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earth resources > oil and gas > Flashcards

oil and gas Flashcards

1
Q

List the factors that affect how oil will flow in a reservoir.

A
  • Rock permeability
  • Presence of faults
  • Relative permeability
  • Capillary pressure
  • Fluid properties: density, viscosity (composition)
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2
Q

What is the average oil recovery factor (RF) worldwide?

A

Between 20% and 40%

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3
Q

What are the main techniques used to recover oil?

A

Primary recovery
Secondary recovery
Enhanced/Improved oil recovery

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4
Q

How much of the total energy use does oil alone account for?

A

Oil 1/3 of total energy

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5
Q

Where is gas energy primarily used?

A

In homes

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6
Q

What is the difference between light or conventional crude oil, and butimen?

A

Light or conventional crude oil flows like cooking oil, while Butimen flows like treacle or is almost solid. Butimen is an unconventional source.

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7
Q

Where does most of the oil come from nowadays and why is this a problem?

A

The Middle East

Political problems

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8
Q

What is light tight oil and why is it so expensive?

A

It’s Oil Shale and it’s expensive because it’s difficult to extract from the rock

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9
Q

Under what conditions does oil form?

A

60° C < T < 160° C (oil window)

at T > 160° C natural gas will form

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10
Q

How does oil migrate and get stored?

A

During burial, as the rock is squashed and heated the oil migrates in droplets out of the source rock, through permeable sedimentary rocks into a reservoir like s.stn or l.stn, where the oil gets trapped beneath a impermeable cap rock like salt or shale.

Gas and oil are less dense than water so they sit on top of it, with the gas sat on top of the oil

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11
Q

What is a good reservoir rock?

A

High permeability palaeodelta channels are good reservoirs for oil and gas because the channel sands are porous and permeable and the delta plain muds are less/im-permeable.

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12
Q

What is the typical reservoir depth, T/P and oil column thickness?

A

Reservoir depth: 500m to >12km

Oil column thickness: 1 to >100m

(50 – 1200 atm, 10° – 140° C)

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13
Q

How do we find oil reservoirs?

A

Using geological knowledge of sedimentary basins, seismic surveying to identify traps and by drilling wells to find out if there’s actually oil there

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14
Q

Why is the extemely high pressure in the oil column a problem?

A

The pressure in oil column >> surface pressure. There is a higher P in the reservoir than expected for that depth.

This can cause a “blowout” which has severe environmental problems (toxic) as well as it being a waste of valuable oil and gas.

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15
Q

Where do we find sedimentary basins suitable for oil recovery today?

A

Off-shore basins around margins of Brazil, West Africa and the Austrailian North Shelf

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16
Q

What drilling techniques are used to FIND oil?

A

Rotary drills
• Depths of 12,000 m achievable
• Water depths of 2.5km
• Horizontal distance of 19km from initial point of entry
• Down hole surveying every 50-300 feet drilled

Drilling mud
• Complex chemistry
» Oil based
» Water based
• Cooling
• Transporting rock cuttings
» Thixotropic
• Balances formation pressure

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17
Q

What can the presence of faults do to the flow of oil/gas in a rock?

A

Faults are barriers to flow although they can occasionally assist flow

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18
Q

What sets of data do we need in order to tell if the discovery will be economic?

A

The volume of oil in place

Need to know how the oil will flow

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19
Q

What data do we need to estimate the volume of the oil in place?

A

o Dimensions of reservoir (area, thickness, geometry), A, H
o Porosity
o Oil/water, Swc /gas saturations,
o Net-to-gross, NTG
o Oil formation volume factor, Boi

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20
Q

What data do we need to know how the oil in the reservoir will flow?

A

o Permeability
o Faults
o Relative permeability
o Capillary pressure
o Fluid properties: density, viscosity (composition

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21
Q

What makes developing an oil field expensive?

A

Offshore platforms are capital intensive
• Shell’s Perdido platform: $3 billion

Drilling a well is costly
• Deepwater off shore: $500 million
• Shallow, onshore: $1 million

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22
Q

What is Primary Recovery and what is the the average oil recovery factor (RF) for Primary Recovery?

A

Uses natural pressure of the reservoir to push crude oil to the surface

Recovery: 5-10%

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23
Q

What are the problems associated with primary oil recovery? Give an example

A

As the oil is “sucked” out and groundwater is withdrawn, the reservoir rock compacts and subsidence can occur

Ekofisk platform subsided by >3.5m and it cost >$1 billion to fix it to protect it against 100 year wave.

24
Q

How is Secondary Recovery achieved? What RF is typical?

A

By injecting pressurised water or gas (water/gas-flooding) from injection wells to displace and drive the residual crude oil and gas remaining after the primary oil recovery phase to the surface wells.

