Quick estimation of product yields from downhole gas sample

Abstract

The estimation of product yields from gas and gas condensate reservoirs is often the subject of multi-million-dollar studies, requiring gas and condensate samples from production tests, lab analysis of the samples, and complex process engineering models. An accurate estimation of sales product yields can, however, be determined simply from the composition of a reservoir sample and a basic material balance calculation.

Sales gas, LNG, and condensate have fairly consistent specifications across the world, based on various properties such as heating value, vapour pressure, etc. This consistency allows the determination of product yields from simple material balance calculations and properties of the individual components found in the reservoir gas. In this case, material balance simply means the allocation of each component to a particular product stream.

The lighter components, C1 through C4 (methane through butane), comprise the LNG and/or sales gas product, and all C5+ (pentane and heavier) components make up the condensate product. The yields can then calculated for each unit of reservoir gas, for example MJ/scm, BTU/scf, bbl/MMscf, etc. Inerts such as CO₂ and N₂ have no heating value and are not included in the yield calculation.

Likewise, contaminants such as H₂S must be removed from the product stream and are not included in the yield. In actual practice, a perfect separation of the individual components is not achieved—that is, the condensate product will contain small amounts of C3 and C4, but experience has shown that the simple method described above gives an accurate estimattion of product yields from a simple gas analysis.