Transport Phen. 3 - M.Lennox

Question 1

What is cap rock?

Answer 1

A relatively impermeable rock (shale, anhydrite, or salt) that forms a barrier or seal above and around reservoir rock so that the fluids cannot migrate beyond the reservoir.

Question 2

What are Darcies?

Answer 2

A unit of permeability, representing the flow (at 1 atm, of 1 cm3 of fluid with 1 cP viscosity in 1 second through 1 cm2 cross section of porous medium 1 cm long.)

k = qmuL/(A*dP)

Question 3

What are conventional and unconventional reservoirs?

Answer 3

Conventional - where the naturally occurring hydrocarbons, e.g. crude oil or natural gas, are trapped by overlying rock formations with lower permeability.

Unconventional - the rocks have high porosity and low permeability which keeps the hydrocarbons trapped in place, thus not requiring cap rock.

Question 4

What does primary oil recovery refer to?

Answer 4

Primary oil recovery refers to the process of extracting oil either via the natural rise of hydrocarbons to the surface of the earth or via pump jacks and other artificial lift devices.
Since this technique only targets the oil, which is either susceptible to its release or accessible to the pump jack, this is very limited in its extraction potential.

In fact, only around 5% - 15% of the well’s potential are recovered from the primary method.

Question 5

What does secondary oil recovery refer to?

Answer 5

This method involves the injection of gas or water, which will displace the oil, force it to move from its resting place and bring it to the surface. This is typically successful in targeting an additional 30% of the oil’s reserves, though the figure could be more or less depending on the oil and of the rock surrounding it.

Question 6

What is tertiary oil recovery also known as?

What does it refer to?

Answer 6

Enhanced oil recovery

Enhanced oil recovery seeks to alter its properties to make it more conducive to extraction.
There are three main types of enhanced oil recovery:

Thermal Recovery - This works by heating the oil to reduce its viscosity and allowing easier flow to the surface.
This is most commonly achieved by introducing steam into the reservoir, which will work to heat the oil.
Less commonplace is the practice of burning part of the oil in order to heat the rest (fire flooding or in-situ burning).

Gas Injection - Either natural gas, nitrogen or carbon dioxide (increasingly the most popular option) are injected into the reservoir to mix with the oil, making it more viscous, whilst simultaneously pushing the oil to the surface (similar to secondary oil recovery).

Chemical Injection - The least common method of EOR, chemical injection works by freeing trapped oil in the well.
This is done by lowering surface tension and increasing the efficiency of water-flooding.

Question 7

What are the 3 most common types of enhanced oil recovery?

Answer 7

Thermal Recovery - This works by heating the oil to reduce its viscosity and allowing easier flow to the surface.
This is most commonly achieved by introducing steam into the reservoir, which will work to heat the oil.
Less commonplace is the practice of burning part of the oil in order to heat the rest (fire flooding or in-situ burning).

Gas Injection - Either natural gas, nitrogen or carbon dioxide (increasingly the most popular option) are injected into the reservoir to mix with the oil, making it more viscous, whilst simultaneously pushing the oil to the surface (similar to secondary oil recovery).

Chemical Injection - The least common method of EOR, chemical injection works by freeing trapped oil in the well.
This is done by lowering surface tension and increasing the efficiency of water-flooding.

[Chemical, miscible, thermal]

Question 8

What are the 4 main factors effecting oil recovery %?

Answer 8

Pore scale Displacement: how much of the oil pushed out by rocks by injected fluid.

Sweep: Calculates how much reservoir rock has been reached by the injected fluid

Drainage: the extent to which the wells can access all the separate segments of the reservoir

Commercial cut-off: indicates the limits of economic production.

Question 9

What is waterflooding (oil recovery)?

Answer 9

Waterflooding is a form of oil recovery wherein the energy required to move the oil from the reservoir rock into a producing well is supplied from the surface by means of water injection and the induced pressure from the presence of additional water.

Question 10

How is chemical injection used for enhanced oil recovery?