The injected water occupies the pore space that the displaced oil originally occupied.

Allows additional 25% to 30% of the oil in the reservoir to be extracted (RF = 30-40%). 60% of the oil remains in the reservoir.

25
Q

Define ‘oil reserve’

A

Those quantities of hydrocarbons which are anticipated to be commercially recovered from known accumulations from a given date forward

26
Q

Define “oil resource”

A

Those estimated quantities of petroleum contained in the subsurface, as well as those quantities already produced

27
Q

What are the factors affecting RF from water flooding (secondary recovery)?

A

RF = Eps ×Es ×ED ×Ec

Eps, pore scale/microscopic displacement efficiency
Ems, macroscopic sweep efficiency
Ed, drainage efficiency/connected volume factor
Ec, economic efficinecy factor/commercial cut-off

where Ed indicates how much of the reservoir is in contact with the wells

28
Q

Define Recovery Factor (RF)

A

the volume of oil recovered over the volume of oil initially in place (OIIP), both measured at surface conditions

29
Q

Define microscopic displacement efficiency

A

the fraction of oil displaced from the pores by the injected water, in those pores which are contacted by the water

30
Q

Define macroscopic sweep efficiency

A

the proportion of the connected reservoir volume that is swept by the injected fluid(s).

This is principally affected by heterogeneity in rock permeability and by gravitational segregation of the fluids

31
Q

Why do we only get 40% RF with water flooding (secondary recovery)?

A

RF=EPS ×ES ×ED ×EC

if each of the efficiency factors is a very respectable 80% then the overall RF is only 41%. Increasing RF requires each of these factors to be increased to close to 100%.

EOR methods are targeted at increasing EPS and ES while IOR methods also aim to increase ED and to some extent ES

32
Q

What factors influence the microscopic displacement efficiency?

A

the capillary and relative permeability characteristics (the geometry/topology of pore space) of the rock control the relative mobility of the oil and water when moving through the pore space

These are influenced by the wetting behaviour of the rock. The rock wettability.

10-30% of oil is trapped in pores (Eps<=70%)

33
Q

Distinguish between water-wet and oil-wet sand grains

A

If the rock is water-wet then there is a higher proportion of oil permanently trapped between the pores. This is caused by growth of the water film on the grain surface which ultimetly bridges at the pore throats.

If the rock is oil-wet then much less oil is trapped between the pores and oil continuity is maintained over the grain surfaces and through pore throats

34
Q

What factors influence macroscopic sweep efficiency

A

1) principally affected by the geological heterogeneity in the reservoir, which controls the spatial distribution of porosity and permeability which is variable on all length scales.

The injected water flows preferentially through higher permeability layers, bypassing volumes of oil contained in the lower permeability zones

2) Gravitational segregation of water and oil. The oil will rise and sit above the water due to the density difference, and the water will flow beneath the oil
3) Well pattern also causes non-uniform sweep between injection and production wells

35
Q

Define EOR

A

Enhanced Oil Recovery focusses on increasing the Microscopic and Macroscopic displacement efficiencies in order to raise the RF. This involves modifying the injected water or gas to make it more effective.

36
Q

Define IOR

A

Improved Oil Recovery

is achieved via better engineering and project management, e.g. identifying volumes of oil that have been bypassed during water injection using 4D seismic surveying, reservoir modelling and then drilling new wells to access those oil pockets

37
Q

How do we increase Macroscopic Sweep Efficiency during EOR?

A

– Recomplete well
– Flow diversion - Polymers or polymer gels used to reduce permeability in zones where water or gas preferentially flow
– Deep reservoir flow diversion - BrightTM water
– Introduce or stimuate microbes that, by their actions, release extra oil
– “Foams”

38
Q

How is flow diversion used to improve Macroscopic sweep efficiency? And give an example of where this is used.

A

The objective is to reduce the permeability in the zones in which water or gas is preferentially flowing by injecting a polymer solution with a cross-linker into the higher permeability zones and that then forms a gel.

This allows the water or gas to then flow into and displace the oil in prevously less permeable, adjacent oil-bearing zones.

e.g. Prudhoe Bay

39
Q

How is Deep reservoir flow diversion used to improve Macroscopic sweep efficiency?

A

To prevent the diverted water flowing back into the theif zone soon after treatment (like in flow diversion) this technique was developed so that temperature-triggered particulates would expand and block the rock pores of the theif zone upon heating to the orginal reservoir temperature by the unswept layers

40
Q

How do you improve microscopic sweep efficiency during EOR?

