Mechanisms of overpressureMechanisms of overpressure

Disequilibrium compactionDisequilibrium compactionOngoing sedimentation increases overburden (vertical stress) faster than �uid di�usesout of zone

Characteristic time of di�usion in porous mediumCharacteristic time of di�usion in porous medium

τ =(ϕ + )ηβf βr l2

k

low-permiability sand (~1 md) on the order of years for kmτ l = 0.1

low-permiability shale (~10 nd) on the order of 100,000 years for kmτ l = 0.1

Common in Gulf of MexicoCommon in Gulf of Mexico

© John Wiley & Sons, Inc. Gordon and Flemmings, 1998

Techtonic compactionTechtonic compaction

Occurs in areas where large-scale tectonic stress changes occur over geolocgically shortperiods of time.

Hydrocarbon column heightsHydrocarbon column heights

Image Source

Hydrocarbon column heightsHydrocarbon column heights

© Cambridge University Press (Fig. 2.11, pp. 42)Zoback, Reservoir Geomechanics

Centroid e�ectsCentroid e�ects

© Cambridge University Press (Fig. 2.12, pp. 43)Zoback, Reservoir Geomechanics

Other Other mechanismsmechanisms

Aquathermal pressurizationAquathermal pressurizationTemperature increases due to radioactive decay and upward heat �ow from mantle

Hydrocarbon generationHydrocarbon generationFrom thermal maturation of kerogen

Direct measurement of pore pressureDirect measurement of pore pressureVia wireline samplers that isolate formation pressure from annular pressure in a smallarea at the wellbore wall.Mud weight

Estimation of pore pressure at depthEstimation of pore pressure at depth

Con�ned compaction experimentCon�ned compaction experiment

© Cambridge University Press (Fig. 2.13, pp. 46)Zoback, Reservoir Geomechanics

Shale compaction relationShale compaction relation

ϕ = ϕ0e−β( − )Sv Pp

Use with caution!Use with caution!

© Cambridge University Press (Fig. 2.14, pp. 48)Zoback, Reservoir Geomechanics

Porosity inference from Porosity inference from -waves-waves

Compaction -wave Pore pressures

© Cambridge University Press (Figs. 2.16a,b, pp. 48, Fig. 2.8b,pp. 36)

P

= + ( ln( )) ϕ = 1 −Pp Sv1

β

ϕ

ϕ0

( )Δtma

Δt

1

f

P

Zoback, Reservoir Geomechanics