Geochemical Studies - 2: Temporal Changes in Crude Oil Quality from Individual Wells.

Although it is widely believed that crude oil quality, as measured by density and sulphur content, does not vary with time from an individual well, there is no published information, at least for Alberta, to support this belief. Since the advent of block price scheduling for special old oil under the NEP, possible temporal changes of crude oil quality from an individual well have become of economic importance. The files of the ERCB and the author were searched for all examples of multiple analyses from individual wells for all scheduled fields and pools.

The resulting ninety nine data sets represent a full range of crude oil quality, areal distribution, size of reserve and discovery date, for sampling periods of up to 23 years. They were examined for temporal changes in crude oil quality with respect to several factors which might cause variations in quality. These included original variations within the pool; sampling conditions, analytical techniques and a wide spectrum of engineering recovery methods, with special emphasis on water flooding and the inadvertent injection of deleterious bacteria which might induce biodegradation and the production of hydrogen sulphide.

Subjective interpretation of the data sets shows that for more than three quarters of the sets, there is no change, effectively no change, or no significant change of crude oil quality with time. The balance were either indeterminate or the changes observed could be attributed to variations in sampling conditions or analytical techniques. Subject to a database better representative of the more than four hundred approved oil conservation projects in Alberta, it is suggested that the injection of water during the operation of an oil conservation project does not appear to affect crude oil quality, in any gross sense.