Mechanisms of overpressure

Disequilibrium compaction

• Ongoing sedimentation increases overburden (vertical stress) faster than fluid diffuses out of zone

Characteristic time of diffusion in porous medium

$${}$$$$ \tau = \frac{(\phi \beta_f + \beta_r) \eta l^2}{k} $$

• low-permiability sand (~1 md)
  • $\tau$ on the order of years for $l=0.1$km

• low-permiability shale (~10 nd)
  • $\tau$ on the order of 100,000 years for $l=0.1$km

Common in Gulf of Mexico

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

Techtonic compaction

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

California

Hydrocarbon column heights

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Image Source

Hydrocarbon column heights

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

Centroid effects

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

Can cause "kicks" in drilling, because pressure will exceed mud weight pressure and formation fluids will get into the wellbore. If this increases in severity uncontrolled, it could lead to a blowout.

Other mechanisms

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Aquathermal pressurization

• Temperature increases due to radioactive decay and upward heat flow from mantle

Hydrocarbon generation

• From thermal maturation of kerogen

Direct measurement of pore pressure

• Via wireline samplers that isolate formation pressure from annular pressure in a small area at the wellbore wall.
• Mud weight

Estimation of pore pressure at depth

Confined compaction experiment

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

Shale compaction relation

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$\phi = \phi_0 e^{-\beta (S_v - P_p)}$

Use with caution!

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

Porosity inference from $P$-waves

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$P_p = S_v + \left(\frac{1}{\beta} \ln\left(\frac{\phi}{\phi_0}\right)\right) \quad \quad \phi = 1 - \left(\frac{\Delta t_{ma}}{\Delta t}\right)^{\frac{1}{f}}$

Compaction	$P$-wave	Pore pressures

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