Answer 10

Surface tension is reduced to help the oil droplets move through the reservoir.

Polymer flooding:
Polymers are used to increase the effectiveness of waterfloods.
This increases the sweeping efficiency as viscosity of the water is increased. This reduces interfacial and capillary forces between the oil and water.

Surfactant flooding:
Detergent-like surfactants help to recover oil trapped by capillary forces at the microscopic scale.
Miscibility of the oil in the displacing fluid is increased.

Alkaline chemical flooding:
Oil-water emulsions are formed

Question 11

How is miscible gas flooding used for enhanced oil recovery?

Answer 11

Via gas injection, the gases expand in a reservoir to push additional oil to a production wellbore.
Some gases dissolve in the oil to lower its viscosity and improve its flowrate.

Question 12

What is oil swelling?

Answer 12

An expansion in oil volume that can occur when a solvent contacts a reservoir fluid. The swelling is due to the complete or partial dissolution of the solvent molecules into the reservoir fluid.

Question 13

What operating problems may arise during gas flooding / injection for oil recovery?

Answer 13

Formation of carbonic acids (corroding the injection pipes)

Formation of deposits

Costs of CO2 recovery from produced gas is high

CO2 and oil can form solid asphaltanes which cause problems with permeability and plugging in production tubing

Question 14

How are thermal techniques (hot fluid injection) used for enhanced oil recovery?

Answer 14

Steam injection is widely used.

Recovery by steam drive - oil is swept by steam from injection wells to producing wells. High quality steam is then injected and then soaked (heated up, reducing viscosity) The heated oil and water is then pumped to the surface.

Recovery by cyclic injection: (same as above) - the same well is used for production and injection.
(Huff and puff - Slang term for a cyclic process in which a well is injected with a recovery enhancement fluid and, after a soak period, the well is put back on production.)

Question 15

How does thermal steam flooding enhance oil recovery?

Answer 15

Temperature is increased, reducing oil viscosity sharply

Interfacial tension is reduced

Relative permeability to oil is increases

Mobility ratio (the ratio of the mobility of an injectant to that of the oil it is displacing) is improved

Question 16

Summarise and explain ‘Fire flooding’:

Answer 16

Oil in the well is ignited. Combustion and the changes in physical properties of the oil (e.g. viscosity, surface tension etc.) then lead to improved oil recovery.

Fire flooding, or in-situ combustion, is an EOR process where a portion of the oil-in-place is oxidised and used as a fuel to generate heat.

Air is compressed and injected into the well to produce a combustion zone in the petroleum. A special heater in the oil well ignites oil in the reservoir and starts a fire.

The heat produced from burning the heavy hydrocarbons (in situ) results in hydrocarbon cracking and the vaporisation of lighter hydrocarbons.

In fire flooding, as the fire moves through the well underground, the burning front forces ahead a mix of hot combustion gases, steam, and hot water. In turn, these then reduce the oil viscosity and displace oil towards production wells.

Question 17

What is ‘Dry-Forward Combustion’?

Answer 17

A form of in-situ combustion where air is injected into a heavy oil reservoir, the crude is ignited in situ, and the resulting combustion front moves away from the injection well, in the same direction as the injected air.
As air is continuously supplied at the injection well, the fire ignited at this location moves toward the production wells.

The term “forward combustion” is used to signify the fact that the flame front is advancing in the same direction as the injected air.

Question 18

What is ‘Wet-Forward Combustion’?

Answer 18

Air and water are injected concurrently or alternately. (The purpose of injecting water is to recuperate and transport heat from the burned zone to the colder regions downstream of the combustion front.)

Wet-forward combustion begins as a dry process - once the flame is established, the oxygen stream is replaced by water; water meets the hot zone from the combustion stream, forms steam, and assists in the displacement of oil.

Question 19

What is ‘Reverse Combustion’?

Answer 19

The combustion front moves in the opposite direction to the flow of injected air.

