ABSTRACT

Conventional resistivity, sonic, and density logs are now used to determine formation pressures in Gulf Coast wells. These determinations are based upon observations that log characteristics of shale formations vary more or less uniformly with depth through zones of normal pressure. When abnormally pressured formations are encountered, they are usually associated with shales that depart from the established trend.

Shales exhibit log characteristics along the normal trend when the contained fluids are free to escape at a normal rate during accumulation of overburden. However, when the formations are sealed such that further fluid loss is prevented, continued accumulation of over burden leads to overpressuring. In such formations, the shales exhibit log values typical of the depth at which the formations were sealed.

The apparent depth of formation sealing is obtained by relating the shale log values to the normal trend. For this sealing depth, a normal pressure of the fluids in the pores is computed assuming 0.465 psi per foot of depth. Fluid pressure in the zone of interest is then computed assuming an increase at 1.0 psi per foot of overburden accumulation subsequent to sealing. Charts simplify these computations and provide values for both the estimated pore pressure and the mud weight needed for a balance.