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High-pressure zones commonly make drilling of wells most difficult in a belt about 50 miles wide along the coastal plain northwest of the Gulf of Mexico from the Rio Grande to the Mississippi Delta. This study is an attempt to link geological factors with occurrences of abnormal pressure in order to provide a better understanding of their origin.
Abnormal pressure has been defined as any pressure which exceeds the hydrostatic pressure of a column of water containing 80,000 parts per million total solids.
Dangerously abnormal pressures occur commonly in isolated porous reservoir beds in thick shale sections developed below the main sand series. Their locations are controlled by the regional facies change in the Gulf Coast Tertiary province, and they appear to be independent of depth and geological age of the formation.
The high pressures are caused by compaction of the shales under the weight of the overburden which is equivalent to approximately one pound per square inch per foot depth. Difference in density between gas and water causes abnormal pressure where hydrocarbon accumulations occur above water, irrespective of whether the water is at normal or abnormal pressures. The magnitude of this pressure depends on the structural elevation above the source of pressure in the water and may cause very high pressure gradients in isolated sand bodies. However, the trend of pressures in the Gulf Coast region indicates that maximum pressures probably do not exceed 90 per cent of the overburden pressure.
The abrupt increase in pressure above normal hydrostatic pressure commonly occurs over a very short vertical interval which makes control difficult. Successful drilling through abnormal pressures involves cementing casing below the main sand series and above the high pressure zones so that heavy mud may be used without loss of circulation.
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