Pressure prediction: analysis of cuttings

	Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Critical elements of the petroleum system
Chapter	Formation fluid pressure and its application
Author	Edward A. Beaumont, Forrest Fiedler
Link	Web page
Store	AAPG Store

The physical appearance of drill cuttings may be a clue to subsurface pressure conditions as reflected in the state of compaction or induration of the rock. This applies to cuttings from permeable as well as impermeable rocks.

Evidence of overpressure

When the formation fluid pressure is greater than the mud pressure, the cuttings tend to explode into the well bore. Cuttings are liberated promptly into the mud with a minimum of abrasion by the bit. Consequently, the cuttings have sharp edges and look fresh; they may even be somewhat larger than normal. This case also applies to shale, so we don't have to wait for a mud kick from a permeable zone to anticipate significant overpressure. Cuttings tend to be large and fresh looking when they are from overpressured, low-permeability rocks because the pore pressure cannot readily bleed out of the pore system when permeability is low.

Evidence of underpressure

When mud pressure exceeds formation fluid pressure, the mud tends to plaster the hole (one of the things mud is designed to do). An undesirable byproduct of this attribute is that cuttings are held in place by the pressure differential between the well bore and the formation. Such cuttings, even if broken away from the host rock, are likely to be struck several times by the bit teeth before being carried away in the mud. The cuttings will be further broken (smaller than normal) and may look worn or abraded. This process is more likely to take place in permeable zones where there is a finite flow of mud filtrate into the formation.

Mud properties caveat

Any discussion about cuttings must be tempered with a consideration of the type of mud used. Consult an experienced drilling and mud engineer to determine the specific properties of a given mud system and its effects on cuttings.

Compaction and fluid pressure

Abnormal fluid pressure causes abnormal pressure within the rock matrix; this directly affects compaction. Fluid carries some of the overburden stress. When fluid pressure is abnormally high, the intergranular stress is lower than normal, resulting in undercompaction. Abnormally low fluid pressure increases intergranular stress, allowing overcompaction (unless the framework is exceptionally rigid, as with a coral reef or well-cemented sandstone).

The table below summarizes the relationships of fluid pressure, intergranular stress, and rock matrix appearance.

Pore pressure	Intergranular stress	Rock matrix appearance	Porosity
Abnormally high	Abnormally low	Evidence of undercompaction	Higher than normal for depth of burial
Abnormally low	Abnormally high	Evidence of overcompaction, i.e., grains may be broken and/or sutured	Lower than normal for depth of burial

Density of shale cuttings

Rocks in overpressured sections have lower bulk densities than rocks in normally pressured sections; rocks from underpressured sections have higher bulk densities. Only differences in shale densities, however, can be detected in cuttings for the following reasons. When cuttings enter the mud column, the fluids in the pore space try to equilibrate with the new pressure environment. If the cuttings are permeable, the equilibration takes place continually as the cutting rises in the mud column. If the cuttings are impermeable, however, equilibration is impeded—very strongly, in the case of shale.

Measuring shale cuttings density

Shale cuttings densities are measured at the well site by dropping chips into a glass cylinder containing a stratified sequence of liquids whose density increases downward. Beads of known density are dropped into the cylinder. The level at which the beads settle indicates the density of the liquid at that level, and the different liquid densities are labeled on the cylinder. The level at which the shale chips settle indicates their density.

Unusual authigenic minerals

Side pressure seals, in particular, are thought to be caused by mineralization adjacent to faults. Top seals form by mineralization that cuts across stratigraphic units. Top seals of major overpressured compartments are usually cemented by calcite. When unexpected pore-filling calcite is seen, a pressure seal should be considered a possibility. Any unusual pore-filling minerals seen in cutting samples should alert the geologist to the potential of a pressure seal.

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Pressure prediction: analysis of cuttings

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