Subsurface Pressure Flashcards
What does pressure data allow us to determine? What does pressure in the hydrocarbon reservoir provide?
- Can determine fluid content
- Pressure in reservoir provides the driving force for primary production
What are the advantages of low mud overbalance and high mud overbalance? How is a compromise chosen?
- Low mud overbalance: drilling efficiency and formation evaluation
- High mud overbalance: well control and hole stability
- Compromise mud weight based on expected formation pressures
What does overburden pressure mean?
Pressure exerted from weight of overlying rocks
What is meant by lithostatic pressure?
Pressure exerted by grains
What is the fluid pressure?
Pressure caused by weight of overlying fluid and in closed system, where the fluid supports the grains
What is the equation for overburden pressure (S)?
S = lithostaic pressure (o) + fluid pressure (p)
What is meant by hydrostatic fluid pressure? How is it expressed?
Pressure imposed by weight of overlying water column at rest
Related to composition of fluid and its density (less dense fluids exert smaller pressure than dense fluids)
Expressed in terms of the gradient of pressure with depth
What is hydrodyamic pressure?
Fluid pressure caused by fluid flow
What is the potentiometric surface? When might it be/not be horizontal?
Potentiometric surface is level to which fluid in well will rise if hole is empty
- Horizontal: when hydrostatic conditions
- Not horizontal: when there is a fluid flow and an associated hydrodynamic pressure
What role could a hydrodynamic flow have in the petroleum system? Name an example where this is the case
May give tilted OWC contacts, can provide a trapping mechanism for oil
An example of this is the Valhall/Hod tilted contact in the North Sea
List potential effects of underpressure (3) and overpressure (2) during drilling operations
Underpressure:
- loss of drilling mud
- formation damage
- differential sticking
Overpressure:
- kicks
- facilities design
What do overpressure and underpressure mean in terms of mean matrix pressure?
Overpressure: matrix pressure is lower, porosity is higher as grains loose
Underpressure: matrix pressure higher, can cause rock failure
What is the normal hydrostatic pressure gradient?
0.458 psi/ft
Of subsurface formations have an open pore system to surface then how will the pressure gradient appear?
Normal, effected only by density/salinity of fluid
What formation pressure constitutes an overpressured formation?
What is this caused by?
Give examples of the kind of locations this may occur
Overpressure if exceeds 0.458 psi/ft
Caused when fluid pressure cannot be transmitted out of the pore system through impermeable barriers (i.e. shale/evaporites)
Common in Tertiary deltas and young subsiding basins
