A very large proportion of reported historical laboratory PVT data is not representative of the actual formation fluids. This study underscores the necessity of performing in depth QC on reported studies. Further, it becomes apparent that performing step wise QC on current studies by an expert is essential to ensuring accurate fluid properties and compositions are collected.

Some years ago, I was asked to look at a suite of PVT data which had been collected over some fifty years on an undersaturated oil reservoir. There were significant variations in bubble point pressures both aerially and vertically. These differences were not consistent with the geological model. The primary objective was to determine whether the differences in bubble point pressure were correct; if so, then it would have a major positive impact on initial oil in place.

It became evident that all the samples exhibiting lower bubble point pressures had been collected during open hole logging of the well. Further investigation revealed that oil-based drilling fluid had been used to drill through the reservoir in all the sampled wells; the main component of the base oil was diesel.

A fundamental part of our QC procedures is to examine the fluid compositions in detail. This entails splitting the compositions into several different ranges; this helps to highlight any anomalies. Once this was done, it was evident that all the open hole samples were contaminated with diesel. In one extreme case, the laboratory had analysed for contamination and reported zero percent; our examination of the renormalised compositional range showed thirty percent.

All the open hole samples were mathematically decontaminated and EOS calculations were used to correct the reported data so that it represented the initial in place fluid. These calculations largely removed the variations in bubble point pressure.

The client decided to validate the above by collecting representative single-phase samples from a producing well and performing some basic PVT measurements including fluid compositions and bubble points. The analysing laboratory then added several known increments of diesel to the fluid and repeated the measurements. The results successfully confirmed the above diesel contamination of the open-hole samples, the mathematical corrections and the EOS predictions used in the decontamination process.