After the burning front has advanced some distance from the production well, air is supplied only near the injection well. The burning front advances toward the injection well while the oil moves toward the production well.

In the reverse combustion process, the vaporized and mobilized fluids move through the heated portion of the reservoir behind the combustion front.

Question 20

Pros and cons of dry forward combustion:

Answer 20

Pros:
- Enhanced oil recovery

Cons:

• Issues arise if flame front is too close to injection site
• The temperature behind the burning front is high, indicating a great amount of heat stored in the formation matrix. The injected gas heats on contact with the matrix and recovers only a small amount of the heat, with considerable losses to the surrounding formations.
• The presence of a highly viscous oil zone surrounding the production well. The fluid in this zone remains at the original reservoir temperature and its forward displacement by the heated oil is normally difficult.

Question 21

Pros and cons of wet forward combustion:

Answer 21

Pros:

• The addition of water during the combustion process means that heat is transferred more effectively than with air alone.
• The steam zone ahead of the combustion front is larger, and the reservoir is swept more efficiently than with air alone.
• The improved displacement from the steam zone results in lower fuel availability and consumption in the combustion zone, so a greater volume of the reservoir is burned for a given volume of air injected.

Cons:
- Lower O2 content (than air alone) which encourages more cracking. In turn, this means less combustion takes place, and this method is less suitable for light oils.

Question 22

Pros and cons of reverse combustion:

Answer 22

Pros:
- Allows the recovery of heavy oils (more suitable for heavy oils)

Cons:

• Spontaneous ignition. Spontaneous ignition would result in oxygen being consumed near the injector, and the process would change to forward combustion
• Inherent instability of the process, which results in narrow combustion channels being formed and therefore an inefficient burn

Question 23

Explain the THAI process:

Toe-to-Heel air injection

Answer 23

The key difference to other combustion methods is that a horizontal production well is used.

Steam is injected into a vertical well to heat the horizontal well and condition the reservoir around the vertical well (over a period of 3 months). Air is then injected, and combustion is initiated.

A combustion front is produced where part of the oil in the reservoir is burned, generating heat which reduces the oil viscosity. This allows the oil to flow by gravity to the horizontal production well.

At the temperatures reached (400-600oC), both thermal cracking and coking occur - about 10% oil is consumed, and the thermal cracking causes the remaining oil to be upgraded.
The combustion front sweeps oil from the toe to the heel of the horizontal production well (recovering 80% of the original oil-in-place).

Question 24

Pros and cons of THAI enhanced oil recovery:

Toe to heel air injection

Answer 24

Pros:

• The distance from combustion front to production well is always very short
• Very high recovery - close to 80% of oil in place
• Water usage is much lower than the above methods
• Increased control over combustion propagation

Cons:

• It takes at least 3 months to prepare the well (relatively long lead time)
• The method is not widely tested

Question 25

How is instantaneous turbulent velocity characterised?

Answer 25

It is separated into a time-averaged component, ū, and a fluctuating component, u’.

Question 26

What do εm, εH, and εi show?

Why can they be related?

Answer 26

These are the eddy diffusivities for momentum, heat, and species respectively.

They are not molecular properties of the fluid; they depend on flow and vary spatially.

Each of the turbulent fluxes is convective in nature and arises from the same eddy motion.
Thus, the simple (but extreme) assumption can be made that:
εm = εH = εi

Question 27

What does Prandtl’s mixing length model (PML) show?

Answer 27

The mixing length model is a method attempting to describe momentum transfer by turbulence Reynolds stresses within a Newtonian fluid boundary layer by means of an eddy viscosity.

It is a method to describe the movement of trace species in the turbulent scale.

Features of PML:

• Eddy diffusivity is proportional to the local velocity gradient
• l is the scale length for eddy motion and is a function of position within the flow domain, so εm is also a function of position
• l*|dū/dy| is the velocity scale for eddy motion
• Eddy viscosity is zero at the centre of the pipe

Question 28

What are the limitations of the PML (Prandtl’s mixing length model)?

