Methods used in real-time pore pressure prediction can be subdivided into two general categories: quantitative and qualitative techniques. Quantitative pore pressure prediction techniques normally used at the rigsite work with resistivity, sonic, DXC and shale density datasets and involve the use of formulae like Eaton. Qualitative techniques work with ROP, flowline temperature, cavings, background and produced gas, and torque, drag and fill datasets. These qualitative datasets are sometimes combined to come up with new techniques. Confining the analysis to one category without the other severely reduces the chances of success at correctly predicting the pore pressure profile.
In quantitative pore pressure analysis a normal compaction trend line (NCT) has to be drawn to calculate the pore pressure. The problem with drawing normal compaction trend lines is that available literature neglects to inform the reader on how to draw the normal compaction trend lines, how many trend lines are to be drawn and at what angle are they to be drawn. Examples project the simplistic idea that in most instances only one trend line is all needs to be drawn. Experience and the preponderance of drilling events, like kicks and stuck pipe, esp. in areas where minimum data is available point to the fact that compaction history is not that simplistic at all and that there are other factors at play. This now begs the question: How do you know that the normal compaction trendline being drawn, hence the calculated pore pressure is correct?
Incorporating two case studies, this paper demonstrates that qualitative techniques could be used to constrain the normal compaction trendlines that will be drawn in the quantitative study of pore pressure and in the process increase the degree of success in determining the pore pressure regime in real time.