The PSI software was developed by E. Fjær in the framework of joint industrial projects (JIP) (Fjær et al., 2008), (Fjær et al., 2002).
The PSI software takes into account both the mechanical, chemical and thermal environment the shale is exposed to under drilling conditions (Nes et al., 2012). It returns as output depth- and time dependent mud weight windows (Fig. 3), indicating the range of well pressure where safe drilling can be performed, with low probability of experiencing well wall collapse or fracturing of the shale formation (the two limits indicated by the stability window).
Rock mechanical tests, run in various projects at SINTEF since the 1980’s, have resulted in proprietary correlations between many parameters, allowing one to obtain for example the compressive strength of a shale type as a function of its porosity, the deposition depth or other parameters such as water content. The correlation database has been continually updated as soon as new tests on received field cores were carried out.
As new logs from the currently drilled well become available, the PSI input parameters are updated; in particular, an important update source is the pore pressure prediction obtained by running the SINTEF PRESSIM software prior to drilling and updated with resistivity log readings. Using the pore pressure prediction for the whole planned well path together with the mean difference between old and new input values, a forward looking prediction can be made for the remaining drill path.