Answer 28

It assumes that the turbulent energy and species fluxes vanish in a uniform mean velocity (i.e. when velocity gradient is 0, e.g. at the centreline, which is wrong)

The model is of little use due to complications in specifying the mixing length, l, for complex flows.

It cannot be used for processes with significant convective and diffusive transport of turbulence.

Only suitable for simple, 1D flows.

Question 29

What is the Universal Velocity Profile, UVP?

Answer 29

A description of the mean velocity within a turbulent boundary layer.

The velocity profile is divided into the viscous sublayer (VSL), buffer layer (BL), and turbulent core (TC).

Question 30

What are the 3 layers of the UVP?

Answer 30

The universal velocity profile is divided into the viscous sublayer (VSL), buffer layer (BL), and turbulent core (TC).

Question 31

Regarding the UVP and PML, (Universal Velocity Profile and Prandtl’s mixing length model), what assumptions are made when considering the region near the wall / the viscous sublayer?

Answer 31

i) No effect of the curvature (radius) of the pipe. u+ = fn(y+) only. [R+ not important]
ii) Viscous effects dominate (thus εm ~ 0)
iii) Shear stresses in the fluid are approx. the same as the at the wall

Question 32

Regarding the UVP and PML, (Universal Velocity Profile and Prandtl’s mixing length model), what assumptions are made when considering the turbulent core?

Answer 32

i) Turbulent effects&raquo_space; viscous
ii) At the edge of the core, 𝜏 = 𝜏0 (since VSL and BL are relatively thin)
iii) l is a transverse length scale, proportional to the largest eddy size at position y.

Question 33

What are Von Karman’s UVP relationships for turbulent flow?

Answer 33

VSL: for y+ < 5
u+ = y+

BL: for 5 < y+ < 30
u+ = 5*ln(y+) - 3.05

TC: for 30 < y+ < R+/2
u+ = 2.5*ln(y+) + 5.5

Question 34

Regarding dry-forward combustion (EOR), how do the temperature and ‘% pore volume’ profiles vary with distance from the igniter?

Answer 34

Temperature:
Increases with distance in the direction of the flow of air before cooling again (before reaching the production well).

% Pore Volume:
Initially only contains gas (assuming oil has already been displaced).
As combustion reactions occur, coke is deposited and oxidised also, and the fraction of oil in the pores increases.
Towards the end (closer to the production well / far from ignition), condensation water also begins to form.

Question 35

What are some of the potential zones / situations that may be found along the length of a dry-forward in-situ combustion process?

Answer 35

Zone 1 - Oil from this zone has already been displaced. The hot porous matrix pre-heats the air stream injected into the well.
Temperature increases in the matrix as the combustion zone is approached.

Zone 2 - Combustion reactions occur and the coke deposited in the porous structure is also oxidised.

Zone 3 - Hot gases from the previous zone displace the oil and increase its mobility. Lighter components are vaporized.
As a result of the high temperature and low O2 concentration, cracking reactions take place which contribute to the formation of coke.

Zone 4 - Hot gases from the combustion zone vaporise some of the light/medium hydrocarbons in the oil. The higher temperatures facilitate the flow of oil.
At these temperatures, some of the water in the vapour start to condense in the porous structure.

Question 36

What are some advantages and disadvantages of fire flooding / in-situ combustion?

Answer 36

Pro:
- This method improves the sweep and displacement efficiencies.

Cons:
- Methods (e.g. dry forward) lead to the formation of a highly viscous oil zone surrounding the production well.

Question 37

What are some of the potential zones / situations that may be found along the length of a wet-forward in-situ combustion process?

Answer 37

Zone 1 - Contains little or no hydrocarbon as the zone has already been swept by the combustion front.
Approximately 50% of the pore volume is occupied by the injected water and the remainder of the pore volume is occupied by injected air.
The temperature of the water increases, and it is vaporized.

Zone 2 - The water is in the vapour phase; the pores contain a mixture of air and steam.