A
  • Reduce oil-displacing fluid IFT
    – Miscible gas injection (CO2, hydrocarbon gas, air)
    – Add surfactant to water
    – Microbes to produce biosurfactant in situ
    – Alkali added to produce surfactants
  • Alter wettability of mineral
    – Low salinity water injection
    – Steam injection
41
Q

How does Miscible gas injection improve microscopic displacement efficiency? What gases can be used?

A

By reducing the interfacial tension (IFT) between the oil and the displacing fluid. Miscible gas can typically displace 95% of the oil in the rock.

When injected into a water-wet rock after waterflood, the gas merges with the oil and re-esablishes a hydrocarbon pathway allowing the oil to move to the production well. When gas flow ceases, the miscible gas is trapped in the pores.

Carbon dioxide, nitrogen, hydrocarbon gas

42
Q

What are the pros and cons of using carbon dioxide during miscible gas injection?

A

PROS:

  • Low minimum miscibility P and T;
  • Also can be stored in oil reservoir (mitigate climate change)/sequestration

CONS:

  • Corrosion of steel pipes
  • Not always available/requires source
43
Q

What are the pros and cons of using nitrogen during miscible gas injection?

A

PROS
Readily available from air

CONS
High minimum miscibility pressure
Requires equipment to separate from air

(as result, not widely being used)

44
Q

What are the pros and cons of using hydrocarbon gas during miscible gas injection?

A

PROS
Usually readily available from field
Multi-contact miscibility

(thus most widely used)

CONS
May be better to sell it

45
Q

What is the drawback to miscible gas injection?

A

The gas is both less viscous and less dense than the oil, resulting in a lower macroscopic sweep efficiency as they are adversely affected by viscous fingering, heterogeneity and gravity

46
Q

What is WAG injection? How and why is it used in EOR?

A

Water Alternating Gas (WAG) injection

Involves the injection of slugs of water alternately or simultaneaously with gas (termed SWAG)

  • Improves macroscopic sweep
    • Water reduces miscible gas mobility so reduces viscous fingering
    • Water heavier than gas so improves vertical sweep
  • Improves microscopic displacement
    • Miscible gas, so low residual oil saturation
  • Need less gas to maintain reservoir pressure

CON: This is however a very complex operation

47
Q

EOR technologies can be very effective at improving recovery, however they have some practical complications, such as:

A

they are often complex to design (require specialists in phase behaviour, thermodynamics and chemistry), develop and operate

and the response to the application of these techniques, in terms of oil production rate, is usually slow, typically months or years after the process is initiated. Whereas the benefits from drilling water injection wells are usually seen within months.

New EOR technologies are needed that are easier to design, require less specialist eqipment and produce a quicker response time in terms of oil rate.

48
Q

What is the problem with (trying to) implement EOR in many mature off-shore fields?

A

It is impossible to implement EOR because of the lack of space on the platforms for additional equipment needed to inject different fluids and/or process the produced fluids

49
Q

What are the technical challenges associated with EOR?

A

Maximum oil recovery is only achieved if EOR is deployed as soon as production begins. The traditional approach of moving to EOR after oil rates from waterflood drop results in slow development, significant volumes of oil being bypassed and high water production

50
Q

What are the practical challenges associated with EOR?

A

The use of large quantities of expensive chemicals or valuable hydrocarbon gases, means that EOR is only economical when the oil price is high

Transportation of products and machinery can also be a complication as the locations are so remote.

51
Q

There are other challenges associated with EOR, what are they?

A

National vs. Independent Oil Companies
Licence to operate

52
Q

What are the associations of EOR with the environment and its impact?

A

increasing or even maintaining crude oil production to help supply the world’s energy demand is likely to adversely affect climate change unless it is associated with geological carbon sequestration.

Oil reservoirs are ideal candidates for secure storage of anthropogenic CO2 because they have trapped oil for millions of years

Oil spills have also been an issue lately eg BP oil-spil, Gulf of Mexico

Development in Arctic, Antarctic, Amazon

53
Q

What socio-politicalincentives are associated with EOR?

A

Incentives to improve recovery

Responsible development benefitting local population

54
Q

What does the future hold for oil?

A

Oil will be important in world energy mix for > 20 years

Plenty of conventional oil still in existing fields

There are technologies to improve recovery factor

55
Q

In summary, what are the PROS and CONS of WATER FLOODING vs EOR/IOR

A

Waterflooding

PROS:

  • Relatively cheap

Off shore there is lots of water
Injection and production wells
Pumping
Separation

  • Fast response

EOR/IOR

PROS:

  • Higher recovery factor (Best applied early in field life)

CONS:

  • Expensive
  • Slow response
  • Extra equipment/ chemicals required
  • Need to plan ahead, often not space available on old offshore platform

earth resources (2 decks)

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