In "Porosity Reduction in Sandstone by Quartz Overgrowth," by Frederic Leder and Won C. Park (AAPG Bulletin, v. 70, no. 11, November 1986, p. 1713-1728), the fluid velocities reported are low by about 35%. However, because of a cancellation of errors, the results published for porosities are correct. The following derivation should be substituted for the published text, beginning with equation 11 (p. 1716):

EQUATION (11)

where ^ngr = fluid viscosity and ^nablaP = pressure gradient.

EQUATION (12)

where

EQUATION (13)

Equation 14 is correct as published.

EQUATION (15)

Neglecting <fr>^dgrvv</>^dgrvy</fr> and <fr>^dgrv²P</>^dgrvy²</fr>,

EQUATION (16)

Rearranging and integrating gives equation 17 as published. The left side of equation 18 is correct as published; the right side, however, should be:

EQUATION (18)

and equation 19 becomes

EQUATION (19)

Equations 20-22 are correct.

EQUATION (23)

EQUATION (24)

Considering fluid velocities, equation 40 should be

EQUATION (40)

The subscript zero is required since ^Thgr is a function of time. Equation 42 should be

EQUATION (42)

and

EQUATION (43)

Using equation 39,

[EQUATION]

Therefore, substituting equation 24 into equation 19,

[EQUATION]

and

[EQUATION]

By equation 40,

[EQUATION]

This is the correct expression for use in calculating porosity.

Equation 40, as it was published, substituted as above into the published equations 19 and 24, will produce the same equation. This fortuitous result preserves the value of the calculations; however, the derivation presented in this erratum is correct and should be used.

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