Hydrocarbon Habitat part 2 Flashcards
What is porosity and what is it a measure of?
Fraction of rock bulk volume occupied by pore space
It is a measure of proportion of pore space to grains
How can effective porosity be subdivided? What is ineffective porosity?
Cul-de-sacs (coordination number of 1)
Caternary pores (coordination number of 2)
Ineffective porosity: no openings, coordination number of 0
Describe primary porosity in terms of when it is formed and how it can be reduced?
Formed during deposition
Reduced by compaction and diagenesis
When is secondary porosity developed? Give examples
Dissolution
Fenestral (caused by early cementation)
Dolomotisation
Vuggy/mouldic
How is porosity measured in the subsurface?
Wireline logs
Cores
Seismic
How is porosity calculated?
Porosity = (volume of gas/bulk velocity of sample) x 100
What is permeability measured in? How is it measured?
Measured in Darcy’s, ususally mD
Measured by:
- Drill stem tests
- Core analysis
- Minipermeameter
- Petrophysical analysis
What controls permeability? How does permeability compare in layered sedimentary rocks?
Permeability controlled by:
- Grain size
- Pore throat size
- Direction of flow
In layered sedimentary rocks Kh is greater than Kv
How might dual porosity occur?
Poroperm in matrix enhanced by fractures providing high permeability corridors to deliver high flow rates
Why might permeability measured from a well be incorrect?
Reservoir damage, e.g. fluid invasion during drilling
What are the 8 key considerations when evaluating a seal?
- Lithology
- Depositional Environment
- Efficiency
- Lateral Variability
- Thickness
- Faults and Fractures
- Geometry
- Control Data
Describe the two different types of seals
Membrane: prevent hydrocarbon migration through pores
Hydraulic: hydrocarbons can only migrate through fractures
What properties do seals commonly have?
Very fine grained, very low poroperm
Under what circumstances are seals ductile?
If composed of mudrock or evaporites
What do the best seals exert?
Significant capillary pressure to stop flow of oil/gas
What is seal permeability a measure of?
Rate of fluid flow through porous medium
What are crucial properties of a seal?
Thickness and lateral continuity
How can stacked seals form?
Interbedded units i.e. mudrocks
What parameters control sealing capacity and hydrocarbon entrapment?
Rock properties:
- Pore distribution, ductility, continuity
Fluid properties:
- density difference of water and hydrocarbons and interfacial tensions of the fluids
Under what circumstances would hydrocarbons enter the reservoir pore space or leak into the seal?
If buoyancy pressure created by the hydrocarbon and water density difference exceeds the capillary entry pressure
