Interpretation of Crude Assay Information

The refining industry trades many different crude oils. Properties for crude oils vary dramatically between production fields. The geochemical history of the crude makes an enormous difference in crude oil properties. At the limit, every oil well produces crude with a different composition. While known for many years, the differences between wells in the same field were relatively minor. Modern production techniques for tight oil production may have large differences in crude oil character between adjacent wells. They may also have rapid changes in quality over time. Trading crude, predicting products, operating plants, and designing plants all require some way to characterize crude oil. The crude oil assay fills this purpose.

Different users of assays have different needs and expectations. What may be an acceptable assay for one purpose may not meet the needs of other purposes. This paper explores the different assay purposes, compares some typical crudes, and examines the meaning and significance of common assay tests.

This discussion covers what information crude assays reveal. The major emphasis is on distillation properties of the crude and how they affect the crude distillation unit products.

Assay information may be available for the entire crude or specific distillation cuts from the crude. With enough cuts, curves can be generated to allow for interpolation of properties across different possible product ranges.

The most important inspection properties of the crude include API gravity (or density) and sulfur content. The most important distillation information is the crude TBP (True boiling point) curve. This gives a breakdown of distillation versus boiling point for the crude.

Cuts on specific distillation range may have a variety of properties. The properties measured will vary depending upon the cut. Assays with narrower cuts improve estimates of product properties. Many crude oil properties blend in complex ways. A too-wide cut can lead to significant miss-prediction of product properties.

Understanding the crude assay and how it affects refinery economics and operation is critical to a profitable refinery.