Hydrocarbon Habitat part 2

Question 1

What is porosity and what is it a measure of?

Answer 1

Fraction of rock bulk volume occupied by pore space

It is a measure of proportion of pore space to grains

Question 2

How can effective porosity be subdivided? What is ineffective porosity?

Answer 2

Cul-de-sacs (coordination number of 1)

Caternary pores (coordination number of 2)

Ineffective porosity: no openings, coordination number of 0

Question 3

Describe primary porosity in terms of when it is formed and how it can be reduced?

Answer 3

Formed during deposition

Reduced by compaction and diagenesis

Question 4

When is secondary porosity developed? Give examples

Answer 4

Dissolution

Fenestral (caused by early cementation)

Dolomotisation

Vuggy/mouldic

Question 5

How is porosity measured in the subsurface?

Answer 5

Wireline logs

Cores

Seismic

Question 6

How is porosity calculated?

Answer 6

Porosity = (volume of gas/bulk velocity of sample) x 100

Question 7

What is permeability measured in? How is it measured?

Answer 7

Measured in Darcy’s, ususally mD

Measured by:

• Drill stem tests
• Core analysis
• Minipermeameter
• Petrophysical analysis

Question 8

What controls permeability? How does permeability compare in layered sedimentary rocks?

Answer 8

Permeability controlled by:

• Grain size
• Pore throat size
• Direction of flow

In layered sedimentary rocks Kh is greater than Kv

Question 9

How might dual porosity occur?

Answer 9

Poroperm in matrix enhanced by fractures providing high permeability corridors to deliver high flow rates

Question 10

Why might permeability measured from a well be incorrect?

Answer 10

Reservoir damage, e.g. fluid invasion during drilling

Question 11

What are the 8 key considerations when evaluating a seal?

Answer 11

• Lithology
• Depositional Environment
• Efficiency
• Lateral Variability
• Thickness
• Faults and Fractures
• Geometry
• Control Data

Question 12

Describe the two different types of seals

Answer 12

Membrane: prevent hydrocarbon migration through pores

Hydraulic: hydrocarbons can only migrate through fractures

Question 13

What properties do seals commonly have?

Answer 13

Very fine grained, very low poroperm

Question 14

Under what circumstances are seals ductile?

Answer 14

If composed of mudrock or evaporites

Question 15

What do the best seals exert?

Answer 15

Significant capillary pressure to stop flow of oil/gas

Question 16

What is seal permeability a measure of?

Answer 16

Rate of fluid flow through porous medium

Question 17

What are crucial properties of a seal?

Answer 17

Thickness and lateral continuity

Question 18

How can stacked seals form?

Answer 18

Interbedded units i.e. mudrocks

Question 19

What parameters control sealing capacity and hydrocarbon entrapment?

Answer 19

Rock properties:
- Pore distribution, ductility, continuity

Fluid properties:
- density difference of water and hydrocarbons and interfacial tensions of the fluids

Question 20

Under what circumstances would hydrocarbons enter the reservoir pore space or leak into the seal?

Answer 20

If buoyancy pressure created by the hydrocarbon and water density difference exceeds the capillary entry pressure