The THAI� process uses horizontal production wells paired
with vertical air injection wells to recover bitumen.
Horizontal production wells are drilled to the desired depth.
Air injection wells are drilled to the ¶toe· of the horizontal wells. Steam is injected through the vertical and horizontal wells for
up to three months to heat the oil sands reservoir.
Once the reservoir reaches the required temperature and
mobility, air is injected into the formation through the airinjection well. When oxygen in the air contacts the heated oil
sands, a combustion reaction starts.
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THAI� Process
As additional air is injected into the formation, thecombustion front moves slowly from the toe to the heelof the horizontal production well, partially upgradingthe bitumen, producing THAI� oil.
The THAI� oil, along with water and gases that areproduced during combustion (primarily nitrogen andcarbon dioxide), drain into the horizontal well.
Once at the surface, the produced fluids are treatedand the THAI� oil is sent to market.
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Suitable reservoir parameters used to
model the THAI� process
Oil Saturation = 50% - 80%
Oil Quality = 8° API or greater
Oil Viscosity At Reservoir Temp. = <250,000 cP
Vertical Permeability = 0.5 D Net Pay = >10m
Shale Content = Shale breaks are not expected to beproblematic
Clay Content = Clay is beneficial to catalyze crackingupgrading reactions.
Thief Zones = Not expected to be problematic
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THAI� Advantages
The THAI� process offers a number of potentiallysignificant resource, economic and environmentalbenefits when compared to other in situ oil sandsrecovery processes, including:
Increased resource recovery Production of a partially upgraded bitumen product
Reduced natural gas use
Less water production
Reduced diluents requirements Reduced greenhouse gas emissions and Smaller surface
footprint.
Lower capital and operating costs.