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
List 7 potential causes of overpressure
- High water table
- Structural deformation
- Compaction
- Hydrocarbon column
- Diagenesis: causing reduced pore space
- Release of fluids during mineralogical change: dehydration of gypsum to anhydrite, releasing water during compaction, alteration of volcanic ash to clay
- Artesian pressures (hydrodynamic flow)
What is the effect of overpressure on porosity?
Preserved with depth
Why do shales form mud diapirs in overpressured systems?
Overpressured shales are less dense than any surrounding normal pressured shales, can be drilling hazard
How are do underpressure reservoirs occur?
How can natural underpressures occur? (4)
Occur in isolated reservoirs where has no water drive
Can occur naturally by:
- Uplift and removal of overburden, decrease in overburden = increase in pore volume, lowering fluid pressure
- Decrease in reservoir temperature, cooling of fluids, reduction in volume
- Production of fluid from a sealed reservoir
- Low water table
How is pressure measured in wells?
- Repeat Formation Tester/Modular Formation Dynamic Tester
- Drill stem tests and production pressure gauges
- Wireline RFT can sample across various parts of reservoir, gradient shows density of fluid in pore system, can determine what fluid in reservoir and if charged
Since fluid pressure is a function of depth, for any fluid type pressures should plot on straight line
Why would inaccuracies occur?
Supercharging Low pressure (bleeding)
What calculation is used to obtain pressure at depth?
Hydrostatic equation:
p - pgh
Pressure = density x gravity x height
What does RFT/MDT data tell us?
- Reservoir pressures and layering
- Estimate vertical permeability
- Can take lab quality samples
- Essential to interpret data honouring reservoir fluid gradients
- Can determine contact depths in discovery wells
