Effective stress laws and their application are not new, but are often overlooked or mis-applied. As we observe in deepwater GoM Lower Tertiary (LT) sands, the effective stress coefficient deviates significantly from one and can be quite different for different rock properties of interest.
As petrophysicists, log and core analysts, we need to be aware that pore pressure can have a significant impact on reservoir properties in ways not easily described using a simple effective stress law. These effects must be taken into account when applying measurements made in the laboratory for calibration of reservoir engineering models and calculation of rock mechanical properties from acoustic logs.
An effective stress law is a means to convert two variables, external stress (σ) and pore pressure (Pp), into one equivalent variable (σ effective). One such expression would be σeffective = σ- a Pp, where a is the "effective stress coefficient".
Every rock property; e.g. permeability, compressibility, and acoustic velocities, has its own effective stress coefficient. This coefficient is found to be less than 1.0 for many rock properties, is commonly thought to be 1.0 for strength and static elastic constants, and can be greater than 1.0 for permeability. Typically, the magnitude of the effective stress coefficient is dependent on the stiffness of the rock. Rocks having higher bulk compressibility tend to be characterized by effective stress coefficients closer to a value of 1.0 for a wider range of properties. However, when one combines stiff rock with very high pore pressures, as we observe in LT sands, the effective stress coefficients for many properties can be quite different from 1.0, and can be variable for different properties of interest. The effective stress coefficient for bulk volume compressibility, the Biot Coefficient a, is probably the most recognized. Often we find that it is substituted for the effective stress coefficient in effective stress laws for other rock properties such as S and P velocities. We will show that in many cases this may not be appropriate, and can lead to erroneous estimates of insitu stress, pore pressure, wellbore stability, and permeability
The intent of this paper is to draw attention to the potential impact of very high pore pressures on petrophysical measurements and interpretations. For the Lower Tertiary Paleogene play in the ultra deep water (water depths > 5,000 feet) Gulf of Mexico, the wells are drilled to depths in excess of 25,000 feet (7620m). Some wells have set near depth records of about 33,000ft (TVD). Pore pressures can exceed 20,000 psia (138MPa). These wells are very expensive to drill and complete, with costs up to about $100 million per well. Production scenarios require large pore pressure depletions. Thus it is very important to quantify rock strength, pore compressibility, and changes in permeability during large pore pressure depletions.
The very high pore pressures in the Lower Tertiary Paleogene of the ultra deep water Gulf of Mexico cause unique challenges for rock and fluid property measurements.
