Formation Pressure Methods
Shale Density
Shale density determination has often proved to be very effective in determining the degree of under compaction and consequent abnormal pore pressure in shale bodies. Shale density determination can be of great value since it provides information on the compaction of the shale. Under normal conditions, shale density should increase with depth. Any sudden decrease in shale density (as porosity increases) may indicate abnormal pressure does exist.
Shale Factor
Shale factor depends on the release of bound water, associated with changes in clay minerals from Montmorillonite to illite. A plot of shale factor versus depth would show a continuous decline in Montmorillonite with depth. However, many anomalies in the declining shale factor plot are observed wherever compaction of the phase of diagnosis (i.e. Montmorillonite to illite) has been slowed or interrupted. Under these conditions, higher than normal Montmorillonite clay and water contents can be expected. The shale factor under these conditions will be higher than normal and would indicate impending abnormal pressure.
AGIP Sigmalog
The AGIP Sigmalog is a rock strength calculation intended for use in shale lithologies, much like DC Exponent. The aim is to solve the shortcomings of the dc exponent while drilling overpressure sequences of carbonates. The Sigmalog calculation, however, allows pressure calculation for formations that are not pure shales. The sigmalog is the variation with depth of the sigma factor, also called "total rock strength". The Sigma Factor takes the same factors into account as the DC exponent.
D Exponent
A strong relation exists between ROP and the hydrostatic differential pressure. Maintaining a constant hole size, bit weights, RPM, and correcting for mud weights, a plot of incremental rate of penetration rates should define an ever-increasing trend in the normal pressured zone. The trend should reverse when drilling into an over-pressured zone.

DXC Exponent
DXC is the corrected DC value for mud weight and can be related directly to formation pressure rather than to differential pressure. The correction factor gives better results and permits graphic determination of Pressure gradient in abnormally pressured zones. The correction pressure gradient for the area in equivalent mud weight divided by the Pressure gradient for the area, in equivalent mud weight divided by the weight of the mud being used.

The Result of Evaluation
We use DC & DXC exponent, mud temperature, total gas, sigma and other ways to monitor real time data, detect and analyze the changes of formation pressure, and predict any abnormal formation pressure. We use shale density, shale factor, LWD and wireline data to synthetically evaluate formation pressure and output the result of reports, data sheets and logs. We can also use the result for future well designing.

The logs mainly include the following:

• Overburden Pressure
• Formation Pore Pressure
• Mud Hydrostatic Pressure
• Equivalent Circulating Density
• Fracture Pressure