BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
Abnormal Pressure
Charged Sand
Normal Pressure
Figure 12 - 2 Structural Feature
• Diagenesis is the physical/chemical transformation of one rock or mineral into another. Many
minerals will undergo a chemical metamorphosis at relatively low temperatures. A classic
example is the conversion of gypsum to anhydrite (CaSO 4·2H 2 O to CaSO 4 ) in which there is a
total volume change of about 50% with the expulsion of water. If this water is not allowed to
migrate this could generate substantial pressure increases within a sealed zone.
Diagenesis of clay minerals is often associated with over-pressured and troublesome
shales/mudstones/claystones. Younger argillaceous sediments are often rich in smectite
clay – montmorillonite. These clays have a high surface area to which up to ten layers of
water can bond. The result is a low density “swelling clay”. Smectite clays go through a
number of changes with burial. Initially, increasing pressure will drive out the loosely
bound water, but as the number of layers is reduced, the pressure required to drive out the
remaining layers increases. Ultimately, only high temperature and chemical processes
will release the last layer, which can be bound with metallic cations. When smectites
within a shale are subjected to lithostatic pressure and a temperature of 67 – 81°C (153 -
178ºF) the penultimate (next to last) layer will be displaced. A further rise in temperature
to 172 – 192°C (342 - 378ºF) is required to drive off the last layer, which is very closely
bound between the clay plates. Interfering with the migration of freed water may lead to
the generation of over-pressures. At the threshold temperature for the loss of the
penultimate water layer is also the point at which smectites can turn into illites.
Depending on the type of smectite, the availability of available cations like K + will satisfy
the surface charges in place of water and collapse the clay into more compact illite. The
remaining water is released into the porosity created by the reduction in clay volume. It
appears that the denser illite can reduce vertical permeability and thus hinder the natural
migration of fluids. This may explain why over-pressures are often associated with zones
containing 40 to 50% illite.
• Formations that have been charged from water floods, casing leaks, or underground blowouts.
HOW TO RECOGNIZE ABNORMAL PRESSURE ZONES
Abnormal pressure zones are often associated with under-compacted shales. This change in
shale characteristic generally results in,
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-3