Under normal conditions fluid pressures in underground reservoirs will be hydrostatic, i.e., in equilibrium with the weight of a (salt) water column extending from reservoir level to surface. Such hydrostatic pressures can be easily controlled by using a mud flush on slightly heavier than water.

Reservoir pressures higher than hydrostatic are not uncommon, however, and a theory is given explaining the why and when of their occurrence. Composition and previous sedimentation rate of overburden are controlling factors, as are the age and geologic history of formations concerned. Maximum possible reservoir pressure to be encountered is petrostatic, i.e., in equilibrium with weight of overburden. The control of such excessive reservoir pressures in drilling wells requires mud weights, which, in extreme cases, may be equal to or even in excess of overburden weight. These latter pressures will unavoidably cause incurable circulation losses by squeeze action, and under appropriate conditions even less high mud weights may have the same effect. A very serious problem is thus created.

Critical situations of this nature are not uncommon and would appear to require greater attention and more study. A few practical experiences of more recent date (North Germany, Netherlands New Guinea, West Pakistan) are discussed.

From both theory and field experience it appears that with proper care and precaution no unsurmountable difficulties need generally be anticipated in drilling through rocks with reservoir pressures not exceeding 80-90% of overburden pressure. For cases of still higher pressures (90-100%), however, the problem seems still far from solved.