Cementation Exponent, m. Function and Calibration
The objective of m is to convert the log evaluation’s Ø, with a*Rw, into the log evaluation’s estimate of Ro (water saturated formation resistivity), not the core analysis Ro or the “true” Ro! The laboratory is an inadequate analogy of in-situ conditions – witness the 100’s of learned papers telling us so, the latest tranche informing us that electrical anisotropy further invalidates our core data a and m’s. With certain water zones (Sw100 zones from side-walls, cuttings and chromatograph), spanning a range of Ø in rocks of the same pore-type (similar Ø-k xplots) we have the actual log data’s value for Ro and m – the actual data we need. Why do we choose to ignore Nature’s own reservoir conditions m laboratory and worry instead about reservoir to laboratory inequalities? This particular faith in core data is misconceived.. m must predict the log data’s Ro not the lab data’s Ro which is often irrelevant and misleading. To repeat.. the lab data’s Ro can be misleading. Think for yourself – consider the objective and focus on the data that will actually be used in the evaluation.
Saturation Exponent, n. Function and Calibration
The objective of n is to convert the log evaluation’s value of Ro/Rt into the actual reservoir Sw. The log evaluations metre scale estimate of Rt/Ro departs from the lab’s cm scale measurement of Rt/Ro as heterogeneity increases. The value of n is determined by the n definition plot, commonly seen in core analysis reports, such that n is the negative slope of Log(Sw) vs. Log(Rt/Ro). This plot is conventionally populated by core data, with its numerous shortcomings, notably that Rt/Ro is not measured at the scale of the log data’s Rt/Ro which will actually be used in the evaluation. This n definition plot can be used more effectively if we .. consider the objective and focus on the data that will actually be used in the evaluation.