Zone 3 - Partial combustion/oxidation occurs in this zone, and oxygen is consumed.
Because of the elevated T at the combustion front and low O2 levels, hydrocarbon cracking reactions occur and carbon (coke) is deposited in the porous structure.

Zone 4 - The hot gases from the combustion zone vaporize some of the light/medium hydrocarbons in the oil and the higher temperatures facilitate flow of oil.
At these temperatures, some of the water in the vapour starts to condense in the porous structure.

Zone 5 - Temperatures are lower in this zone.
As the pores already contain oil, and water is also condensed in these pores, this creates a restriction in flow (high back-pressure).
The formation gradually approaches the initial conditions encountered in this reservoir.

Question 38

What are some of the potential zones / situations that may be found along the length of a reverse in-situ combustion process?

Answer 38

Zone 1 - The porous matrix consists of oil and some water, which is swept with air.
Depending on temperatures and pressures, partial oxidation reactions may occur in this zone.

Zone 2 - Heat conduction through the porous matrix from the hot combustion zone helps to vaporise the oil and increase its mobility.

Zone 3 - Partial combustion/oxidation occurs here, and O2 is consumed. Elevated temperatures and low O2 levels (at the combustion front) lead to hydrocarbon cracking and more coke deposit in the pores.

Zone 4 - As O2 levels are too low to oxidise the residual carbon, it remains present in the porous matrix.
Temperature decreases with distance from the combustion zone and condensation of heavier oil fractions occur.

[In reverse combustion, the flame front is moving away from the producing well.]

Question 39

What is fracking?

Outline the process:

Answer 39

Fracking (hydraulic fracturing) is a drilling technology used for extracting fossil fuels from deep underground.

Large amounts of water, sand, and chemical additives are pumped into subterranean rock formations at high pressures. (Small explosions and a mix of sand / water / chemicals are used to break up shale rock formations containing fossil fuels).

This causes the rock to fracture, freeing up oil and gas that have been trapped there.

Question 40

Why are chemicals and sand added to fracking fluid?

Answer 40

Chemicals are added to dissolve minerals, reduce friction, prevent corrosion, thicken the fluid (to transport the sand), clean out debris, prevent the clay from swelling, and kill bacteria.
(Common ingredients include methanol, ethylene glycol, and propargyl alcohol.)

Sand helps hold open the channels formed by cracking, and prevents the channels collapsing/closing back on themselves.

Question 41

What are the pros and cons of fracking?

Answer 41

Benefits:

• High potential to provide fuel gas
• Lowers dependency on countries exporting gas
• Offers long term gas/energy security
• Better to use natural gas than coal (fewer harmful particles released into the air)
• Lowered energy prices

Risks:

• Large volumes of water are needed, increasing likelihood for spillages
• Can lead to contamination of water supplies (for humans, due to inadequate treatment of waste or aquifer contamination)
• Can generate mercury waste, which must be treated and disposed of due to the environmental and safety considerations.
• Can cause earthquakes
• Air pollution and methane leaks
• Sound pollution

Question 42

What is permeability (regarding oil and gas wells)?

What are the units of permeability?

Answer 42

Permeability measures the capacity and ability of the porous rock to transmit fluid.

It is a measure of the connectivity of pores in the subsurface.

Permeability is measured in Darcies.

Rock permeability, k, is significant since it controls the directional movement and flowrate of reservoir fluids.

Question 43

What is cyclic injection?

Answer 43

Cyclic injection - a method of thermal recovery where the well is injected with steam and then put back on production.

Firstly, a ‘slug’ of steam is injected into the reservoir. The well is then shut for a period of time to allow uniform heat distribution within the oil. Finally, the thinned oil is produced through the same well, and the cycle is repeated. (Also referred to as the Huff ‘n’ Puff method).

Question 44

How does (steam) cyclic injection differ from steam drive?

Answer 44

Steam drive - steam is continuously injected

Steam soak (cyclic steam injection) -involves repeated injection of steam and water